Question

A closed switch has a resistance of:
A) Zero ohm
B) About $50\Omega $
C) About $500\Omega $
D) Infinity

Answer 1

Hint: The ratio of voltage to current is constant. This constant ratio is called resistance. Resistance causes loss of power in a circuit. More the resistance of the circuit more is the power loss. However, we know that the only purpose of the switch is to open and close the circuit as per our requirement.

Complete step by step solution:
When current flows through a conductor or circuit, the electrons collide with each other and make the flow of current difficult known as the resistance. In other words, the measure of the opposition to the flow of current is known as resistance of the circuit. It is measured in ohms and is denoted by the Greek letter omega ( $\Omega $ ).

Mathematically, resistance is given by Ohm's law which states that the current flowing through is circuit is equal to the ratio of the voltage of the circuit by the resistance in the circuit.
$I = \dfrac{V}{R}$
Where $I$ is the current through the circuit
$V$ is the voltage of the circuit
$R$ is the resistance in the circuit
The resistance of a material depends on the material of the resistor, length of the resistor, thickness and temperature of the resistor. A closed switch symbolises that the circuit is closed or complete and is equivalent to a short circuit. Current flows from one point to other in the closed circuit

Whereas, an open switch symbolises that the circuit is disconnected or incomplete and thus no current flows through the circuit. Since, the current flows in the closed switch or closed circuit, there is no opposition or minimal opposition in the flow of current through the circuit. This implies, that the resistance of the circuit will be minimal or negligible i.e. zero ohms.

Thus, option (A) is the correct answer.

Note: Resistance and resistivity are not the same. Do not confuse between resistance and resistivity. Resistance is the property of the material that opposes the flow of current whereas resistivity is a physical property and has a particular value for a specific material which defines the resistance for one unit of length having one unit of cross sectional area. SI unit of resistance is $\Omega $ and that of resistivity is $\Omega .m$ .