(1) State Joule's law of heating. Name two gases which are filled in electric bulbs and explain why these gases are filled in electric bulbs. (2) An electric iron consumes energy at a rate of 840 W, when heating is at the maximum rate. The voltage of electric source is 220 V. Calculate the value of current and the resistance.


(1) Joule’s law of heating states that the power of heating obtained by an electrical conductor is directly proportional to the

(a) Product of its resistance and

(b) Square of the electric current passing through the conductor.

(c) Duration for which the conductive current flows.

Following are the two gases that are filled in filament type electric light bulbs:

  • Argon
  • Nitrogen

The filament in an electric bulb is made using tungsten which would burn quickly when it comes in contact with the oxygen in the air. This is the reason why unreactive gases such as argon and nitrogen are used for filling an electric bulb. These gases are chemically interactive.


Given parameters

Power (P) = 840 W

Voltage (V) = 220V

To calculate I =? and R =?

Power input (P) = VI

I = P/V

I = 840/220

I = 3.82 A

The resistance of the electric iron (R) = V/I

R = 220/3.82

R = 57.59 Ω

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