Short Circuits

Basic electronics part 7

This page is about short circuits, check out the main index for the rest.

There is a YouTube video that demonstrates this article in practise, or scroll down for the written version. Here is a simple circuit, with a battery and a bulb. The bulb has a resistance of 100 Ohms, so Ohms Law shows us that the current draw at 5 volts would be 50mA. If we put a short across the circuit as shown, then the circuit will no longer function.

But why is this?. When I was at school many years ago, I was told that this is because ‘the electricity will take the shortest route’, which is partially true, although in theory we still have 5 Volts across the bulb, so why doesn’t it light? Let’s consider the diagram below. Everything has a resistance, so the ‘short’ circuit still has some resistance to electricity in reality, perhaps 0.01 Ohms, which means that in fact we are trying to draw a current of 500 Amps through it. Most power supplies or batteries are not capable of sourcing this much current, and so their Potential Difference, measured in Volts, will drop to practically nothing, which is why the circuit doesn’t function in reality. This is demonstrated in the video above. If the power supply was capable of maintaining 5 volts with a load of 500 Amps, then the circuit would still function, and we would see that in fact not all of the electricity takes the shortest route. Ultimately the resistances in the circuit will determine the current draw throughout.