HSC Physics Motors and Generators

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With the old Motors and Generators topic being examined for the last time this November a list of true-false questions on concepts that are often misunderstood is given below. This list will be updated during the year.

DC Motor

  1. A common answer is "a DC motor converts electrical energy into kinetic energy". This means that the heat energy released by the current flowing in the coils is equal to the kinetic energy gained by the coils.
  2. A 12 V battery has zero internal resistance. It is connected to a coil of resistance 4Ω that can spin freely in a magnetic field. In one second the coil gains 36 J of kinetic energy.
  3. The back emf in a motor increases when the speed of the coils decreases.

AC Generator

  1. A common answer is "an AC generator converts kinetic energy into electrical energy". This means that the kinetic energy of the coils is transformed into the heat energy given off by the current flowing in the load resistor.
  2. The work done by an external force in turning the handle of an AC generator in one second is 40 J. If there is no friction in the axle as the coil turns the potential difference across the generator terminals is 40 V.
  3. The kinetic energy of the coils of a generator is doubled. The size of the induced emf is doubled.

Eddy Currents

  1. The induced eddy currents in a spinning metal disk in a uniform magnetic field experience a magnetic force that slows down the disk.
  2. A metal disk is spinning in a uniform magnetic field with all of its area in the field. The plane of the disk is perpendicular to the field lines. No eddy currents are induced in the disc.
  3. A copper disk is spinning clockwise. The north pole of a magnet is held above the disc. The induced eddy currents circulate clockwise in the disc.
  4. A copper disc is at rest and can spin on a smooth axle. The north pole of a bar magnet is moved clockwise over the disc. The disc moves counterclockwise.

AC Induction Motor

  1. In an AC induction motor a rotating magnetic field passes through the squirrel cage. The squirrel cage turns because it is repelled by the increasing magnetic flux.

Transformer

  1. The iron core of a transformer increases the magnetic flux entering the secondary coil.

Electromagnetic Induction

  1. A straight copper wire is moving with its length and velocity vector both perpendicular to a uniform electric field. An induced emf occurs in the wire.
  2. When a conductor moves relative to a magnetic field with its velocity vector parallel to the magnetic field vector an emf is induced in the conductor.

  3. When a reference frame moves relative to a magnetic field there is no electric field in the reference frame.

  4. When a conductor moves through a magnetic field a current is induced in the conductor that produces a magnetic field that exerts a force on the conductor.

  5. Imagine a rapidly flowing salt water river flowing from west to east through the Earth's magnetic field. A voltmeter is placed on a bridge over the river. One terminal of the voltmeter is connected to a wire placed in the water at the north bank and the terminal is connected to a wire placed in the water at the south bank. The voltmeter gives zero reading.