Heating Effect of Electric Current

When an electric current passes through a bulb, the filament gets so hot that it glows and emits light. When a current passes through the filament of an electric iron, the iron becomes very hot. This increase in temperature is due to what is called 'the heat is produced due to current.

When an electric current is passed through a high resistance wire like nichrome wire, then the wire becomes very hot and produces heat. This is called the heating effect of current. The heating effect of current is obtained by the transformation of electrical energy into heat energy. e.g. An electric fan becomes warm, if it is used continuously for longer time, etc.

Factors on Which Heating Effect Depends :

There are two factors on which heating effect of current depends :

1. Resistance of wire : Greater is the resistance of wire, greater will be heat produced in it by a given current. e.g. if we choose two wires, one of the copper and other is nichrome of equal length and equal thickness and pass equal amount of current through them for the same duration, then we will notice that nichrome wire will become more hot in comparison to the copper wire. It is due to the reason that resistance of nichrome wire is more than that of copper wire.

2. Magnitude of current passed through given wire :  If the magnitude of the current passed through the given wire is greater, then the heat produced in it will also be greater. For e g : If a normal amount of current flow through a copper wire of household electric wiring then the wires do not become hot but if a large current flows in the same wiring accidently , then the wires become extremely hot and a fire may be started.

Uses of Heating Effect:  

Some of important applications of heating effect of electric current  are :

1. For production of light, heating effect of electric current is utilized in the electric bulbs.

2. For the working of electrical heating appliances such as water heater, electric room heater, electric iron, etc. the heating effect of electric current is utilized.

3. The heating effect of electric current is utilized in a safety device called' electric fuse'. 

Calculation Of Heat Generated In a Conductor:

Assuming a conductor as a resistance wire which resists the flow of current through it. So, work must be done by the current source for continuous flow of the current. Now, we calculate the work done by the source when the current I flows through a wire of resistance R. When an electric charge q moves against a potential difference V. Then      Amount of work done is given by,

                  .....(i)

From definition of current, we know that,

                     .......(ii)

From Ohm s law,

                           ......(iii)

Substituting the values of q and V in Eq. (i), we get

Assuming that all electrical work done or electrical energy consumed is converted into heat energy, i.e. heat produced. So, heat produced is given by

Thus, it is known as Joule s Law of Heating.

This law implies that heat produced in a resistor is

(i) directly proportional to the square of current for a given resistance.

(ii) directly proportional to the resistance for a given current.

(iii) directly proportional to the time for which the current flows through the resistor.

Illustration:  200 J of heat is produced 10 s in a resistance. Find the potential difference across the resistor.

Solution: Given,Heat, H = 200 J

 Resistance, R =

Time, t = 10 s, Potential difference, V = ?

We know that,

Heat,

So, the potential difference across the resistor is