# What happens in circuits?(Introduction)

### Describing circuits

When a battery is connected to a bulb to make a complete circuit, the bulb lights up. If you are interested in understanding how electric circuits work, this familiar event raises a number of questions.

While the bulb is lit energy is shifted or transferred (see the SPT: Energy topic), to the surroundings as the filament of the bulb glows and light is emitted (see the SPT: Energy topic).

• Where does this energy come from in the first place?
• How is the energy shifted to the surroundings by the bulb?
• How does the bulb come on so quickly?
• You probably have some pretty good ideas about answers to these questions. What is needed is a way of thinking about electric circuits that allows us to picture what's going on inside them (inside the battery, wires and bulb). You can see the effect of what's going on (the bulb lighting up); what is needed is a model for the electric circuit to help you understand why that happens.

### So what actually happens?

What happens when a battery is connected to a bulb to make a circuit? The bulb lights up very quickly as the circuit is completed. The situation switches from equilibrium, where there is very little happening, to a steady state, where the bulb is glowing, very quickly. We're going to suggest that you concentrate on the steady state, where the bulb is glowing steadily.

The energy must come initially from the battery which acts as an energy store (see the SPT: Energy topic). This being the case, what happens in the circuit to enable the energy which originates in the battery to be shifted to the surroundings via the bulb? The energy story gives us an one level of description of what happens but provides no details about the mechanism.