Electricity

Question 1

What current is mains supply?

Answer 1

Alternating current

Question 2

What current is battery supply?

Answer 2

Direct current

Question 3

What is the voltage of the UK mains supply?

Answer 3

230 volts

Question 4

What is the frequency of the UK mains supply?

Answer 4

50 Hz

Question 5

What is a cathode ray oscilloscope?

Answer 5

A voltmeter

Question 6

What happens if you plug a DC supply into a CRO?

Answer 6

The trace is a straight line.

Question 7

What does the vertical height of the AC trace show?

Answer 7

The input voltage.

Question 8

How can you tell what the voltage is from a DC trace?

Answer 8

The distance from the trace to the centre line.

Question 9

What is the formula for frequency?

Answer 9

Frequency = 1 / Time period
Hz = 1 / s

Question 10

What are the nine electrical hazards?

Answer 10

1. Long cables
2. Frayed cables
3. Cables touching hot or wet things
4. Water near sockets
5. Things in sockets
6. Damaged plugs
7. Too many plugs
8. Light with bulb
9. Appliance without cover

Question 11

What do three-core cables have inside?

Answer 11

Three wires, each with a copper core and plastic coating.

Question 12

What is the brown wire?

Answer 12

Live wire, alternates between high +ve and -ve voltage.

Question 13

What is the blue wire?

Answer 13

Neutral wire, 0V all the time.

Question 14

What is the green and yellow wire?

Answer 14

Earth wire, protects wiring for safety.

Question 15

What are the wire safety features?

Answer 15

1. Connected to correct pin, firmly screwed in
2. No bare wires
3. Cable grip tightly fastened

Question 16

What is the significance of thicker cables having less resistance?

Answer 16

They carry more current.

Question 17

Why are the metal parts of plugs made of copper or brass?

Answer 17

Good conductors

Question 18

Which parts of the plug are made of rubber or plastic?

Answer 18

Case, cable grip and cable insulation because of insulation and flexibility.

Question 19

How do fuses prevent electrical overloads?

Answer 19

1. Surge in current melts fuse
2. Live supply cut off
3. Circuit broken

Question 20

What causes a fault?

Answer 20

Live wire touches metal case.

Question 21

How should fuses be rated?

Answer 21

Just above the normal operating current.

Question 22

Why does fuse rating needed for tables usually increase with cable thickness?

Answer 22

Larger currents need thicker cables.

Question 23

What does a double insulated appliance mean?

Answer 23

Plastic casing and no metal parts showing.

Question 24

What do two-core cables carry?

Answer 24

Live and neutral wires.

Question 25

How do circuit breakers work?

Answer 25

1. Current surge detected

2. Circuit broken by opening a switch

Question 26

What is the advantage of circuit breakers?

Answer 26

Easily reset, no replacement.

Question 27

What is the disadvantage of circuit breakers?

Answer 27

More expensive

Question 28

How do Residual Current Circuit Breakers work?

Answer 28

1. If someone touches a live wire, small current flows through them
2. Neutral wire will carry less current than live wire
3. Difference detected, circuit broken

Question 29

What are the advantages of RCCB?

Answer 29

1. Act more quickly - safer

2. Detects small current changes - better at protecting against electrocution

Question 30

What is the power rating formula?

Answer 30

Energy transferred = Power rating x Time

J = W x s

Question 31

What is the electrical power formula?

Answer 31

Power = Current x Potential difference
W = A x V

Question 32

What is the formula for energy transformed?

Answer 32

Energy transformed = Charge x Potential difference

J = C x V