If you’re reading this on a laptop that’s plugged in, the chances are that between the end of the cable that’s in the back of your computer and the plug that’s in the socket or the wall, there’s a box that gets hot enough to warm your feet up.
That’s what I use mine for sometimes, anyway. But have you ever wondered what that box in your charging cable actually does?
Last time, we looked at electricity in circuits, using water as an analogy for the flow of tiny charged particles called electrons. Each electron carries a small charge, so when lots of them flow very quickly, you get a current of charge that we call electricity.
Voltage is essentially a measure of how much energy they carry, and it can be thought of as the difference between two points on a circuit. For example, water flows faster down a steeper hill, so the greater the difference between the top and the bottom, the greater the current.
Lightning too, is the connection between two differently charged points. A flow of electrons streams across the voltage difference between the clouds and the earth, and ‘discharges’ a huge amount of energy on the way.
These are all examples of what we call direct current (DC). The electrons, the electricity, the flow and the current only ever moves in one direction - from the source (like a battery, for example) to the load. On the back of your laptop, just above where you plug in the charger, you might even be able to see where it says DC/IN or something similar.
That’s because laptops and stereos and powered alarm clocks and musical keyboards (just like the light bulbs we talked about last time) are powered on direct current.
But there’s a problem with direct current. The further away the source is from the load, the harder it is to get the current flowing fast enough to power anything when it gets there. In fact, it’s really difficult. The wires get hot as the electrons flow through them. Energy gets dissipated and lost, and (despite Thomas Edison trying hard to get it going in the 1880s) DC is just inefficient at getting energy from, say, a huge power station, all the way to your laptop. You need a current that DC can’t give you over long distances.
So, how do you get a low enough current to travel long distances, that’s somehow able to get to your home without blowing up the wires as it goes?
Remember Ohm’s law? It stated that:
V = I x R
where V is the voltage of a system, I is the current (flow) and R is the resistance encountered by that current.
If you need a low current to travel long distances through wires, you need a high voltage. Remember, for the equation to be true, if one of the quantities on the right goes down, the quantity on the left has to go up! In fact, the higher the voltage (V), the better, because the current will travel much more efficiently through those cables at a lower current.
This is where Alternating Current (AC) comes in! AC is carried along high voltage power lines (usually suspended by electricity pylons) and it arrives with a voltage of 250 Volts (120 Volts in the USA) at your plug socket. It’s called Alternating Current because of the clever way it’s generated, which we’ll talk about next time. The high voltage of course, makes it potentially very dangerous - a little bit like the potential difference between those two points of a lightning strike. Or perhaps like standing in front of a water pistol (DC voltages), and standing in front of a fireman’s hose (AC).
That’s why you should never misuse or unwire a socket, or put anything in it other than a tested plug on a properly wired electrical device.
Speaking of electrical devices, your laptop runs on low voltage DC of course, so something between the wall and your computer needs to convert the high voltage AC that could fry your laptop and jolt you into the stratosphere... to low voltage DC which will let you get on with checking Facebook and sending emails instead.
And that’s exactly what that foot-warming box does. It’s called a ‘rectifier’ because it rectifies or converts alternating current into direct current in a very elaborate way. And it’s brilliant on cold nights when you can’t find your slippers.
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