4. Practical Electricity

Question 1

Heating Effect of Electricity in Appliances

Answer 1

Appliances such as electric kettles, ovens, and heaters use the heating effect of electrical current.

They have a heating element made of nichrome.

Nichrome has:

1. high resistivity
2. high melting point
3. does not oxidise easily at high temperature.

Question 2

Lighting Effect of Electricity in Appliances

Answer 2

Filament lamps convert electrical energy to light and heat.

Filament wire is made of tungsten. Tungsten wire is heated to about 2500°C when an electric current passes through it and generates light.

Tungsten has:

1. high resistivity
2. high melting point
3. does oxidise easily at high temperature (light bulb is filled with argon or nitrogen which is unreactive)

Question 3

Power

Equations

Answer 3

P = IV
power = current x voltage

P = I²R

P = V²/R

Question 4

What are the hazards of using electricity?

Answer 4

1. Damaged insulation
2. Overheating of cables
3. Damp environment

Question 5

Damaged Insulation

How is it caused & why is it a hazard?

Answer 5

How is damaged insulation caused?

• Due to age, prolonged use and gnawed by rats

Why is it a hazard?

• Exposed live wire can cause electric shock when touched
• Damaged insulation between live and neutral wires may result in a short circuit leading to a fire

Question 6

Overheating of Cables

How is it caused & why is it a hazard?

Answer 6

How are overheating circuits caused?

• The electrical cable passes current that exceeds its limits
• A large current flows through the wires due to too many plugs being connected to a socket (overloading)
• Poor contact between a plug and a socket
• Inapporiate wires are used - a thinner wire has a high resistance and generates more heat, compared to a thick wire (appliances that require high power to function need thicker wires)

Why is it a hazard?

• Large amount of heat generated can start a fire

Question 7

Damp Environments

Why is it a hazard & how to prevent?

Answer 7

Why is it a hazard?

• Water from taps and other sources usually contain charged ions and impurities that makes it a good conductor of electricity.
• Water in contact with exposed electrical wires provides a conducting path for electric current and can lead to electric shock.
• Wet skin has a relatively low resistance. A large current may pass through the user and result in an electric shock.

How to prevent?

• Electrical appliances should be kept in dry places and handled with dry hands: Dry skin has a relatively high resistance of ≥ 100 000Ω while wet skin has a relatively low resistance of a few hundred Ω
• Wear rubber shoes when carrying out electrical repairs or inspection of electrical faults: Rubber shoes act as insulators to cut off the closed path to the ground

Question 8

Use of Circuit Breakers in Electrical Circuits & Advantage

Answer 8

When there is excessive current flow (exceeds the rating) through a circuit (caused by overloading or short circuit), the circuit breaker trips, resulting in an open circuit that will prevent the current to flow any longer.
This prevents the heating effect of current which might lead to the outbreak of fire.

Advantage: Can be manually reset without having to replace wires, as in the case of fuses.

Question 9

Use of Fuses in Electrical Circuits

Answer 9

When the current through the fuse exceeds its rated value (caused by overloading or short circuit), the fuse will melt, resulting in an open circuit that will prevent the current to flow any longer.
This prevents the heating effect of current which might lead to the outbreak of fire.

Fuse is connected to the live wire.
Typical household fuse ratings are: 1A, 2A, 3A, 5A, 10A, 13A.

Question 10

Why are switches, fuses, and circuit breakers fitted to the live wire?

Answer 10

In the event there is an electrical fault, the switch can be flipped / the fuse will blow / the circuit breaker will trip to disconnect the appliance from the high voltage of the live wire.

Question 11

How does the earth wire / earth metal casing protect the user?

Answer 11

1. The earth wire is connected to the metal case of the appliance.
2. If the live wire touches the metal casing of the appliance, the large current will flow to the earth through the earth wire.
3. The fuse in the live wire will blow / circuit breaker will trip, resulting in an open circuit.
4. The appliance will be disconnected from the high voltage of the live wire.
5. The user will avoid an electrical shock if they touch the metal case.

Question 12

What does an appliance with double insulation mean?

Answer 12

Appliances with double insulation are enclosed in a plastic casing which is not earthed.
2 layers of insulation are present to enclose all exposed metalwork and “live” parts.

Question 13

Why do appliances with double insulation use two-pin plugs?

Answer 13

Two-pin plugs only have the live and neutral wires, and no earth wire.
As the casing of these appliances are made of plastic (an electrical insulator), there is no need for an earth wire to be in contact with the casing to prevent electric shock by touching the casing.

Question 14

Live, Neutral & Earth Wires

Colour + Purpose + Others

Answer 14

Live wire (brown):

• Delivers current to the appliance
• 240V in SG
• Switches, circuit breakers & fuses are connected to live wire

Neutral wire (blue):

• Completes the circuit by providing a returning path for the current to the supply
• 0V

Earth wire (green and yellow):

• Prevents electric shock in the case of electrical fault
• Connected to the metal casing of appliance