Electrical Circuit
Electricity and Magnetism

Power and brightness of bulbs

Physics Narrative for 11-14 Supporting Physics Teaching

Current and voltage leading to power

To predict the brightness of bulbs you need to think about the power in the electrical pathway. The brightness is the result of two electrical factors:

  • The flow – that is the current
  • The push – that is the voltage

To make this concrete, let's start with a circuit, in which a 12 volt battery is connected to a single bulb:

This circuit is similar to that suggested for the big circuit in episode 01, where a car headlamp bulb was connected to a 12 volt supply. Assume that an ammeter connected into the circuit shows a current of 2 ampere and start by thinking about the bulb:

Current in the bulb is 2 ampere.

So 2 coulomb of charge pass through the bulb each second.

Voltage across bulb is 12 volt (assuming that each coulomb of charge shifts all 12 joule of energy in the bulb).

12 joule of energy are shifted to the surroundings for every coulomb of charge passing through the bulb.

If these current and voltage figures are taken together, we can see that in the bulb there's continuous activity:

  • 2 coulomb of charge pass through every second.
  • Each coulomb of charge shifts 12 joule of energy.

So 24 joule of energy are shifted by the bulb each second.

In other words, as the filament of the bulb warms up and radiates, 24 joule of energy are shifted to the surroundings each second. The power of the bulb is 24 watt.

Energy is shifted (or transferred) by the circuit: from the chemical store of the battery to the thermal store of the surroundings; via heating and lighting pathways (see SPT: Energy topic).

What you have calculated here is the energy shifted every second by the electrical working pathway – that is the power input.

All of this ends up heating or lighting.

The power input is equal to the power output: the bulb switches power from one pathway to another.


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