HSC Physics Motors and Generators

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.

  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. 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.
  4. 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.
  5. The induced eddy currents in a spinning metal disk in a uniform magnetic field experience a magnetic force that slows down the disk.
  6. 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.
  7. more to come