# Using an energymeter to measure power in electrical circuits

##### 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.

#### Procedure

**a** 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.

**b** 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)?

A B C

**c** Try making predictions about the behaviour of the circuits if just a single battery is used, then measure the power.

**d** 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.

**e** Predict how much energy would be used 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 (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.

**1** 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.

**2** 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.