Class practical

Calculating the energy transferred per second from a lamp.

Apparatus and Materials

For each group of students...

• Power supply, LV
• Lamp 12V 6W
• Lamp holder on base
• Ammeter (0 - 1 amp), DC
• DC voltmeter (0 -15 volt)
• Variable resistor, optional

Health & Safety and Technical Notes

Read our standard health & safety guidance

The rating of the lamp is chosen so as to provide reasonable current and voltage readings. Any lamp that produces similar values to a (12 V 6 W) lamp is suitable. Remember that, on switch-on, a lamp draws several times the rated current: the power supply must be able to supply this.

Procedure

1. Connect the circuit shown and take readings of the ammeter and voltmeter. Calculate the energy transferred electrically each second.

Teaching Notes

• To give more practice in making calculations of power, a variable resistor can be included in the circuit. Students take a series of readings and compare them with the brightness of the lamp.
• The table could be labelled as shown:
• Current in amps (charge flowing in coulombs per second)
• Potential difference in volts (energy transferred in joules by each coulomb)
• Power (= energy transferred electrically from the power supply per second, in joules/second or watts (Power = VI)
• Working out the units is a useful check on what is happening in the circuit in terms of the physics.
• How Science Works extension: Students could be asked to design an experiment whereby they calculate the efficiency of the energy transferred electrically to light. Discussion will likely identify the difficulty in quantifying the amount of light produced. What should also emerge is that the amount of light radiated can be inferred by calculating the energy stored thermally. A possible approach is to put the lamp in a sealed polystyrene cup filled with air (or even water) and measure the temperature rise.
• The specific thermal capacity of air at constant pressure is about 1,000 J/kg 'C, and that of water is 4,200 J/kg 'C

This experiment was safety-checked in January 2007

Demonstration

The intensity of illumination from different lamps is compared.

Apparatus and Materials

• Lamp holders (SBC) on base, 2 (Safety pattern batten lamp holders are best)
• Mains lamp, 230V 25W, pearl
• Mains lamp, 230V 60W, pearl
• Screen and post

Health & Safety and Technical Notes

Safety pattern lamp holders incorporate a switch so that when the bulb is removed the contacts become dead. The lamp holders must use a double-insulated mains lead fitted with a proper (13 A) mains plug, and a suitable fuse.

Read our standard health & safety guidance

Procedure

1. Switch the lamps on and compare the brightness of their illumination.
2. Stand the lamps in front of the post so that the shadow of the post falls on the screen. If the lamps are lit one at a time, the apparent intensity of the shadow gives a good comparison of the lamp intensities.

Teaching Notes

• The two lamps are rated at different wattages. This means that they will transfer energy at different rates. A different number of joules per second are carried from the power supply to the lamp.
• Students may wish to see the markings on the lamps to confirm the power ratings.

This experiment was safety-tested in February 2006

Demonstration

Measuring how the light intensity of a lamp varies with power input.

Apparatus and Materials

• Power supply, LV
• Lamp, 12 V 24 W
• Lamp holder (SBC for LV use)
• Rheostat (10 - 15 ohms)
• Demonstration meter (0 - 15 volts DC)
• Demonstration meter, 0 - 5 amp DC
• Light meter (or photographic exposure meter)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Digital light meters suitable for education use can be obtained from several electrical equipment suppliers for £30 or so.

Procedure

1. Set up the circuit shown with the light meter some 10 - 15 cm from the lamp.

2. Record values from the light meter, together with ammeter and voltmeter readings.
3. Calculations of the power input (energy transferred electrically per second to the lamp) should range from 10 to 30 watts (corresponding to a change of 7-14 volts).
4. Plot a graph of light intensity against power input.

Teaching Notes

• The graph of light meter readings against power input is a very important one when the efficiency of lamps is being investigated. This graph should give approximately a straight line, not passing through the origin.
• This is an opportunity to discuss the power rating of devices. This is the power input when the device is connected to the potential difference for which it has been designed. It will not transfer energy at the same rate if connected to a lower (or higher) potential difference.
• The light intensity is a proxy for the power output (the energy transferred per second by the light).
• The light meter may need to be placed further from the lamp. For a 100-watt lamp, meters will be needed recording up to 250 volts AC and 500 mA. AC.
• Energy efficient lamps could also be tried but if high voltages are needed, then suitable precautions need to be taken.

This experiment was safety-tested in January 2006

Class experiment

Measuring the power input to a motor.

Apparatus and Materials

• For each group of students
• LV power supply, variable DC
• Motor/generator unit
• Ammeter (0 - 1 amp), DC
• DC voltmeter (0 - 5 volt)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

Power input

1. Set up the circuit as shown in the diagram with no load on the motor other than friction.

2. Adjust the voltage supply to give the appropriate voltage for the motor.
3. Note the readings of the ammeter and voltmeter.
4. Calculate the number of joules transferred each second.
Efficiency of the motor

5. The experiment can be extended by using the motor to drive a generator which lights 1, 2, or 3 lamps in a lamp unit. Connect the voltmeter across the voltage supply and use the ammeter to measure the current to the motor. Connect 1, 2 or 3 of the lamps into the circuit. Note the change in input power. Now place the voltmeter across the generator terminals, and the ammeter in series with the lamps. Record the results in a table and calculate output power.

Teaching Notes

• The power transferred to the motor is calculated by measuring the current in the circuit for various potential differences across the motor. You might wish to use the heading of ‘charge flowing per second, in coulombs per second’ as well as ‘current in amps’. Similarly you might want to use ‘energy transferred in joules by each coulomb’ as well as ‘potential difference in volts’.
• Electrical power is calculated from P = potential difference x current = VI
• The power, transferred to the motor from the power supply, is then expressed in joules/second or watts.
• In step 3, the efficiency of the energy transfer is calculated from

This experiment was safety-tested in January 2006

Demonstration

Comparing the power output of two motors.

Apparatus and Materials

• Electric motor, small
• Switch unit
• Mass, 1 kg
• Line shaft unit
• Power supply, 0-12 V
• Fractional horse-power motor (or the largest DC motor available)
• G-clamps
• Cord
• Rubber band or driving belt
• Knife switch
• Stopwatch
• Metre rule

Health & Safety and Technical Notes

The person controlling the switch must stop the motor before the load reaches the line shaft. (If the load spins round the shaft, the string may break and the load go flying.)

To stop the motor, the current must stop flowing in both field and armature coils. The power supply may incorporate a smoothing capacitor which discharges slowly when the mains switch is operated.

Read our standard health & safety guidance

Procedure

Small electric motor

1. Connect the small electric motor to the line shaft using the drive belt. Attach the mass to the line shaft with the cord.
2. Connect the motor to the variable DC supply using a switch and set at the appropriate voltage for the motor so that the load rises slowly. (Although the knife switch is not essential, it allows the power to be connected for a more precise time.)
3. Time the load being lifted a fixed distance, say 1 metre.

4. Connect the field and armature terminals of the fractional horse-power motor in parallel to the DC terminals of the power supply set at 6 volts (or the rated voltage for the motor).
5. Attach the mass to the shaft by cord, so that it is lifted when the motor is switched on, as before.
6. Note the time for raising the load through 1 metre and compare this with time taken for the small motor.

Teaching Notes

• The time taken to raise the load is shorter when a higher potential difference is used. The motor is said to be ‘more powerful’. The loads can be changed and the potential difference kept constant; larger loads take longer to raise. There is only so much energy per second that a motor can transfer for a given potential difference, and so it works more slowly with heavier loads.
• Change in energy stored gravitationally = m x g x Δh joules
• Power output to raise the load = m x g x Δh / t watts
• Where m is the mass in kilograms, Δh is the vertical distance in metres, g is the gravitational field strength in newtons per kilogram, and t is time taken in seconds.
• Energy transferred from a power supply to a component, such as a motor = V x Q joules
• Power input due to current in motor = V x Q / t = V x I watts
• Where V is potential difference in volts, Q is charge in coulombs, I is current in amps and t is the time taken in seconds.
• The field coils of the fractional horse-power motor also have energy transferred to them. For a more accurate comparison with other motors, measure the potential difference across the armature and the current passing through it.
• If you compare the small motor and the fractional horse-power motor, both running on 6 volts and hauling up the same load, the larger motor is faster. Measuring the current through the armature will show that the current is also greater through the heavier motor.
• Another variation is to run the two motors at the same voltage, but with different loads, so that the speed with which the load is raised is the same for both motors. The bigger motor will have a larger current passing through it, and will haul up a larger load.
• The motors can also be compared by measuring the braking force to stall the motor. Increase the load until the motor stops turning. (Do not keep it long in a stalled position.)

This experiment was safety-tested in January 2006

Demonstration

Comparing the power output of two motors.

Apparatus and Materials

• Small electric motors of different powers, 2
• Line shaft unit
• Knife switch
• Mass, 1 kg
• Power supply, LV
• Rubber band or driving belt
• Cord
• Stopwatch & metre rule

Health & Safety and Technical Notes

The person controlling the switch must stop the motor before the load reaches the line shaft. (If the load spins round the shaft, the string may break and the load go flying.)

In all activities where loads may fall on toes, precautions (such as using cardboard boxes full of waste material) should be taken.

Read our standard health & safety guidance

The motors suggested are part of the Malvern Energy Conversion Kit, now obtainable from the supplier: Beecroft and Partners.

Procedure

1. Clamp the two electric motors to the bench on either side of the line shaft unit, so that the driving belt can be connected to either of them. The line shaft is used for lifting the load of 1 kg.
2. Link one of the motors to the line shaft and connect it to the power supply.
3. Measure the time taken to raise the load from the floor to the bench.
4. Repeat for the second motor.

Teaching Notes

• The same amount of work has to be done in both cases. The same amount of energy will be stored gravitationally in the raised weight in both cases. The energy is transferred electrically from the power station by the power supply and motor to the raised weight, but the faster motor is the more powerful.
• Labels on the motors may indicate the horse-power or wattage. The motor does not always supply this power, (the maximum recommended). Instead the motor adjusts itself to the load. If more power than that is demanded the motor will overheat and perhaps burn out. Its efficiency is also likely to be lower.
• It may be worth indicating that animals, including humans, adjust to the load as well; watch how slowly someone staggers when they have to lift something heavy. Safety rules try to prevent people hurting themselves. (Injuries to backs may not manifest themselves for many years.)

This experiment was safety-tested in January 2006

Class practical

Estimating the change in energy stored gravitationally in climbing a flight of stairs.

Apparatus and Materials

• Bathroom scales
• Tape measure, 5 m

Health & Safety and Technical Notes

Since this activity does not require the time taken to be measured, a delegation could measure the height while the class remains in the laboratory or classroom.

Read our standard health & safety guidance

Procedure

1. Each student must know their mass in kilograms, use the scales to determine it, or use an 'mass of an average person'.
2. Measure the height of the stairs from the ground floor to the top of the highest step.
3. Each student climbs the flight of stairs and calculates the increase in the energy stored gravitationally.

Teaching Notes

• When someone climbs the stairs energy is transferred from their muscles (energy stored chemically in food and oxygen) to be stored gravitationally. The change to the energy stored gravitationally can be calculated using the equation: change in energy stored gravitationally = mgΔh.
• Where mg is the weight of the climber and QuantitySymbol{Δh} is the vertical distance climbed.
• You may need skills in crowd control if other classes are not to be disturbed and students are not to get hurt. Students may try to turn this experiment into a demonstration of power, by timing how fast they can get to the top of the stairs!
• Students may also ask why they feel fatigued when they hold up a load but the load doesn't move. In order to grip the load, muscles are kept tensed; they squeeze the blood vessels and restrict blood flow. As a result the chemical products of muscular activity accumulate and are not washed away so quickly by the blood. This accumulation of chemical products makes the nerves give a sense of fatigue. So the feeling of fatigue is chiefly an indirect result of the muscle tension.
• The continuing demand while you hold a load at rest arises from the muscle fibres developing tension very rapidly. In a large muscle, fibre after fibre is fired into tension as a nerve impulse arrives but each fibre soon relaxes. Later on it renews its tension again. So the steady pull of a muscle is really the result of many brief tugs, a dynamic force. The sum total of these pulses of force shows tiny statistical fluctuations like a trembling effect.
• There is a transfer from energy stored chemically to energy stored thermally. Releasing the tension does not transfer energy back to the muscles.

This experiment was safety-tested in January 2006

Class practical

Students measure their personal power by running up a flight of stairs.

Apparatus and Materials

• Stopwatch
• Metre rules, 2
• Bathroom scales, calibrated in kg (or N)
• Flight of stairs, not less than 3m in height, and preferably much more

Health & Safety and Technical Notes

Those with health difficulties may need to be given other jobs to do. Check on students in advance of the lesson.

Before using this activity, the teacher must assess carefully the likelihood of the runner slipping or any other accident on stairs. If the risk is serious, consider substituting a properly-supervised climbing activity in the gym.

Read our standard health & safety guidance

Procedure

1. Measure the height of the staircase in metres.
2. One student runs from the bottom to the top of the stairs, whilst another student times them with the stop-watch.
3. Students who do not know their weight in newtons can use the scales to find their mass in kilograms and hence calculate their weight.
4. Repeat the experiment, but with students walking up the stairs at a speed which they guess they could keep up for an 8-hour day.

Teaching Notes

• This experiment will generate noise and discipline problems unless it is well organized. Students will wish to compete against one another to find the most powerful person. Plan in advance where students will stand and how other staff and/or students can pass by whilst the experiment is in progress.
• Students measure their own useful power from:
• Power in watts = (weight in newtons x height of stairs in metres) / time in seconds
• Where the weight in newtons = mass in kilograms x g (take g to be 10 N kg^-1)
• A very short staircase and a flying start can give astounding values of power which are quite misleading. A short burst of effort cannot be sustained. Adult males may come close to one kW for a short spurt to about 75 watts for continuous labour.
• Look at dietary charts (or packets of crisps or whatever) for energy stored in food and calculate how many cream cakes it takes to climb the stairs or do a manual job for 8 hours. Or how many stairs can be climbed on one cream cake.

This experiment was safety-tested in January 2006

Class practical

Measuring the power used in cycling uphill.

Apparatus and Materials

None listed

Health & Safety and Technical Notes

This must be done in a safe place, such as the school grounds.

Read our standard health & safety guidance

Procedure

1. Students time themselves cycling up a rise.
2. Determine the vertical distance climbed from an appropriate map or using a handheld GPS device. Alternatively, estimate the height with a sighting pole, a plumb line, and a large cardboard protractor.
3. Knowing their own weight and estimating that of their bicycle, students calculate the output of useful power.

Teaching Notes

• Useful power is calculated from:
• Useful power = energy transferred/time taken = (total weight in newtons x height rise)/time taken
• The mass of a bicycle is very unlikely to be less than 5 kg nor more than 20 kg . 10 kg would be an acceptable estimate, particularly as the mass of the student is probably of the order of 70 kg.
• This experiment could be set as a class activity if a suitable site is available.

This experiment was safety-tested in January 2006

Class practical

This activity explores factors which affect the power output of a solar panel, and may lead to an investigation of quantitative aspects of some factors.

Apparatus and Materials

Each group will need

• Solar panel unit
• Small motor unit
• Desk lamp (40 or 60 W tungsten lamp)
• Digital multimeter (or voltmeter)
• Plug-plug 4 mm leads (red), 2
• Plug-plug 4 mm leads (black), 2
• Metre rule
• Piece of cardboard
• Translucent sheets (e.g. tracing paper) cut a suitable size to cover the solar panel
• Coloured filters
• Clamp stand

Health & Safety and Technical Notes

Desk lamps with metal shades can get very hot. Take care when moving them.

Place the colour filters far enough away from the lamp so they don’t burn.

Be careful if considering other types of lamp such as halogen lamp and fluorescents because they can emit significant UV.

Read our standard health & safety guidance

The activity works best if done in a room with blackout or low light levels. Otherwise ambient light may overwhelm the variations in light levels being controlled.

If students are to measure the distance of the lamp from the panel, make a reference mark on the lamp casing level with the centre of the light bulb.

You can obtain solar panels and a small motor unit from:

Middlesex University Teaching Resources

• SEP 050 Solar panel unit
• SEP 051 Large solar panel unit
• SEP 052 Small solar panel unit
• SEP 054 Small motor unit

Procedure

1. Connect a solar panel to an electric motor. Shine a desk lamp on the panel so that the motor turns.

2. Connect a voltmeter across the solar panel.

3. The voltmeter can be used to get an idea about the output of the solar panel. A voltmeter does not measure power (power = voltage x current), but the voltage can be used to make comparisons.
4. Explore the effects of:
• Moving the lamp closer and further away
• Partially covering and uncovering the solar panel
• Tilting the panel backwards and forwards
• Putting a translucent sheet between the lamp and the panel
• Putting different coloured filters between the lamp and the panel.

Teaching Notes

• A solar power system for a house is not always going to give the same power output. It will depend on the time of day, the season, the weather, and so on.
• In this activity, students use a range of investigative skills, including the identification of different kinds of variable, the tabulation of data and drawing different kinds of graphs and charts.
• This can be done in small groups as a qualitative activity to get a feel for the factors that affect the output of a solar panel, in which case the activity may only take up part of a lesson.
• Students should find that the maximum output is given with a high light intensity and the biggest surface area, and with no tilt in relation to the light source. The translucent sheet and coloured filters reduce the power output.
• Student could go on to a quantitative investigation of particular factors, done in pairs and taking longer. Different groups could each look at a different factor, and then report back to the whole class. Possible relationships to investigate:
• How does the distance of the lamp affect the voltage across the panel? A suitable range of distances is from 10 to 25 cm (if using just the motor); with the voltmeter, a suitable range of distances would be from 10 to 50 cm.
• How does the area of the panel exposed affect the output voltage?
• How do different filters affect the output voltage? for example: glass, clear plastic sheet, white paper
• Ask students to explain how their results relate to real conditions with the Sun. You could have students take their panel outside and look at the effect of tilting it, of partially covering it or of using different kinds of filters.

This experiment is copyright of Gatsby Science Enhancement Programme, and is reproduced with permission.