Class practical

Using the hand as a detector to compare the absorption of infra-red radiation.

Apparatus and Materials

• Radiant heater
• Insulating sheet with hole
• Retort stand, boss, and clamp
• Crystallizing dishAluminium leaf (not kitchen foil)
• Vegetable black
• Paint brush
• Methylated spirit

Health & Safety and Technical Notes

Only aluminium leaf should be used. Do not use the thicker aluminium kitchen foil, which has a higher thermal capacity.

Some pupils may burn their hands without realizing the danger, unless an appropriate warning is given. Appropriate words are given in the "Procedure" below.

Read our standard health & safety guidance

Procedure

1. Set up the heating element with the insulating sheet in front of it. Ask a student to place the back of one hand near the hole for about 5 seconds. You might say:
"Hold your hand close to the hole in the screen, with the back of your hand towards the red-hot heater. Notice what you feel. Only do this for a short time to decide what you feel: one . . . two . . . three . . . four . . . five ... about that long. Do not hold your hand longer than this in case you burn it."
2. Cover the back of the student's hand with aluminium leaf and ask them to repeat this for a further 5 seconds. Now coat the aluminium leaf with a vegetable black paint. When the paint is dry, ask the student to place the back of the hand again near the hole for about 5 seconds.

Teaching Notes

• To put the leaf on the student's hand, get them to clench their fist and moisten the back of their hand with clean water until wet all over. Lay a sheet of aluminium leaf gently on the wet skin, blowing on it to push it on to the skin. Ask the student to unclench their hand a little to stop the leaf from cracking.

• To blacken the aluminium leaf, mix some vegetable black with methylated spirit to the consistency of thick soup. Apply with a soft paint brush on top of the leaf.

• To get rid of the paint and leaf, students should hold their hand under a running tap. Students should not try to rub off the paint or leaf. This will rub soot into the hands which will be difficult to remove.
• When infra-red radiation is absorbed by any surface, the surface is warmed up.
• The hand absorbs most infra-red radiation when it is painted black, a medium amount when it is not coated with anything, and the least when coated with a shiny surface. In the last case the infra-red radiation is being reflected away.
• White paper rather than aluminium foil stuck on the back of the hand and brought up to the heater may give a surprising result. The white paper and blackened foil absorb nearly the same amount of infra-red radiation, even though the white paper reflects most of the infra-red radiation. What is it about the respective properties of paper and aluminium foil?
• How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and the absorption of infra-red radiation. You could:
• Use the experiment to illustrate the concept of valid evidence.
• Point out that, in a comparative analysis, numerical data is not always necessary.
• Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
• Ask students to illustrate the findings of this data-free experiment in an appropriate way.

This experiment was safety-checked in January 2007

Class practical

Comparing the infra-red radiation from a dull black and a shiny surface.

Apparatus and Materials

• Copper sheet, mounted, (or use Leslie's cube, with mains immersion heater)
• Thermopile
• Light-spot galvanometer
• Retort stand, boss, and clamp
• Bunsen burners, 4
• Methylated spirit
• Vegetable black

Health & Safety and Technical Notes

Supervise this experiment closely. Watch that students do not get too close to the hot plate.

Read our standard health & safety guidance

The mounted copper sheet is made from a sheet of 5-mm copper, blackened on one side, with 1-cm iron rod handle secured rigidly with two nuts and bolts.

Give one side of the copper sheet a coat of vegetable black mixed with methylated spirit. Allow it to dry so that it has a dull black surface. Polish the other side of the plate so that it is bright. (Tarnishing in the flame is inevitable.)

Procedure

1. Secure the copper sheet rigidly to the retort stand, using a boss. The sheet should be horizontal with the bright side downwards.
2. Heat it vigorously with four Bunsen burners underneath until it is as hot as possible.
3. Remove the Bunsen burners and turn the plate so that it is vertical. Avoid burning hand when doing this.
4. As quickly as possible ask students to hold the back of their hand first near but not touching the bright side of the plate, then the black side, and then back near the bright side. (Teacher supervision essential).
5. Alternatively, use a thermopile connected to a light-spot galvanometer as a detector of infra-red radiation.

Teaching Notes

• The plate will need to be re-heated after every 6 to 8 students have tried it.
• Radiation appears to come from hot surfaces. Copper is a good conductor and the two sides of the plate will be at the same temperature. You can bring your hand much closer to the shiny side than to the black side without burning it. More radiation is coming from the black side even though it is at the same temperature as the shiny side.
• An alternative experiment is the Leslie cube, made of copper, whose four vertical faces are finished differently. Boiling water is placed in the cube and the radiation detected with the cheek as the sensitive detector. The temperature is too low for this to be impressive using the back of the hand: a thermopile or other detector is better. (The water may be kept boiling with a mains immersion heater.)
• To make the distance comparison clear, both hands are brought up to either side of the plate. The hand on the shiny side can approach closer to the plate.
• How Science Works extension: No measurements are taken in this experiment, but it is still possible to draw a valid conclusion about the nature of a surface and infra-red radiation. You could:
• Use the experiment to illustrate the concept of valid evidence.
• Point out that, in a comparative analysis, numerical data is not always necessary.
• Ask students whether this experiment would still provide valid evidence if three or more colours were investigated. If they say it would not, ask them how the experiment could be adapted so that it did.
• Ask students to illustrate the findings of this data-free experiment in an appropriate way.

This experiment was safety-checked in August 2007

Class practical

An introductory experiment showing that electromagnets can conveniently be switched on and off.

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends

• Iron filings in a pepper pot

• Nail, large, made of iron
• Tintacks or paper clips, supply of

• Power supply, low-voltage

• Chemical balance OPTIONAL
• Ammeter, 0-1 A OPTIONAL

Health & Safety and Technical Notes

Read our standard health & safety guidance

The nail should be made of iron which is magnetically soft (cut nails are suitable).

The nail may also have gained some magnetism while it has been lying in a cupboard in the Earth's magnetic field. This is easily remedied by heating the nail to cherry red heat and allowing it to cool in the East-West direction. Alternatively use a demagnetising coil, in which an alternating potential difference is connected to a solenoid, and the nail is then slowly withdrawn from the coil to a distance from it.

Procedure

1. Wind a few dozen turns of insulated wire around an iron nail. (Leave enough wire free at either end to make connections to the power supply.)
2. Connect the ends of the wire to the low-voltage DC power supply, so that a large current flows round the coil.
3. To find out if the nail is a magnet, test it with iron filings. What happens if you turn the current off?
4. Offer your electromagnet some larger bits of iron, such as tintacks or paper clips.
5. What happens each time you turn the current off?

Teaching Notes

• Soft iron is a good temporary magnet. A steel nail will retain a lot of its magnetism once the current in the coil is switched off.
• Iron filings are chips of soft iron which become temporary magnets when in a magnetic field, and so they line up north to south indicating the direction of the magnetic field.
• How Science Works extension: This experiment can produce a valid relationship between the number of coils and the strength of the electromagnet without any measurements, only counting.
• After a demonstration of the procedure above, students could be asked to design a version of the experiment which would allow them to investigate two factors affecting the strength of the electromagnet: the number of coils and the current flowing in the wire. The number of paper clips held by the electromagnet could indicate the strength of the electromagnet.
• This provides an opportunity to discuss the concept of a discrete variable and whether evidence based on discrete variables can lead to a valid conclusion.
• The scope of variables here is limited, so this would be suitable as a first investigation that students might plan and carry out themselves, with little or no guidance. Encourage students to find appropriate ways in which to present their results to make them clear and easy to understand.
• If students use the mass of iron filings picked up as a measure of the strength, making measurements can prove problematic. One solution is to have a mass of iron filings on a balance pan, use the electromagnet to remove whatever it can and then record the drop in the balance reading.

This experiment was safety-checked in January 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

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

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