not just described, Explained

Lesson 2: Inside Circuits

Introduction

Last lesson we saw that electricity is all to do with moving energy around.  In this lesson we'll introduce a way of how energy is transferred in a simple electric circuit.

Black blobs are charges, red stuff is energy

In our animation we can see that there are two main features.  There are little black blobs that move around the circuit and there's some red stuff that moves round with them.  We've used the little black blobs to represent charges.  The red stuff represents energy.

This is only a model

It's important to state straight away that this is not what's 'really happening'.  A very low-level explanation involves talking about surface charges and the Poynting vector.  What our model does is simply allows us to keep track of how charges and energy move in an electric circuit.  This makes it much easier to explain current, voltage and power.  But there are some perfectly reasonable criticisms of the Furry Elephant electricity animation.

Our basic story is this: charges carry energy from the battery to the bulb.  In the bulb the energy changes from electrical energy into heat and light energy and this spreads out into the surroundings.

Charges are already there and move very slowly everywhere at once

If you look at the animation you can see that the charges move very slowly.  The typical speed that charges move down a wire is of the order of a centimetre per hour.  This seems very strange.  How is it that lights come on straight away if the charges only move very slowly?

The answer is that the charges don't come from the battery and then move very quickly through empty wires.  The charges are already there in every part of the circuit and they all start moving everywhere at the same time.  Just like a wheel, there's no part of the circuit that begins moving first.

Charges go round and round, energy gets shifted

The next point to note is that the little black blobs don't get used up.  They just go round and round.  So charges aren't lost in an electric circuit.  The stuff that gets lost is the red stuff: energy.

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