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Lesson 4: Complete Circuits

Introduction

In this lesson we'll look at why a circuit needs to be unbroken for it to work.  We'll also look at switches and short circuits.  This may seem like a simple topic but there are quite a few subtleties to grasp.

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Why you need a complete conducting path

Remember that the electrons that flow in an electric circuit are already there.  They come from the atoms that make up the wires so the wires are already full of electrons.  It's best to think that charges just go round and round a circuit, even flowing through the battery.

If the circuit isn't complete then there is nowhere for the electrons to go to.

Remember that electrons are negatively charged and that like charges repel.  The closer you bring electrons together the more they repel.  So you can squash the electrons together a bit but after a while their mutual repulsion means you can't squash them any more.

What sort of materials can make a complete conducting path?

If a material is a conductor then this means that it has lots of little charged particles that are free to move about.  In metals the little charged particles are electrons.  Some liquids can also conduct, for example salty water.  In this case the little charged particles are the positive and negative ions that make up the salt.

Pure water isn't a very good conductor.  But most tap water has enough mineral salts of one kind or another dissolved in it to make it conduct well enough to give you a shock if you touch a live connection with wet hands.

Even the air can be made to conduct if you strip electrons off the atoms and leave them free to move about.  Lightening, sparks and fluorescent lights all work like this.

Conducting paths in series and parallel circuits

We can follow a conducting path by starting at one terminal of the battery and following the wire, through any components, like bulbs or motors, back to the other terminal of the battery.

As you do this it's important to think to yourself.  'The charges don't start from the battery.  The wires are already full of charges and they all start moving very slowly everywhere at the same time, like a wheel.'

A series circuit has only one conducting path.  That's pretty much the definition of a series circuit.  A parallel circuit has more than one conducting path.  In other words there's more than one route you can take to get from one terminal of the battery to the other.

Why all components have to have two connections

The reason why circuits are always loops is that the charges have to have somewhere to go.  This means that for every charge that enters a component another one has to leave.

This means that every electrical component must have two connections, and so two wires.  One for charges to come into the component and the other for charges to leave.  This is fairly obvious with school apparatus because it's designed to make it easy to understand how it works.

But in the home it sometime appears that appliances only have one connection.  A bulb is a case in point.  It's quite difficult to see how a bulb has two connections.

All we ever see of most electrical appliances is a single lead.  It's only by looking inside the lead that we can see it consists of at least two wires.  The third wire is an earth connection.

Setting up a circuit slowly

When a circuit is broken then no charges flow anywhere.  But when the circuit is completed how do the charges 'know' to start moving?  How do they detect that the circuit is complete?

The most basic answer is that energy is passed quickly from charge to charge even though the charges themselves move very slowly.  It's like when a locomotive starts pulling a train of carriages.  The carriages find out that the locomotive has started moving very quickly as the couplings take up the tension, but the train still starts moving very slowly.

Using springs to imagine how a battery works

You can think of a battery storing energy as being a bit like coiling a spring.  Though this analogy has its flaws it can be used to explain how energy is passed quickly as a circuit is set up, even though the charges themselves don't start moving round the circuit.

Switches in series and parallel circuits

If a series circuit is broken anywhere then none of the charges can move because they have nowhere to go.  That's why you can put a switch anywhere in a series circuit and it will always turn a bulb on and off.  Again, the reason for this is that the charges DON'T start from one terminal of the battery and then go round the circuit to the other.  The electrons are already there in the circuit and they all start moving very slowly everywhere at the same time.

The same idea works for parallel circuit but this time you can think of each loop as a completely separate circuit.  If you break the conducting path around one loop then there'll still be a conducting path around the other so the other bulb will still work.  This is why everything in your home is connected in parallel.  It means you can turn the microwave off but still watch the TV.

Short circuits

If you short out a bulb then it goes out.  The explanation for why this happens is quite complex and it's not because the current takes the easiest path.  But it's easy to tell whether you'll have a short.  Just see if there's a conducting path from one side of the battery to the other without going through any components.

You can deliberately short out a component but in the real world this can happen accidentally for example if a circuit gets wet.

Shorts are generally bad news because the current in the circuit suddenly increases a lot.  This normally blows a fuse, which at least keeps everything safe, even though everything stops working.  But if there's no fuse then the wires can get very hot and this can cause a fire.

If your power supply is just a battery, rather than the mains then the battery will get very hot and can be permanently damaged, or even explode!

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