Chapter 19 - Practical Electricity

1.   

Explain why the metal case of electrical equipment which is operated from the mains supply should be earthed.

  • If the live wire touches the metal casing accidentally, current will flow through the casing and the user touching the casing will get an electrical shock.
  • The earth wire has a resistance much lower than the human body.This will allow the current to flow through it to the ground instead of through the user’s body.
  • This protects the user from getting electrocuted.

 

2.   

Explain why thin wires should not be used in electrical systems, even through this would be cheap.

  • Since resistance is inversely proportional to cross-sectional area of wire, thin wires have high resistance.
  • Since the heating effect of the wire is given by P=I2R, a thin wire with large R will result in excessive heating. This results in loss of energy and in more serious cases, overheating which may lead to a fire hazard.

 

3.   

Explain why switches should be placed in the live wire, not the neutral wire, in household electrical systems.

  • Switches must be fitted onto the live wire so that opening it will disconnect (isolate) the appliance from the high voltage live wire. (If the switch is placed in the neutral wire, the electrical appliance is still connected to the high voltage live wire even when the switch is opened.)
  • This may cause the user to get an electric shock if he touches the live wire accidentally.

 

4.   

Explain why a fuse should have a rating a little higher than the current normally expected in the circuit.

  • This is to ensure that the appliance will operate when the normal operating current flows in the circuit.
  • The fuse will only melt when the current is higher than the rated value of the fuse.

 

5.   

What is the function of a fuse?

·         A fuse will prevent excessive current from flowing in a circuit. This protects the electrical appliance from being damaged.

 

6.   

How does a fuse work?

  • The wire in a fuse will melt and break the circuit if the current exceeds the rating of the fuse.
  • This ensures that the circuit is broken.

 

7.   

Why should a fuse be placed in the live wire of a mains circuit?

  • The fuse is connected to the live wire so that the appliance will not become charged (have a potential difference of 230 V) after the fuse has melted due to excessive current.
  • Fuses must be fitted onto the live wire so that when it blows, it will disconnect (isolate) the appliance from the high voltage live wire. (If the fuse is placed in the neutral wire, the electrical appliance is still connected to the high voltage live wire even when the switch is opened.)
  • This may cause the user to get an electric shock if he touches the live wire accidentally.