Demonstration

An introduction to what voltmeters measure and how they are connected in circuits.

Apparatus and Materials

• Cells, 1.5 V, with holders, 3
• Lamps with holders, 3
• Leads, 4 mm, 8
• Demonstration voltmeter (0-5 V)
• Digital multimeter with multiple voltage ranges (optional)
• Digital and analogue voltmeters with varying ranges (optional)

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Read our standard health & safety guidance

Procedure

1. Connect three cells in series. (Don’t complete the circuit.)
2. Attach two leads to the demonstration voltmeter of a different, distinctive colour, e.g. green.
3. Connect the meter, reading 0-5 volts, first across one cell, then across two, then across three. Show that the meter reading increases in equal steps – the meter is ‘counting the cells’. (You might wish to mark the meter face to indicate ‘1 cell’, ‘2 cells’, ‘3 cells’.)
4. Now connect three lamps in series. Connect one cell across the three lamps - the demonstration meter should read approximately ‘1 cell’. Repeat with two and three cells.

5. Finally, with three cells and three lamps, make readings across one, two and then three lamps to show how the voltage of the cells is shared between the lamps when they are in series.

Teaching Notes

• As a first introduction to the voltmeter there is no need to define the volt. Instead use it like a ruler or a watch is first used - without defining the metre or the second. Eventually the volt will be used as a measure of the energy which a cell is able to provide. (A potential difference in volts is defined from the energy transferred to or from each coulomb of charge flowing between the two points in the circuit, where the voltmeter is connected.)
• You could start by introducing the cell as a device that stores energy chemically. This may help students to understand that the current is not used up, but the chemicals in the cell are used up. So the current is the same all round a series circuit and is not used up in devices. (The current does electrical work - the lamps heat up and warm up the surroundings.) We are paying for the chemicals in the cell, and, on a large scale, for the coal (and infrastructure) that enables a current to flow when appliances are connected to the mains. We pay for the fuel, power stations, and National Grid when we pay an electricity bill.
• Students could carry out a similar experiment, but they are likely to be less confused by their observations if the experiment is performed as a demonstration. They could go on to practise using voltmeters to measure voltages in any circuits which they have previously studied.
• How Science Works extension: Use this demonstration as an opportunity to raise some of the issues relating to the selection of appropriate equipment for practical work. Students may think that there is only one type of voltmeter. By demonstrating that there are voltmeters with different ranges, you can reinforce the importance of selecting appropriate equipment. With two or more meters on different ranges, take the same measurement: the pointer of a meter with a more sensitive scale will be deflected further.
• In some experiments, students will not get a single, fixed reading on their voltmeter but will get a constantly fluctuating value. Deciding what the ‘right’ reading is provides an excellent opportunity to discuss:
• the relative merits of analogue and digital meters
• uncertainty in measurements
• how to select a meter with an appropriate range and sensitivity
• Point out that care needs to be taken to avoid meters being overloaded or damaged. If an electrical meter has more than one range, students should always select the highest range first and then more sensitive ranges, if appropriate. They should also select the lowest voltage on the power supply.

This experiment was safety-tested in December 2006

Class practical

An opportunity to measure the electric current and introduce the ampere unit. There is no need to define an ampere.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Lamps with holders, 3
• Ammeter (0-1 amp), DC, preferably moving-coil
• Leads, 4 mm, 6
• Digital and analogue ammeters with varying ranges (optional)
• Digital multimeter with multiple current ranges (optional)

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Read our standard health & safety guidance

Procedure

1. Set up a circuit in which a cell, a lamp and an ammeter are connected in series.
2. To record what you observe, draw a circuit diagram. Beside the lamp, note its brightness. Beside the ammeter, note its reading.

3. Set up a second circuit with two lamps connected in series with the cell and ammeter. Record your observations.
4. Repeat this with the two lamps connected in parallel with each other (side-to-side).
5. Repeat these observations using two cells in place of one.
6. How does the reading on the ammeter relate to the brightness of the lamps?
7. Investigate how the reading on the ammeter depends on its position in the circuit.

Teaching Notes

• With two lamps in series, less light is produced and the ammeter will show that the current is less. This is where high power cells are needed. Otherwise the result is spoilt by internal resistance.
• When two lamps are connected in parallel, twice as much light will be produced (two lamps of equal brightness), and the ammeter will show that twice as much current is flowing.
• Note that the results of this experiment may not match up to an idealized view of current flow. There are two reasons for this:
• when two lamps are connected in series, the voltage across them is halved but the current is likely to be more than half the previous value (because the lamp's resistance is lower when it is cooler)
• the voltage provided by a cell is likely to be less when it is making a bigger current flow, as a consequence of the internal resistance of the cell.
How Science Works extension:
• Use this demonstration as an opportunity to raise some of the issues relating to the selection of appropriate equipment for practical work. Students may think that there is only one type of ammeter. By demonstrating that there are ammeters with different ranges, you can reinforce the importance of selecting appropriate equipment. With two or more meters on different ranges, take the same measurement: the pointer of a meter with a more sensitive scale will be deflected further.
• In some experiments, students will not get a single, fixed reading on their ammeter but will get a constantly fluctuating value. Deciding what the 'right' reading is provides an excellent opportunity to discuss:
• the relative merits of analogue and digital meters
• uncertainty in measurements
• how to select a meter with an appropriate range and sensitivity.
• Point out that care needs to be taken to avoid meters being overloaded or damaged. If an electrical meter has more than one range, students should always select the highest range first and then more sensitive ranges, if appropriate. They should also select the lowest voltage on the power supply.

This experiment was safety-checked in December 2006

Class practical

A simple investigation of the factors affecting the resistance of a wire.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 2
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 5
• Wire available for class use (see technical notes)
• Power supply, 0 to 12 V, DC (OPTIONAL)
• Metre rule (OPTIONAL)
• Insulating tape (OPTIONAL)
• Digital and analogue ammeters, 0-1 A (OPTIONAL)
• Digital and analogue voltmeters, 0-12 V (OPTIONAL)
• Micrometer (OPTIONAL)

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

When using a power supply, high currents will cause the safety cut-out on the power packs to automatically switch it off. If short lengths of wire are used with relatively high currents and voltages, then significant electrical heating may also occur. Students should be encouraged to adjust the voltage to keep currents small with every set of readings. At each stage they can connect the circuit, take readings quickly and then disconnect the power supply.

If you use a mains power supply, use one that is designed to limit the output current to about 1 amp, and preferably with a current overload indicator.

Read our standard health & safety guidance

The following apparatus should be available for class use:

• Selection of reels of Eureka wire (also known as Constantan or Contra) of different gauges, e.g. 0.71 mm (22 SWG), 0.46 mm (26 SWG), 0.32 mm (30 SWG) and 0.24 mm (34 SWG).
• Selection of reels of different wires (e.g. copper, Eureka, iron) of same gauge (e.g. 34 SWG).

Procedure

1. Connect up a series circuit of two cells, and the ammeter, with a 30 cm length of one of the wires closing a gap between two crocodile clips. Note the reading on the ammeter.
2. Replace the specimen of wire with another of the same length but different gauge or material.
3. Investigate how the current depends on the thickness of the wire, its length and the material from which it is made.

Teaching Notes

• Use fine gauge wires. If too thick a wire is used, the results may be affected by warming of the wires.
• If coils of copper and Eureka wires of the same gauge can be prepared so that they have equal resistances, the effect is very striking. However, this would then lose its value as an open investigation.
• Students should come to understand that the resistance of a wire depends on its length, its cross sectional area, and the material out of which it is made. With some students you could go further and introduce the concept of resistivity ρ, through the relationship R = ρ l / A where R = resistance, ρ = resistivity, l = length and A = cross-sectional area.
• This may also be an opportunity for a large scale demonstration of the effect by the teacher. But note: if the current is too large, the voltage of the cells will fall due to their internal resistance. For this reason, it is important to keep the current very low - copper wire is effectively a short.
• How Science Works extension: This experiment can be used as a more open-ended investigation. Students can select the variables, the ranges of results and the equipment used. The amount of guidance will depend greatly upon the teaching group. Investigating the effect of length on resistance is common but some students may wish to investigate the effect of the thickness of wire. In either case, different wires should be made of the same material. Students may need to know the conversion between SWG (standard wire gauge) and wire diameter/radius.
• Students will find it easier to measure at a prescribed length if they tape the wire to a metre rule with insulating tape and make connections with flying leads rather than crocodile clips.

This experiment was safety-checked in August 2007

Class practical

This is a useful introduction to the use of ticker-timers.

Apparatus and Materials

For each student or student group

• Ticker-timer
• Ticker-tape
• Stopwatch or stopclock
• Mechanics trolley or wind-up/pull-back toy car OPTIONAL

Health & Safety and Technical Notes

In crowded laboratories, estimate the space needed by the tape puller and arrange the groups to avoid collisions.

Read our standard health & safety guidance

The ticker-timer should come with a recommended power supply unit (low voltage AC as specified, with on/off switch).

Some ticker-timers use light-developed tape rather than carbon discs. After the tape has been struck with the vibrating arm, it takes a few minutes for the dots to become visible.

Procedure

1. Thread a short length of ticker-tape through the ticker-timer. If there is a carbon paper disc, make sure the tape goes underneath the disc.
2. Turn the ticker-timer on for a few seconds. It vibrates rapidly and hits the top of the carbon paper. It makes a lot of dots on the tape, at regular intervals.
3. Remove the tape from the ticker-timer. If the tape didn't move when the ticker-timer was switched on, then all the dots will be in the same place.
4. Thread a longer piece of ticker-tape, about 1 metre long, through the ticker-timer. Switch the ticker-timer on. Pull the tape slowly through the ticker-timer.
5. Check the tape to see if you can see each individual dot, with a space between. We can say that each dot-to-dot space stands for a tick of time.
6. Thread another 1 metre piece of tape through the ticker-timer.
7. You need a start signal and a stop signal. These could be handclaps by one of your group or by your teacher. They should be just a few seconds apart. Pull the tape slowly and switch the ticker-timer on at the start signal. Switch it off at the stop signal.
8. Count the number of dot-to-dot spaces between the start and the stop. That is the time between the signals, measured in ticks.
9. Use a fresh piece of tape, and a stopwatch or stopclock. Pull the tape through the ticker-timer for 3 seconds. Find out how many ticks there are in 3 seconds. Find out how many there are in 1 second. Work out the time in seconds that is the same as 1 tick.

Teaching Notes

• All students could use the same start and stop signals, such as two handclaps by the teacher. This provides the opportunity for comparison of the times obtained by different students. Times can be tabulated, and used for discussion of range, mean and estimated error.
• You could make a bar chart of students' answers for the time, in seconds, that is equivalent to one tick. Most ticker-timers vibrate at 50 Hz, and thus make 50 dots per second. For these, the expected mean value of one tick is 1/50 second, or 0.02 s . (Some ticker-timers will give dot-making frequency of 100 Hz, and for these the value of one tick is equivalent to 1/100 seconds or 0.01 s .)
• You can make a model of the action of the ticker-timer and the ticker-tape. Move a roll of wallpaper along the bench or floor and ask a student to put blobs of paint or ink onto the wallpaper at regular times. These could be indicated by a steady handclap or a metronome. Fast and slow motion by the wallpaper puller will produce blobs at different separations for equal time intervals. Each blob-to-blob space represents a tick of time.
• How Science Works extension: This activity can be used as a prompt to discuss the relative merits of the ticker-timer as a timing device compared to a stopwatch. Make a simple speed measurement for a moving object such as a mechanics trolley or a pull-back car using the ticker-timer and conventional stopwatch. Encourage students to discuss or write about the strengths and weaknesses of each method. The key teaching point is how to select appropriate equipment, by using the concepts of accuracy and uncertainty in measurements.

accuracy and uncertainty

• Many students find the ticker-timer an awkward piece of equipment. Some struggle to get a time reading from the paper tape and get confused about what the dot spacings represent. If you plan to use ticker-timers regularly in motion experiments, give students extra time with early investigations so they become familiar and confident with the equipment.

This experiment was safety-checked in January 2007

Class practical

Practice in estimating time and measuring time.

Apparatus and Materials

As many different time measuring devices as possible, including:

• Wall clock
• Wrist watch (to allow the use of the human pulse as a timer)
• Laboratory timers, electronic (ideally ones with different scale intervals, e.g. 0.1 s , and 0.01 s )
• Sand hourglass / egg timer
• Kitchen timer (either wind-up or digital)
• Electronic timers or datalogging package (OPTIONAL)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Some mobile phones also have in-built timers which can be used.

Procedure

1. Ask students to make some rough measurements of time, to gain familiarity both with time intervals, e.g. of the time between hand claps, the time for a book to drop, the time to walk or run a given distance and stopwatches and other measuring devices.
2. Have them measure (or attempt to measure) the same time interval with a variety of devices.

Teaching Notes

• This activity can begin with students estimating time intervals between various activities and then by measuring time intervals to check how good their estimates are. Young students will find five minutes of silence to be a life sentence!
• Some real tasks will improve motivation, e.g. measuring the time to run a 100 m race. Average speed measurements might also be included, e.g. the time taken for a ball to fall from a high window.
• Discuss the difference between timing an unexpected time period (e.g. between hand claps hidden under a desk) compared to something that they can predict (e.g. a falling object).
• There are many electronic timers and datalogging packages available for measuring time, which will make a change from stopwatches and provide an introduction to electronic time measurement.
• How Science Works extension: The selection of appropriate equipment to carry out a particular measurement is an important skill that is often overlooked, particularly if the ‘correct’ equipment for any experiment is generally made available for students. Students need practice choosing equipment and justifying their choice if they are to become confident experimenters.
• This activity allows students to look at different ways of measuring time and to compare uncertainties in measurements. They could also discuss measurement errors. Point out that no single timing device will suit all situations.

This experiment was safety-checked in July 2007

Class practical

Introducing the idea of measuring multiple objects and finding an average.

Apparatus and Materials

For each student group

• Large supply of coins (all of one denomination)
• Ruler with graduations in mm (e.g. metre rule)
• Micrometer (OPTIONAL)

Health & Safety and Technical Notes

Read our standard health & safety guidance

There is a significant risk that the coins will go missing! You could use steel washers as an alternative if you are concerned about this.

Procedure

1. Estimate the thickness of a single coin.
2. Measure one with a ruler.
3. Measure a pile of coins stacked on each other. Calculate the average thickness.

Teaching Notes

• Each student should try to measure the thickness of a single coin with a ruler with millimetres marked on it. The teacher should ask for results and also ask how reliable students think they are. Then ask for suggestions of improvement. Some may suggest measuring the height of a pile of coins and then calculate the thickness of one.
• Discuss the general idea of accuracy behind the method by saying:
• "Suppose you have just one good coin and this ruler marked in millimetres, how thick would you find the coin if you could measure it very carefully? Yes, we do now know that the thickness is, say, 1.3 mm but could you really see that if you had just one coin to measure? Even if you thought you could see it, would that be a safe and fair answer to give? With just one coin what would be the fairest thing to say? If you wanted to be quite safe, what would you say? Yes I agree; all we can say is somewhere between 1 and 2 millimetres. Now suppose you have 10 coins in a pile and you measure the pile. Even if you make a mistake of 1 millimetre in that measurement, how much of a mistake is that in the thickness of one coin? So if you measure 10 coins you could say that you think each coin is 1.3 millimetres thick. What would you say if you measured 100 coins in a pile?"
• At an introductory level, you might leave this problem there and come back to it later. Big numbers and small decimals are not easy, and the problem of accuracy is not a particularly interesting one yet.
• If a student points out that worn coins are thinner than new coins, then it might be worthwhile sorting them into two stacks, using the faces on the heads and the date on the coins. Compare the heights of the stacks. Of course, this kind of experiment is of far greater value if pupils suggest it themselves, or even if the teacher can coax it out of them in a way that makes them feel it is their own suggestion. Then they are doing science. Measuring many atoms in order to find the size of one atom is a skill which scientists have.
• How Science Works extensions: Point out that it is worthwhile changing and improving an experimental method as you carry out an experiment and that deciding on a method does not preclude subsequent changes.
• Collecting the data from the class for the three different measurements of the thickness can be used to prompt discussion about data spread, variation and accuracy.
• You could ask students to use the micrometer on a single coin and compare the value measured this way with the value obtained from the stack of coins. If students have not used a micrometer before, allow time for teaching them how it works and have them take sample readings before expecting them to use it confidently.

This experiment was safety-checked in July 2007

Class practical

The behaviour of springs provides a topic through which students can learn about simple relationships between pairs of variables, in a practical context. Seventeenth-century scientists, like Robert Hooke and Robert Boyle, helped to lay the foundations for physics and for other sciences by working in this way.

Apparatus and Materials

• Extendable steel springs, 2 or 3
• Stand, clamp and additional boss
• Flat-headed nail, large
• Metre rule
• Mass hanger and slotted masses (100g)
• Eye protection for each student
• G-clamp
• Rubber bands OPTIONAL
• Set square OPTIONAL

Health & Safety and Technical Notes

Students should clamp their stand to the bench to prevent it from toppling.

Students must wear eye protection. Eyes may be at the same level as clamp and the nail. Also, steel springs store more energy elastically than copper springs and can fly off their supports.

Read our standard health & safety guidance

Provide spare springs. Students will stretch springs beyond their elastic limit and replacements will be necessary. This is not willful destruction but, rather, good science.

If the springs are supplied close-coiled it is better to have the coils separated before issuing them to the students. Hanging about 500-600 g gently on the tightly coiled springs will do this.

Procedure

1. Fix the nail horizontally, with its point in the boss on the stand. Hang a spring from it and secure it so that it does not fly off.
2. Hold the metre rule vertically in the clamp, alongside the spring.
3. Record the metre rule reading level with the bottom of the spring. The number of masses hanging from the spring is 0 and the extension of the spring is 0 cm.
4. Hang a mass hanger from the bottom of the spring. Record the new metre rule reading, the number of masses (1) and the extension of the spring.
5. Add a mass. Record the new metre rule reading, the number of masses (2), and the total extension of the spring from its unstretched length.
6. Repeat this until after the spring has become permanently stretched.
7. Describe the pattern in the results. To do this fully, you will need to plot a graph. Plot the number of masses on the horizontal axis, since it is the input (or independent) variable. The extension of the spring is the output (or dependent) variable and you should plot it on the vertical axis.

Teaching Notes

• This is a more formal variation of this experiment:

  Home-made springs

  There is benefit in doing both, since it will invite discussion and thought on the nature and use of graphs.
• You could discuss whether doubling the load on a spring sometimes or always doubles the extension. This relates to the shape of the graph, whether it is sometimes or always a simple straight line passing through the origin. It thus leads to the concept of proportionality. Proportionality, or linearity, describes a simple form of relationship between variables. This relationship is common in nature.
• Much of physics is devoted to seeking such simplicity. Hooke's law states that, up to a limit, extension is proportional to load. (When the load is doubled then the stretch is doubled.) Robert Hooke noticed this very simple pattern in 1676. Since he was worried that others, maybe even Newton, would steal the credit for this he wrote in code at first, and created an anagram: ceiiinosssttuv. This is taken to mean ut tensio sic vis, which is Latin for: as the stretch, so the force. The fact, though, that Hooke's law is only obeyed by materials up to a limit highlights the fact that nature does not always offer simplicity.
• Invite students to think about applications of springs, in systems from door catches to vehicle suspensions. Point out that engineers must understand the behaviour of springs.
• Extension activity can include investigation of other springs, elastic bands and any other elastic materials (e.g. polythene strips). Comparison of graphs provides opportunity for discussion.
• How Science Works extension: Include among the equipment available for this experiment a second boss and clamp as well as a set square for each student group. Either prompt a discussion initially or leave the students to work out how these extra items might be useful.
• Students can improve the accuracy of their measurements by clamping the metre ruler in place and then using the set square to make the length/extension measurement. They can also use the set square to make sure that the clamped ruler is vertical in relation to the bench. Students might set the clamped ruler at 0 cm when no masses are added and so read the extension directly. This procedure helps them avoid simple mistakes that arise when measuring lengths and then calculating extensions. These refinements provide good illustrations of improving an experimental method.

Further ideas:
• Give students access to extra springs so that they can try series and parallel arrangements. You could also ask them to predict what they expect to happen qualitatively and perhaps even quantitatively.
• Investigating whether the same results are obtained when a materials is loaded and unloaded, particularly if rubber bands are used. Stretched rubber exhibits elastic hysteresis.

This experiment was safety-checked in January 2007

Demonstration

By stretching confectionery laces, students learn that extension is not always proportional to load. They also gain experience in adopting consistent procedures to make and record measurements.

Apparatus and Materials

For each student group

• Strawberry, apple or cola laces (preferably not sugar-coated)
• Retort stand
• Clamp
• Felt-tip pen (dark colour)
• Metre rule
• Clotted massess, 100 g set
• Slottee masses, 10 g set

Health & Safety and Technical Notes

Make sure that students do not eat the laces since eating anything in a laboratory is hazardous.

Read our standard health & safety guidance

If the laces are sugar-coated, take care to avoid getting sugar into other equipment (open the packet over a sink). Wash off the sugar under a cold tap and allow the laces to dry before use.

Procedure

1. Tie one end of a lace around the clamp and the other to a mass-holder. Make two marks on the lace a measured distance apart (approx 0.5 m).
2. Add masses singly or a few at a time. Observe how the lace behaves over a short period after the load is increased.
3. Observe how the lace behaves if the load is removed. For each load, record the distance between the two marks.
4. Continue until the lace breaks.
5. Plot a graph to show how extension varies with load.

Teaching Notes

• This activity can be used for a variety of purposes, depending on the ability, age and experience of the students.
• For some students, it will be a useful exercise in making measurements and displaying them graphically.
• For others, it will provide an example of a material whose load-extension graph is not a straight line (it does not obey Hooke's law) and which exhibits creep (gradual deformation under a steady load). They can be asked to discuss when they should record the extension for a given load (immediately? or after the sample has stopped stretching?). There is no right answer, but students should be consistent and state clearly what strategy they have adopted.
• You might want to discuss the role of tests such as these in the food industry. Measurements can be directly related to how a confectionery product feels when eaten, and samples are tested before a batch of products leave the factory to ensure they are of suitable quality.
• How Science Works extension: If students have obtained a graph from one lace, they may assume that this will describe the behaviour of all laces. A nice extension is to ask them to investigate the variation in stretchiness (or spring constant) within a packet of fruit laces. Terms such as variation and range could be introduced and used, if appropriate.
• Students could carry out a similar process as seen in the experiment Investigating simple steel springs and possibly go on to compare the variation in springs behaviour with the variation in confectionery laces.
• This experiment comes from Salters Horners Adanved Physics©, University of York Science Education Group.

This experiment was safety-checked in January 2007

Class practical

Controlling the object you are measuring.

Apparatus and Materials

For each student group

• Metre rules, 2
• Ruler, 15 or 30 cm
• Set squares, 2
• Marbles, a supply of
• Supply of ball bearings of different sizes (at least 5 of each size) OPTIONAL

Health & Safety and Technical Notes

Care needs to be taken as there is a trip hazard if marbles fall and roll around the floor.

Read our standard health & safety guidance

Procedure

1. Give students a single marble and ask them to measure the radius using a ruler.
2. Discuss with the class the difficulties with this method, in particular the shape of the marble.

3. Give the students the opportunity to devise an improvement before suggesting the method illustrated above. Two metre rules provide a trough which the marbles sit in so that they do not roll away. The rules are held next to each other and the pair of set squares makes the ends of the line of marbles clear.
4. Students then can carry out this revised and improved measurement of the marbles and work out the radius of one.

Teaching Notes

• In step 1 students should be able to get a reading but the variation across the class may be considerable. Check that they have not given the marble diameter as their result, instead of its radius.
• This experiment is designed to support the development of practical skills and confidence, not simply finding the radius itself. Students may ask 'how many marbles should we use?', to which a non-committal reply such as 'how many do you think?" or 'enough is appropriate'. Using increasing numbers of marbles reduces the uncertainty in the diameter measured, but there is no right number. It is worth mentioning that errors may be introduced when more marbles are used, e.g. not all the marbles may be in a perfectly straight line.
• How Science Works extension: This experiment can support the skills involved in devising experimental methods which improve the accuracy of measurements. If students have carried out the experiment Measuring paper or Measuring the thickness of a coin, then they will already have encountered some of these ideas. In both of those experiments, a micrometer can be used to collect an accurate answer from a single coin or piece of paper.

Measuring paper

Measuring thickness coin

This experiment was safety-checked in December 2006

Class practical

Measuring many sheets to find the thickness of one.

Apparatus and Materials

For each student group

• Pack of paper containing a known number of sheets (e.g. 500)
• Sheet of the same paper
• Ruler with millimetre scale (e.g. a metre rule)
• Chemical balance
• Micrometer (OPTIONAL)
• Selection of rulers/tape measures with different scale divisions (OPTIONAL)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Attempt to measure the thickness of a single sheet of paper. Fold the paper in half, in half again, and so on, to obtain multiple thicknesses. Measure the thickness and calculate the thickness of a single sheet.
2. Measure the thickness of a pile of a known number of sheets (say, 100 or 200). Calculate the thickness of a single sheet.
3. Compare these two methods for measuring the thickness of a single sheet. Which is better?

Teaching Notes

• A book could be used instead of a pile of sheets of paper. Students should make a rough measurement of a pile of paper or the thickness of the book (remember the book is numbered on both sides of the paper).
• As well as the length, width and thickness of the paper, some students may attempt to measure its density. Often paper is sold with a g/m ² value on the packet (which is not a density) and so this may provide a value against which they could check their measured value.
• Alternatively, a textbook can be used, but beware of the factor 1/2 since a book has half as many leaves as pages.
• To encourage order of magnitude estimates, you could go on to give pupils a rough value for the diameter of an atom so that they can calculate how many atoms would sit next to each other in the thickness of a piece of paper. For paper made of cellulose (which contains carbon, hydrogen and oxygen) then the average atomic diameter is probably only about 1.5 x 10 ^-9 cm. Students who do not delight in using large numbers should not be dragged through these calculations.
• There are excellent books, videos and web sites on Powers of ten:

  Powers of ten

  Classroom displays of large and small distance measurements with pictures of the objects measured will create a good background to this work.
• You could follow this up with the activity Practice in using large and small numbers, which can be downloaded from:

  here

• How Science Works extensions: Although the measurement methods here are relatively straightforward, this provides an opportunity for students to design and carry out their own experiment. Emphasise that they should measure as accurately as possible and clearly explain how their method improves the quality of data collected. Get them to try and estimate the uncertainties in their results and to identify which measurement they feel has the greatest uncertainty.
• You could encourage students to write a full plan/method before they begin. If they are to do this, make relevant teaching points about the importance of clear and unambiguous instructions. Each group/student could write a procedure, which is then passed on to a different group/student to carry out, exactly as written. The second group/student can then evaluate the information that they have been given.

This experiment was safety-checked in October 2007

Demonstration

Measuring the half-life of a radioactive isotope brings some of the wonder of radioactive decay into the school laboratory. Students can witness one element turning into another and hear (or see) the decrease in the radiation it gives out as it transmutes.

This demonstration uses a protactinium generator to show the exponential decay of protactinium-234, a grand-daughter of uranium. It has a half-life of just over a minute, which gives students the chance to measure and analyze the decay in a single lesson.

Apparatus and Materials

• tray
• Holder for Geiger-Müller tube
• Geiger-Müller tube, thin window
• Scaler
• Stopclock
• Retort stand, boss, and clamp
• Ratemeter (OPTIONAL)

• Protactinium generator

Health & Safety and Technical Notes

See the following guidance note:

Managing radioactive materials in schools

To limit the risk of radioactive liquids being spilt, there should be special instructions in the local rules for handling (and preparing) this source.

Read our standard health & safety guidance

Preparation of the protactinium generator

It is now possible to purchase the chemicals already made up in a sealed bottle. One supplier is TAAB Laboratories Equipment Ltd, 3 Minerva House, Calleva Park, Aldermaston, RG7 8NA. Tel: 0118 9817775. However, you can make your own if you prefer.

These quantities make a total volume of 20 cm3. You can scale them up if you have a larger bottle. (A '30 ml' bottle has a capacity of about 35 ml, so there is still room to shake the solution when the total volume is 30 ml.)

1. Dissolve 1 g of uranyl nitrate in 3 cm3 of water. Wash it into a small separating funnel or beaker with 7 cm3 of concentrated hydrochloric acid.
2. To this solution, add 10 cm3 of iso-butyl methyl ketone or amyl acetate.
3. Shake the mixture together for about five minutes. Then run the liquid into the polypropylene bottle and firmly screw down the cap. It can help to shield the lower half of the bottle with some lead.
4. Place the bottle in a tray lined with absorbent paper.

Once you have made the protactinium generator, you can store it with other radioactive materials, taking care to follow your school code of practice and local rules: see the Managing radioactive materials in schools guidance note:

Managing radioactive materials in schools

A polypropylene bottle is preferable to polythene because it is somewhat more resistant to attack by the acid and ketone. Nevertheless, polythene bottles can be used, provided no attempt is made to store the liquid in them for more than a few weeks.

The organic layer which separates out contains the protactinium-234. This decays with a half-life of about 70 seconds.

An alternative to protactinium: A new, effective and extremely low hazard system for measuring half-life is available from Cooknell Electronics Ltd, Weymouth, DT4 9TJ. This uses fabric gas mantles designed for camping lights. Each mantle contains a small quantity of radioactive thorium. More details are available on the Cooknell Electronics website:

Cooknell Electronics

Procedure

1. Support the Geiger-Muller tube holder in a clamp, so that the tube is facing downwards towards the neck of the bottle.
2. Allow the bottle to stand for at least ten minutes. Take the background count by running the counter for at least 30 seconds. This is done with the bottle in position, because some of the count will come from the lower layer. You can do this before the experiment or some time after it has finished.

3. Alternatively, the GM tube can be clamped horizontally with the window close to the upper layer.

4. Shake the bottle vigorously for about 15 seconds to thoroughly mix the layers.
5. Place the bottle in the tray.
6. As soon as the two layers have separated, start the count and start the stop-clock.
7. Record the time from the beginning of the experiment - i.e. the time of day for the sample.
8. Record the count every 10 seconds. Or record it for 10 seconds every 30 seconds.
9. Run the experiment for about five minutes, ample time to reveal the meaning of the term half-life and to illustrate the decay process.
10. Provided you leave a few minutes between each attempt, you can repeat the experiment. In 5 minutes the activity of the protactinium in the aqueous layer grows to 15/16 of its equilibrium value.
11. It is possible to record the growth to equilibrium. Do this by moving the GM tube so that the aqueous layer at the bottom of the bottle is immediately above the end window of the GM tube.

Teaching Notes

The chemistry of the experiment:

• The first stages of the uranium-238 series are involved in this experiment.
• 
• The aqueous solution (at the bottom of the bottle) contains the uranium-238, its daughter thorium-234 and the short-lived granddaughter protactinium-234.
• Uranium and protactinium both form anionic chloride complexes but thorium does not. At high hydrogen ion concentrations, these complexes will dissolve in the organic layer (which is floating on top of the aqueous solution).
• When you shake the bottle, about 95% of the short-lived granddaughter (protactinium) and some of the uranium will be dissolved in the organic layer. The thorium stays in the aqueous layer.
• Since radioactivity is a property of the innermost nucleus of the atom it is not affected by chemical combination.
• The granddaughter (in the organic layer) decays without any more being produced by its parent (thorium) all of which is still in the aqueous layer. It emits beta particles which travel through the plastic wall of the bottle. Isolating the protactinium in the top (organic) layer allows it to decay without any top-up from its parent (thorium).
• The radiation from the thorium and uranium should not interfere with the results, for two reasons:
  1. The counter does not detect the alpha particles from the uranium or the low energy beta particles from the thorium. It only records the high energy (2 MeV) beta particles from the granddaughter (protactinium).
  2. The uranium-238 decays with an extremely long half-life. It yields a meagre, almost constant, stream of low energy alpha particles. Its daughter, thorium-234, decays with a half-life of 24 days. During the length of this experiment the decay rate can be assumed to be constant. If these two isotopes contribute to the count at all, it will be accommodated in the background count. The stockpile of thorium is also constantly topped up in the aqueous layer as long as the protactinium is present with the thorium.

Table of count rate: Get the students to make a table of count rate against time, and correct it for background count. The first 10-second reading should be allocated to a time of zero.

Plot a graph: Get the students to plot a graph of count rate against time. They should draw a smooth curve through the points.

• First point out the general pattern - that the count rate decreases with time. Then look for an exponential trend - that the best fit curve always takes the same amount of time to halve.
• Get students to measure the half-life from the curve.
• Point out the random nature of the points: although the decay follows a pattern, there is an element of randomness and it is not perfectly predictable.

How Science Works extension This experiment provides an opportunity to assess the accuracy of the measured half-life value and how the random nature of decay affects the answer.

The accepted value for the half-life of protactinium is about 70 seconds.

Explore different ways in which a half-life value can be obtained from this apparatus:

• Amend the procedure described above so that, instead of a scaler (counter), a ratemeter is used. One student just records the time it takes for the count-rate to halve. This will provide a very approximate value.
• Repeat the experiment with several members of the class timing how long it takes for the count-rate to halve. There is likely to be considerable spread in results across the group and the mean result may differ from the accepted value for half-life. In each case, ask students to identify errors and uncertainties in their measurement(s) and to suggest ways in which these could be reduced.

For example, ask: "how does the random nature of the decay affect the measured count-rate when the count is low, or high, compared the background count?"

• Either you or your students may suggest a graphical method as an improvement. The procedure described in the main experiment above could then be carried out, and then the accuracy of the half life value assessed and evaluated.

Radioactive materials raise significant safety issues, providing an opportunity to discuss the value and use of secondary data sources.

This experiment was safety-tested in February 2007

Class practical

An example of the behaviour of a simple component, giving students opportunities to construct a circuit, gather data and perform some analysis.

Apparatus and Materials

• Power supply, 0 to 12 V, DC
• Carbon film resistor - e.g. about 100 ohms, 1 W
• Leads, 4 mm
• Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges
• Rheostat, e.g. 200 ohms, 2 W

Health & Safety and Technical Notes

Read our standard health & safety guidance

Some components may become hot enough to burn fingers.

Procedure

1. Set up the circuit as shown below.

2. Use the variable power supply and the variable resistor to vary the potential difference across the resistor, from 1.0 V to 4.0 V, in intervals of 0.5 V. Record pairs of potential difference and current values in the table (see below).
3. You can record results for currents in the opposite direction by reversing the connections on the resistor.

Analysis: Plot a graph of current/A (y-axis) against potential difference/V (x-axis). Remember to include the readings for ‘negative’ voltages.

The resistance of the resistor is equal to the ratio of potential difference to current.

Use the graph to calculate the resistance of the resistor at a number of different currents.

Describe how the resistance changes with current. Is the resistance of the resistor the same for current in both directions?

The conductance of the resistor at a particular potential difference = current/potential difference.

Use the graph to calculate the conductance of the resistor at a number of different potential differences.

Teaching Notes

• The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. If you decide that all students should attempt all the experiments in this collection, it may be sensible to start with this very simple one. The analysis is straightforward but students may well need reminding to convert mA into A where necessary. The graph should be a straight line through the origin. Many students will realize that, if the gradient of the line is constant and if it passes through the origin, all V/I values will be the same. However, there is much confusion about such ideas! See point 2 below.
• It is often stated that the resistance of a component is the gradient of a V against I graph. Only for ohmic conductors (as in this experiment) does this happen to be true. Resistance is the ratio of V/I, so it is generally best to encourage students to take V/I ratios at specific points.
• In this case a higher potential difference raises more electrons into the conduction band so the use of the term conductance is probably helpful.
• Using a potential divider, as shown below, will enable students to get a full range of readings.

How Science Works extension: This experiment provides an excellent opportunity to focus on the range and number of results, as well as the analysis of them. Typically it yields an accurate set. The rheostat enables students to select their own range of results. You may want to encourage them to initially take maximum and minimum readings with the equipment and then select their range and justify it.

If they don’t think of it themselves, suggest that students take pairs of current and voltage readings as they increase the voltage from 0 V to the maximum. They then repeat these readings while reducing the voltage from the maximum to 0 V. This may help them to identify whether the resistance of the resistor remains constant when it is heated. (Turning the equipment off immediately after readings are taken and allowing the resistor to cool provides an alternative to this procedure but will considerably lengthen the time needed for the experiment. It is also possible to put the carbon resistor into a beaker of water to maintain the resistor at constant temperature.) Students could also change the direction of the current and repeat the other procedures.

You can use the fact that resistors are sold with a specified tolerance (and thus a variation in value) as the basis for a discussion about what a ‘true’ value really means in this case. Compare calculated resistance values with the manufacturer’s stated value or value range. Students can also be encouraged to identify the sources and nature of errors and uncertainties in the experimental method.

This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

This experiment was safety-tested in January 2007

Class practical

A simple method for comparing the density of liquids.

Apparatus and Materials

For each group

• Measuring cylinders, 100 ml or 250 ml, clean and dry, 2 or more
• Chemical balance
• Access to water and vegetable or olive oil
• Any other liquids that are safe to handle (OPTIONAL)

Health & Safety and Technical Notes

Take care with any spillages, particularly with the oil, which can create a slip hazard.

Read our standard health & safety guidance

Procedure

1. Take the measuring cylinder and measure its mass, in grams, as accurately as possible.
2. Take the measuring cylinder off the balance and add the water carefully, either by careful pouring or with a pipette until the level is as close to the 10 ml mark as possible. Put the measuring cylinder back on the balance. Measure and record the new mass (cylinder plus water), in grams.
3. Repeat the procedure, adding 10 ml at a time as accurately as possible and recording the volume and total mass, until the measuring cylinder is full. Then, for each volume calculate the mass of the liquid alone.
4. NOTE: If a 250 ml measuring cylinder is being used you may wish to use 20 ml or 25 ml intervals.
5. Repeat steps 1 to 3 for the oil (and any other liquids being tested).
6. Draw a graph of mass of liquid (y-axis) against volume (x-axis). Try to scale the graph so that you can plot all your data sets on a single graph.
7. For each set of data try and draw a straight ‘best fit’ line passing through the origin. Calculate the density of each liquid from the gradient of its graph line.

Teaching Notes

• Students will need to have studied density previously and be familiar with the density equation. Examples may have used cm ³ as the unit of volume and g/cm ³ as the unit of density, or m ³ and kg/m ³ . Either sets of units are generally acceptable, but all length measurements must use the same unit. Students may need to be told that with a measuring cylinder 1 ml =1 cm³.
• The density of water is measured before the oil because water can be easily and quickly rinsed out of the measuring cylinder and oil cannot. When adding the oil to the measuring cylinder, instruct students to try and avoid pouring it down the side otherwise it will form a coating on the sides which will increase the mass without raising the level from which the volume is read, so dry the measuring cylinder before weighing.
• If there are limitations to the number of balances available then it is still possible to carry this out with students sharing a balance, although care needs to be taken that there are no spillages. If students are not familiar with the meniscus that is formed, show them how to take volume readings correctly.
• How Science Works extension: If asked to find the density of a liquid, students may take only a single set of readings. The ease with which water and other liquids can be poured allows the refinement of this method to collect multiple results and use a graphical method to minimize the effect of any systematic error in the measurements.
• Finding densities of liquids and their behaviour is important to food scientists. You could illustrate this by having students measure the density of vinegar, making and measuring the density of a vinaigrette, and then predicting which of these will sit on top when they are poured into a single container.

This experiment was safety-checked in January 2007

Class practical

Students make a timer using a simple pendulum.

Apparatus and Materials

For each student group

• Pendulum (e.g. Plasticine bob on string/thread)
• Stopclock
• Chemical balance (0-100 g)
• Stand, clamp and boss
• Protractor

Health & Safety and Technical Notes

If large masses are used then the stands may need to be clamped to the bench.

Read our standard health & safety guidance

Procedure

1. Show a demonstration pendulum and ask students to think about the variables that may affect the time period for one oscillation.
2. Ask students to select one independent variable, collecting a set of data to investigate its effect on the oscillation time.
3. After students have completed an initial investigation and drawn conclusions, ask them to evaluate their method in terms of its accuracy and improve on it.

Teaching Notes

• You may need to explain what one oscillation for a pendulum is (motion there and back again). Variables to investigate include the mass of the pendulum bob, length of the pendulum (best measured to centre of bob), and initial amplitude (angle or displacement).
• A discussion following students' first attempts might lead to the following ideas for improving their measured value.
• Reduce the uncertainty in a measurement of periodic time by:
• measuring many oscillations to calculate the average time for one oscillation
• increasing the total time measured for multiple swings
• There is some uncertainty when measuring both the start time and also the stopping time, resulting from the experimenter's reflex time (as much as 0.2 s each, i.e. totalling 0.4 s ). The percentage uncertainty which this 0.4 s represents decreases as the total time measured increases. Students could carry out simple error calculations to discover, for example, the effect of a human reaction time of 0.2 s econds on timings of 2 s 20 s and 200 s.
• You may wish to get them to estimate the human reaction time or measure it as a separate activity. There are many web-based activities freely available.
• Improve the accuracy of a measurement of periodic time by:
• making timings by sighting the bob past a fixed reference point (called a fiducial point)
• sighting the bob as it moves fastest past a reference point. The pendulum swings fastest at its lowest point and slowest at the top of each swing.
• The periodic time for a swinging pendulum is constant only when amplitudes are small. Its period of oscillation is then T =2π √ _(l /g)_where
• T = Time period for one oscillation (s)
• l = Length of pendulum (m)
• g = acceleration due to gravity (m s^-2)
• Students investigating the effect of bob mass or pendulum length should keep the maximum angle of swing under 5 °.
• How Science Works Extension: This provides an excellent opportunity for planning, carrying out and evaluating an investigation using multiple skills. The number of variables is limited but there is enough scope to allow students to develop an approach and select appropriate ranges and intervals.
• Students often assume that any sensibly selected independent variable will always have an effect upon the dependent variable. Many may decide to investigate the effect of the mass of the bob, which yields an unexpected (counter-intuitive) result: the mass has no effect on periodic time. Proving that there is no link between two variables can be just as significant as finding one.
• The introductory discussion can put the pendulum into a scientific and historical context by describing the development of timing devices. Start with the hours of a day as one of the simplest units of time, easily measured with a sundial. Use this to introduce Galileo Galilei (1564-1642) and the (possibly apocryphal) story that his understanding of the behaviour of pendulums was spurred by observing the bronze chandelier or incense burner in the cathedral at Pisa. Galileo's pendulum introduced a method of measuring short periods of time that improved on the use of the human pulse. You could extend this timeline by describing further developments in timing devices, right up to the atomic clock (usually containing caesium) which is accurate to within 10-9 seconds per day.
• For students who take a particular interest in the measurement of time, suggest the book Longitude by Dava Sobel (ISBN 0007214227), which provides further background about the development of clocks and their use in navigation. It also has some examples of the struggles that can happen in the development of science and technology.

This experiment was safety-checked in January 2007

Class practical

To introduce ideas of energy transfer by heating and thermal capacity.

Apparatus and Materials

For each student group

• Datalogger with temperature sensor
• 1 litre beaker
• 250 ml beaker
• Insulating jacket
• Immersion heater
• 1 kg metal block (e.g. aluminium) with bores drilled for heater and temperature sensor
• Electric kettle or Bunsen burner to heat water rapidly
• Mug(s), ceramic OPTIONAL
• Cup(s), paper, polystyrene and plastic, with lids if possible OPTIONAL
• Different insulating materials (e.g. expanded polystyrene, newspaper, wool) OPTIONAL
• Instant coffee and tea bags OPTIONAL

Health & Safety and Technical Notes

An electric kettle is a much safer source of hot water than a Bunsen burner, tripod and gauze. However, immersion heaters also get hot and must be handled with care.

Read our standard health & safety guidance

Procedure

There are a number of things you can do with just temperature sensors.

1. Cooling curves. Fill a beaker with hot water from a kettle. Record its temperature once a second for a few minutes. If possible, produce a graph directly.
2. Compare cooling curves for beakers with different insulation, lids etc. Start each with water at the same temperature and record information from several sensors on the same graph.
3. Heating curves. Place sensors and heaters in beakers with 1 litre water and 250 ml water, and a 1 kg metal block. Start the heaters at the same time and with the same voltage and record the temperature-time graphs, all on the same display.

Teaching Notes

• These activities are excellent to emphasize the value of datalogging as the display is much easier to read than normal thermometers. Readings can be taken more often and with less chance of recording errors. Suitable software can produce an immediate graphical display to confirm that the data are being collected correctly.
Specific teaching points:
• This experiment can be used to calculate cooling rates in °C per second. The flattening curve shows that the rate of decrease of temperature is lower as the temperature falls.
• Without being quantitative, cooling curves which are produced live provide at-a-glance evidence for the effectiveness of different insulations.
• Comparing different masses of the same material (water is easiest) shows how the same amount of energy transferred causes different changes in temperature that depends on the mass. This is an introduction to thermal capacity and to the difference between energy transferred and temperature.
• Comparing the different materials (but same mass) is a further step on this road. The temperature of the aluminium will rise much more quickly than the 1 kg of water (1 litre). This is also partly because it will dissipate energy more slowly: it will take longer for energy to be transferred to the surface of the aluminium by conduction, and then be transferred to the surroundings by radiation, compared with time for convection currents to be set up in water. Hence the ratio of the rate of temperature rises is not the same as the ratio of the specific thermal capacities.
• If you want to use these methods to measure specific thermal capacities, then you need to ensure that you minimize energy dissipated to the surroundings with good thermal insulation.
• How Science Works extension: You could either set students a structured investigation and then follow with questions based on this or offer an open-ended investigation.
• Students could:
• identify and select the variables that they wish to measure and control
• produce their own experimental procedure, including the selection of appropriate time intervals.
• The amount of guidance given will very much depend on your students’ level of confidence and skills with designing their own experiments.
• Some groups could be set a very open-ended brief, ‘investigate cooling’. With others you might set the investigation in a real world context, suggesting some of the possible variables e.g. you could tell them that they are to investigate which is better to keep a cup of hot coffee warm for longest - a ceramic mug, a paper or a polystyrene cup? Most takeaway coffee cups have a lid, so this could be extended to investigating how effective the lid is at reducing energy dissipation. More advanced students could investigate whether tea and coffee behave in exactly the same way as water.
• Collecting data for cooling curves for cups of different materials is relatively straightforward, so students need only minimal guidance in the specifics of what they are to do.

Heating and cooling worksheet (Word, 58 KB)