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Concepts Of Physics MCQ Edition [Volume 2]PhysicsElectromagnetic Waves

A compass needle is positioned within the gap of a parallel plate capacitor. This capacitor is wired to a battery via a resistance. The compass needle

Options

  1. Adeflects for a very short duration and then returns to its original position
  2. Bdeflects and gradually returns to its original position over a time that is large compared to the time constan
  3. Cdeflects and stays deflected for as long as the battery is connected
  4. Dshows no deflection

Correct answer

A. deflects for a very short duration and then returns to its original position

Step-by-step solution

When the capacitor is connected to a battery through a resistance, it starts charging. During the charging process, a varying electric field is produced between the plates of the capacitor, which gives rise to a displacement current. According to the Ampere-Maxwell law, this displacement current produces a magnetic field in the gap between the plates. The magnetic field exerts a torque on the compass needle, causing it to deflect. The charging current (and hence the displacement current) decays exponentially as I =

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