Lenz's law - Magnet Falling Through a Copper Tube (Stephen Herr)

Author

Stephen

Principle(s) Illustrated

1. Conservation of Energy

2. Conservation of Charge

3. Newton's 3rd Law

4. Newton's 2nd Law

5. Lorentz Force Law

6. Lenz's law

7. Electric Generator

8. Magnetic flux

9. Electric field

Standards

• HS-PS2-5. Plan and conduct an investigation to provide evidence that an electric current can produce a magnetic field and that a changing magnetic field can produce an electric

• PS2.B: Types of Interactions

  • Newton’s law of universal gravitation and Coulomb’s law provide the mathematical models to describe and predict the effects of gravitational and electrostatic forces between distant objects. (HS-PS2-4)

  • Forces at a distance are explained by fields (gravitational, electric, and magnetic) permeating space that can transfer energy through space. Magnets or electric currents cause magnetic fields; electric charges or changing magnetic fields cause electric fields.

• PS3.A: Definitions of Energy  “Electrical energy” may mean energy stored in a battery or energy transmitted by electric currents. (secondary)

Questioning Script

Prior knowledge & experience:

• objects in free fall accelerate towards the earth at a rate of 9.8 m/s^2.

• gravitational potential energy is converted to kinetic energy as an object falls through a gravitational field

• the gravitational field does work on a system as it falls

• copper pipe is not magnetic

• copper is conductive

• Newton's laws

Root question:

• Sketch a velocity vs. time and acceleration vs. time graph for the magnet as it falls through the copper pipe.

• Sketch a graph of what the tension force holding the pipe will look like as the magnet falls through it

Target response:

• A current is induced in the copper pipe by the changing magnetic flux caused by the relative motion between the pipe and the magnet (Faraday's Law). The current induced in the pipe creates a magnetic field opposite to the rate change of flux caused by the falling magnet (Lenz's Law)

• The magnet falls at a constant velocity when the

• Energy is transferred from the gravitational field to the current moving in the pipe and eventually dissipated as heat.

• The faster the magnetic moves relative to the pipe, the greater the resistive magnetic force acting on it and the greater the rate of energy transfer to the pipe (power)

• The sketch of the velocity vs. time should initially increase and then level off (a=0 m/s^2).

• The tension force holding the pipe will increase to a constant value while the magnet is falling through the pipe.

Common Misconceptions:

• Objects can't move if they are in equilibrium

• Gravity is the only significant long range force

Photographs and Movies

References

• MIT Physics Demo - Lenz's Law with Copper Pipe

• Lorentz force

• Faraday's Law - HyperPhysics

• Video of a large neodym magnet magnet