Question

Assertion: A voltmeter and ammeter can be used together to measure resistance and power.
Reason: Resistance and power both are proportional to voltage and current.
A. Both assertion and reason are true and reason is the correct explanation of assertion
B. Both assertion and reason are true but reason is not the correct explanation of assertion
C. Assertion is correct but reason is incorrect
D. Both assertion and reason are incorrect

Answer 1

Hint: The voltage between two points of the circuit is proportional to the current in the circuit and the constant of proportionality is known as resistance. The power is the product of voltage and current.

Complete step by step answer:
A voltmeter is the electric device used to measure the voltage between the two points in the electrical circuit. An ammeter is the electric device used to measure the current in the electric circuit when connected in series with the electric component.
We know the relation between resistance, voltage and current. The voltage is proportional to current in the electric circuit. Therefore,
\[V \propto I\]
\[ \Rightarrow V = RI\]
The constant of proportionality R is known as resistance of the electric circuit. Therefore, rearrange the above expression for R as follows,
\[R = \dfrac{V}{I}\] …… (1)
Therefore, if we measure the value of voltage and current in the circuit using voltmeter and ammeter respectively, we can determine the resistance of the circuit.
Power is defined as a product of voltage and current. Therefore,
\[P = VI\] …… (2)
Therefore, we measure the voltage and current using voltmeter and ammeter together, we can determine power in the electric circuit.
So, the assertion is correct.
From equation (1), the resistance is proportional to the voltage but inversely proportional to the current. The power is proportional to both current and voltage.
Therefore, the power is proportional to current and voltage but the resistance is not proportional to the current.
Thus, the reason is not the correct explanation of assertion.

So, the correct answer is “Option C”.

Note:
To use voltmeter and ammeter together in the circuit, the voltmeter always connected parallel to the circuit and ammeter in the series with the circuit.
The power is proportional to both current and voltage.