For most people, the basic concepts of current and voltage remain baffling, even after the best efforts of their teachers. They nevertheless play an important part in our everyday lives. Watts on our microwave knob, amps in the fuse cupboard and volts on our batteries – what do they mean? It’s not as hard as you fear!
All materials are made of atoms which are themselves made of particles that are positively or negatively charged or neutral. In most materials these particles stay stuck tightly together and balance out exactly, so the majority of things are not electrical – chairs carpets, windows etc. In metals however some of the negatively charged particles, called electrons, are not stuck like this – they are free to move. But it takes an external push to do so.
The voltage is a measure of this push. More volts, more push on each electron. And the greater the push (or voltage) on the electrons, the greater the energy each of them acquires – just as a stronger push on a bike pedal gets you going faster.
The current is the flow – the number of electrons that pass by each second. It’s measured in amps. In a wire (or any piece of metal) there is resistance to being pushed, just as there is pedalling a bike. So, the flow of electrons in a wire is impeded, to a greater or lesser extent, depending on the properties of the wire. Where you have a lot of resistance to the flow, as in the special wires of an electric heater, you need a strong push (or high voltage) to get a reasonable flow. But in a cell phone, where there is very little resistance, a low voltage is enough – hence the low voltage battery. Thus, a given voltage will produce a big current when applied to something that doesn’t resist much (like a bright light bulb), or a smaller current when applied to something more resistant (like a dim light bulb).
To recap, volts measure the push or pressure driving a current, and amps measure how many particles are flowing past each second. What interests us most, however, in everyday life, is the amount of energy that gets used every second – that’s what we pay for. Washing machines consume a lot of energy every second to keep spinning, but LED lights need very little to keep shining. The energy being used or supplied each second is called the power of a machine and is measured in Watts. A powerful vacuum cleaner may consume 1000 units of energy every second, a light bulb, just 40. One is rated at 1000 Watts (called a kilowatt) and the other at 40 Watts.
In summary:
Volts – the push exerted on particles moving in a current – or the energy each carries.
Amps – the rate at which the particles move past.
Watts – the amount of energy transported by the particles each second as they pass by.
There’s a useful link between these. The total amount of energy passing by each second must depend on how much each particle carries and on the number that are passing by each second. Thus, the Watts are given by the Volts times the Amps.
Of course, what you pay for each month is not exactly either of these. It’s not the current or the voltage or the power that matters – it’s the total amount of energy you’ve consumed. In domestic bills, this is measured in kilowatt- hours. It means that if you have a washing machine rated at 1000 Watts and it runs for one hour, you have used 1 kilowatt-hour (kWh) of energy. So, all the wattages of all your machines and devices multiplied by the times they are on for, gives you the energy you have consumed and have to pay for each month. Fortunately, the energy suppliers do this calculation for you!
A small matter of interest – the use of capital letters. Electrical units are mostly named after great scientists – Alessandro Volta, André-Marie Ampere and James Watt – so the formal names for the units begin with capital letters. In everyday writing however the lower case is often used.
Wider considerations
The basic concepts outlined here are universal – they apply in every country, in every part of the universe, at all times. However electrical things don’t just happen in wires in the home. Lightning is the discharge of electrical energy from molecules in the air when the voltage gets very high; pain is the result of electrical signals passing though nerves to the brain. Electricity appears in many guises, all underpinned by the same fundamentals.
© Andrew Morris 17th July 2024
Getting to Grips with Science 