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A: When an electric motor is started, a ...

A: When an electric motor is started, a variable resistance (that decreases with time) is used in series. This resistance is known as motor slarter,
R: The back-emf in the beginning, when motor starts, is very small.

A

If both Assertion & Reason are true and the reason is the correct explanation of the assertion.

B

If both Assertion & Reason are true but the reason is not the correct explanation of the assertion.

C

If Assertion is true statement but Reason is false.

D

If both Assertion and Reason are false statements.

Text Solution

AI Generated Solution

The correct Answer is:
To solve the assertion and reason question regarding the electric motor and back-emf, we will analyze both statements step by step. ### Step 1: Understand the Assertion The assertion states that when an electric motor is started, a variable resistance (known as a motor starter) is used in series. - **Explanation**: When an electric motor starts, it draws a large amount of current due to low back-emf (electromotive force). To limit this starting current and protect the motor, a variable resistance is introduced in series. As the motor speeds up, the back-emf increases, which reduces the current flowing through the circuit. ### Step 2: Understand the Reason The reason states that the back-emf at the beginning, when the motor starts, is very small. - **Explanation**: Back-emf is generated in a motor due to its rotation in a magnetic field. At the moment the motor starts, it is not yet rotating, so the back-emf is minimal. As the motor gains speed, the back-emf increases, which opposes the applied voltage and reduces the current. ### Step 3: Analyze the Relationship Now, we need to determine if the assertion and reason are true or false. - **Assertion Evaluation**: The assertion is true because a variable resistance (motor starter) is indeed used to limit the starting current when the motor is started. - **Reason Evaluation**: The reason is also true since the back-emf is very small when the motor starts. ### Step 4: Conclusion Both the assertion and the reason are true. However, the reason correctly explains why the assertion is true, as the low back-emf at startup necessitates the use of a motor starter to manage the initial current. ### Final Answer Both the assertion and the reason are true, and the reason is a correct explanation of the assertion. ---
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