Question

(a) 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.
(b) 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.

Answer 1

(a) Joule's law of heating states that the power of heating generated by an electrical conductor is proportional to the product of its resistance and the square of the electric current passing through the conductor.

Electric bulbs are usually filled with argon or krypton.

In Light Bulb, tungsten filament (due to its high melting temperature) is heated to extreme temperature so as to emit enough amount of visible light. However to prevent tungsten from catching fire (combustion) at these high temperatures, tungsten filament is sealed in oxygen free chamber. But creating vacuum, i.e., lack of surrounding air pressure to avoid tungsten filament combustion leads to evaporation of tungsten at extreme high temperatures which gets collected inside portion of the glass of electric bulb and reduces light output. Hence in electric bulb inert gases typically argon is filled to avoid loss of tungsten due to evaporation. Also as argon is inert gas there is no chance of combustion of tungsten filament.

(b) The power input is $\left(P\right)=VI$

Thus, the current, $I=\frac{P}{V}$

When heat is at the maximum rate, $I=\frac{840}{220}=3.82A$

The resistance of the electric iron, $R=\frac{V}{I}==\frac{220}{3.82}=57.59\Omega$