4.Main Content
experiments
Lamps in parallel
Demonstration
The current flowing in a circuit increases as more lamps are added in parallel with each other.
Apparatus and materials
- Lamps (12 V 6 W) in holders, 4
- Switches, single-pole, 4
- Ammeter, 0 - 5 A (a demonstration one would be good for this)
- Power supply, low-voltage
Technical notes
Calculate the current required by each lamp (current = power / voltage) and ensure that the power supply can provide sufficient current to operate 4 such lamps. Ensure also that your ammeter can show this maximum current.
Safety
Read our standard health & safety guidance
Procedure
a Connect the ammeter and the four lamp holders with the switches to the low voltage supply.
b Note the current as first one, then two, then more, lamps are switched on.
Teaching notes
1 Any energy that you transfer can be measured in joules. As long as a power supply maintains the electric current through the lamp, you continue to obtain a stream of energy which can be measured:
• by catching the light radiation on light-sensitive paper
• by warming up water
• by measuring the energy delivered by a motor.
So you can find out how many joules of energy are being transferred per second from the power supply to some other component.
2 As more lamps are connected across the power supply, the amount of energy transferred by the lamps to the environment in joules/second increases. So does the current in coulombs per second, registered by the ammeter. The rate of energy transfer increases in proportion to the current drawn from the power supply.
This experiment was safety-checked in October 2006