Question

A lamp rated 100W at 22V is connected to the main electric supply. What current is drawn from the supply line if the voltage is 220V?

Answer 1

Hint: The voltage and power of the lamp is given so we can find the current flowing through the bulb by using the formula of power. Power is given as the product of current and voltage. Here we can draw a circuit diagram for the given set-up to find the current flowing in the circuit and the current drawn by the lamp.

Formula used:
\[P=VI\]

Complete step-by-step answer:
Let us first draw the circuit diagram for the given set, according to the question a lamp is connected to the main supply line. The voltage and power is given for the lamp and the voltage of the supply line. The circuit diagram for the question can be drawn as follows

As there is a single lamp connected to the main supply, therefore current flowing through the lamp will be the same as the current associated with the supply line. Hence current drawn from the supply line can be given as the current flowing through the lamp.
Now power consumed by an electrical device is given as the product of voltage and the current flowing through it. The formula for power is
\[P=VI\]
Here we have given the value of voltage and power for the lamp and by using the above formula we can calculate the current flowing through the circuit.
Substituting \[P=100W\text{ and }V=22V\]in the above formula we get
\[\begin{align}
  & 100=22I \\
 & I=\dfrac{100}{22} \\
 & I=4.54A \\
\end{align}\]
Hence current drawn from the main supply will be 4.54A.

Note: In case there were two lamps connected in parallel to each other from the main supply then the current drawn by each lamp will be different or we can say the sum of current flowing through each lamp will be equal to the current from the main supply. One can also get confused that current will be different across the lamp as voltage is different but that will be the case when the resistance of the main supply line is the same as the lamp, which is not the case here. Therefore current will be the same flowing through the circuit.