Question

Assertion: Out of galvanometer, voltmeter and ammeter, resistance of ammeter is lowest and resistance of voltmeter is highest.
Reason: An ammeter is connected in series and a voltmeter is connected in parallel, in a circuit.
A. Both (A) and (R) are true and (R) is the correct explanation of A.
B. Both (A) and (R) are true but (R) is not the correct explanation of A.
C. (A) is true but (R) is false.
D. (A) is false but (R) is true.

Answer 1

Hint: Recall how a galvanometer is converted into an ammeter or voltmeter. Hence, check the resistance of which of the electrical components out of galvanometer, voltmeter and ammeter is highest and lowest. Also recall the reason that ammeters are always connected in series and voltmeters are always connected in parallel.

Complete answer:
The galvanometer is converted into ammeter by when a shunt resistor of low resistance is connected in parallel with the galvanometer. The shunt resistance connected in parallel makes the effective resistance of the system of resistors lower than the resistance of the individual resistors. Hence, the resistance of the ammeter formed is less than the resistance of the galvanometer.

The galvanometer is converted into voltmeter by when a shunt resistor of very high resistance is connected in series with the galvanometer. The shunt resistance connected in series makes the effective resistance of the system of resistors higher than the resistance of the individual resistors. Hence, the resistance of the voltmeter formed is more than the resistance of the galvanometer.Thus, out of galvanometer, voltmeter and ammeter, resistance of ammeter is lowest and resistance of voltmeter is highest.Hence, the assertion is true.

The ammeter is always connected in series because the function of the ammeter is to measure the accurate current. If the ammeter is connected in series, whole current flowing in the circuit flows through the ammeter with a very low resistance. Hence, we get the correct reading of the ammeter.

The voltmeter is always connected in parallel because the main function of the voltmeter is to measure the potential difference between the two ends of the wire. When the voltmeter of high resistance is connected in parallel in a circuit, the current flowing through it is very low and being connected in parallel, we can have the same potential difference in the voltmeter as that of the electrical instrument across which we want to measure the potential difference. This gives us the correct reading of the potential difference.Hence, the reason is true.The reason is the correct explanation of the assertion.

Hence, the correct option is A.

Note: From the above information, one may also think that the reason for low resistance of ammeter and high resistance of voltmeter is to measure the correct readings of current and potential difference. This is correct but the same thing is obtained by connecting the ammeter always in series and voltmeter always in parallel in a circuit. Hence, the reason gives the correct explanation of assertion.