Problem #0253 Electricity & Magnetism Sub-menu Problem #0255 Chapters Chapters

524.

Problem 38.35 (RHK)

The coil of a superconducting electromagnet used for nuclear magnetic resonance investigations has an inductance of 152 H and carries a current of 32 A. The coil is immersed in liquid helium, which has a latent heat of vaporization of . (a) We have to calculate the energy in the magnetic field of the coil. (b) We have to find the mass of helium that is boiled off if the superconductor is quenched and thereby suddenly develops a finite resistance.

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The energy stored in the magnetic field in an inductor of inductance L when a current i is flowing through it is

In our problem

Therefore, the energy stored in the magnetic field

The latent heat of vaporization of helium is .

The amount of helium vaporized by 778 kJ of energy will be

Mass of 1 mole of helium is 4 g. Therefore, the mass of 915.6 mole of helium will be This amount of helium will be boiled off by the energy stored in the inductor when the superconductor is quenched.