Electricity - Class 10th Science
Heating Effect of Electric Current
When an electrical appliance is connected to electric source and put on work, electric current, i.e. electrons starts flowing between its two terminals. To put electrical appliance in working, electric source has to continue to supply the electric current, i.e. energy.
Some of electrical energy supplied is consumed in working of the electrical appliance, i.e. in useful work, and rest of the electrical energy is used in raising the temperature of the electrical appliance.
Example: When an electric current is supplied to an electric fan, its blades start moving. Along with moving of blades of fan, motor of electric fan gets warmed also.
If electric current is supplied to an electric circuit with resistor only, the electric current is consumed in raising the heat of the resistor. This means that electric current dissipated entirely in the form of heat.
We cannot use electrical energy directly. To use first electrical energy is converted into heat, light or mechanical power.
Thus, conversion of electrical energy into heat energy is called the HEATING EFFECT OF ELECTRIC CURRENT.
In other words, generation of heat because of dissipation of electrical energy, when electric current is passed through a conductor or resistor, is called the HEATING EFFECT OF ELECTRIC CURRENT.
Joule's Law of Heating
Let an electric current I is flowing through a resistor having resistance = R
Let the potential difference across two ends of resistor = V
Let electric charge Q flows across the resistor in time t.
Therefore, work done in moving the electric charge Q through a potential difference V = VQ
Therefore, the electrical source must supply this energy equal to VQ in time t.
Therefore, power input to the circuit by electrical source is
Since, energy supplied to the circuit by the source in time t
Threfore, P x t = VI x t
Thus for a steady current (I), the amount of heat (H) produced in time t is equal to VIt
Or, H = VIt ------- (i)
According to Ohm's Law, V = IR
By substituting V = IR in equation (i) we get
Where, H = heat produced
I = electric current
R = resistance of resistor
And t = time
This expression (ii) is known as Joule's Law of Heating.
Thus, Joule's Law of Heating states 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, and
(iii) directly proportional to the time for which the current flows through the resistor.
Practical Application of Heating Effect of Electric Current
Whenever electric current is supplied to electric circuit, a part of the electrical energy is consumed in generation of heat. This generation of heat in a conductor is an inevitable process. But this heating effect of electric current is used in many useful applications.
Electric Bulb
In an electric bulb electric current is passed through a conductor called filament. Filament gets heated because of electric current and becomes red hot. Bulb is filled with inactive gas, such as argon or inactive nitrogen to prevent filament from getting burnt. After getting heated filament produce light.
A metal with very high melting point is used as filament. Generally filaments are made of tungsten, which has melting point near 33800 C. If metals with low melting point will be used in bulb filament, this will be melt even at low temperature and because of circuit break bulb will stop working.
Electric Iron
A rod made of alloy of metal with high melting point is used in electric iron as heating rod. When electric current is passed, heating rod of electric iron produce heat, which is supplied to the base of electric iron, which is used to get rid of wrinkles in clothes.
Electric heater
Similar to electric iron, heating filament or rod made of metals having high melting point is used to produce heat. When electric current is passed through this filament or heating rod, because of heating effect of electric current, filament generates heat and gives us comfort in winter seasons.
Heating elements are generally made of nicrome which is an alloy of nickel and chrominum.
Electric Fuse
Electric fuse is a device used in house hold to break electric circuit when electric current more than required potential difference reaches.
Electric fuse consists of a piece of wire made of a metal or alloy of appropriate melting point. Fuse wire is encased generally in a porcelain or similar materials cartridge. Electric fuse is connected in series with electric circuit. When electric current higher than the specified value flow through the circuit, fuse wire melts due to increase in heat and breaks the electric circuit. This prevent other electrical appliances connected to the circuit from getting damaged.
Electric fuse wire rated as 1 A, 2 A, 3 A, 5 A, and 10 A, etc. is generally used in households.
Example: An electric iron operated at 220 V of potential difference of electric current.
An electric current of (1000/220 ) A = 4.54 A generally flows though the circuit.
In this case a fuse wire of 5 A rate is used in electric iron.
Geysers, hair dryer, toaster, electric cooking plate, etc. are many other appliances which work on the heating effect of electric current.