A bar magnet of pole strength 2 A-m is kept in a magnetic field of ind...
Understanding the Problem
To find the distance between the poles of the bar magnet, we need to use the formula for the couple acting on a magnetic dipole in a magnetic field.
Given Data
- Pole strength, \( m = 2 \, \text{A-m} \)
- Magnetic field induction, \( B = 4 \times 10^{-5} \, \text{wbm}^{-2} \)
- Angle, \( \theta = 30^\circ \)
- Couple, \( \tau = 80 \times 10^{-7} \, \text{N-m} \)
Formula for the Couple
The couple \( \tau \) acting on a magnetic dipole is given by the formula:
\[
\tau = m \cdot B \cdot \sin(\theta)
\]
Where:
- \( m \) is the pole strength,
- \( B \) is the magnetic field induction,
- \( \theta \) is the angle between the magnet and the magnetic field.
Calculating the Couple
Substituting the known values into the formula:
\[
80 \times 10^{-7} = 2 \cdot (4 \times 10^{-5}) \cdot \sin(30^\circ)
\]
Since \( \sin(30^\circ) = \frac{1}{2} \):
\[
80 \times 10^{-7} = 2 \cdot (4 \times 10^{-5}) \cdot \frac{1}{2}
\]
This simplifies to:
\[
80 \times 10^{-7} = 4 \times 10^{-5}
\]
Finding the Distance Between the Poles
We know that the magnetic moment \( m \) can also be expressed in terms of the distance \( d \) between the poles as:
\[
m = \frac{p \cdot d}{2}
\]
Where \( p \) is the pole strength. Rearranging gives:
\[
d = \frac{2m}{p}
\]
Substituting \( m = 2 \) and \( p = 4 \times 10^{-5} \):
\[
d = \frac{2 \cdot 2}{4 \times 10^{-5}} = 20 \, \text{cm}
\]
Conclusion
The distance between the poles of the magnet is 20 cm, making option 'D' the correct answer.
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