Mains electricity Flashcards
Direct current (DC)
Electric current in a circuit that is in one direction only
Alternating current (AC)
Electric current in a circuit that repeatedly reverses its direction
Frequency
The number of wave crests passing a fixed point every second
Live wire
The wire of a mains circuit that has a potential that alternates from positive to negative and back each cycle
Neutral wire
The wire of a mains circuit that is earthed at the local sub-station so its potential difference is close to zero
Oscilloscope
A device used to display the shape of an electrical wave
f=1/t
frequency of an arc (Hz) = 1 ÷ time for one cycle (s)
Socket
A mains socket is used to connect the mains plug of a mains appliance to a mains circuit
Cable
Two or three insulated wires surrounded by an outer layer of rubber or flexible plastic
Three-pin plug
A three pin plug has a live pin, a neutral pin and an earth pin - the earth pin is used to earth the metal case of an appliance so the case cannot become live
Fuse
A fuse contains a thin wire that melts and cuts off the current if too much current passes through it
Circuit breaker
An electromagnetic switch that opens and cuts the current off if too much current passes through it
Residual current circuit breaker (RCCB)
An RCCB cuts off the current in a live wire when it is different from the current in a neutral wire
P=IV
power (W) = current (A) x potential difference (V)
Q=It
charge (C) = current (A) x time (s)
E=VQ
energy (J) = potential difference (V) x charge (Q)
What is mean by direct current and alternating current?
Direct current is in one direction only, whereas alternating current repeatedly reverses its direction
What do we mean by the peak voltage of an alternating potential difference?
The peak voltage of an alternating potential difference is the maximum voltage measured from zero volts
What is the frequency of the UK mains system?
50 Hz
What is the peak voltage of the UK mains system?
±325 V
What is the direct voltage of the UK mains system?
230 V
What do we mean by the live wire and the neutral wire of a mains circuit?
A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts
How do we use an oscilloscope to measure the frequency of an alternating current?
Measure the time period taken by the waves then use the formula f=1/t
What is the casing of a mains plug or socket made from and why?
Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical conductor
What is a mains cable?
Mains cables are three core cables consisting of two or three insulated copper wires surrounded by an outer layer of flexible plastic material
What colour is the live wire?
Brown
What colour is the neutral wire?
Blue
What colour is the earth wire?
Green and yellow striped
Why does a 3-pin plug include an earth pin?
The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance
Why is brass better than copper for the parts of three-pin plug?
While copper is more conductive, brass is harder, and so more durable
What do we use a fuse for?
A fuse contains a thin wire that heats up, melts and cuts off the current if the current is too large
Why is a fuse always on the “live” side of an appliance?
A fuse is always fitted in series with the live wire, thus cutting the appliance off from the live wire if the fuse blows
What is a circuit breaker
A circuit breaker is an electromagnetic switch that opens (“trips”) and cuts off the current if too much current passes through the circuit breaker
Why are appliances with plastic cases not earthed?
A mains appliance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live
What is the relationship between power and energy?
The power supplied to a device is the energy transferred to it each second
How can we calculate the correct current for a fuse?
The correct rating for a fuse, in amperes, equals the electrical power divided by the potential difference
What is an electric current?
An electric current is the rate of flow of charge
What energy transfers take place when charge flows through a resistor?
When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot
What can we say about the electrical energy supplied by the battery in a circuit and the electrical energy transferred to the components?
When charge flows round a circuit for a certain time, the electrical energy supplied by the battery is equal to the electrical energy transferred to all the components in the circuit
Why are electrical faults dangerous?
Electrical faults are dangerous because they can cause electric shocks and fires
How can we prevent electrical faults?
Never touch a mains appliance with wet hands, and never touch a bare wire or a terminal at a potential of more than 30 V. Check cables, plugs and sockets for damage regularly, and check smoke alarms and infrared sensors regularly
When choosing an electrical appliance, what factors should in addition to cost should we consider?
When choosing an electrical appliance, the power and efficiency rating of the appliance need to be considered
How do different forms of lighting compare in terms of cost and energy efficiency?
Filament bulbs and halogen bulbs are much less efficient than low energy bulbs
