Lenz's law - Magnet Falling Through a Copper Tube (Stephen Herr)
Author
Stephen
Principle(s) Illustrated
Conservation of Energy
Conservation of Charge
Newton's 3rd Law
Newton's 2nd Law
Lorentz Force Law
Lenz's law
Electric Generator
Magnetic flux
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