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James Prescott Joule
The English scientist James Prescott Joule, born in 1818, conducted studies about the salient features of heat energy and about the mechanical movements caused by heat. He became famous for his two laws related to the magnitude of heat due to current and how the energy of gas influences pressure.James Prescott Joule measured and proved that mechanical energy could be converted into heat energy and vice versa.
Before Joule, it was thought that heat energy could neither be created not destroyed. But Joule discovered that it could be created, and was often created, by mechanical sources.
What is Joule’s Law
The heat generated (H) in a current carrying conductor is directly proportional to the product of the square of the current (1) in the conductor, the resistance of the conductor (R) and the time (t) of flow of current.
H=I2 RT
l is the current in ampere, R is the resistance in ohm and t is the time in second.
Complete the following table on the basis of Joule’s Law
Analyse the table and find out the factor that influences heat the most.
Let’s solve some mathematical problems which are related to Joules Law.
•How much will be the heat developed if 0.2 A current flows through a conductor of resistance 200 Ω for 5 minute?
R=200 Ω
I=0.2A
t=5×60s=300 s
∴ H=I2 RT
=(0.2)² ×200×300
=2400 J
∴ Heat generated is 2400 J
If 4.2 J is one calorie then H = …….. calorie
H=I2 RT is used to find out the heat developed when current flows through a conductor.
Let’s try to write down the equation in some other forms as well.
According to Ohm’s Law, I= V/R. If we substitute this in the equation according to Joule’s Law H=I2 RT,
we get H= (V/R)² Rt
H= (V÷R)² Rt
=…………….
Some Questions for Students
- State Joule’s law of heating.
- How does a bulb work based on this law?
Answers
- Joules law of heating- Joule’s law of heating states that when a current I passes through a conductor then some amount of energy is given out as heat the heat produced is directly proportional to square of the current, resistance and the time for which the current flows ,hence, H=I2 RT.2.a bulb work based on this law is the heating of filament of the bulb has high resistance and high melting point which is made up of tungsten when a high amount of current passes through the filament it becomes hot because of high resistance offered by filament and high current flowing through it and hence it becomes hot and start glowing.
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