The Physics
Opus in profectus

Lenz' Law

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  1. Write something.
  2. Write something.
  3. Write something.
  4. Write something completely different.


  1. Indicate the direction of induced current in the circular loop of wire if the strength of the magnetic field…
    1. increases
    2. decreases
    3. remains the same

    A circular loop of wire in a magnetic field represented by many 'x' symbols.

  2. Indicate the direction of induced current in the solenoid as the bar magnet is moved to the right.

    A bar magnet on the left. A coil of wire wrapped around a cylinder on the right. The north pole of the magnet is approacing the coil.

  3. Why do electric motors burn out if they are switched on but not allowed to rotate?


  1. It has been said that large numbers of payloads could be launched relatively economically from the surface of a moon, asteroid, or planet with a tenuous atmosphere using an electromagnetic rail gun. Work out the details of such a device located on Earth's moon. Determine…
    1. the escape velocity from the surface of the moon.
    2. the current needed for a 1 m wide, 0.1 Ω rail gun to reach the velocity calculated in part a in a 5 T magnetic field (disregarding friction)
    3. the power consumed when the rail gun is operating at top speed
    4. the minimum length of the rail gun if its average acceleration should not excede 3 g