# Measuring the power of a lamp

##### 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

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 a Connect the circuit shown and take readings of the ammeter and voltmeter. Calculate the number of joules transferred each second to heat and radiation.

#### Teaching notes

1 To give more practice in making energy calculations, 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 transferred from the power supply 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.

2 How Science Works extension: Students could be asked to design an experiment whereby they calculate the efficiency of the energy transfer 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 thermal transfer. 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.

This experiment was safety-checked in January 2007