When charging your phone has your charging cable suddenly stopped working no matter which way you twist it and how many different sockets you try and plug it into? Well, this typically occurs because of how the different materials that form your charger were put together, not only to charge your phone but also to ensure that no electrical fault will jeopardise your health and safety.
Many aspects of our modern-day lives require electricity, whether this is the phones we use to communicate with or the desktop computers that we use to study. To power our electronics we require cables, sockets and plugs to connect us to the national grid and to do so safely. In this article, we will explore how these electrical components work, their safety features and how they work together.
- Firstly, we will define exactly what a cable is for your foundational knowledge.
- Then we will explore the properties and design of cables that make them work as well as ensure user safety. Short circuits and their relationship to cables will also be discussed.
- Afterwards, we will look at plugs, including what materials they are made from and their features.
- Similarly, we will investigate plug sockets and the different wires contained within.
- Finally, we will briefly discuss how cables connect our electrical devices to the national grid.
Cables definition
You have likely heard the word cable and use it fairly often, but it is probably helpful to define the term cable as a scientist would.
A Cable is an insulated wire or group of wires used to transmit energy.
Cables are used to connect multiple objects together to facilitate the flow of electrical current between them. The flow of electric charge (electricity) allows us to power our devices from electric generators. Most cables you are familiar with are made out of insulated copper wires, that are then coated by a flexible plastic material or a layer of rubber called a cable jacket, which protects the cable from physical damage.
If you have seen a broken cable before, you may have peeked inside to see that there are many colourful wires twisted together inside the cable jacket. You can see this more clearly by looking at our image above. As you can see there are multiple stripped wires encased in wire insulation of different colours. (The meaning of these colours will be explained later in the article).
Cables are used for a wide variety of applications. Depending on their use, they may have slightly different design features. Let us then investigate the different properties of cables in the next section.
Properties of cable wires
Now that we have defined what a cable is, let us look at the different properties that not only allow it to work, but to also be safe.
Firstly, the thin shiny wires are made out of copper. We use this material because:
- as a metal, it is a good electrical conductor,
- it is cheap relative to other alternatives,
- copper is highly ductile so can be drawn easily into a thin wire shape,
- it is flexible, so can be deformed elastically inside the cable.
Flexibility is a very important property of a wire, as the material must be able to be bent and twisted without breaking the transmission of electrical energy from the source to the object.
The materials that we use to encase these wires are plastics. The reason why we use plastics is that they are good electrical insulators, so do not readily transmit electricity across them. (This is the opposite of the conducting material of the wire itself). As well as the plastic wire insulation, the wires are further encased by a plastic cable jacket This ensures that nobody can touch the copper wires directly, which could cause harmful and even potentially life-threatening electric shocks! The cable jacket also helps to keep the different wires inside the cable bundled together. The plastics used to encase the conductive wires are also chosen for their flexibility so they can be shaped into the desired configuration.
Cable thickness is another important property of cables. The chosen thickness of a cable and the wires within depends on what it is being used for.
If a constant voltage is applied across a conductor, then having a relatively high resistance will decrease the current flow proportionally. Refer to Ohm's law below, where \(V\) is the voltage, \(I\) is the current, and \(R\) is the resistance
\[V=IR.\]
Making the cables thicker and thereby increasing resistance will decrease the current in the wire. This is very useful as a high value of current generates a lot of heat energy. Thick cables prevent the wires from becoming too hot and melting the fuse or becoming a potential fire hazard. If you compare the cabling behind plug sockets to the wiring of a light hanging on the ceiling you will see that they are considerably thicker.
Cables and plugs
The insides of our plugs and cables contain a variety of things as seen in the diagram of a UK three-pin plug below.
The casing and grip of the plug are made out of hard plastics which are electrical insulators, their shape is designed in a particular way to ensure that all the wires inside do not touch each other.
Pins
In the UK we have three pins in our plugs that connect to the wires in our cables. The top pin connects to the earth wire, the lower right pin goes to the live wire, and the lower left pin to the neutral wire. These three pins insert easily into any UK plug socket.
All three pins are made out of brass, which is an alloy of copper and zinc. Brass is the perfect material for the pins which are exposed to the surrounding environment by design (so they can be inserted into an external plug socket) as:
- brass is nearly as good of an electrical conductor as copper,
- it does not corrode easily when exposed to air,
- it is a stronger material than pure copper,
- brass is relatively cheap,
- they also have a good visual appeal as brass somewhat resembles gold.
Not all countries have 3 pins in their plugs, and those that do not, do not contain an earth wire. All these features work together to transmit energy in a safe way.
Wires
Let's begin with the Earth wire which connects to the ground in your home. It only carries a current if there is an electrical fault. The Earth wire is made out of copper, so it provides a low-resistant path to the ground. Modern Earth wires in the UK are covered in plastic insulation that is coloured yellow and green. The Earth wire is a great safety feature if a problem occurs. The transmission of electricity will go to the ground instead of anyone touching faulty wiring, preventing an electric shock which can sometimes be fatal.
A neutral wire is covered in blue insulation, this wire is also made out of copper and helps complete our circuit by carrying electricity from our electronics back to the power supply.
Our final wire is the live wire. This is the wire that carries the current used to actually power our electronics. The live wire is cased in brown plastic and carries a voltage of \(230\,\mathrm{V}\).
An alloy is a metal that is made by combining two or more metallic elements together.
Fuse
Finally, we have the fuse, which is nestled between the live pin and the live wire. Fuses melt when too much current passes through, this breaks the connection been the live pin and the live wire and causes a short circuit.
Short Circuits
We previously mentioned the role of the fuse, and how it melts and cuts the current. Well, this is where short circuits enter our journey. If a live wire and a neutral wire come into contact with an appliance a very large current will pass both of them, which is called a short circuit. When this takes place, the fuse melts or blows, preventing the flow of current and any accidents.
It is important to remember that we should never touch any exposed wires in a cable regardless of whether an appliance is plugged in. This is because our bodies are not charged and are at \(0\,\mathrm{V}\), if by accident you touch a live wire, a large potential difference will pass through your body, which we feel as an electric shock. This can be very dangerous, sometimes leading to death.
Cables and sockets
Let us define what a socket is.
Sockets are electrical devices that a plug or bulb fits into to make an electrical connection.
Sockets are made out of a hard plastic material with three slots for the pins in a UK plug to fit into. They contain wires that connect to cables (usually inside walls). The cables on the other side of a building's plug socket first connect to a fuse box, which then connects to the national grid.
A fuse box acts as a central control and monitoring system for the electrical circuits in a building.
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