In this activity, students explore how high a ping pong ball bounces when dropped by itself and then with a golf ball. You can use it to show how an energy analysis allows us to put limits on possible outcomes.

Equipment

Each group of students will need:

• 30 cm ruler
• Golf ball
• Ping pong ball
• Bench/table to bounce off
• Sticky tape
• A4 sheet of clear plastic (eg document wallet)
• Access to a mass balance (capable of measuring to nearest g or better)

Procedure

Ask students to:

1. Roll the clear plastic A4 sheet into a tube with a diameter slightly wider than the golf ball. Measure the length of the tube (this should be 30 cm).
2. Use sticky tape to hold the tube in shape and stand it upright on a bench or table. Ask an assistant to gently grip the bottom of the tube (or use a clamp stand at the top to keep it upright).
3. Hold the ping pong ball so that the bottom of the ball is at the top of the tube. Let go. Measure the height the ping pong ball bounces to.
4. Repeat for the golf ball.
5. Measure masses of golf ball and ping pong ball.
6. Hold the ping pong ball directly above the golf ball and drop the two together so that the ping pong ball bounces straight up. Measure the height the golf ball reaches.

Discussion prompts

• What percentage of its original height did the ping pong ball reach when it bounces off the bench?
• How high would it bounce if the bounce efficiency was 100%?
• What is the maximum possible height the ping pong ball can reach in the two-ball drop?

Teaching notes

The start and end points of an energy analysis for the stacked ball drop are shown below.

The length of the tube is 𝐿 and, for the ping pong ball and golf ball respectively, the rebound heights are 𝐻 and ℎ and masses are 𝑀 and 𝑚. For a perfectly elastic collision, we can say that the energy stored gravitationally before the drop would be the same as the energy stored gravitationally afterwards. Therefore:

(𝑀+𝑚)𝑔𝐿 ≥ 𝑚𝑔ℎ + 𝑀𝑔𝐻

And so the height of the ping pong ball can be predicted using:

ℎ ≤ 𝐿 + 𝑀𝑚 (𝐿−𝐻)

Substituting in experimental values should give a maximum value for ℎ of up to a few metres. The actual height will be lower as the real bounce efficiency will be less than 100%.

Learning outcome

Students use an energy analysis to put an upper limit on the height an object can reach after a collision.

Home learning

For a version of this activity for younger pupils to try at home, see Do Try This at Home: episode 13

This experiment was safety-checked in March 2020.

In this activity, students explore how an arrow can look bigger and reversed through a glass of water.

Learning outcome

Students use the terms object, image, magnified, inverted and diminished when describing images formed by a converging lens.

Equipment

Each group of students will need:

• A clear, straight-sided glass or beaker
• A4 white paper or card
• A felt tip pen and ruler
• A jug or bottle of water for pouring

Procedure

Ask students to:

1. Draw two short identical arrows on the A4 paper. They should be of a length equal to about a third of the diameter of the glass and pointing the same way, one above the other.
2. Stand the paper upright – lean it against a book or wall if necessary.
3. Place the empty glass/beaker so that it is touching the paper.
4. Partly fill the glass so that one of the arrows is visible through water in the glass and the other can be seen through the air above the water.
5. Gradually move the glass away from the paper.

Discussion prompts

• How is what you see different from what you drew on the paper?
• How does it change as you move the glass away from the paper?

Teaching notes

Introduce the terms below to help students describe what they see.

To start, they will see a magnified image that is the same way around as the object. As they increase distance the image will becomes ‘left-right reversed’ – a bit like the image they see of themselves when they look in a mirror. As they move the glass away from the paper the inverted image will initially be a magnified one, then become the same size as the object, before becoming a diminished image.

Extension

Challenge students to film the reversing arrow trick. They will need to position their glass of water so that inverted image is the same size as the object.

This experiment was safety-checked in March 2020.

In this demonstration students predict which kettle boils first. You can use it to illustrate that power is the rate at which energy is transferred and to introduce the relationship P = IV.

Equipment

• 3 kW electric kettle
• Small low-power electric kettle (eg 1 kW travel kettle of type found in hotel rooms)
• 2 plug-in energy meters with voltage, current and power functions
• 500 ml measuring jug or beaker
• Thermometer (optional)

Preparation

A 3 kW kettle draws a large current. Don’t use an extension lead. Plug each kettle into its own wall outlet and check that the RCD circuits in your lab do not trip when you switch on both at the same time.

The energy functions on your meters are likely to be calibrated in kilowatt-hours. They are not needed for this activity. If you do decide to use them remember to reset meters to zero before you start and explain how to convert to joules (1 kWh = 3 600 000 J).

Procedure

1. Plug the large kettle into a power meter and into the mains supply. Do the same for the small kettle.
2. Set both meters to the voltage function and note values on board. They should see that both kettles give the same reading (240 V).
3. Pour 0.5 litre of water into each kettle. Measure water temperature (optional).
4. Switch on both kettles at the same time.
5. Switch both meters to the current function and note values on board.
6. Switch to the power settings and leave both kettles switched on.

Discussion prompts

• Which kettle will boil first?
• The kettles are plugged into the same mains supply. Which kettle transfers energy to the water more quickly?
• Can you see a relationship between power, current and voltage?

Teaching notes

Students may think that the smaller kettle will come to the boil first because there is less casing material to warm. Remind them that inside there is a large mass of water. It is not possible to work out which kettle will boil first from its appearance alone.

The mass of water poured into each kettle is the same. The starting temperature for the water is also the same and so is the end temperature because both kettles switch off automatically when the water reaches 100°C. The energy required to raise the temperature of the water in each kettle is equal.

The power reading in watts is the energy in joules transferred electrically in one second by the heating element circuit. For the small kettle the rate is about 1,000 joules per second. For large the kettle it is close to 3,000 joules per second.

The current readings reveal the reason that the larger kettle will warm up more quickly. The current is three times as great and so energy is transferred at three times the rate. We’d expect the kettle with a 3 kW heating element to boil about three times as fast as one with a 1 kW element.

Explain that the electrical power depends on both voltage (ie energy transferred per charge) and current. Multiply current (I) and voltage (V) to introduce the relationship for electrical power (P). Show that P = IV for both kettles.

If students ask why the larger kettle has a higher current, calculate resistances by dividing voltage by current. The large kettle’s heating element has a lower resistance and so for the same ‘push’ (voltage) from the mains the current inside it will be larger.

Learning outcome

Students define power as the rate at which energy is transferred and can use the relationship P = IV to calculate power for an electrical appliance.

This experiment was safety-checked in March 2020.

This activity allows students to investigate up to three bulbs without them having to rebuild their circuit. You can use it to test their understanding of current in parallel circuits.

Equipment

Each group of students will need:

• Low voltage power supply (e.g. 1.5 V electrical cell)
• Three identical bulbs
• Two ammeters
• Eight 4 mm leads
• Glass of drinking water and straws (optional)

Preparation

Before the start of the lesson, set up one circuit as an exemplar so students can refer to it if required.

Drinking water through straws is a useful analogy for parallel circuits, but students should not eat or drink in labs. Only provide straws if you carry out the activity in a classroom.

Procedure

Ask the students to:

1. Set up the circuit below. To start only one bulb should be lit.

2. Record the readings on both ammeters (A1 and A2).
3. Predict what will happen when they connect lead Y.
4. Connect lead Y and record observations.
5. Repeat for lead Z.

Discussion prompts

• Why must the ammeter readings always be the same?
• Why do they go up when you connect leads Y and Z?

Teaching notes

Most students should be able explain why the readings on the ammeters are the same. The meters show the rate at which charges flow in and out of the cell. The two values must be the same because charge isn’t used up in a circuit.

For students that struggle to explain why readings increase when they connect leads Y and Z, a useful analogy is drinking water through straws. Adding more bulbs in parallel is like increasing the number of straws: the overall rate of flow increases because there are more parallel paths along which the flow can happen.

Learning outcome

Students predict and explain how the current will change when two or more bulbs are connected in parallel.

This experiment was safety-checked in March 2020.

This activity shows that a loud sound is capable of making small grains jump. You can use it to introduce the idea that sound is a vibration of the air.

Equipment

Each group of students will need:

• Bowl
• Cling film
• Hundreds and thousands sprinkles of the type used for cake decorations
• Large spoon or drumstick
• Metal baking tray, drum or similar to hit to make a loud noise

Procedure

Ask the students to:

1. Cover the top of the bowl with cling film. Stretch it tightly.
2. Scatter some of the hundreds and thousands sprinkles on the cling film.
3. Hold the baking tray close to – but not touching - the cling film and strike it sharply with the spoon.

Discussion prompts

• Why does the baking tray make a sound?
• How do the sprinkles move when they haven’t been touched by anything?

Teaching notes

Students will probably have heard of a ‘sound wave’ but, based on everyday experience (e.g. shouting or whistling), think it involves air travelling en masse from source to detector. In this activity there is no obvious source of moving air. Identify the source, medium and detector in your explanation and introduce the idea that a sound is a vibration of the air.

The baking tray is a sound source because it vibrates when it’s struck. The vibrations are transmitted through the air (the medium) to the bowl and cling film (the detector). The incoming sound wave makes the surface of the cling film move up and down and the sprinkles on its surface dance in response.

Learning outcome

Students describe sound waves as vibrations of the air, initiated by the vibrating source of the sound.

This experiment was safety-checked in March 2020.

In this demonstration students are introduced to a mechanical model of how sound displays on an oscilloscope.

Learning outcome

Students can describe what an oscilloscope shows when displaying a sound wave and determine the time period from the trace.

Equipment

• Slinky spring
• Metre rule
• Rubber band
• Clamp stand
• Felt tip pen
• Sticky tape
• A few sheet of A3 squared or graph paper (eg made by sticking two A4 sheets together)
• Stopwatch (optional)

Preparation

Build and test your slink-o-scope before the lesson. For instructions, watch the video above.

Procedure

1. Ask for a volunteer. They will be in your assistant in charge of holding the paper under the pen.
2. Holding one end of the slinky in place move the other end back and forth to generate longitudinal waves. The pen will move on the paper.
3. Show the resulting graph to the class - they should see that that it is close to a straight-line.
4. Place a new sheet of paper under the pen.
5. Ask for another volunteer. They will be in charge of timing (they can use a stopwatch or count - eg “one thousand, two thousand..”).
6. Send longitudinal waves down the slinky again. Ask the timing volunteer to shout “start” so that your assistant can start moving the paper at a steady speed in a straight line towards the clamp stand.
7. Ask the assistant to stop when they near the end of the paper and to shout “stop”.
8. Display the resulting trace to the class – they should see that it is close to a sine shaped curve.

Discussion prompts

• What labels should I use for the graph axes?
• What does the distance between two peaks show?

Teaching notes

Students may think that the distance between two peaks represents the wavelength. Encourage them to think about what caused the motion of the pen across the paper. In the vertical direction it was driven by the motion of a slinky coil, in the horizontal your assistant pulled the paper at steady speed. It’s a displacement-time graph. The distance between two peaks represents the time period T.

Discuss how to find T by averaging over a number of peaks. For example, the graph in the video above took 6s to plot and has 12 peaks. So T = 6/12 = 0.5 s.

Introduce an oscilloscope as the electronic equivalent of a slink-o-scope.

Extension

Model increasing an oscilloscope’s time-base setting by increasing the speed of the paper. Model increasing its vertical sensitivity by increasing the distance between pen and pivot.

This experiment was safety-checked in March 2020.

In this demonstration students observe oil floating on water and ice floating on oil. You can use it to test understanding of density.

Equipment

• Two 50 ml measuring cylinders
• 250ml beaker
• Two electronic balances (capable of measuring to nearest g or better)
• 400 ml vegetable oil
• Water in a glass or clear plastic jug (at least 100 ml)
• Blue food colouring
• Ice cube tray
• Tissues (to mop up any spillages)

Preparation

The night before the activity prepare blue ice cubes by adding a few drops of food colouring to water in an ice-cube tray and freezing. The blue food dye will make it easier for the students to distinguish the ice and resulting meltwater from the oil.

When carrying out the activity avoid getting oil on the bench or floor where it may cause a slipping hazard. Afterwards, dispose of the oil in the non-recycling waste by putting an absorbent material (e.g. newspaper or cat litter) into a strong bin bag and pouring the top layer of oil from the cylinders and beaker into the bag. The remaining coloured water can be washed down the sink with the tap running.

Procedure

1. Add a few drops of blue colouring into the water in the jug.
2. Pour 40 ml of the blue water into a measuring cylinder and add 10 ml of oil.
3. Pour 40 ml of oil into the other measuring cylinder and add 10 ml of blue water.
4. Put the beaker on a balance and zero it.
5. Pour in the remainder of the oil into the beaker and add the ice cube.

Discussion prompts

• The total volumes of liquid in each cylinder are the same. Are the masses?
• When the ice in the beaker melts what will happen?

Teaching notes

Students may talk about ice and water being lighter or heavier than oil. Encourage them to think in terms of the density of these materials. Some may think that the mass or volume of an individual ice cube is important, show them this isn’t the case by floating both large and small ones in the beaker.

In both measuring cylinders the water settles at the bottom because it has a higher density than oil. The oil-water mixtures have the same volume, but the one with a greater percentage of water will have the larger mass because water contributes more mass per volume than oil. Confirm this by putting each measuring cylinder in turn on a balance (the mass difference should be about 3g).

The ice floats on the oil because it’s less dense than oil. When it melts, it turns to water and so we would expect it to sink. If they look at the beaker, they can confirm this. Blue water droplets are detaching from the bottom of the ice cube, dropping through the oil and collecting at the bottom of the beaker (if the ice cubes haven’t started melting use a ruler to submerge them to speed up the process).

During the change of state the mass doesn’t change (the reading on the balance under the beaker remains constant). The increase in density must be due to a decrease in volume. The molecules must pack more closely together when the ice melts (water is very unusual in this regard as most solid substances are denser than when they’re liquid).

Learning outcome

Students describe density changes during a change of state in terms of a rearrangement of the molecules.

This experiment was safety-checked in March 2020.

In this activity students observe a crate being lifted by two different methods. You can use it to introduce horizontal and vertical force components.

Equipment

• Crate filled with books (or other objects) to provide a total mass of approx. 2.5 kg
• Two lengths of rope each about 2 m long

Preparation & safety

Before the activity attach a length of rope to each end of the crate. Ensure that the ropes are tied securely and that the crate doesn’t tip over when lifted. During the demonstration discourage volunteers from trying to impress their classmates by pulling on the ropes with exaggerated force.

Procedure

1. Tuck the ropes into the crate and use your hands to lift it off the floor or bench.
2. Hold crate stationary with your arms straight and then put it down.
3. Ask for two volunteers to pull on the ropes to lift the crate and hold it stationary above the ground with the ropes at angles of about 45°.
4. Now challenge them to try to pull firmly on the ropes until they are horizontal.

Discussion prompts

• Which forces act on the crate?
• Are the forces balanced?
• Why is it impossible to get the ropes horizontal?

Teaching notes

Most students should be able to identify forces acting on the crate lifted by hand and explain why they balance. The forces are vertical. Each hand provides half the upward force required to balance the pull of gravity.

For the crate lifted by ropes some may struggle with the direction of the lifting forces. Explain how they arise. When the crate is lifted off the ground, the ropes stretch slightly, exerting forces along their length and at an angle of 45°.

To explain how the forces balance, introduce force components. They can think of each force as being made up of two parts one sideways and one upwards known as the horizontal and vertical components. In the horizontal direction the components are of equal size but in opposite directions and so cancel each other out. Similarly, gravity is balanced by the vertical components.

No matter how hard your students pull, it’s impossible to get the ropes completely horizontal because you always need a vertical component to balance gravity.

Learning outcome

Students explain equilibrium situations in terms of vertical and horizontal force components.

This experiment was safety-checked in March 2020.

In this activity students are introduced to the idea of the centre of gravity by comparing the balance points of a ruler and a wooden spoon.

Equipment

Each group of students will need:

• Wooden spoon
• 30 cm ruler
• Electronic balance (capable of measuring to nearest g or better)

You will also need:

• A saw
• Masking tape

Preparation & safety

Choose spoons with cylindrical handles and oval-shaped bowls and check that they balance at a point on their handles. Cut them through their balance points and connect the handle and bowl back together using masking tape.

Procedure

Introduce the term ‘centre of gravity’ as the point around which the weight of an object is evenly distributed and the point at which an object will balance.

Ask students to:work in pairs to:

1. Use their outstretched finger as a pivot to find the balance point of the ruler.
2. Repeat for the spoon.
3. Remove the masking tape and balance each part of the spoon separately.
4. Measure distances from balance positions to the cut edge and calculate the mass x distance for both handle and bowl.
5. Draw force diagrams to show where the gravitational forces act on ruler and spoon.

Discussion prompts

• Why does the ruler balance at its midpoint, but not the spoon?
• How does the mass left and right of the pivot compare?
• Where should I draw gravity force arrows on a diagram?

Teaching notes

Students will accept that the centre of gravity for the ruler is at its midpoint because it has a uniform shape. The mass left and right of the pivot is equal and so gravity pulls downs on each side with equal effect. Their measurements for handle and bowl should illustrate the more general case for an irregularly shaped object: it is the mass x distance from the pivot that must be equal for an object to balance.

They may suggest drawing one, two or many arrows to represent the pull of gravity on a spoon (or ruler). All options are correct. It depends on many they view the object: a single object, two sections or many stuck together. But whichever they choose, they must always start their arrow(s) at the centre of gravity position(s).

Learning outcome

Students can explain why the centre of gravity of an irregular object is not half way along its length.

This experiment was safety-checked in March 2020.

In this demonstration, students see that objects with a lower centre of gravity are more stable.

Learning outcome

Students can relate the stability of an object to the position of its centre of gravity relative to its base.

Equipment

• 3 identical clear plastic bottles with caps - bottles with flat bases are best as their is less ambiguity about the area of the bottle in contact with the board.
• Water in a jug or large beaker - enough to fill two bottles
• Food colouring (so students can see the mass distribution in the bottles more easily)
• Strong cardboard or wooden board
• Pencil
• Sticky tape

Procedure

1. Add a few drops of food colouring to the water
2. Fill one bottle with coloured water to the very top.
3. Half-fill another bottle (leaving the last one empty).
4. Using sticky tape, secure the pencil to one end of the board to make a lip.
5. Place the three bottles in a row in increasing mass order along the board next to the lip.
6. Raise the other side of the board slowly.

Discussion prompts

• What force makes a bottle fall over?
• Can you predict the order in which the bottles will topple?

Teaching notes

Students may be surprised that the bottles with the least and most mass topple at the same time. Encourage them to think about the how the mass is distributed in the bottles.

If students are unfamiliar with the terms 'the centre of gravity' and 'stable object' introduce them.

• The centre of gravity is the point at which we can consider the gravity force to act. A sort of average position for the mass in an object.
• A stable object is one that returns to its original position when disturbed.

Explanation

In the full and empty bottles the mass is distributed evenly and so the centre of gravity is at half way up the bottle. The half full bottle is different because it has an uneven distribution of mass and a lower centre of gravity.

When the bottles are upright, the gravitational force acts downwards through the base of the bottle (the area that is in contact with the board). As the bottle is tilted, it will remain in contact with the board until the line of action of the gravitational force falls outside the base -at which point it will topple over and so will no longer be stable. The full and empty bottles topple first because they have a higher centre of gravity and so reach their tipping points first; the half full bottle is more stable because it has a low centre of gravity.

This experiment was safety-checked in March 2020.

This class activity allows students to investigate circuits with up to three bulbs without having to take their circuit apart. You can use it to test their understanding of current in series circuits.

Equipment

Each group of students will need:

• Low voltage power supply (e.g. 1.5 V electrical cell)
• Three identical bulbs
• Two ammeters
• Eight 4 mm leads

Preparation

Before the start of the lesson, set up one circuit as an exemplar so that students can refer to it if required.

During the activity students will need to bypass some of the bulbs by connecting leads around them. To avoid damaging the ammeters, ensure they don’t short circuit all the bulbs. There must be at least one bulb in the circuit to avoid the current becoming too high.

Procedure

Ask students to:

1. Set up the circuit below. To start, they should connect leads around two of the bulbs so that only the first bulb is lit.

2. Record the readings on both ammeters (A1 and A2).
3. Predict what will happen when they remove lead Y.
4. Disconnect lead Y and record observations.
5. Repeat for lead Z.

Discussion prompts

• Why must the ammeters readings always be the same?
• Why do they go down when you disconnect the leads?

Teaching notes

Students may not understand how leads Y and Z allow them to change the number of bulbs in the circuit. Explain this in terms of the current taking the path of least resistance. The leads have a much lower resistance than the other components. Connecting a lead around a bulb means (almost) all the current will go through the lead, not the bulb.

There is only one loop in a series circuit and so an ammeter placed anywhere in the circuit will read the same. Disconnecting a lead adds another bulb in series, increasing the circuit’s overall resistance and so reducing the current throughout.

Learning outcome

Students predict and explain what will happen to the current when another light bulb is added in series.

This experiment was safety-checked in March 2020.

This demonstration shows that the forces of attraction between two magnetically interacting objects are equal and opposite. You can use it as an example of Newton’s third law.

Equipment

• Two identical small, strong neodymium magnets with holes in – the diameter of the hole should be large enough to thread string through (e.g. 2.5 mm)
• Three 30 cm lengths of string
• Two clamps and stands
• Small bulldog clip
• Two G-clamps(optional)

Preparation & safety

Rare-earth magnets are brittle and shatter easily. Don’t drill a hole into an existing magnet. Source neodymium magnets with pre-made holes or make a harness out of string or wire. When handling or moving magnets towards each other ensure that they don’t collide.

Before the activity, thread strings through each of the magnets and secure with a knot. Also tie one end of a string to the bulldog clip. Suspend the two magnets from a clamp stand so they hang around 10 cm below where the string is tied.

Setting the distance between the magnets and magnet and clip can take a bit of practice. Try it out beforehand. Mark positions on the bench and/or secure stands with G clamps to allow a quicker set-up next time.

Procedure

1. Attach one magnet by its string to a clamp and secure the string tightly so that the magnet hangs around 10 cm below where the string is tied.
2. Hang a second magnet from the second clamp. Arrange the stands so that the two magnets are close and attracting each other strongly with their strings almost horizontal.
3. Repeat step 2 but replace one of the magnets with the bulldog clip.

Discussion prompts

• What causes the magnetic force on the left magnet? What about the right magnet?
• What causes the forces on the magnet and clip?
• How do the size and direction of the forces compare?

Teaching notes

Students will be aware that magnets can attract each other and so will accept that two identical magnets pull equally on each other. The force on the left magnet is due to the right magnet; the one on the right is due to the left.

They may be surprised to see the same effect with the magnet and clip. These need to be closer to produce the same size forces but as previously the size of the forces are equal in size and opposite in direction. Like all interactions, magnetic interactions create Newton’s third law force pairs.

Learning outcome

Students identify Newton’s third law force pairs for objects that interact magnetically.

This experiment was safety-checked in March 2020.

In this class activity, students see that after it’s rubbed against your clothes a balloon will attract a drinks can and make it roll. You can use it to introduce why charged objects exert forces on uncharged objects.

Equipment

Each student will need:

• Empty aluminium soft drink can
• Rubber balloon
• Cloth or woollen clothing

Procedure

Ask students to:

1. Inflate the balloon and tie its neck.
2. Place the empty can on its side on a flat surface.
3. Hold the balloon close to the can. They should see that nothing happens because the balloon is initially uncharged.
4. Rub the balloon on their clothing or a piece of cloth so that it becomes charged.
5. Bring the balloon close to the can. They should see the can start to move towards the balloon.
6. Move the balloon gradually away from the can so that the can rolls along.

Discussion prompts

• After it’s been rubbed, the balloon attracts the can. Have you seen this sort of thing before?
• How can you tell that the forces on the aluminium can are unbalanced?
• How do you think the balloon creates a force on the can?

Teaching notes

Charged objects attracting other objects may be familiar from, for example, a comb attracting hair. You could rub the balloon and show that it also attracts a student’s hair. To help them visualise charging processes, introduce electrons as negatively charged particles that move between the materials.

The balloon becomes charged when it’s rubbed because it’s made of a material that attracts electrons more strongly than the cloth. Electrons are transferred from the cloth to the balloon and so the balloon gains a negative charge overall. Explaining that the cloth is left with a positive charge will help students appreciate that charge is conserved, but there is no need to discuss atomic structure or the nature of the positive charge in the objects.

The charging process for the aluminium can is different. The two objects do not come into contact. Instead, electrons in the can are repelled by the balloon and so move to the part of the can furthest away. The back of the can becomes negatively charged and the front positive, but overall the can remains electrically neutral. The reason the aluminium can starts rolling is because the back of the can is further away and so the repulsive force on the back of the can is smaller than the attractive force on the front.

If students use the phrase ‘static electricity’, explain that it can be a misleading one. The charging process for the balloon involves the transfer of charge between cloth and balloon, and the process for the aluminium can involves charges moving within the can. The charging processes may be different, but in neither are the charges ‘static’.

Learning outcome

Students describe how an object made of an insulating material becomes charged when we rub it and also why it then attracts other objects.

This experiment was safety-checked in March 2020.

In this activity students shake sweets to model the radioactive decay of a large number of unstable nuclei. You can use it to introduce decay curves.

Preparation & safety

Sweets or chocolates provide a colourful analogy for radioactivate decay, but there should be no eating or drinking in labs. Consumption will also skew results. If you think the temptation to eat sweets might be too great for your students you may want to consider alternatives such as coins or small dice. Whatever you choose source a large number.

Equipment

Each group of students will need:

• Enough sweets or chocolates with a logo on one side so that each student can have four (eg 100 sweets for a class of 25 students)
• Four extra sweets for yourself
• Seven or eight measuring cylinders each with a capacity large enough to hold half the total number of sweets

Procedure

1. Line up measuring cylinders in a row and ask for a volunteer to be your assistant for counting and collecting sweets.
2. Write up the total number of sweets on the board and distribute four sweets to each student. Keep four for yourself.
3. Ask the class to hold sweets in cupped hands like you are and when you shout ‘shake’ to shake them so that the sweets move around inside their hands. You should do the same with yours.
4. After 5 seconds shout ‘stop’. They should open hands palm up, remove any sweets that are logo-side up. You should do the same.
5. Ask your assistant to collect all discarded sweets and put them into first measuring cylinder.
6. Repeat steps 3 to 5 to fill the other cylinders. Time your shouts of ‘shake’ to be at regular intervals for a better model of radioactive decay.

Discussion prompts

• How many sweets do you think are in the first cylinder? What about the second?
• For each shake, what are the chances for an individual sweet landing face up?
• Is it possible to predict when a particular sweet will land face-up?

Teaching notes

Students could ‘place bets’ by writing down predictions for sweet numbers on mini-white boards or post-it notes. Refer back to them at the end of the activity. Discuss results before removing sweets from the cylinders to do any final counts.

The chance of being face-up after a shake for a sweet is 1 in 2, or they could say there is a 50% probability. Emphasise that each shake is an independent event. What happens in one does not depend on what happened in the last or affect a future one. The probability of the single sweet landing face-up is 50% whether it is your first or last shake.

Unstable nuclei in radioactive sources behave in a similar way. The probability of a decay is fixed, but it is not possible to predict when a particular nuclide will decay. Provide an example of a decay curve for a radioactive source to show that it sweeps downwards just like the sweet simulation.

Learning outcome

Students describe a sweet/coin model for unstable nuclei and sketch a decay curve.

This experiment was safety-checked in March 2020.

In this demonstration students see a simple rocket in action. You can use it to illustrate Newton’s third law of motion.

Equipment

• Balloon
• Drinking straw
• Clothes peg or other clip
• Length of string
• Sticky tape
• Scissors

Preparation

Locate suitable fixed points in the room (eg cupboard handles) to tie the length of string to.

Procedure

1. Pass the string through the straw.
2. Attach the two ends of the string to the fixed points in the room.
3. Inflate the balloon and use the clothes peg to close the mouth.
4. Attach the balloon to the straw using sticky tape.
5. Undo the peg to release the air.

Discussion prompts

• What keeps a balloon inflated?
• Are the forces balanced or unbalanced?
• Which force causes the balloon to speed up?

Teaching notes

Students may refer to ‘action and reaction’ force pairs when describing the motion of the rocket. Emphasise that these can be misleading terms. They imply that one of the forces in Newton’s third law appears in response to the other. Discuss what’s happening inside the balloon to illustrate how the forwards force on the balloon arises at the same instant as a backwards push on air.

When the peg is attached, the balloon remains inflated because the air particles inside it are colliding with the inside surface. They push equally to the left, right, up and down and so the forces on the balloon are balanced (as are those on the air inside it).

When the peg is removed, the air particles no longer push on the open end of the balloon. The forward force on the front end of the balloon is no longer balanced by a backward force and so the balloon accelerates forwards. Similarly, if we consider the forces acting on the air in the balloon, we can see that there is a resultant force acting on it to the left, and so the air accelerates backwards.

Emphasise that, as with all Newton’s third law force pairs, the two forces that arise act on different objects (balloon and air).

Learning outcome

Students describe how an air-filled balloon propels itself and identify the Newtons third law force pairs involved.

Home learning

For a version of this activity for families and younger pupils to try at home, see Do try this at home: episode 7

This experiment was safety-checked in March 2020.

In this demonstration students see that if there is temperature difference between the bottom and top of a coloured liquid, the top surface moves. You can use it to introduce solar convection.

Equipment

• Electrical hotplate
• Flat bottomed aluminium food tray or pie tin
• 5 coins all of the same denomination
• 50 ml liquid soap (eg moisturising face wash)
• A few drops of red food colouring
• 500 ml of water
• Torch (or other white light source)

Preparation & safety

The liquid soap/shampoo will need to contain glycol stearate, glycol distearate, or glycerol stearate in order to make the convection cells visible. Moisturising products with a pearlescent appearance often contain one of these. Check ingredients on the bottle.

Be careful not to touch the hotplate when it is on. The liquid temperature should not exceed 50°C (check with a thermometer).

Procedure

1. With the hotplate off, place the five coins in a cross pattern on top of the plate.
2. Place the food tray on top of the coins.
3. Pour in cold water until the tray is half full.
4. Add 50 ml of liquid soap and a few drops of food colouring; mix well using a finger.
5. Switch the hotplate on to a low setting. Leave for a few minutes.
6. Shine a torch at an angle onto the tray to make it easier for the students to see the water rise and sink.

Discussion prompts

• Why does the surface of the water move?
• The Sun’s surface also moves. Why do you think that is?

Teaching notes

Students may talk about heat or energy rising. Emphasise that neither energy nor heat are substances. Convection is mechanical process that it is best described in terms fluids at a higher temperature expanding and floating, and then cooling and sinking. In this experiment it is driven by the hotspots created by the coins at the bottom of the tray.

The columns of rising and falling fluid are called convection cells. When we look down on the tray we see the top of the cells: the liquid appear as it rises to the surface, moves across the surface and then disappears at is sinks back below. The process is a repeating one so the water gets circulated continuously as long as there is a temperature difference between the bottom and top.

Link the demonstration to stellar convection by providing an image of the sun’s surface. The giant granules they can see are the top of very large convection cells formed by plasma rising upwards from the hot interior to the cooler surface.

Teaching notes

Students describe how convection cells are formed and why they are responsible for the grainy appearance of the Sun.

This experiment was safety-checked in March 2020.

Show that if hot water is below cold water, they mix, but if the situation is reversed they do not. Students can use their knowledge of floating and sinking to explain this. An introduction to convection.

Equipment

• 2 trays
• Red and blue food colouring
• Stirrer
• Water from hot tap (eg in a thermos flask)
• Water from the cold tap at room temperature (eg in a jug)
• Four large identical jars or bottles
• 2 pieces of card (the lids used for foil containers work well)
• A cork (optional)

Preparation & safety

Practise the demonstration in a large sink or basin before performing it in front of the class. If you use glass jars/bottles ensure that they can be set up and taken down safely without danger of breakages. Alternatively, use plastic containers.

Procedure

1. Place two jars on a tray.
2. Put a few drops of red dye in one jar and fill it with hot water. Make sure that the water reaches the open mouth of the jar.
3. Put a few drops of blue dye in another jar and fill it with cold water. Make sure that the water reaches the open mouth of the jar.
4. Place the piece of card over the mouth of the hot (red) water jar and press firmly in place. Turn this jar upside down and place it directly on top of the jar of cold (blue) water.
5. Carefully slide the card out from between the two bottles so that their mouths are in contact.
6. Repeat steps 1-5 for the other tray, but this time place the blue jar on top of the red.

Discussion prompts

• What will happen if I remove the card?
• Why do some things float and others sink?
• Why does the water not mix when the hot water is on top?

Teaching notes

Students should be familiar with the idea of objects (eg a cork) floating in water if their density is less than that of water. Extend this idea to liquids floating by explaining that water expands slightly when you warm it. The density of the cold water (998 kg m^-3 at 20°C) is a little higher than that of the hot water (988 kg m^-3 at 50°C). If they imagine a small volume of hot water surrounded by cold water it will rise up to the surface and float just like a cork would if it were submerged underwater and then released.

On the first tray the hot water is on top. There is very little mixing because the hot water is already floating. On the second tray, the hot water is below and so it rises and cold water flows downwards to replace it and the two mix.

Provide a simple diagram of the arrangement of particles in the two jars. Emphasise that the density of water decreases when you warm it because the (average) space between the particles increases. The particles themselves do not expand.

Learning outcome

Students explain why hot water rises and cold water sinks in terms of differences in the distance between particles that make up the water.

This experiment was safety-checked in March 2020.

In this activity students observe a rope loop being circulated. You can use it as a model to introduce circuits.

Equipment

• A 3 m length of rope - preferably made of nylon with speckles (alternatively draw dots on a normal rope with marker pen)
• Duct tape (optional)
• Leads, a cell and a bulb (optional)

Preparation & safety

Tie the rope into a loop, or if you are using a nylon rope melt the ends together then cover the join with duct tape.

Procedure

1. Set up or draw a diagram of a series circuit with one cell and one bulb.
2. Show the class the rope loop and explain that that the dots on the loop represent charged particles (electrons) in the circuit.
3. Ask a student volunteer to lightly grip the rope with one hand so that it can slip through easily. They will be representing the light bulb in the circuit
4. Hold the opposite end of the loop and check your volunteer isn’t gripping too tightly (to avoid rope burns)
5. Circulate the rope by pulling it at a steady rate, hand over hand. You, the teacher represent the cell, and the speed of the moving speckles/dots show the size of the current.

Discussion prompts

• Where did the 'current' start flowing first?
• How does the current entering and leaving the bulb/battery compare?
• How can I increase the current?

Teaching notes

Many students believe that electrons must travel from the cell to the bulb in order for a lighting circuit to work. Some also think that current gets used up in a circuit. Emphasise that the speckles/dots (electrons) in this model all start moving at the same time and flow in a continuous loop.

Another common misconception is that a cell will provide the same current irrespective of the circuit it is placed in. Discuss or demonstrate how the speed of the dots (current) increases if you increase the size of the push (voltage) or your volunteer decreases friction (resistance).

Learning outcome

Students describe current as a flow that happens throughout a circuit that’s size depends on voltage provided by a cell and resistance of the component(s).

This experiment was safety-checked in March 2020.

Show that only certain colours of light produce a voltage when shone onto an LED. An example of light behaving as a particle.

Equipment

• Green LED (clear type, without a coating)
• Torch (or other white light source)
• Red and green laser pointers
• Digital voltmeter, internal resistance 10 MΩ or greater
• 2 connecting leads and crocodile clips
• Black card
• Diffraction grating (optional)

Preparation & safety

Any sunlight or room light falling on the LED will produce a reading. Carry out the demonstration in a darkened room and/or shield the LED using a black cardboard tube.

The voltmeter should have a resistance of 10 MΩ or greater. Otherwise the small current produced when light is shone onto it will leak away too rapidly to give a reading.

Use only class 2 lasers from reputable suppliers. Fix them firmly in a clamp and direct them away from students towards a screen.

Procedure

1. Use a pencil to make a hole in a small piece of card, push the LED through and mount it in a clamp stand.
2. Connect a voltmeter across the LED. Use a black cardboard tube to block any ambient light so that voltmeter reads zero.
3. Mount the torch in a clamp stand and aim it directly onto the domed end of the LED. The voltmeter should show a small reading.
4. Repeat with the red laser pointer. The voltmeter should read zero.
5. Repeat with the green laser pointer. The voltmeter should once again provide a non-zero reading.

Discussion prompts

• Why do you think we get a reading with white and green light, but not red?
• Is light behaving as a wave or a particle?

Teaching notes

Students may suggest the red light does not produce a reading because the red laser isn’t bright enough. Emphasise that both lasers produce a much more intense beam than the torch. The results can’t be explained in terms of of wave amplitude. It is the frequency of the light that is important.

Discuss how a particle model of light can be used to explain the results. When a 'particle', (photon) strikes the LED it is absorbed. Each photon has an energy directly proportional to its frequency so only those with a high enough energy will release an electron. Red photons are ineffective because they have the lowest frequency and so least energy. Green photons are more energetic. White light is made up of all visible frequencies and so will contain some photons with high enough energy.

You could shine the torch through a diffraction grating onto a wall to discuss how the energy of photons varies across the spectrum.

Learning outcome

Students describe an experiment that shows light behaving as a particle.

This experiment was safety-checked in March 2020.

In this activity students see how total internal reflection makes a coin seem to vanish.

Learning outcome

Students can explain why a coin under a beaker of water is not visible when viewed through the side of the beaker.

Equipment

Each student will need:

• A small empty beaker (or drinks glass) with straight sides
• A larger jug/beaker of water
• A coin
• Paper or card
• Scissors
• Tissue or cloth to mop up spillages

Procedure

Ask students to:

1. Put the beaker upside down on the paper. Draw around it and cut out to make a lid.
2. Cut a hole in the lid so that water can be poured through it.
3. Place the coin on bench, making sure that both are completely dry.
4. Place the beaker on top of the coin and add the lid.
5. Pour water into the beaker so that it is completely full. The coin should no longer be visible.
6. Lift the lid. An image of the coin should be now be visible on the inside vertical surface of the beaker.

Discussion prompts

• What needs to happen for us to be able to see the coin?
• Where does the light go when the beaker is full?

Explanation

For the empty beaker, the light refracts but still passes through the side of the glass. For the full glass, light cannot escape because of total internal reflection and so the coin seems to vanish to anyone viewing it through the side of the beaker.

Teaching notes

This trick works because a full beaker forms a five layer air-glass-water-glass-air structure. The first layer of air is created by a gap between coin and bottom of beaker due to the ridges on the coin. If the coin is wet, it will not vanish. Students can check this for themselves by putting a few drops of water on the bottom of their beaker.

This experiment was safety-checked in March 2020.

In this activity students see that a ruler supported by two fingers remains balanced when they slide their fingers towards its centre. You can use it to introduce the co-efficient of friction.

Apparatus and Materials

• A metre rule

Procedure

Ask students to:

1. Use the index finger of each hand to support a meter rule at either end. Gently slide their fingers closer together until they meet at the midpoint – the rule should remain horizontal and balanced throughout.
2. Then, support the meter rule with one finger close to the 10 cm mark and the other finger close to the 70 cm mark. Note which one of the downward forces they feel on their fingers is larger. Then slide their fingers closer together and note which finger moves first.
3. Repeat step 2 with one finger at the 30 cm mark and the other at the 90 cm mark.

Discussion prompts

• Which finger experiences the greater contact force due to the rule?
• Which finger experiences the greater frictional force when you try to move it?
• Can you draw a force diagram for the rule? What is the direction of the resultant force?

Teaching notes

For many students it will seem counter-intuitive that the ruler remains horizontal as the fingers are moved inwards. When the two fingers are moved towards one another, first one sticks and the other slips. Then the second finger sticks and the first slips.

The finger that slips is the one further from the midpoint of the rule. You can feel that the (downward) contact force on this finger is less than the contact force on the other. This can be explained by considering moments about the midpoint of the rule. The diagram below illustrates step 2 in the procedure, force A is further from the midpoint than force B and since the rule is balanced, A must be less than B.

The finger that slips is the one further from the midpoint which indicates that this finger experiences less friction than the one that sticks. So the horizontal forces on the rule are unbalanced (F_B is greater than F_A) and the rule is pushed sideways (to the left, in the diagram).

This experiment shows that the frictional force between two objects is greater when the contact force between them is greater. This can be used to introduce the idea of the coefficient of friction µ, where

µ = frictional force / contact force

that is, the frictional force is proportional to the contact force (and depends on the nature of the surfaces in contact).

Learning outcome

Students recognise that frictional forces are proportional to the contact force and identify the co-efficient of friction as the constant of proportionality.

This experiment was safety-tested in March 2020.

In this activity students build a simple motor. You can use it to illustrate Fleming's left hand rule.

Apparatus

Each group of students will need:

• Neodymium magnet
• Screw
• Short length of cable with two bare ends
• 1.5 V cell

Preparation and safety

Rare-earth magnets are brittle and shatter easily. Students should not lift the magnet too high off the bench.

Preparation and safety

Ask student to:

1. Put the head of the screw onto magnet so that they attach to each other.
2. Put the negative terminal of cell onto sharp end of screw so that it also attaches.
3. Lift assembly off the bench by gripping the cell so that there is a small gap between bench and magnet.
4. Hold one end of cable onto the top of cell and touch the other end to edge of the magnet. The magnet and screw should start to spin.

Discussion prompts

• Which direction is the current in the magnet?
• Which direction is the force that makes it spin?
• Which direction is the magnetic field?

If students struggle to identify the direction of current, remind them it flows from the positive to the negative terminals of the cell. Inside the magnet the current is radially inwards from the edge to the centre.

To work out the direction of the force they can look at whether their magnet spins clockwise or anticlockwise. To work out the direction of the magnetic field they can use Fleming’s left hand rule. If the magnets spins anticlockwise the magnetic field is downwards, if it spins clockwise it is it upwards.

Learning outcome

Students apply Fleming’s left hand rule to determine the direction of a magnetic field.

This experiment was safety-tested in March 2020.

Students download an oscilloscope app onto their phones to investigate pitch and loudness of sounds.

Equipment

Each student will need:

• A smartphone
• Two different sized bottles with long necks (optional)

Preparation

Students will need download an oscilloscope app with a pause function.. For example, they could use Oscilloscope (xyz apps) from the Play store (tap screen to stop trace) or Oscilyzer from the App Store (use pause button to stop trace).

Procedure

Ask students to:

1. Download the oscilloscope app on to their phone.
2. Whistle and observe the trace on the screen. If they can’t whistle, blow over a bottle to make a sound.
3. Whistle a loud, steady note pause the trace. If they missed it, repeat to try to catch the waveform mid-whistle.
4. Repeat, but this time whistle more quietly.
5. Whistle with a high pitch and then a low pitch.

Discussion prompts

• How does the waveform change with loudness?
• How does the waveform change with pitch?

Teaching notes

If students can’t whistle, they can blow over bottles or use a musical instrument such as a guitar or a recorder. They should see that when the sound is louder the peaks of the waveform are bigger, and that when the pitch is higher the peaks are closer together.

Learning outcome

Students sketch waveforms for sounds with different volumes and frequencies.

In this activity students build a model universe using washers and elastic bands. You can use it to introduce Hubble’s law.

Preparation

The student worksheet below includes information on how to make a model universe. Alternatively, to save time you may want to make these for each group before the lesson.

Equipment

Each student will need:

• 6 assorted washers (or paper clips)
• 5 elastic bands of the same thickness (and ideally of different lengths)
• Small sticker to indicate ‘home’
• Ruler or tape measure
• Graph paper (or laptop with Microsoft Excel or similar)
• Sticky tape

Procedure

Ask students to:

1. Choose one washer to be the home galaxy and label it with a sticker. Label the other galaxies with letters A to E.
2. Measure the distance from the home galaxy to galaxy A. Repeat for the other galaxies.
3. Expand the universe until it is twice its original length and then tape down the ends to a table or the floor to hold it in place.
4. Measure the new distance from the home to the other galaxies.
5. Calculate the change in distance for each of the galaxies.
6. Plot a graph of “change in distance” against “initial distance” and draw a line of best fit.

Discussion prompts

• How does the change in distance depend on how far away the galaxy is?
• Does it matter which galaxy we label home?

Teaching notes

When students plot a change in distance against distance graph they should find that it is a straight line. The galaxies move away from them us at a speed that is proportional to their distance from our galaxy. This is known as Hubble’s law.

As an extension students can follow the instructions on the worksheet (below) to explore the viewpoint from other galaxies. The gradient of the graph is the same irrespective of which washer they consider to be ‘home’. Like real galaxies, the galaxies in the model seem to move away from home, but home is not the centre of the expansion.

Learning outcome

Students explain why galaxies move away from our galaxy with a speed that is proportional to their distance.

With thanks to the Perimeter Institute of Theoretical Physics for permission to adapt their activity

Use a lamp and a transparent dome attached to a globe to show how the path of the Sun across the sky varies over the year.

Preparation

This activity works best in a darkened room.

Equipment

• Approx. 40 cm diameter globe
• A small transparent dome (eg half of a 4 cm clear plastic bauble)
• Lamp
• Blu Tack or sticky tape
• Books to adjust height of lamp (optional)

Procedure

1. Use blu-tac or sticky tape to attach the dome to the globe so that it covers the UK.
2. Place the globe about 1 m from the lamp (the Sun). Adjust the lamp's height so that it is the same as the globe’s equator.
3. Position the globe so that the northern hemisphere is tilted away from the Sun.
4. Spin the globe anticlockwise about its axis so that the reflection of the lamp appears on the base of the eastern edge of the dome, travels up the dome and sets on the western edge.
5. Repeat, but this time tilt the globe's Northern Hemisphere towards the Sun (the arm of the globe may get in the way when you spin. Detach and re-attach dome as required).

Discussion prompts

• Which lasts longer: day or night?
• What season is it in the UK?

Teaching notes

This demonstration tackles the common misconception that the path of the Sun across the sky does not vary over a year. Students should see that when the northern hemisphere is tilted away from the Sun (first day of winter in the UK) sunrise to sunset takes less than half a spin, day is shorter than night and the Sun follows a low path across the sky. When the northern hemisphere is tilted towards (first day of summer in UK), the Sun follows a high path across the sky, days are longer than night and it is warmer because the sun's radiation warms the ground for more time.

You could also demonstrate the path of the Sun across the sky on the first day or spring/autumn to show that day and night lasts equal times and the Sun follows an intermediate path across the sky.

Learning outcome

Students explain why days are longer in summer and how this contributes to it being warmer.

This experiment was safety-checked in March 2020.

Students listen to how the loudness of a tuning fork varies as they rotate it. An introduction to destructive interference.

Equipment

Each pair of students will need:

• A tuning fork

Procedure

Ask students to:

1. Strike the tuning fork on a suitable surface and hold the fork upright next to their ear.
2. Repeat but rotate the fork slowly about its axis while they listen. They should hear the loudness vary.
3. Repeat but this time identify the number of times there is silence per rotation.

Discussion prompts

• How many sources of sound does a tuning fork have?
• How can two sound waves cancel each other out?

Teaching notes

The tuning fork has two identical prongs. As they vibrate, each act as a source of sound.

Sound waves from two sources can arrive in step (in phase) and constructively interfere to produce a louder sound or they can arrive out of step (out of phase) and destructively interfere. As students rotate the tuning fork, they should hear four regions of silence.

It is challenging to draw diagrams to illustrate destructive interference for a tuning fork. The distance between the two prongs (a few centimetres) is smaller than the wavelength of the sound (typically a metre or so). If you want to draw diagrams to illustrate overlapping wavefronts use an example in which the sources are separated by a distance greater than a wavelength (eg two loudspeakers).

Students are likely to be familiar with noise-cancelling headphones. They could research how these work. Like the tuning forks they use destructive interference to cancel sounds. The headphones include a microphone which receives sound waves from the environment and an electronic circuit generates an inverted version of these sound waves so that when this is played into the listener’s ears, the two sets of waves cancel out.

Learning outcome

Students describe how sound from two sources can cancel out through destructive interference.

This experiment was safety-checked in March 2020.

A simple demonstration to introduce the idea of a pendulum using everyday objects.

Equipment

• Students‘ bags
• Timer with large display
• Simple pendulum made of string with a metal bob at one end (optional)

Preparation

When selecting students’ bags look for those which have a loop at the top for holding, or which have a long strap. You will need a varied selection of bags to give a range of periods of oscillation when they swing.

Procedure

1. Select 3 or 4 students’ bags from the class . Choose one bag and hang it from a finger.
2. Pull the bag to one side and release it so that it swings from side to side. Repeat, showing that it swings with the same period.
3. Repeat with another bag to show that it has a different period of oscillation.

Discussion prompts

• Why does one bag swing at a different rate to another?
• How could we measure the time period?
• How could we change the period of a swinging bag?

Teaching notes

This demonstration introduces pendulums by drawing on students’ everyday experience. Explain that the time period (T) is the time for one complete back-and-forth motion and that it is difficult to measure the time for a single swing and so is better to time a number (eg 10T) to find an average. Also discuss whys it is better to count from the point where the bag passes through the midpoint of the swing rather than at the ends (it is instantaneously stationary at the ends so the time difficult to judge).

Students may suggest changing mass, length or amplitude as ways to alter the period of a swinging bag. To illustrate that period doesn’t depend on the mass add books to the bag.

This activity can used as a precursor to a more formal investigation into the factors that affect the period of a pendulum. Make the link to simple pendulums by explaining that in science we try to design experiments so that we remove superfluous complications. The swing of a bag can be modelled using a pendulum made of string with a heavy bob as the moving mass.

Learning outcome

Students measure the period of a pendulum

This experiment was safety-tested in March 2020.

Build a train with a cell, two magnets and a coil to test their understanding of electromagnetic forces and Lenz’s law.

Equipment

• AA cell
• A small nut (that fits over the positive terminal of the cell)
• Two spherical neodymium magnets with a diameter of 15 mm or greater
• Bare copper wire
• 50 cm long pole with a diameter similar to magnets (eg broom handle or copper tube)
• Sticky or duct tape

Preparation

Before the lesson wrap the copper wire around a pole to make a 40 to 50 cm long coil.

You can use standard cylindrical neodymium magnets to build your train, but they may get caught in the coil. For a more reliable demonstration, source spherical magnets.

Procedure

1. Place the small nut over the positive terminal of the cell. Attach a magnet.
2. Attach a magnet to the negative terminal of the cell to complete your train. Like poles of the magnet should face each other.
3. Insert train into coil. If it moves backwards, turn the train around. If it doesn’t move at all, turn one of battery around. Explain that like poles need to be facing the cell for this demonstration to work.
4. Make a circular track shape out of the coil, joining the ends by slotting the end coils into each other. Secure your track to the bench with tape.
5. Separate the end coils, insert the train and re-join the coil again. Your train should go around in a circle.

Discussion prompts

• What part of the coil does current flow through?
• Why does the train accelerate?
• Why does it reach a constant speed?

Explanation

The train consists of two permanent magnets at either end of a cell. The magnets touch the bare copper wires of the coil, thereby completing the circuit so that there is a current in a section of the coil.

The current produces a magnetic field inside the coil which exerts forces on the two permanent magnets. It attracts the N pole of the left hand magnet and repels the N pole of the right hand magnet. These two forces act in the same direction on the train, and so it accelerates to the right.

If students ask about forces on the S poles of the magnets, explain that these will be in the opposite directions to the one on the N poles. These forces will be weaker than those on the N poles because the S poles are outside of the region where a current flows. The resultant force on each magnet will be to the right.

The train quickly reaches a constant speed around the track. This is because the moving magnets induce a magnetic field in the coils that acts to oppose the motion of the magnet (an example of Lenz’s law). The train reaches terminal velocity when the forces accelerating it forwards are balanced by the forces arising from electromagnetic induction.

Learning outcome

Students explain how a simple magnetic train works.

This experiment was safety-checked in March 2020.