Worksheet: Magnetic Field Energy

In this worksheet, we will practice calculating the energy stored in a changing magnetic field of a self-inductive conductor.


A rectangular toroid has 2,000 windings around its core and the core has a height of 0.10 m. The toroid has a self-inductance of 0.040 H. What is the current through the toroid when the energy in its magnetic field is 2.0×10 J?


A 10 H inductor carries a current of 20×10 mA. Calculate how much ice at 0.0C could be melted by the energy stored in the magnetic field of the inductor. Use a value of 334 J/g for the latent heat of fusion of ice.


There is a current of 1.2 A in a coaxial cable whose outer radius is five times its inner radius. The copper in the coaxial cable has a magnetic permeability of 𝜇=1.26×10/Hm. What is the magnetic field energy stored in a length of 3.0 m of the cable?

  • A8.5×10 J
  • B5.5×10 J
  • C6.2×10 J
  • D5.0×10 J
  • E7.0×10 J


At the instant a current of 0.50 A is flowing through a coil of wire, the energy stored in its magnetic field is 8.0×10 J. What is the self-inductance of the coil?


A coil with a self-inductance of 5.0 H and a resistance of 200 Ω carries a steady current of 3.0 A. What is the energy stored in the magnetic field of the coil?


A 7,000 µF capacitor is charged to 200 V and then quickly connected to a 70.0 mH inductor.

Determine the maximum energy stored in the magnetic field of the inductor.

Determine the peak value of the current.

Determine the frequency of oscillation of the circuit.

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