Question

What is the resistance of a torch bulb rated at $2.5V$ and $500mA$?

Answer 1

Hint: The resistance can be simply found out using Ohm’s law, which states that the resistance of a resistor is the ratio of the voltage across it to the current flowing through it. With the values of voltage and current given in the question, the resistance can be found out using ohm’s law.

Formula used:
The resistance $R$ of a resistor, when current $I$ flows through it due to a voltage $V$ applied across it, is given by,
$R=\dfrac{V}{I}$ (Ohm’s Law)
Complete step by step answer:
Since the rated value of the voltage and the current through the torch bulb is given, we can find out the resistance using Ohm’s law.
The resistance $R$ of a resistor, when current $I$ flows through it due to a voltage $V$ applied across it, is given by,
$R=\dfrac{V}{I}$ (Ohm’s Law) ---(1)
Hence, let us analyze the question.
The voltage of the torch bulb is $V=2.5Volt$
The rated current of the torch bulb is $I=500mA=0.5A$ $\left( \because 1mA=0.001A \right)$

Therefore, using (1), we get the resistance $R$ of the torch bulb as,
$R=\dfrac{2.5}{0.5}=5\Omega $
Hence, the resistance of the torch bulb is $5\Omega $.

Note: Students must be aware that the Ohm’s law is not valid for all electrical elements. In fact, the electrical elements that follow this linear dependence of current and voltage (that is, Ohm’s Law) are called linear elements.
There are many other electrical devices such as semiconductor diodes that do not follow Ohm’s law, that is, the current flowing through them is not directly proportional to the voltage applied across them. Ohm’s law cannot be used to find out their resistance in such cases. So, students should read the question carefully to decide whether the device in question is one that follows Ohm’s law or not.