Mains Electricity Flashcards
What is direct current (DC) and what is alternating current (AC)?
Direct current (DC) is a current that flows in one direction only (such as from a battery). Alternating current (AC) changes direction periodically. The UK mains electricity supply is AC: the current and voltage alternate 50 times per second (50 Hz frequency). In the UK, the mains voltage is about 230 volts (V) RMS.
What are the frequency and voltage of the UK mains supply?
The UK mains supply has a frequency of 50 Hz and a potential difference of about 230 V (RMS). Students are expected to remember these values.
50 Hz means the AC waveform completes 50 cycles per second.
Why does AC supply not have fixed positive and negative terminals?
AC supply alternates, so it does not have permanent positive or negative terminals. Instead, we use the terms live and neutral for the two supply wires. In AC, the polarity of the live wire switches, so one moment it is positive relative to neutral, then negative, repeatedly.
What are the live, neutral, and earth wires used for in a UK mains cable, and what are their colors?
A UK mains cable typically has three wires inside with color insulation:
- Live (brown): Carries the AC from the supply to the appliance. It is at about 230 V relative to earth and is the dangerous wire.
- Neutral (blue): Completes the circuit and carries current back to the supply. It is close to earth potential (0 V).
- Earth (green/yellow stripes): A safety wire. It does not normally carry current. It is connected to the metal casing of the appliance to keep it at 0 V. Its purpose is to protect users if a fault occurs.
What is the role of the live wire? Why is it dangerous?
The live wire brings the high potential (about 230 V) from the supply into the appliance. Touching the live wire can be dangerous because it can give your body a path to earth, causing current to flow through you. Even if the appliance switch is off, the live wire is still at high voltage relative to earth. Therefore, contact with the live wire can result in an electric shock.
What is the role of the neutral wire?
The neutral wire completes the electrical circuit by carrying current back to the power supply. It is at or near earth potential (0 V), so it is generally at low risk. However, in a fault situation it can still carry current (e.g. if live is shorted to neutral).
What is the role of the earth wire?
The earth wire is a safety feature. It is connected to the metal body of an appliance (or the earth pin in a plug). Its purpose is to keep the appliance at 0 V. If a live wire inside the appliance accidentally touches the metal casing, the earth wire provides a low-resistance path to ground. This causes a large surge of current, which quickly melts the fuse (or trips a breaker) and cuts off the supply. Thus, the earth wire prevents the casing from becoming live and protects from electric shocks.
Why can touching the live wire give you an electric shock?
Your body is normally at earth potential (0 V). If you touch the live wire (at ~230 V), a potential difference is established between live and earth through your body. This causes current to flow through you to earth, which you feel as an electric shock. The larger the potential difference, the more dangerous the shock.
Why is the live wire still dangerous even when a switch in the circuit is open?
When a switch is open, it only breaks the circuit path, but the live wire up to the switch is still connected to the power supply. It remains at ~230 V relative to earth. If you touch the live wire downstream of an open switch and provide a path to earth, current will flow through you. Thus, an open switch does not make the live wire safe to touch.
What happens if the live wire is accidentally connected to the earth wire?
If live is connected to earth (a short circuit), a very large current flows because the resistance path is very low. This huge current will cause the fuse in the live wire to melt (or a circuit breaker to trip) almost instantly, disconnecting the live supply. This protects the wiring and prevents fire or further hazards. However, it also means a significant spark and potential danger if someone is nearby, so such faults are very dangerous and must be prevented.
What is a fuse and how does it work?
A fuse is a safety device inserted in the live wire (often inside plugs). It contains a thin metal wire or strip designed to melt at a specific current. If the current exceeds the safe rating of the fuse (for example, if a fault causes too much current), the wire heats up and melts. This breaks the circuit and stops the flow of electricity, protecting the circuit and preventing overheating or fire.
What is a circuit breaker and what are its advantages?
A circuit breaker is another safety device that automatically cuts off (breaks) the circuit if the current exceeds a set limit. It typically uses an electromagnet to trip a switch. Advantages over a fuse are: it can be reset and reused (no replacement needed), and it usually trips faster than a fuse melts. Circuit breakers are commonly used in homes (in consumer units, sometimes called ‘fuse boxes’) instead of or alongside fuses.
What is double insulation? Why don’t double-insulated appliances need an earth wire?
Double insulation means an appliance has two layers of insulating material (often plastic casing plus insulation on wires) so that no live parts can touch the user. A double-insulated appliance has no metal exposed parts, so even if a fault occurs, the user cannot touch a live conductor. Because the case is insulated, it does not need an earth wire for safety. Such appliances often have a symbol (a square within a square) and no earth pin.
How do you calculate electrical power in a circuit?
Electrical power P (in watts, W) can be calculated using P = V × I, where V is the potential difference across a device (in volts) and I is the current through it (in amperes). Using Ohm’s Law, power can also be written as P = I² × R (where R is the resistance). These formulas are essential for finding how much energy per second an appliance uses.
How is electrical energy consumption related to power and time?
The energy transferred (or consumed) by an appliance is equal to its power multiplied by the time it is on. Energy = Power × time (E = P × t). If P = V × I, then energy E = V × I × t. In everyday terms, an appliance’s power rating (in W or kW) multiplied by usage time (in hours) gives the energy used (often measured in kilowatt-hours, kWh).
Give examples of how domestic appliances transfer electrical energy into other forms.
Everyday appliances convert electrical energy into various useful forms. For example:
- A heater or toaster converts electrical energy into thermal (heat) energy.
- An electric motor (in fans, drills, etc.) converts electrical energy into kinetic (motion) energy and often some heat.
- A light bulb (incandescent or LED) converts electrical energy into light (and some heat).
- A loudspeaker converts electrical energy into sound energy.
- A mobile phone charger converts electrical energy into chemical energy stored in the battery.
What color is the live wire in a UK plug?
Brown.
What color is the neutral wire in a UK plug?
Blue.
What color is the earth wire in a UK plug?
Green and yellow stripes.
What is the function of a fuse in a plug?
To protect the appliance by melting and cutting off the current if too much current flows.
What is the function of a step-up transformer in the National Grid?
To increase the potential difference for efficient transmission (reduces current and energy loss).
What is the function of a step-down transformer in the National Grid?
To decrease the potential difference to a safe level for domestic use.
Why does the National Grid transmit electricity at high voltages?
To reduce current and minimize energy loss as heat in cables.
What is double insulation?
An appliance design where the casing and wiring are both insulated so no earth wire is needed.
