not just described, Explained


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Short circuits

What is a short circuit?

A short circuit is a fault.  It means there is a very low resistance conducting path from one side of a component to the other.  For example a wire might have come loose which connects two sides of a circuit together.  Or perhaps there's some moisture on the surface of a component that means current can bypass it.

The wire or the moisture 'shorts' the circuit because the length of the conducting path back to the battery has decreased.

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A short makes the circuit behave as if the component wasn't there.  The component stops working (it isn't there, after all) and the current everywhere in that circuit will increase, which can damage other components or, in extreme cases, cause a fire.

So how can we explain shorts?  A very misleading way of explaining them is to say that current takes the easiest path.

Shorting out a single component which is in series with others

When you connect a wire across the terminals of a bulb you're effectively creating a little parallel circuit.  The thing with parallel circuits is that the effective resistance is less than the smallest resistance.  In this case the smallest resistance is just the wire, and this has a very low resistance indeed.

Now you've reduced the resistance of the series circuit and so the current everywhere increases.  The current through the unshorted bulb increases and so it gets brighter.

But brightness isn't just a function of current.

You have a low resistance (the shorted bulb) in series with a higher resistance (the unshorted bulb) and this changes the way the voltage is shared around the circuit.  The bigger resistance takes a bigger share of the total voltage.  So the second bulb is bright for these two reasons, bigger current through it AND bigger voltage across it.  Remember that voltage and current are connected.  The current through the second bulb can only increase because the voltage across it is bigger.

In the same way the shorted bulb has a very low voltage across it so the current through it is very small and that's why it's out.  The wire doing the shorting has the same voltage across it as the bulb but it also has a very low resistance so the current through the wire is big.  The current through the wire and the current through the bulb add up to the current through the unshorted bulb.

Shorting out a whole circuit

This is exactly the same as saying that the power supply is shorted out.  In this case the explanation of why the bulb goes out is slightly different.

Again we've introduced a parallel circuit and the parallel circuit has an effective resistance of a little bit less than the wire.  This means that there isn't really any resistance anywhere in the circuit and so the current supplied by the battery becomes very big.  So the battery has to work very hard.

When you work very hard you sweat a lot and this is similar to what the battery does.  The chemical reactions in the battery take place very rapidly and lots of the energy released gets turned straight into heat rather than being given to the charges in the circuit.  This means the voltage is a lot less than it should be.  The voltage across the components is very low and so none of them work.

This type of short can cause a battery to get very hot.  It may even explode!

back to Lesson 4: Complete Circuits