Electrical Circuit
Electricity and Magnetism

Energy: battery store to stores of surroundings via bulb

Physics Narrative for 11-14 Supporting Physics Teaching

Shifting energy in a circuit

Read more about the mechanisms by which energy is shifted in a circuit.

When we say that it is the charged particles moving round the circuit, that enables the shifting of energy from battery to surroundings, what exactly do we mean? The answer to this question is not straightforward! It may be of interest to you, but is certainly not appropriate for pupils in the 11–16 age range.

The essence is that when the simple circuit is complete all of the charged particles in the circuit are set into motion as they are pushed away from the battery terminal of the same charge and attracted towards the battery terminal of the opposite charge by the distributions of charge set up by the battery. (In metallic wires negatively charged electrons throughout the circuit are pushed away from the negative terminal and attracted towards the positive).

An alternative way of thinking about this is to say that the battery creates an electric field around the circuit between its positive and negative terminals, and the charged particles both start, and are kept, moving due to the effect of this field (just as a mass has a force exerted upon it in a gravitational field). All of the charged particles in the circuit experience the force or push of the battery, even if they are not in direct contact with either of its terminals (just as objects with mass experience the pull of the Earth without being in direct contact with it). This is the phenomenon of action-at-a-distance, which is discussed in detail in the SPT: Forces topic. Almost as soon as the electric circuit is completed, an electric field is created throughout the circuit.

These electrical fields are not equally great everywhere in the circuit. Where the fields are greater the charged particles shift most energy in moving a small distance. (See the SPT: Energy topic for more detail).

Remember that the comparative thickness of the wires affects how much the charged particles are impeded (thin wires have greater resistance). The charged particles are moving faster in this thinner wire. To keep the charged particles moving at a greater steady speed, there is a larger field where the wires are thinnest. That is where most energy gets shifted.

As a charge (along with the countless other charged particles) moves round the circuit under the influence of the electric field, it is impeded by the fixed ions, that make up the connecting wires and bulb filament. As both the moving charged particles (often electrons) and the ions are charged, there is a retarding force between the moving charged particles and the fixed ions, leading to a series of interactions between the moving charged particles and the fixed ions.

As a result of each interaction, the mobile charged particles are slowed down and the ions vibrate more. The charge is then accelerated by the battery's electric field and moves off once again before undergoing another interaction with a different ion. In the thin, highly resistive wire of the bulb filament, there are many such interactions, so the ions here vibrate a lot. In the connecting wires, there are fewer interactions and so the ions vibrate rather less.

The difference in the rate of interactions leads to far less electrical working in the connecting wires and far more in the filament. The thermal store of the filament is therefore filled and the filament warms up and glows. The connecting wire does not glow, as its thermal store is much emptier.

So most energy is shifted by the interactions of the ions and mobile charged particles in the filament wire, not the connecting wires.

What happens in the battery?

When the charged particles reach the positive terminal of the battery, energy is shifted as they move across onto the negative terminal. Again, a large force (and so a strong electric field) is necessary in this section of the circuit loop, as a large quantity of energy is shifted as each charge moves across this short section of the circuit. All this happens within the battery, but only at the cost of emptying the chemical store of the battery. When this store cannot do its job any more, we say it has gone flat.

There is no mystery here – you simply engineer the parts of the circuit so that the forces on the charged particles vary as they drift around the circuit at more or less constant speed. A greater retarding force caused by the material that the charged particles are moving through is balanced by a greater driving force from the electric field set up by the battery.

This description underpins the two jobs for the battery outlined earlier in this episode.

It is not the case that charged particles must actually pass through the battery to shift energy. All of the charged particles in the circuit can shift energy due to the electric field created by the battery.

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