Question

(a) State Joule’s law of heating. Name two gases which are filled in the electric bulbs and explain why these gases are filled in electric bulbs.
(b) An electric iron consumes energy at a rate of 840 W when heating is at the maximum rate. The voltage of the electric source is 220 V. Calculate the value of current and resistance.

Answer 1

Hint: (a) Joule’s law of heating relates the heat produced in a conductor with the current and resistance of the conductor. Electric bulbs require an inert atmosphere to keep the filament safe.
(b) By using the expression for power in electrical circuits and Ohm’s law, we can calculate the required values of current and resistance.

Formula used:
Power is defined for an electrical circuit by following expression:
$P = VI$
Ohm’s law is given as
$V = IR$
Here symbols have their usual meanings.

Complete step-by-step solution:
(a) The Joule’s law of heating can be stated in the following way.
A current flowing through a conductor produces heat in the conductor. The amount of heat produced by the conductor is equal to the product of the square of the current flowing through the conductor and the resistance offered by the conductor to the flow of current.
$H = {I^2}R$
Here I is the value of current and R is the resistance.
The commonly used gases in the electric bulb are argon, neon, krypton, etc. These are inert gases and are used in the electric bulbs because we want to provide an inert atmosphere to the filament of the bulb. The filament is highly reactive with other gases in the air which can damage it and affect the working of the bulb.
(b) We are given that an electric iron consumes energy at a rate of 840 W when heating is at the maximum rate.
$P = 840W$
The voltage of electric source is given as
$V = 220V$
Therefore current can be calculated.
$I = \dfrac{P}{V} = \dfrac{{840}}{{220}} = 3.82A$
Resistance can be calculated by Ohm’s law:
$R = \dfrac{V}{I} = \dfrac{{220}}{{3.82}} = 57.59\Omega $

Note: (a) Argon is readily available in nature and more commonly used in the electric bulbs.
(b) Power consumed by a circuit has the same meaning as that in mechanical systems. It represents the work done per unit time in terms of current and resistance.