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Circuit and electromagnetism basics

How much energy is stored in a coil of inductance 2 mH carrying a DC current of 3 A?

  1. A18 mJ
  2. B9 mJ
  3. C3 mJ
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Answer: B. 9 mJ

The energy in an inductor is W = (1/2)LI²: (1/2) × 2×10⁻³ H × (3 A)² = 9×10⁻³ J = 9 mJ. This energy is stored in the magnetic field. If the current is cut abruptly the energy has nowhere to go, producing a high voltage that can cause sparks at switch contacts or destroy devices, so a diode or similar path is provided for the current.

  • A. 18 mJ forgets the factor of 1/2.
  • B. Correct. (1/2) × 2 mH × (3 A)² = 9 mJ.
  • C. 3 mJ forgets to square the current.
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