Question

ASSERTION: A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
REASON: Sometimes a rheostat is made from resistance wire wound on a heat-resisting cylinder, with the slider made from a number of metal fingers

(A) both assertion and reason are correct and reason is the correct explanation for assertion
(B) both assertion and reason are correct but reason is not the correct explanation for assertion
(C) assertion is correct but reason is incorrect
(D) both assertion and reason are incorrect

Answer 1

A potentiometer is a resistor device used for finding value of unknown resistors. It is in fact a resistor. A rheostat is a variable resistor and is a box consisting of many resistances and is used in labs. The resistance which is taken out from it gets attached into the circuit.

Complete step by step answer:
Resistors can be of two types- variable and fixed, fixed resistor has a single fixed value of resistance, whereas a variable resistor has resistance value over a defined range. Potentiometer is a three terminal variable resistor. Out of its three terminals, two of them are fixed and one is a varying terminal. When only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. So, the assertion is correct.
Reason talks about the rheostat. As the name says it is also a variable resistance device. The reason talks about making one kind of resistor. There are numerous ways of making the rheostat. The reason is also correct but it is not the explanation for the assertion. Hence, the correct option is (B)

Note: A potentiometer is a three terminal variable resistor, but a rheostat is a two terminal variable resistor. One important point to make here is that potentiometers can work as a rheostat but opposite of this cannot be done. Potentiometers are used to vary voltages while later for varying current.