Making energy real: using the SEP Energymeter

Energy and Thermal Physics

Getting to know the joule and the watt

Practical Activity for 14-16

Class practical

Students use a hand-turned generator to gain direct experience of measuring energy transfer and to get a ‘feel’ for the size of a joule and the size of a watt.

Apparatus and Materials

SEP Energymeter and mains adaptor
Hand-turned generator and small (low voltage) electric motor (e.g. SEP Energy transfer unit)
2 plug-plug leads, red
2 plug-plug leads, black

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

Before using the energymeter, first connect the hand-turned generator to an electric motor. Turn the generator by hand, and note what happens to the motor as it is turned at different speeds.
Now include the energymeter into the circuit as shown in the diagram. Plug the mains adaptor into the energymeter. Set the knob on the energymeter to measure energy transferred.

Press the start/pause button of the energymeter and turn the generator. Watch the display to see how the value of energy transferred increases cumulatively as the handle is turned.
Note how the value of energy transferred increases faster as the handle is turned faster. This could be done by seeing how long it takes to transfer 10 J of energy when the handle is turned at different speeds.
A graph showing how the energy transferred changes over time can be made by measuring the total energy transferred after 10, 20, 30, 40 and 50 seconds. Two sets of measurements could be made by turning the handle first at a slow speed and then at a faster speed. This should give a graph showing two lines of different slopes.
Now set the knob on the energymeter to measure power. Turn the handle again and note how the value of power increases when the handle is turned faster and decreases when it is turned slower.

Teaching Notes

The key ideas that can be taught through this activity are that:

Energy is measured in joules
Power is the rate at which energy is transferred
Power is measured in watts.

A hand-turned generator is a good way to make a start on this, because it provides an experience through physical activity. Before making measurements, it is helpful to explore the effects qualitatively. Turning the generator by hand gives a very direct ‘feel’ for the way in which energy is ‘shifted’ from the generator to the motor; if the leads to the motor are removed so that there is no load, it is very striking how easy it is to turn the generator.
The SEP Energy transfer unit is a convenient piece of equipment for carrying out this experiment, though any similar hand-turned generator and motor could be used. For the generator on the energy transfer kit, transferring 10 J of energy takes about 25 seconds when turned at moderate speed and 15 seconds at a fast speed.
When students draw a graph plotting energy transferred against time, they should find that they get a steeper slope for the values when they turn the handle at a faster speed. This can form the starting point for a discussion of power (the slope of the line represents the rate at which energy is transferred, i.e. the power).
It is worth making a comparison between the energy transferred in physical activity and the energy values of foods. A typical energy value for a 100 g chocolate bar is 2000 kJ. It would take 5 000 000 seconds (25/10 x 2000 x 1000) or about 1400 hours to transfer this amount of energy from the generator to the motor when turned at moderate speed.

Total Energy of a System

Energy and Thermal Physics

Using an energymeter to measure efficiency of energy transfer

Practical Activity for 14-16

Class practical

Students make measurements on the energy transfers involved when a falling mass drives an electrical generator, and calculate the efficiency of the process

Apparatus and Materials

For each student group

SEP Energymeter and mains adaptor
Pulley / generator and small (low voltage) electric motor (e.g. SEP Energy transfer unit)
1 slotted mass hanger (100 g) and 1 slotted mass (100 g)
Thread for attaching slotted mass to pulley
Metre rule
2 plug-plug leads, red
2 plug-plug leads, black

Note: The apparatus should be placed on top of a box on the lab bench so that it gives a 1 metre drop above the floor.

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

Before using the energymeter, first connect the generator to an electric motor and wind the thread attached to the slotted mass around the pulley. Let the slotted mass fall, and note that the motor turns around.
Now include the energymeter into the circuit as shown in the diagram. Plug the mains adaptor into the energymeter. Set the knob on the energymeter to measure energy transfer.

Turn the pulley to wind up the thread so that the 100 g mass is at the top of the metre rule. Press the ‘start/pause’ button on the energymeter and let go of the pulley. The mass should fall and the motor should turn round. From the energymeter display, make a note of the energy transferred from the generator to the motor. Repeat the experiment to see if you get consistent readings.
Calculate the energy transferred from the falling mass to the generator (work done by the falling mass) using the relationship: work done (J) = force (N) x distance (m)
From the values of the energy transferred from the falling mass to the generator and from the generator to the motor, calculate the efficiency of the process. efficiency = (energy transferred to motor / energy transferred from falling mass) x 100%
Repeat the experiment with different amounts of work done by the falling mass, for example a 100 g mass and a 0.5 m drop, a 200 g mass and a 1 m drop.

Teaching Notes

The key ideas that can be taught through this activity are that:

work done can be calculated from the force applied and the distance moved
the efficiency of a process can be calculated from the energy input and the energy output.
In most contexts, energy input means the energy transferred electrically from a power station or cell. Fuel or chemicals are depletd. Energy output means the work done in the device to do a job, such as heating, or lighting. Energy ends up being stored thermally in the surroundings.

If students have not used this kind of arrangement before, then it is helpful if they explore the system qualitatively first before adding the energymeter. The SEP Energy transfer unit is a convenient piece of equipment for carrying out this experiment, though any similar generator and motor could be used.
To avoid damage to the generator, the thread should be long enough for the masses to reach the ground to avoid jerking on the generator. Note that the thread needs to be wrapped clockwise on the pulley, so when the leads to the energymeter are connected with the correct polarity (i.e. red to red, black to black), the energymeter will record the energy transferred correctly. If the thread is wound the wrong way, the meter displays ‘Source current wrong direction’. The message ‘Source current wrong direction’ will also be displayed when the pulley is turned backwards to wind the thread back after the mass has fallen – students should ignore this.
A mass of 1 kg has a weight of about 10 N, so a mass of 100 g has a weight of about 1 N, i.e. a 100 g mass is pulled towards the Earth with a force of 1 N. When it drops by 1 metre, the mass does 1 J of work on the generator. This is the energy input. Of course, we would expect the energy output measured using the energymeter to be less that the 1 J that was put in, but how much less will depend of the efficiency of the generator. Typically, the reading on the energymeter is about 200 mJ, so the efficiency is about 20%.

Total Energy of a System

Energy and Thermal Physics

Using an energymeter to measure power in electrical circuits

Practical Activity for 14-16

Class practical

Students make direct measurements of power in various electrical circuits using an energymeter, and use values of voltage and current to calculate power.

Apparatus and Materials

For each student group

SEP Energymeter and mains adaptor
2 batteries in holder (1.5 V, AA or D size)
2 lamps in holders (2.5 V, 0.2 A)
3 plug-plug leads, red
3 plug-plug leads, black
Spare bulbs

Health & Safety and Technical Notes

Keep in mind that some types of battery (e.g. NiMH, nickel metal hydride) can give high currents if accidentally shorted.

Read our standard health & safety guidance

Procedure

To measure the power in a circuit with two batteries and a lamp, the energymeter needs to be included in the circuit as shown below. Plug the mains adaptor into the energymeter. Set the knob on the energymeter to measure power.

The energymeter can be used to measure the power in other arrangements of batteries and lamps. Try to make predictions about the power before making measurements. For example, will the measurement in (b) with two lamps in series be greater or less than in (a)? Will the measurement in (c) with two lamps in parallel be greater or less than in (a)?

Try making predictions about the behaviour of the circuits if just a single battery is used, then measure the power.
The energymeter can measure power because it acts as both a voltmeter (measuring the voltage across the source) and an ammeter (measuring the current in the circuit). It then uses this equation to calculate the power:
power (W) = voltage (V) x current (A)
Turn the knob on the energymeter to measure ‘V, I and P’. Note how the power is calculated from voltage and current. Use the values of voltage and current to explain the differences in power in each of the circuits.
Predict how much energy would be transferred in each of these circuits in a period of 20 seconds, using the formula below. Turn the knob on the energymeter to measure energy and test your predictions.
energy transferred (J) = power (W) x time (s)

Teaching Notes

The key ideas that can be taught through this activity are that:

the power in simple electrical circuits is dependent on the numbers and arrangements of the lamps
power can be calculated from measurements of voltage and current
energy can be calculated from measurements of power and time.

Though the energymeter can measure voltage and current, it is certainly not a substitute for the traditional voltmeter and ammeter. An essential idea needed for understanding electrical circuits is the distinction between voltage and current. The use of two separate instruments emphasizes that, for example, in a circuit containing, a battery and a bulb, the voltmeter measures voltage across the battery, while the ammeter measures current through the circuit. If the energymeter is introduced before this distinction is made, then it simply becomes a ‘magic box’ that measures everything. However, once the concepts are differentiated, then being able simply to turn a knob on the energymeter to move between displays of different values can be a very effective way for students to see how the concepts of voltage, current, power and energy relate to each other.
With the apparatus specified above, a typical value for the power with two batteries and a lamp is about 600 mW. The power would be lower for two lamps in series (greater resistance, less current) and higher for two lamps in parallel (less resistance, greater current). To calculate values of power from voltage and ammeter, it would be possible to use a separate voltmeter and ammeter instead of using the energymeter to obtain values, but there may be less agreement because of the variations in the accuracy of the instruments.

Total Energy of a System

Energy and Thermal Physics

Measuring energy with the SEP Energymeter

Teaching Guidance for 14-16

One of the problems about teaching energy effectively is that there has often been an emphasis on learning about ‘accepted’ qualitative descriptions; the key idea that energy is a quantitative concept used to solve real problems tends to get lost. Many people have emphasized the importance of introducing younger students to the idea that energy is quantitative, for example, by comparing the energy values given on food labels. Using secondary data like this is very helpful, but is not a substitute for students getting hands-on experience of obtaining their own data through practical measurements.

The SEP Energymeter was developed to enable students to get direct experience of measuring energy and power in low-voltage circuits (it can be used in circuits up to 10 V). The intention was to produce a low-cost instrument that schools could afford to buy in sufficient numbers to maximize the opportunities for hands-on activity, and to complement the existing meters for the domestic market that measure energy consumption for mains appliances.

Concepts such as temperature and force are no less difficult and subtle than energy, but they have acquired an apparent straightforwardness because it is possible to measure them directly. Similarly, for energy, if students can make their own measurements it can deepen their understanding of the concept.

SEP Energymeter

The energymeter has two sets of terminals – ‘source’ (1) and ‘load’ (2). In the example below, the diagram shows how the energymeter can be connected to make measurements on a domestic torch. Two batteries are connected to the ‘source’ terminals and the ‘load’ terminals are connected to the torch (by taking the cover off, removing the batteries and clipping the leads onto the contacts inside).

The energymeter is supplied with a 12 V DC mains adaptor which is plugged into a socket (5). When it is inserted, values will appear on the display panel (3).

The function knob (4) has four positions and allows the following measurements to be made:

Energy
Power
Average power
Voltage, current and power

The display of the energymeter automatically adjusts to use appropriate units for the values involved (for example, mJ, J or kJ), and automatically shifts the decimal point so that the display always shows values to three significant figures.

To measure the energy transferred from the batteries to the torch, the function knob is set to ‘Energy’ and the ‘start/pause’ button (6) is pressed. The display shows the total amount of the energy transferred, and the time elapsed since the button was pressed. Pressing the button again pauses the collection of data; pressing the reset button (7) sets the values back to zero.

With the function knob on the ‘Power’ setting, the display shows the value of the power at a particular moment in time (it refreshes every half-second).

The ‘Average power’ setting can be useful when there is a lot of variation in the power, such as using a hand-turned generator, a wind turbine, or a solar panel in varying light conditions.

To measure the voltage across the source and the current in the circuit, the function knob is turned to the ‘V, I and P’ setting. As well as voltage and current, the display shows the power (calculated from voltage x current).

The energymeter, in fact, works by measuring the voltage across the source and the current in the circuit, and calculating the power from these. From the power, it can then calculate the energy transferred over a period of time. The diagram below represents how the energymeter can be thought of as a combined voltmeter and ammeter.

Though the energymeter starts from measurements of voltage and power, conceptually, the sequence is reversed. Younger students can begin by using the energymeter to measure amounts of energy. They can then go on to measure power, developing an understanding that power is the rate of transfer of energy. Finally, older students can make measurements of voltage and current and relate these to power and energy.

There is a very wide range of experiments in which students get hands-on experience of measuring energy and power. The SEP publications ‘Making Energy Real: Using the SEP Energymeter’ and ‘Energy Storage’ give examples and suggestions for these.

These materials have been adapted from the booklet ‘Making Energy Real: Using the SEP Energymeter’ published by the Gatsby Science Enhancement Programme.

Share this article: