Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Lamp brightness comparison


This experiment provides an excellent introduction to the concept of potential difference (voltage). Students observe that two lamps with the same current give out quite different amounts of light and this sets off a discussion.

Apparatus and materials

'Lamp Brightness Comparison' unit (see Technical notes)

Multimeter with probes, 2

Power supply, low voltage, variable AC, capable of delivering at least 0.5 A

Lamp 12 V, 5 W in SBC holder

Lamp, 230 V 100 W

Leads, 4 mm, 4

Health & Safety and Technical notes

This experiment should only be done by teachers with good knowledge of mains electricity and the dangers. Do remember that you are working with potentially lethal voltages and take extra care. Students should not come near the apparatus when it is being used.

The Electrosound apparatus reduces the risk of electrocution. It includes the use of a safety mains lamp socket, which has a built in switching system that only allows it to be switched on when a lamp is inserted. Also included are two switches, which must be pressed simultaneously to route the mains current through the low voltage lamp. The switches are sited at opposite ends of the case, requiring two hands to operate them.

The main risk arises from the ammeter and connections. The 2 mm sockets on the unit are at mains potential even when the switches are not pressed. It is essential that a suitable multimeter (on ammeter setting) and connections are used. Connections to the ammeter should be made with the unit disconnected from the mains, and no adjustments made to the ammeter or wires once the unit is plugged into the mains.

The multimeter should be a type rated for 230 V, with shrouded 4 mm socket connectors. The multimeter probes should have shrouded 4 mm plugs for connection to the multimeter, and 2 mm plugs for the unit of a type that when inserted, make accidental contact with the conductors highly unlikely. All plugs and cables need to be rated at least 230 V AC.

The 'Lamp Brightness Comparison' unit must have the appropriate lamps inserted into the holders before the unit is used.

Read our standard health & safety guidance

Lamp brightness comparison unit

The 'Lamp Brightness Comparison' unit is obtainable from Electrosound, 30 Goulds Farm, Duckend Green, Rayne, Essex, CM77 6DF (tel 01376 340506) 


a Set up a simple circuit with the12 V, 5 W lamp in series with a multimeter set on the 2 amp AC range. Connect this to the low voltage power supply set at 12 V AC. 

b Switch on the power supply and observe the brightness of the lamp and record the current. Switch off.

c Make sure the 'Lamp Brightness Comparison' unit is unplugged from the mains. Insert a 100 W mains lamp and the 12V, 5W lamp into appropriate sockets of the ‘Lamp Brightness Comparison’ unit. Move the black switch to ‘on’. Connect a multimeter set on the 2 amp AC range to the 2 mm sockets, using the multimeter probes. The use of a meter is essential to the correct operation of the circuit.

d Connect the mains lead from the 'Lamp Brightness Comparison' unit into a suitable mains socket and switch on. Caution: Remember the meter probes are 'live'. Do not touch. The mains lamp should illuminate. Once again, observe the brightness of this lamp and record the current. Switch off.

e Insert the 230 V and 12 V lamps and ask students to predict what will happen (see Teaching notes). Switch mains on again. Connect the two lamps in series by depressing the two black buttons on the sides of the unit, simultaneously.


Teaching notes

1 In step b, the current through the 12 V lamp will be about 0.4 A. 

current though the 12 V lamp

2 In step d, the mains lamp is connected directly to the mains (230 V in the UK) and should operate normally. It is useful at this stage to compare the two current readings. The current through the 100 W mains lamp will also be about 0.4 A.

main lamp is connected directly to the mains 
In other words, the two lamps are each carrying approximately the same current. The question for students is why is the mains lamp brighter?

Stress the fact that the current reading alone does not give enough information to enable the amount of radiation (heat and light) from a lamp to be predicted.

3 Before going on to step e, get students to predict what would happen if the 230 V lamp and the 12 V were connected in series to the 230 V supply. Would anything happen? Would one or both of the lamps blow or explode? Involve as many students as possible in thinking about this before proceeding.

4 In step e, when the two lamps are connected in series with the mains, both lamps operate normally. This will come as a great surprise to most students. (Just a few will notice that the mains lamp is fractionally dimmer than it was, and that the current is slightly reduced.)2 lamps connected in the series
Point out that the energy transferred (radiated) every second by the mains lamp is greater than the 12 V lamp, for the same current. The higher the voltage, the more energy is radiated by a lamp - for the same current.

Introduce the concept of potential difference V, as the energy E transferred by each unit of charge Q flowing through the circuit.

Potential difference (volts) = energy transferred (joules) / charge (coulombs)

V = E / Q

'1 volt' means 1 joule is transferred by each coulomb of charge.

5 Alternatively, you could point out that the rate of energy (power, P) transferred at the mains lamp is much greater than the rate of energy (power, P) transferred at the 12 V lamp, for the same current, I. The ratio of power to current at a transducer has a name: potential difference (voltage, V). In symbols,

V = P / I

This is the same as the ratio of energy E transferred per unit charge Q, or,

V = E / Q,

since V = P / I = (E/ t ) / (Q/ t ) = E / Q

The advantage of introducing potential difference in this way is that it makes sense of something puzzling but easily observed. By contrast, the energy transferred by each unit charge cannot be observed, and is not generally of interest.

You may want some students to note what this means about the rate of energy transferred: it is the product of current x voltage.

6 With intermediate and advanced students, you may want to go on and introduce the idea of a potential divider, i.e. resistances in series share out the voltage in proportion to their relative size.

Consider the resistance of the lamp filaments.

Resistance = Potential difference/Current, so the resistance of each filament can be calculated from the measurements taken.

Using a rough calculation (which may be a little out because of the characteristics of various lamps*), the resistance of the 100 W lamp is 529 ohms and that of the 5 W lamp is 28.8 ohms.

You might re-calculate the resistance from the relationship

Resistance = (potential difference)2/Power.

*See the following note from Electrosound.

7 There are further questions you might ask at advanced level, related to the effect of temperature on resistance of a metal wire.

Using a multimeter to read the resistance of the lamp shows that the resistance is lower than calculated previously. Why should this be? What effect would this have on the current flow?

Why do mains lamps most often 'blow' when they are first switched on, rather than when they have been on for some time?

Acknowledgement: with thanks to Phil Walsh at Electrosound, for devising this apparatus and enabling a valuable experiment to get back into school use.

This experiment was safety-checked in March 2007 

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