Ch 18 Practical Electricity

Question 1

What is the name of 2 types of high resistance wire?

Answer 1

nichrome and tungsten

Question 2

What is the energy change in a high resistance wire?

Answer 2

electrical energy to heat

Question 3

What is the relationship between P, I and V?

Answer 3

P = VI
P: power
V: voltage
I: current

Question 4

What is the relationship between E, I, V and t?

Answer 4

E = VIt
E: energy
V: voltage
I: current
t: time

Question 5

State the 2 other variations of the formula E= VIt

Answer 5

E = I²Rt

E = (V²/R)t

Question 6

State 2 other variation of the formula P= VI

Answer 6

P = I²R

P = V²/R

Question 7

What is the unit of energy used for calculating home electrical consumption?

Answer 7

kWh

(kilowatt-hour)

Question 8

Conversions of unit needed for calculating energy in kWh.

1 s = __________ h

1 W = ____________ kW

Answer 8

1 s = 1/3600 h

1 W = 0.001 kW

Question 9

Why are damaged insulation so dangerous?

(There are 2 answers)

Answer 9

1. Causes electric shock when someone touches an exposed live wire
2. Causes an electric fire when the exposed live and neutral wire touches each other resulting in a short circuit

Question 10

What is overloading?

Answer 10

When there are too many electrical appliances connected to the same power socket.

Question 11

Why is overloading dangerous?

Answer 11

Overloading causes the wires to overheat and eventually causes a fire.

Question 12

Why should we not handle electrical appliances with wet hands?

Answer 12

Water lowers the resistance of our body, making it easier for current to pass through giving us an electric shock.

Question 13

What is the purpose of a fuse?

Answer 13

The fuse melts and breaks the circuit when the current exceeds the current rating of the fuse.

It protects the electrical appliances by preventing overheating of wires.

It protects the user from an electric shock.

Question 14

What is a fuse rating?

Answer 14

The maximum amount of current allowed to pass through the fuse before it melts and breaks the circuit.

Question 15

What is the colour of the live wire?

What is the colour of the neutral wire?

What is the colour of the earth wire?

Answer 15

Live wire: brown (labelled C in diagram)

Neutral wire: blue (labelled B in diagram)

Earth wire: yellow and green stripes (labelled A in diagram)

Question 16

Explain how the earth wire works with a fuse to protect the user from an electric shock.

Answer 16

The earth wire is connected to the metal casing of the appliance.

When there is an electrical fault such as the live wire touching the metal casing, a large current will flow from the live wire to the earth wire.

This causes the fuse in the live wire to melt and break the circuit, protecting the user from an electric shock.

Question 17

How do a double insulated wire help to protect the user from electric shock?

Answer 17

When the inner insulation of the live wire fails, the outer casing will still insulate the live wire from the user thus preventing an electric shock.

Question 18

Why is the switch, fuse and circuit breaker connected at the live wire?

Answer 18

When the switch is off, it isolates high voltage of the live wire from the rest of the appliance.

When the fuse melts and breaks, it isolates high voltage of the live wire from the rest of the appliance.

When the circuit breaker breaks, it isolates high voltage of the live wire from the rest of the appliance.

Question 19

An electrical appliance is rated ‘60 W, 240 V, 0.25 A’.

What is the voltage and current value in the following wires when the appliance is working normally?

Live wire:

V = _____

I = ______

Neutral wire:

V = _____

I = ______

Earth wire:

V = _____

I = ______

Answer 19

Live wire:

V = 240 V

I = 0.25 A

Neutral wire:

V = 0 V

I = 0.25 A

Earth wire:

V = 0 V

I = 0 A