Topic 5 - Mains Electricity

Question 1

What is direct current?

Answer 1

Current that only flows in one direction in the circuit.

Question 2

What is alternating current?

Answer 2

Electric current in a circuit that repeatedly reverses direction.

Question 3

What is the short name for direct current?

Answer 3

D.C.

Question 4

What is the short hand name for alternating current?

Answer 4

A.C.

Question 5

What is a time period?

Answer 5

The amount of time taken for one wave of alternating current to appear.

Question 6

What is amplitude in a wave?

Answer 6

The distance between the centre line and the bottom of a trough or the top of a peak.

Question 7

What is ‘hertz’?

Answer 7

The units at which frequency is measured. 1 hert is 1 cycle per second.

Question 8

What is frequency?

Answer 8

The number of cycles per second.

Question 9

What is the general upper and lower limit of alternating current?

Answer 9

+325V and -325V, a difference of 650V.

Question 10

What is the average UK mains supply in hertz and volts?

Answer 10

UK mains supply is at 50 hertz and 230 volts.

Question 11

Oscilloscopes are what?

Answer 11

Graphs measuring the positive and negative volt values, showing the alternating current.

Question 12

What is earth wire in a mains plug?

Answer 12

The wire used to connect the metal casing to the ground in the event in an overload of current.

Question 13

What is a fuse in a mains socket?

Answer 13

A fuse is a thin wire that melts in the event of a current overload cutting off the circuit.

Question 14

What is a live wire?

Answer 14

The mains wire that has a voltage that alternates in voltage, -325V and 325 V in Europe.

Question 15

What is the neutral wire?

Answer 15

The wire of a mains circuit that is earthed at the local substation so its potential is close to zero. In other words it brings the wire back to its source.

Question 16

What is a plug?

Answer 16

A plug has an insulating case and is used to connect the cable from an appliance to a socket.

Question 17

What is a step-up transformer?

Answer 17

A device used to step-up the potential size of an alternating potential difference. This is normally used to bring electricity from a power plant to the national grid.

Question 18

What is a step-down transformer?

Answer 18

A device used to step down the size of an alternating potential difference. This is normally done to bring electricity down from the national grid to the mains supply instead of from a power plant to the national grid.

Question 19

What is a three-pin plug?

Answer 19

A three pin plug is the plug type used in the U.K. containing an earth wire, live wire and neutral wire.

Question 20

What is the equation linking power, current and voltage?

Answer 20

Power(W) = Current (A) x Voltage (V) or P= I x V

Question 21

What is the equation linking power, current and resistance?

Answer 21

Power (W) = Current squared (A) x Resistance (Ohms) or P= i2 x R

Question 22

What is the effect of using a step-up transformer on energy efficiency and why does this happen?

Answer 22

Step-up transformers increase potential difference and decrease current, reducing energy transfer to the surroundings which decreases wasted energy, which therefore increases efficiency.