Mains electricity and safety
What you'll learn
- How domestic appliances transfer energy from batteries and the mains supply.
- The difference between d.c. and a.c., including the UK mains values.
- What the live, neutral and earth wires do.
- How fuses, circuit breakers and correct switch positions reduce danger.
The starting point: charge, current and voltage
Electricity is about the movement of electric charge. In metal wires, charge can move through the material, making an electric current.
A cell is a single chemical source of electrical energy. A battery is two or more cells connected together, although in everyday speech people often call a single cell a battery too.
Current and potential difference
Current is the rate of flow of electric charge, measured in amperes (A). Potential difference (p.d.), often called voltage, is the energy transferred per coulomb of charge between two points, measured in volts (V). The coulomb (C) is the unit of charge.
A power supply such as a cell, battery or mains socket transfers energy to charges. The charges then transfer energy to components in the appliance.
Energy transfers in domestic devices
A domestic device is an electrical appliance used in the home, such as a kettle, toaster, fan, washing machine or phone charger.
A motor is a component that transfers energy electrically into the kinetic energy store of moving parts. For example, the motor in a fan makes the blades spin.
A heating device is designed to transfer energy electrically into the thermal energy store of a component or material. For example, a kettle’s heating element increases the thermal energy store of the water.
Batteries and cells often power portable devices, such as torches and toys. The a.c. mains supply powers many high-power home appliances, such as kettles, washing machines and electric ovens.
Energy transfer in appliances
A battery or mains supply transfers energy electrically to an appliance. A motor mainly transfers it to kinetic energy stores, while a heating element mainly transfers it to thermal energy stores.
Describing energy transfers in a hair dryer
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Identify the supply: the hair dryer is connected to the a.c. mains, so energy is transferred electrically from the mains to the appliance.
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Identify the motor: the motor transfers energy to the kinetic energy store of the spinning fan, which pushes air through the dryer.
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Identify the heater: the heating element transfers energy to the thermal energy store of the element and the air passing over it.
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Link the outcome to the device’s job: the moving warm air transfers energy to the water in wet hair, helping it evaporate.
Direct voltage and direct current
A direct voltage has one fixed polarity: one terminal stays positive and the other stays negative. In a simple battery circuit, the voltage is roughly constant.
Direct current
Direct current, shortened to d.c., is current where charge moves in one direction only. Cells and batteries supply d.c.
For example, in a torch circuit, charge moves around the circuit in one direction while the cell is connected.
Alternating voltage and alternating current
An alternating voltage repeatedly changes polarity: each side of the supply becomes positive, then negative, then positive again.
Alternating current
Alternating current, shortened to a.c., is current where the movement of charge changes direction repeatedly.
The frequency of an alternating supply is the number of complete cycles each second. It is measured in hertz (Hz), where 1 Hz means one cycle per second.
The graph shape helps you see the difference: d.c. stays one way, while a.c. repeatedly reverses.
UK mains supply
In the UK, the domestic mains supply is a.c., with a frequency of 50 Hz and a potential difference of about 230 V.
Finding the time for one mains cycle
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Use the meaning of frequency: 50 Hz means there are 50 complete cycles every second.
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Use the relationship between time period and frequency:
- Substitute f=50 Hzf = 50\ \text{Hz}f=50 Hz:
- Interpret the answer: one complete cycle of UK mains a.c. takes 0.020 s.
A.c. still transfers energy
Even though the charges move back and forth in a.c., energy is still transferred to the appliance. A mains heater warms up on both halves of the cycle.
The three wires in a mains cable
Many mains appliances use a cable with three wires: live, neutral and earth.
The live wire carries the alternating potential difference from the mains supply to the appliance. It is the most dangerous wire because it has a potential difference of about 230 V compared with the earth.
The neutral wire completes the circuit back to the supply. It is close to 0 V, but it can still carry current during normal operation.
The earth wire is a safety wire connected to the metal case of some appliances. It is normally at 0 V and normally carries no current.
This diagram shows normal operation, the key potential differences, and what happens during a fault.
For GCSE, remember these potential differences:
- Live to neutral: about 230 V
- Live to earth: about 230 V
- Neutral to earth: about 0 V
Thinking neutral and earth do the same job
Neutral and earth are both close to 0 V, but they do different jobs. Neutral completes the normal circuit and carries current; earth is a safety wire and should only carry current during a fault.
Fuses and circuit breakers
A fuse is a safety device containing a thin wire that melts if the current becomes too large. When it melts, it breaks the circuit.
A circuit breaker is a safety device that switches the circuit off if the current becomes too large. Unlike a fuse, it can usually be reset.
Both protect the circuit from overheating and reduce the risk of fire or electric shock.
If a live wire touches the metal case of an appliance, the case could become live. If someone touched it while also connected to earth, current could pass through their body.
The earth wire helps by providing a low-resistance path to earth. This causes a large fault current, so the fuse melts or the circuit breaker trips, disconnecting the live supply.
Fault in a metal-cased appliance
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Identify the fault: the live wire touches the metal case, so the case would be at about 230 V relative to earth if it were not protected.
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Apply the role of the earth wire: the earth wire connects the metal case to 0 V through a low-resistance path.
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Compare the current with normal operation: the low resistance causes a much larger current than normal to flow in the earth wire.
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Apply the safety device: the large current melts the fuse or trips the circuit breaker, so the live wire is disconnected and the case no longer remains live.
Why switches and fuses go in the live wire
Switches and fuses should be connected in the live wire, not the neutral wire.
If a switch in the live wire is open, the appliance is disconnected from the dangerous 230 V supply. The internal parts are not connected to live.
If a fuse in the live wire blows, the appliance is also disconnected from live. This makes the appliance much safer to touch or repair after the supply has been switched off.
Putting the switch in the neutral wire
A switch or fuse in the neutral wire may stop the appliance working, but parts inside the appliance could still be connected to the live wire. That means they could still give an electric shock.
Any connection between live and earth is dangerous because it creates a large potential difference across a low-resistance path. This can cause a very large current, overheating, sparks, fire, or a severe electric shock if the path includes a person.
Power ratings and energy changes
Power is the rate at which energy is transferred. It is measured in watts (W), where 1 W means 1 joule of energy transferred every second.
Power rating
The power rating of an appliance tells you how quickly it transfers energy when it is working normally. A higher power rating means energy is transferred faster.
A 2 kW kettle transfers energy much faster than a 5 W phone charger. That is why high-power appliances often need thicker wires, stronger switches and suitable fuses.
The key relationship is:
ΔE=Pt \Delta E = P t ΔE=Ptwhere ΔE\Delta EΔE is energy transferred in joules (J), PPP is power in watts (W), and ttt is time in seconds (s).
Calculating energy transferred by a kettle
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Convert the power into watts: 2.0 kW is 2000 W.
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Convert the time into seconds: 3.0 minutes is 180 s.
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Use the power equation:
- Substitute the values:
- Interpret the answer: the kettle transfers 360000 J, or 360 kJ, mainly to the thermal energy store of the water and kettle.
Power rating sanity check
If two appliances run for the same time, the one with the larger power rating transfers more energy. If one appliance runs for longer, you must consider both power and time.
In the exam
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Quote mains values accurately: UK domestic supply is a.c., about 230 V, with a frequency of 50 Hz.
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For safety explanations, include the full chain: live wire touches case, earth wire provides a low-resistance path, large current flows, fuse blows or circuit breaker trips, live supply is disconnected.
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For power calculations, convert kilowatts to watts and minutes to seconds before using ΔE=Pt\Delta E = P tΔE=Pt, unless the question tells you to use different units.
Check yourself
- Why is it safer for a fuse to be in the live wire rather than the neutral wire?
- What are the potential differences live-neutral, live-earth and neutral-earth in UK mains?
- A 1.5 kW heater runs for 10 minutes. What equation would you use to find the energy transferred?
