As more and more electric cars start to hit the roads, a basic understanding of electricity can be very useful when trying to understand the benefits and drawbacks of different vehicles, chargers, and suchlike.
This short ‘Electricity 101’ series aims to give you as an EV owner a little bit of an understanding of the physics of electricity. This first article will give a quick overview of what charge, voltage, current, and resistance are, with the following articles giving a little bit of insight into how this is relevant to you as an EV owner.
What is electricity?
A useful place to start out is by asking the question “what is electricity?” – the most common definition is that electricity is the flow of electrons through a conductor.
A conductor is any material that allows electrons to flow through it – in practice this is generally a metal such as copper.
These electrons will have a certain amount of energy (given to them by the battery) which can be used to do work – for example illuminating a lightbulb or turning the electric motor in your car.
For these electrons to be able to flow, the conductors must form a closed circuit with the battery, as shown in Fig. 1. If the circuit is broken at any point, then the electricity will immediately stop flowing.
Charge
This brings us on to our first electrical measurement, charge.
Charge is usually given the mathematical symbol Q and is measured in units of coulombs (C).
Charge is simply the number of electrons. One coulomb is equivalent to roughly a billion billion electrons.
Because one ‘charge’ corresponds to such a large number of electrons, we don’t usually bother calculating this! Instead we just refer to a single ‘charge’ of one coulomb.
Current
The measurement of charge isn’t used very often when talking about circuits. However, current, which is the number of charges per second is often used when talking about the flow of electrons in a circuit.
Current is usually given the mathematical symbol I and is measured in units of amps (A).
For example, we may talk about a circuit having a current of 2A, meaning that there is a flow of 2 coulombs of charge per second in the circuit.
Voltage
Voltage is the driving force in a circuit, or more accurately the amount of energy per unit charge.
Voltage is usually given the mathematical symbol V and is measured in units of volts (V), where one volt is equal to one joule of energy per coulomb of charge.
So if, for example, we had a 5V battery, that would mean that the battery gives 5 joules of energy to each charge passing through it.
Resistance
Resistance is a proprety of circuit components, for example lightbulbs, electrical motors, or other ‘resistors’. Resistance is simply a measure of how much a component resists the flow of electrical current.
Resistance is usually given the mathematical symbol R and is measured in units of ohms (Ω).
Voltage, current, and resistance are related by a fundamental equation called Ohm’s law:
V=IR
This means that if you put a voltage (e.g. 5V) across a component that has a low resistance (e.g. 1Ω) you will see a large current flowing through that component (5A). Conversely, if you put the same voltage across a component that has a high resistance (e.g. 5000Ω) you will see a low current flowing through that component (0.001A).
A little bit confusing…
This might be a lot of information to absorb, so sometimes it helps to think about things a little bit more metaphorically.
Personally, I like to imagine the charges as trucks carrying packets of energy down the motorway: the number of trucks passing each second is the current, the amount of energy that each truck is carrying is the voltage, and the total amount of energy being transported by the trucks per second is the power.
This is all summed up in Fig. 2 below.
