Assertion (A): Magnetic moment of a bar magnet decreases with length.
Reason (R): Magnetic strength decreases as magnetic moment decreases.

Options

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 and (R) is also false.

Explanation

To analyze the given assertion and reason, let's break it down step by step.

### Step 1: Understanding Magnetic Moment
The magnetic moment (m) of a bar magnet is given by the formula:
\[ m = p \cdot 2L \]
where:
- \( p \) is the pole strength,
- \( L \) is the length of the magnet.

### Step 2: Analyzing the Assertion
The assertion states that the magnetic moment of a bar magnet decreases with length.
- From the formula, if the length \( L \) decreases while the pole strength \( p \) remains constant, the magnetic moment \( m \) will also decrease.
- Therefore, the assertion is **true**.

### Step 3: Analyzing the Reason
The reason states that magnetic strength decreases as magnetic moment decreases.
- Magnetic strength (often referred to as magnetic field strength) is related to the magnetic moment, but it is not directly proportional to it. The magnetic strength depends on various factors, including the distance from the magnet and the medium in which the magnet is placed.
- The decrease in magnetic moment does not necessarily imply a decrease in magnetic strength. Thus, the reason is **false**.

### Step 4: Conclusion
- The assertion is true, but the reason is false. Therefore, the correct relationship is that the assertion stands alone without support from the reason.

### Final Answer:
- Assertion (A) is true.
- Reason (R) is false.

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