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Question
Consider the energy density in a solenoid at its centre and that near its ends. Which of the two is greater?
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Solution
In a solenoid, energy is stored in the form of a magnetic field. If a constant current flows through the solenoid, the magnetic field inside is approximately uniform at the centre but decreases near the ends. The energy per unit volume (or energy density) in the magnetic field is given by `mu = B^2/(2mu_0)`.
Since B is stronger at the centre and weaker near the ends, the energy density is greater at the centre than near the ends.
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