Heating effect of electric current

HEATING EFFECT OF ELECTRIC CURRENT 

Electric current flowing through a conductor/wire also produces the heating effect across the length of the wire. You must have observed your electrical devices like television, fan, electric bulb etc get hot after operating them for few hours. This is simply the heating effect produced by electric current flowing in the device.

Work is done by the battery in supplying an electric current in a circuit. A part of the battery’s energy may be consumed into some useful work like in rotating the blades of an electric fan. Rest of the battery's energy may be expended in the form of heat to raise the temperature of gadget.
 
If the electric circuit is purely resistive, that is, we have a system of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This is known as the heating effect of electric current. This effect is utilised in devices such as electric heater, electric iron etc. 

Expression for heating effect of electric current 

Consider a purely resistive circuit, a resistor R connected to a voltage source V. Let current I flows through a resistor of resistance R. Let the potential difference across it be V. Let t be the time during which a charge Q flows across. The work done in moving the charge Q through a potential difference V is

\[W= VQ\]
Since \(Q=It\), therefore,
\[W=VIt\]
Using Ohm's law, \(V=IR\),
\[W=(IR)It\]
\[W=I^2Rt\]For purely resistive circuit, the work done by the battery gets dissipated in the resistor as heat.
\[H=I^2Rt\]

Joule’s Law of Heating 

According to Joule’s law of heating, heat produced in a resistor is 
(i) directly proportional to the square of current, 
(ii) directly proportional to resistance, and 
(iii) directly proportional to the time for which the current flows through the resistor. 
Thus,
\[H=I^2Rt\]

Undesirable effect of heating in electric circuit 

(1) Heating effect of electric current convert useful electrical energy into heat. 
(2) In electric circuits, the unavoidable heating increases the temperature of the gadget and alter their properties.

Practical Applications of Heating Effect of Electric Current 

(1) The electric laundry iron, electric toaster, electric oven, electric kettle and electric heater devices are based on Joule’s heating. 
(2) The electric heating is used to produce light, as in an electric bulb. 
(3) The functioning of fuse in electric circuit is based on joule’s heating.

Working of Electric iron, Electric toaster etc 

Alloys such as nichrome ( an alloy of nickel, chromium, manganese and iron), Constantan ( alloy of copper and nickel) and manganin (alloy of copper, manganese and nickel ) are used as an element of heating devices. Two properties which make these alloys suitable for heating element are: 
(a) High resistivity than metal. 
(b) Do not oxidise at higher temperature. 

When a large current is passed through these alloys then according to joule’s heating a large amount of heat is generated.

Working of Electric bulb 

The filament of electric bulb is made up of tungsten. The melting point of tungsten is very high ( 3380°C) When electric current is passed through tungsten filament it gets very hot and emits light. The filament is thermally isolated from the surrounding. The bulbs are usually filled with chemically inactive nitrogen and argon gases to prolong the life of filament. Most of the power consumed by the filament appears as heat, but a small part of it is in the form of light radiated. 


Two properties of tungsten which makes them suitable as filament of electric bulb 
(a) Very high melting point ( 3380°C) 
(b) Can be drawn into very thin wires 
(c) High resistivity than metals 

Electric Fuse 

Electric fuse protects circuits and appliances by stopping the flow of any unduly high electric current. It consists of a piece of wire made of a metal or an alloy of appropriate melting point, for example aluminium, copper, iron, lead etc. 

Working: If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

Capacity of fuse wire: The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.
Connection of fuse wire in circuit: The fuse is placed in series with the device.

NOW CHECK YOUR PROGRESS!!! 

1. An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater. 

2. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. Calculate the ratio of heat produced in series and parallel combinations. 

3. Two resistors each of resistance 6Ω are first connected in series and then parallel in a circuit across a battery of 10V. Calculate the ratio of heat produced in series and parallel combination. 

4. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.

5.Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V. 

6. 100 J of heat are produced each second in a 4 Ω resistance. Find the potential difference across the resistor. 

7. An electric iron consumes energy at a rate of 840 W when heating is at the maximum rate and 360 W when the heating is at the minimum. The voltage is 220 V. What are the current and the resistance in each case?