Question

When electric current flows in a purely resistive circuit electrical energy gets fully converted into heat energy. The amount of heat produced (H) in the circuit is found to be directly proportional to (i) the square of current (I²)
(ii) the resistance (R) of the conductor and
(iii) the time (t) for which current flows. In other words H = I²Rt. Electrical devices such an electric fuse, electric heater, electric iron etc. are all based on this effect called heating effect of electric current.
&
a) List two properties of heating elements.
b) List two properties of electric fuse.
c) Name the principle on which an electric fuse works. Explain how a fuse wire is capable of saving electrical appliances from getting damaged due to accidentally produced high currents.
OR
c) The power of an electric heater is 1100 W. If the potential difference between the two terminals of the heater is 220 V, find the current flowing in the circuit. What will happen to an electric fuse of rating 5 A connected in this circuit?

Answer 1

Heating Effect of Electric Current
When electric current flows through a purely resistive circuit, all electrical energy gets converted into heat energy. The heat produced is given by:
H = I²Rt
where H = heat produced, I = current, R = resistance, t = time.
Devices like electric iron, electric heater, and fuse operate on this principle.

a) Two properties of heating elements:
1. High resistivity: Heating elements are made of materials like nichrome that have high electrical resistance to produce more heat.
2. High melting point: They must withstand high temperatures without melting or deforming.

b) Two properties of an electric fuse:
1. Low melting point: The fuse wire melts easily when current exceeds the safe limit.
2. Made of a thin wire: It is designed to break the circuit when excessive current flows, preventing damage.

c) Principle of electric fuse:
An electric fuse works on the principle of the heating effect of electric current.

Explanation:
When excessive current flows through the circuit, the heat generated (H = I²Rt) in the thin fuse wire increases. Since the fuse wire has a low melting point, it melts quickly and breaks the circuit. This disconnects the power supply and prevents damage to the connected electrical devices due to high current or short circuit.

OR

c) Given:
Power (P) = 1100 W
Voltage (V) = 220 V

To find: Current (I)
Using the formula: P = VI
⇒ I = P / V = 1100 / 220 = 5 A

What happens to a 5 A fuse?
- The current drawn is exactly 5 A.
- A fuse of 5 A rating is designed to melt when the current exceeds 5 A.
- Since the current is right at the limit, the fuse may not blow immediately but it may melt if there is a slight increase or prolonged use at 5 A due to heat buildup.

Conclusion:
The fuse may withstand the current for a short time but can melt if there is a slight overload or extended operation.