Which of the following quantities is a vector?

Options

A. Magnetic pole-strength
B. Magnetic permeability
C. Effective length of the magnet
D. Magnetic moment

Answer

To determine which of the given quantities is a vector, we need to analyze each option based on the definition of vector quantities. A vector quantity is defined as one that has both magnitude and direction.

### Step-by-Step Solution:

1. **Understand the Definition of Vector Quantities**:
- A vector quantity has both magnitude and direction. Examples include displacement, velocity, acceleration, and magnetic moment.

2. **Analyze the First Option: Magnetic Permeability**:
- Magnetic permeability is a property of materials that indicates how easily they can become magnetized in a magnetic field. It is a scalar quantity because it only has magnitude and no direction. Therefore, this option is not a vector.

3. **Analyze the Second Option: Effective Length of a Magnet**:
- The effective length of a magnet refers to the distance between its poles (north and south). Length is a scalar quantity, as it only has magnitude and no inherent direction associated with it. Thus, this option is also not a vector.

4. **Analyze the Third Option: Magnetic Moment**:
- Magnetic moment is defined as the product of the current flowing in a loop and the area of the loop, and it has both magnitude and direction (the direction is from the south pole to the north pole of the magnet). Therefore, magnetic moment is a vector quantity.

5. **Conclusion**:
- Based on the analysis, the only quantity that qualifies as a vector is the **magnetic moment**.

### Final Answer:
The quantity that is a vector is **magnetic moment**.