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In a vibration magnetometer, the time-period of magnets when the identical poles are in the same directon is 3 s. When the polarity of weaker magnet is reversed, the combination makes 15 oscillations per minute. The ratio of magnetic moments of the magnets is
  • a)
    9:16
  • b)
    16:9
  • c)
    7:25
  • d)
    25:7
Correct answer is option 'D'. Can you explain this answer?
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In a vibration magnetometer, the time-period of magnets when the ident...
Vibration Magnetometer and Calculation of Magnetic Moment Ratio

Introduction:
In a vibration magnetometer, the time-period of magnets when the identical poles are in the same direction is 3 s. When the polarity of the weaker magnet is reversed, the combination makes 15 oscillations per minute. The ratio of magnetic moments of the magnets is to be calculated.

Formula Used:
The time-period of oscillation of a magnet in a vibration magnetometer is given by the formula:
T = 2π √(I/mB)
where T is the time-period of oscillation, I is the moment of inertia of the magnet, m is the magnetic moment of the magnet, and B is the magnetic field strength.

Solution:
Let the magnetic moment of the stronger magnet be m1 and that of the weaker magnet be m2.
The given time-period of oscillation when the identical poles are in the same direction is 3 s.
Hence, for the stronger magnet,
T1 = 2π √(I/m1B) = 3
Squaring both sides and simplifying, we get:
m1/T1^2 = I/4π^2B

When the polarity of the weaker magnet is reversed, the combination makes 15 oscillations per minute.
Hence, for the combination of magnets,
T2 = 60/15 = 4 s
The time-period of oscillation for the combination of magnets is given by:
T2 = 2π √(I/(m1 + m2)B)
Squaring both sides and simplifying, we get:
(m1 + m2)/(T2^2 - T1^2) = I/4π^2B

Substituting the values of T1 and T2, and simplifying the above two equations, we get:
m1/T1^2 = (m1 + m2)/(T2^2 - T1^2)

Solving for m2/m1, we get:
m2/m1 = (T2^2 - T1^2)/T1^2

Substituting the given values, we get:
m2/m1 = (16 - 9)/9 = 7/9

Hence, the ratio of magnetic moments of the magnets is 25:7 (since m1:m2 = 25:7).

Answer:
The correct option is D, i.e., 25:7.
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In a vibration magnetometer, the time-period of magnets when the identical poles are in the same directon is 3 s. When the polarity of weaker magnet is reversed, the combination makes 15 oscillations per minute. The ratio of magnetic moments of the magnets isa)9:16b)16:9c)7:25d)25:7Correct answer is option 'D'. Can you explain this answer?
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