Pathways may be more natural

An easier route

We can also describe the action of a simple circuit in terms of pathways (see the SPT: Energy topic). Electrical appliances are sold on the basis of how much energy they are shifting each second: you buy bulbs on the basis of their power rating – they are marked in watts, not in joules. So, although electric circuits are so prevalent because they shift energy, this effect is through accumulated electrical working, not the steady shifting that is what we detect with our eyes. Describing what is happening now in a circuit is best done in terms of power. If you settle on a duration and so consider the circuit running for a fixed length of time – what has happened – then you can sensibly discuss energy. Indeed that accumulation over time is what your domestic bills record.

What is happening in a simple lamp and battery circuit is happening continuously: only when you consider the accumulated effect of this activity can you ask and answer questions about joules. When the bulb is connected to the chosen battery, the brightness is set by choice of bulb and the current in the bulb. This brightness is a result of power being dissipated.