Unit 2: Chapter 5 - Mains Electricity Flashcards
Alternating Current
DC - Direct Current. It goes in one direction only.
AC - Alternating Current - It flows both ways in successive cycles. Its frequency is the number of cycles it passes through each second.
MAINS CIRCUIT
- In a mains circuit, there is always a LIVE and a NEUTRAL wire. The current through a mains appliance alternates. That’s the mains supply provides an alternating potential difference between the two wires.
- The neutral wire is EARTHED at the local substation. The potential difference between the live wire and the ‘earth’ is usually referred to as the ‘potential’ or voltage of the live wire.
- The live wire is dangerous as its voltage repeatedly changes from + to - back every cycle. It reaches 300V in every direction.
MEASURING ALTERNATING POTENTIAL DIFFERENCE
- We use an OSCILLOSCOPE to measure the peak potential difference and the frequency of a low voltage AC supply.
FREQUENCY OF AC SUPPLY = 1/Time for 1 cycle
MORE ABOUT MAINS CIRCUIT
The live wire alternates between +325V and -325V. This is the equivalent to the direct voltage of 230V. The voltage of mains electricity is 230V.
Cables and Plugs
When you plug in a heater with a metal case into a wall socket, you ‘earth’ the metal case automatically. This stops the metal case becoming live, which would electrocute you.
Plastic materials are good insulators and an a appliance with a plastic case is doubly-insulated.
PLUGS, SOCKETS AND CABLES
- Outer casings of plugs, sockets and cables of all mains circuits and appliances are made of hard-wearing electrical insulators. Thats because they contain live wires.
- Most mains appliances are connected via a wall socket to the mains using a cable and a 3-pin plug.
- Pins are made of brass because it is a good conductor and it does not rust or oxidise.
- The Earth pin of the plug is the longest so it connects first into the socket to ‘earth’ it.
- If there is too much current passing through the live wire, it melts and cuts the live wire off.
- Cables are made of copper because it is flexible and is a very good conductor.
- If more current passes through, the wire is heated more and so a wires must be thicker to stop the heating effect.
Fuses
A fuse is a thin wire that heats up and melts if too much current passes through.
If the fuse ‘blows’, it cuts the live wire off and the appliance will be cut off.
A fuse has a rating. If the rating is too high, the fuse won’t blow as it should. The heating effect could then cause a fire.
CIRCUIT BREAKERS
- A circuit breaker is an electromagnet that opens (switches off or ‘trips’. This stops the current in a live wire flowing. The magnetic field of the electromagnet is strong enough to pull the switch contacts apart.
Circuit breakers are used instead of fuses. They work faster and can be reset easier.
The RESIDUAL CURRENT CIRCUIT BREAKER (RCCB) works even faster than ordinary circuit breakers described above. An RCCB cuts of the current of the live wire if it is indifferent of the neutral wire. The RCCB is more sensitive that the fuse or an ordinary circuit breaker.
Electrical Power and the Potential Difference
Power = Energy Transferred (J) / Time (S)
Power supplied (W) = Current (A) x Potential Difference (V)
CHOOSING A FUSE
Domestic appliances are often fitted with 3, 5, 13A fuses. If there is no label to explain what fuse to use, you can work it out:
E.G.
Calculate the normal current through a 500W, 230V heater.
Current = 500W / 230V = 2.2A
Electrical Energy and Charge
Charge (C) = Current (A) X Time (s)
Coulomb is the rate of flow charge through an appliance.
When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot.
Energy transferred (J) = P.D (V) X C (c)
When charge flows round a circuit for a certain time, the electrical energy supplied by the energy is equal to the electrical energy transferred to all components in the circuit.
Electrical Issues
Overheating in appliances could lead to fires.
Appliances should be used less in extended periods of time.
Smoke alarms and infra-red sensors must be constantly checked if they’re working.
Low energy bulbs are much more efficient than standard filament bulbs, which may damage plastic parts of bulb socket to make them brittle so they are liable to snap.
