Question

(a) State joule's law of heating names 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 maximum rate. The voltage of the electric source is 220 V. Calculate the value of current and the resistance.

Answer 1

The heat produced due to flow of current within an electric wire, is known as the heating effect of Joule.

Complete step by step answer:
(a) Joule's law of heating states that power of heating generated by an electrical conductor is proportional to the product of its resistance (R) and square of the electric current passing through the conductor with time.
$\text{Heat generated }\left( \text{W} \right)\text{ = }{{\text{I}}^{2}}\text{Rt}$
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 visible lights. However to prevent tungsten from catching fire (combustion) at these high temperatures. Tungsten Filament is sealed in oxygen free chambers.
But creating vacuum, that is lack of surrounding air pressure to avoid tungsten Filament combustion leads to evaporation of tungsten at extreme high temperatures which gets collected inside a portion of the glass of electric bulb insert 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) Given, Power consumed (p) = 840 watt
Voltage of Electric source (v) = 220 volt
The power input (P) = VI
$\Rightarrow I=\dfrac{P}{V}=\dfrac{840}{220}=3.82A$
So, the electric resistance of Iron
$R=\dfrac{V}{I}=\dfrac{220}{3.82}=57.59\Omega$

Note:
The amount of heat produced is a type of energy which is produced in current conducting wire is proportional to the square of the amount of current, the Resistance of wire and time of current flowing
$\Rightarrow H={{I}^{2}}RT$