What the Activity is for

Show: current and potential difference both contribute to power.

Here you can show that there are such things as compensated circuits: that is, two circuits where the character of the pathways is different but the power in each pathway is identical.

What to Prepare

• a MES 2.4 V 50 mA bulb
• a MES 12 V 100 mA bulb
• 2 matched ammeters
• 2 matched voltmeters
• a 24 V battery
• a 12 V battery
• a supply of wires to wire up the two loops, coding by colour

What Happens During this Activity

Set up the two circuits and note the similar brightness of the bulbs. Other combinations of bulbs can be substituted, so long as the powers dissipated by each lamp are identical and the currents and potential differences vary.

Then develop a conversation with your class to bring out the following points.

Bulbs are devices that switch power from an electrical pathway to the lighting and heating pathways. It seems that the power in these pathways is identical. However, the character of the pathways is different: one is of large potential difference and small current, and the other is of a smaller potential difference and larger current. The smaller current in one is compensated for by the larger potential difference.

This very simple idea becomes important when dealing with power distribution, and it's best to introduce it here, without the complexity of transformers.

What the Activity is for

Adding some quantitative values to the debate.

In this activity the power input to the wind stream driving a wind turbine is compared with the power delivered to an external load in order to estimate the effectiveness of the device. Inevitably much of the input power is not switched successfully, but the reasons for this may lead to fruitful discussions about the siting of wind turbines in the UK. This in turn will support discussion regarding the practicality of utilising wind turbines to satisfy significant fractions of the power requirements of the UK.

What to Prepare

• an electric fan that can be set in one position (rather than rotating from side to side)
• a model wind turbine with motor attached (ideally of similar diameter to the fan)
• a G clamp
• 4 leads
• a mains energy meter
• an SEP energy meter
• a fixed resistor of 47 ohm (or any value between 10 ohm and 40 ohm)
• a laboratory jack

What Happens During this Activity

Set up the wind turbine directly in front of the fan, and 0.35 metre from it. Ensure the centre of the fan and the centre of the wind turbine are aligned with one another. You may need to raise the height of the fan with a jack or other suitable object. Connect the fan to the mains supply through a mains energy meter set to read power. The wind turbine is wired across a fixed resistor through an SEP energy meter set to read average power.

Introduce the activity with a discussion about using wind turbines in the UK. The students will be familiar with the idea that using renewable resources is one way of helping to reduce the consumption of fossil fuels. They will also know that many places in the UK offer a good windy location for such turbines.

Connecting how much questions to real measurements and concerns

Teacher: Let's imagine we live in a very windy place on the coast of England. A local company has been able to calculate the power available from the wind in the place where we live. We know how much power we are likely to require to keep our heating system working through the winter (around 2 kW). The company tells us that there's more than enough power available from wind for all of our requirements if only we can harness it. The question the company seeks to answer is How much of this available power are we able to usefully divert for our use?

Explain that you'll use a model to investigate the efficiency with which a wind turbine can shift the estimated energy from the kinetic store of the wind along the electrical working pathway to the final thermal store in a load resistor. Suggest working with power rather than energy – perhaps discussing why this is a more practical approach. Using the suggested scenario, the load represents the heating system of the house.

Teacher: Before we try to compare input and output power of this system, could you estimate the fraction that we might divert?

It's a good idea to record some of the guesses before doing the measurements. Revisit them at the end to compare the two sets of values.

Switch the fan on, making sure that it remains stationary.

Teacher: The fan generates a channel of flowing air directly incident upon the wind turbine. We can see from the mains energy meter what power is being delivered to the fan.

Lucy: Yes, the fan is shifting 28 joule inverse second as the power is 28 watt.

Teacher: Good. Now can it be that the fan is shifting all of the 28 joule inverse second into the energy store of the moving wind?

Liam: No, because the internal parts of the fan are getting hot.

Lucy: Also the fan is making a noise.

Teacher: Yes. This means that the thermal and vibration store are also filling. The fan is not shifting all of its energy to the energy store of the moving air. But to get started let's just say that it is. This is a useful strategy in physics – make things simple enough to get started.

Lucy: OK then. So are we saying that the fan is delivering 28 joule inverse second to the energy store of the moving air?

Teacher: Yes. Especially since the diameter of the fan is about the same as the diameter of the wind turbine.

Concluding the discussion – checking the numbers

Now switch the energymeter on, which is connected to the load resistor, to see that the power delivered to the load is much smaller.

Lucy: It's only 19.55 milliwatt.

Liam: That is tiny!

Teacher: It is small.

Lucy: That's so much lower than I expected. It's a tiny fraction.

Teacher: I thought you might find this surprising. Now let's make this calculation more realistic. Again physics at work – after getting started we gradually make things more complicated.

Teacher: Let's now imagine that the fan was able to shift 30 % of the energy from the original store to the energy in the kinetic store of the moving air, and let's imagine that the wind turbine is only able to switch 50 % of the energy in this store to the electrical pathway. Maybe we'd get a bigger fraction?

Lucy: Well the fraction would certainly be higher.

Teacher: Let's work it out. Power available to wind turbine is 28 watt  ×  0.3  ×  0.5, which is 0.42 watt

Lucy: Wow. That is still so small.

Teacher: Yes, but these are pretty realistic when trying to extract the energy from the wind flow over the UK! There's not much energy at all reaching the target store. Most energy will be dissipated and will fill up the store of the surroundings through the heating by particles pathway.

Now you could lead a discussion about the practicality of utilising wind turbines solely to satisfy the energy requirements of the UK.

You might then like to investigate how various factors, such as the number of blades, the wind turbine design and the total blade area, affect the fraction at a given distance from the fan. You could also vary the wind speed by moving the fan closer to or farther from the wind turbine.

What the Activity is for

In this activity a solar cell is illuminated with light from a 12 V bulb. The distance from the bulb to the cell is increased so that the light intensity decreases, although not in a linear way. Measure the power delivered to an external load to produce a graph relating distance to power delivered.

What to Prepare

• a 12 volt power pack
• 8 leads
• 2 crocodile clips
• a fixed resistor below 100 ohm (sample used was 33 ohm)
• 2 SEP energy meters (although one is sufficient if moved from the bulb connection to the solar cell connection during the demonstration)
• 1 12 V incandescent bulb in holder (the bulb glass should be transparent so that the filament is visible)
• 1 solar cell (sample used was 9.5 centimetre by 6.5 cm)
• some black card, large enough to form a closed box around the cell and bulb (sample used was 15 centimetre by 40 cm)
• some Blu-tack
• a strip of graph paper 15 cm long with lines marked at each centimetre along the length

What Happens During this Activity

Set the power pack at 10 volt so that the bulb is near to maximum brightness. When collecting data, the black card surrounding the bulb should exclude light from the room. The solar cell is fixed to stand at right angles to the benchtop. The graph paper strip is fixed to the base of the black card to read the distance between the cell and bulb filament. The filament bulb should be moved along the length of the black card, Blu-tacked into position each time it's moved.

Your own values are likely to differ from those shown and should be used. Show the students the set-up and explain that you will be investigating how well a solar cell can deliver power to a load when illuminated by an external source.

Using, and measuring the output from, solar cells

Teacher: Here we have a solar cell. Has anyone ever seen one in use?

Jenny: Yes. I have one on my calculator.

Julian: And I have seen one connected to the bus stop where I live. It's connected to the sign that says what time the bus is due to arrive.

Teacher: Good. They're designed so that when they're illuminated by light they deliver power to devices such as the ones you have mentioned. During the day the light conditions are changing if we think about the bus sign. What causes this to happen?

Jenny: Loads of things, like how cloudy it is or if a bird is sitting in front of the cell.

Julian: And it is obviously sunnier at midday than in the afternoon.

Teacher: You're right. More generally, the ability of the cell to deliver power to a device depends on the brightness of the lighting on the cell. We'll use the intensity as a measure of the brightness of the light – that'll be a precise enough definition for our use. We'll find the power delivered to a resistor when the distance from the lamp filament to the cell is increased.

Julian: Why did you say filament? Can't we just use the edge of the bulb?

Teacher: No. That's because the source of the lighting is the filament.

Begin by placing the filament 3.5 cm from the cell. Switch the bulb on and have both SEP meters set to read power. It's useful to write these figures on the board:

At 3.5 centimetre, power delivered to bulb is 24 W

At 3.5 cm, power delivered to load is 12.6 milliwatt

Teacher: Any thoughts on these values?

Jenny: The power delivered to the load is half as much as to the bulb.

Linda: Don't be silly! It's much, much less because one number is measured in watts and the other milliwatts which is 1000 times smaller than watts! This means that the power delivered to the load is 2000 times smaller than delivered to the bulb.

Teacher: That's excellent! Now why do you think that's so?

Pulling ideas together

Julian: Well, the bulb is being continuously supplied with energy through the electrical working pathway. It must be that not all of the energy shifted from the original store is being shifted to the light pathway. The bulb is getting hot so the thermal energy store is also filling.

Teacher: That's also an excellent point.

Linda: If we're talking about efficiency, surely the solar cell itself is not 100 % efficient when switching power to the electrical pathway.

Teacher: You're also right. There's another significant point too. The filament is emitting light in all directions away from it over a large area, but the solar cell is only collecting a very small percentage of the total light being emitted over a small area, this being the area of the solar cell. All of the other light is not collected by the cell and so ends up heating the surroundings along the heating by radiation pathway.

Teacher: Now let's see what happens if we double the distance between the filament and the bulb. Any guesses?

Jenny: I think the power delivered will be half that of before because the light intensity must be less, so about 6 mW.

Put the bulb now a distance of 7.0 cm from the cell and notice that the power delivered to the load is around 1.5 mW.

Jenny: That's not what I expected!

Teacher: This is because you imagined that the brightness of the light reaching the cell would halve if we moved the bulb twice the distance from the solar cell. This might seem sensible but actually it's not the case. The light intensity doesn't fall in a linear way as we increase the distance from the cell to the bulb.

It's sufficient to leave this point as it is and not to explain further why this is the case. A more able group might want to know what the relationship is between distance and light intensity. If so, see the note about taking matters further.

Teacher: Let's see how the cell responds as we increase the distance in a regular way.

Begin with the bulb 3.5 centimetre away from the cell and record the power delivered to the load as you increase the distance in 1 cm increments. You'll see that the power delivered will fall quite rapidly as the distance increases. You might want to run through the process two or three times, finding average values of power delivered to the load at each distance. Students should be asked to plot a graph of the data with the distance between the filament and the cell on the x axis and power delivered to the load on the y axis, as in the sample data.

Teacher: We can see from the data that the amount of power being delivered to the load decreases in a non-linear way as the light intensity or brightness falls.

When increasing the distance between the filament and the cell by a factor of 2, say from 3.5 centimetre to 7.0 centimetre, the light intensity will decrease by a factor of 22, which is 4. When increasing the distance between the filament and the cell by a factor of 3, say, from 3.5 cm to 10.5 cm, the light intensity will decrease by a factor of 32, which is 9, and so on.

What the Activity is for

This is an interactive teacher demonstration that can be used to explore and to get students talking about various aspects of electromagnetic induction in the context of a simple transformer.

What to Prepare

• a mounted 2.5 V filament bulb
• a low-voltage AC power supply, 0–12 volt
• 2 (120 + 120)-turn coils
• an iron retort stand
• connecting wires

What Happens During this Activity

This demonstration might be carried out after electromagnetic induction has been introduced. It then provides a way into thinking and talking about transformers. The main aim is to get students talking about what they can see happening. The demonstration can be carried out in a series of steps.

Start simple – with a single glowing bulb (we used a 2.5 volt bulb) connected to the AC low-voltage output of a power supply. Switch on to about 3.0 volt and the bulb lights nice and bright. No surprise there!

Now set the challenge:

Teacher: How can we light the bulb without connecting it up to this kind of power supply or to a battery?

Using coils to make a link

Connect the bulb to a 240-turn coil of wire.

Teacher: How might connecting the bulb to a coil of wire help in making it light up?

Place the second 240-turn coil at the bottom of an iron retort stand and connect it to the alternating supply still set at about 3.0 volt. Now place the coil connected to the bulb over the retort stand. Nothing happens… until the second coil (with bulb) is slid down the vertical stand.

Concluding the demonstration

The bulb lights, getting brighter the further the secondary coil is moved down the iron rod.

This simple demonstration is based on some fascinating physics and provides an excellent opportunity for students to talk through quite a complicated, multi–step explanation based on electromagnetic induction. The first point you might make:

Teacher: Look! The bulb is not connected to the supply and yet it lights! The first coil is connected to the supply but there's no connection to the second coil. What on Earth's going on here? Who can explain?

The explanation to move towards is as follows:

The changing potential difference from the supply drives a changing electric current round the lower coil or primary.

The changing electric current in the lower coil produces a changing magnetic field. When the current is zero, the field is zero; when the current is a maximum, the magnetic field is at maximum strength.

The changing magnetic field is carried by the retort stand core and links the secondary (upper) coil to the primary (lower) coil.

The changing magnetic field linking the upper coil induces a changing voltage across this secondary coil.

The changing voltage across the upper coil drives a changing current in that coil and in the bulb, making it light.

Further questions to ask, and some (smart) students' answers:

Teacher: So why does the bulb get dimmer as the upper coil is moved up the retort stand?

Bill: There must be a weakening of the magnetic field as you go higher.

Teacher: What happens if the bulb is connected to 120 turns rather than across all 240? Try it!

Emily: The bulb goes dimmer. This is because a smaller voltage is induced across the secondary coil as there are a smaller number of terms. We have a simple step–down transformer!

What the Activity is for

Use this set-up to introduce the motor effect: a motor's spinning comes about from the interaction between the current in a conductor in a magnetic field. There's no spinning here, but the geometry is very clear.

What to Prepare

• an eclipse major magnet
• some card and Blu-tack (to cover the magnet, as shown in the photograph)
• a strip of aluminium foil approximately 1.5 centimetre by 15.0 cm
• 2 crocodile clips
• 2 leads
• 2 bulldog clips
• 8 100 gram masses (or similar objects) to elevate the magnet by about 1 cm from bench
• a standard laboratory power pack (vary the p.d. between around 1 volt and 4 V)
• a signal generator

Safety note: If the current is too large then the foil will get hot and the force may be too large and tear the foil. The power supply may suffer if it cannot supply a sufficiently large current. Care is needed, and the potential difference must remain modest.

What Happens During this Activity

Introduce the very strong magnet. It is shaped in such a way that the north and south poles are at each end of the horseshoe: between the two poles there is a magnetic field that's uniform (the strength and direction of the field is the same in that space). Remind students that magnetic field lines run from north to south. You might put an arrow, or a piece of card with field lines drawn on it, across the flat front edge of the horseshoe pointing from north to south to show the magnetic field.

Introducing the apparatus and switching on the supply

Teacher: Between the two poles of the magnet there is a thin strip of aluminium foil, which is a good electrical conductor. Two bulldog clips (one on each side of the magnet) connect it to the power supply, which drives an alternating current through the foil, changing direction 50 times a second.

Now switch on the supply.

Teacher: What do you hear?

Henry: Buzzing.

Teacher: The buzz is caused by the foil: moving in; then out; then in; then out. There must be a resultant force on the foil causing it to move that wasn't there before. The direction of that resultant force must be constantly changing to move the foil backwards then forwards and so on.

This is the motor effect in action. Now for some details on the forces and fields – adapt this dialogue to suit your class (you may need to seek more elucidation to get to these answers).

Teacher: In what direction is the wire moving relative to the direction of the current?

Sally: The direction of the current and the direction of the back-and-forth motion are at right angles.

Teacher: In what direction is the wire moving relative to the direction of the magnetic field?

Elmira: The direction of the magnetic field and the direction of the motion are at right angles.

Now place another arrow in the direction of the back and forth movement (and so in the direction of the resultant force which is causing the wire to move). Notice that every one of the three arrows is at right angles to every other arrow.

Bringing things to a close – and a possible extension

Summarise the discussion:

Teacher: A current-carrying conductor, at right angles to the field lines of a uniform magnetic field, has a magnetic force acting on it. The direction of this force is at right-angles to both the current direction and the field direction. This is called the motor effect.

Why does the strip change direction constantly?

This is because the direction of the current is constantly changing (remember: the supply drives an alternating current). The current alternates – charge moves: up the strip; then down; then up; then down. Each time the current changes direction (up or down) the direction of the resultant force changes (out or in). There is a vibration. This leads to a sound, if of the right frequency.

It's clear that the direction of the current affects the direction of the resultant force. By reversing the direction of the current only, the resultant force direction is reversed/changed.

Increase the size of the current in the strip (increase the applied potential difference a little here) and the size of the resultant force will increase. What do you hear?

A final (optional) step:

Now connect up the signal generator (take care not to draw too much current from this – keep the amplitude small). Now you can vary the frequency and hear different sounds. But the principle is the same.

So now you know how to get movement: arrange a current-carrying conductor at right angles to a magnetic field. There is some more cunning engineering to be done to convert this to a spinning motion.

What the Activity is for

A demonstration that's near magical.

This is an interactive teacher demonstration that can be used to introduce students to the idea of inducing an electrical potential difference by moving a wire through a magnetic field.

What to Prepare

• a very sensitive ammeter (maybe a light-beam galvanometer)
• a 2 m length of wire, stripped a little at both ends
• a strong horseshoe magnet

What Happens During this Activity

This demonstration might be used to introduce students to the essential ideas of electromagnetic induction. The physics explanation involves a new way of thinking and talking for the students (with conductors cutting magnetic lines of force), so care needs to be taken in presenting these ideas. In addition, don't spare your efforts in emphasising the vast importance of this apparently simple effect: this is the basis of all of our electrical supply throughout the world.

The demonstration can be carried out in a series of steps, starting with a way of setting the scene to capture students' interest and imagination.

In setting the scene ask the students to imagine themselves living on a remote island where they need an electrical supply for the basic comforts of life (e.g. lighting and cooking).

Teacher: So where would you get your electricity from?

Sanji: The plug?

Teacher: Oh no! There are no plugs on this island… no electricity supply!

Anya: You could use batteries.

Teacher: None available, I'm afraid.

The solution seems hopeless until you offer an alternative…

Building an understanding through demonstration

Teacher: Well, I can actually make an electrical supply if I have just a magnet and a piece of wire, both of which are available on the island.

It may seem unlikely to the students that you will be able to generate an electric current armed only with a magnet and a piece of wire.

Now introduce the wire and magnet solution, first sitting the students around the demonstration bench. Explain that the light beam galvanometer is simply a very sensitive ammeter capable of measuring small currents. The magnet is very strong. You may wish to ask one of the students to remove the iron keepers for you (which will be a struggle, so choose wisely).

Now, with the galvanometer pre-set on a suitably sensitive range, move the wire down through the jaws of the magnet: the spot moves! Triumph!

What kind of movement is needed to generate an electric current?

First make it clear that:

• If the wire is not moving then there is no electric current.

However:

• If the wire is moved down into the magnet: there is an electric current.
• If the wire is moved up out of the magnet: there is an electric current.

But:

• If the wire is moved across the jaws of the magnet, there is no electric current.

Describe the process like this:

Teacher: If the conducting wire is moved in such a way that it cuts the magnetic field lines running between the poles of the magnet, then a potential difference is induced that drives a small current round the circuit.

The process by which a potential difference is generated across the ends of a conductor when it's moved through a magnetic field is called electromagnetic induction.

Improving the generator

We'd hope that the students will be impressed by this generation of an electric current, but it will be clear that such a small current is hardly going to be your saviour on the small island. The obvious question is:

Teacher: How can we get a bigger electric current by induction?

Discussions will probably settle on three possibilities:

• Use a stronger magnet.
• Move the wire faster through the field.
• Use more coils of wire.

It's easy to demonstrate the second two.

The demonstrations lead to other demonstrations with bicycle dynamos and similar commercial electrical generators. All of these involve moving wires past magnets or vice versa.

Energy descriptions are best offered in relation to a simple bicycle dynamo where it's clear that the quicker you turn the handle, the brighter is the bulb.

Physical: I turn the handle and the bulb lights.

Energy (stores): energy is shifted from the chemical store associated with my body to the thermal stores of the bulb filament and of the surroundings.

Power (pathways): power is switched from the mechanical working pathway by the generator (wire and magnet) and from the electrical working pathway by the bulb.