Lenz's Law (Viktor Genjoyan)

Principles Investigated:

· Students will be able to understand Lenz’s Law.

· Students will be able to review and connect their previous knowledge of kinematics, potential and kinetic energies, free fall, conservation of energy, thermodynamics, inverse square law and electromagnetism to Lenz’s Law.

· Students will be able to use Lenz’s Law to answer questions and predict different outcomes of experiment.

CA Standards:

Electric and Magnetic Phenomena

5 h. Students know changing magnetic fields produce electric fields, thereby inducing currents in nearby conductors.

5. f. Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources.

5 g. Students know how to determine the direction of a magnetic field produced by a current flowing in a straight wire or in a coil.

Heat and Thermodynamics

3. Energy cannot be created or destroyed, although in many processes energy is transferred to the environment as heat. As a basis for understanding this concept:

Conservation of Energy and Momentum

2. a. Students know how to calculate kinetic energy by using the formula E=1/2mv².

2 b. Students know how to calculate changes in gravitational potential energy near Earth by using the formula (change in potential energy) =mgh (h is the change in the elevation). c. Students know how to solve problems involving conservation of energy in simple systems, such as falling objects.

Motion and Forces

1b. Students know that when forces are balanced, no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton’s first law).

Materials:

• Copper or aluminum pipe.
• PVC and iron pipe (optional).
• Magnet. For better effect use strong magnet with diameter slightly less than the pipe’s.
• Plastic and metallic (nonmagnetic) objects that will pass through the pipe.

Procedure:

1. Review questions about free fall, potential and kinetic energy.
2. Drop few objects and show that they fall at the same rate. When dropping the magnet make sure that it doesn’t hit the ground, because this will weaken the magnet.
3. Ask what will happen if you drop the plastic or non-magnetic object trough pipe.
4. Drop the objects to demonstrate that this will not affect the free fall.
5. Now drop the magnet through the pipe, it will fall very slowly (discrepant event).
6. Drop the top onto the base from the approximate height it floats at to show that it should fall.
7. Discussing about what going on that slows down the magnet - the moving magnet induces an electric current in the wire electromagnetic induction.
8. Ask questions why induced magnetic field (B-field) opposes B-field of magnet. What will happen if B-field wasn’t opposing the magnet’s it B-field.
9. Questions - checking for understanding:

a) What will happen if instead of copper pipe we use PVC pipe?

b) What will happen if instead of copper we use an aluminum pipe?

c) What will happen if instead of copper we use iron pipe?

d) What effect will the diameter of the pipe play?

e) Does the magnet accelerate in the copper pipe?

Student Prior Knowledge:

Lenz’s Law is taught after mechanics and thermodynamics, therefore the students should be familiar with electric and magnetic phenomena, thermodynamics, conservation of energy, motion and forces.

Explanation:

Like Faraday's original experiment when magnet passes through a coil of wire it induces current in coil, falling magnet induces current in the pipe. Since the pipe is not a coil this will create random currents - eddy currents. Next, the eddy currents in the conductor generate their own magnetic field, which opposes the magnetic field of the magnet. This is the Lenz’s law. According to Lenz's law, the induced magnetic field opposes the cause, which is the moving magnet. Hence the induced magnetic field is in a direction to try to stop the moving magnet. Lenz's law is a consequence of the law of conservation of energy. According to the law of conservation of energy (First Law of Thermodynamics) the total amount of energy is conserved. The connection between Lenz's law and the conservation of energy is a good example of the unity of physics. Many laws stem from a few very fundamental principles.

Similarly if magnet brought close to spinning pizza cutter disk will slow down it. Or if a metal disk placed under the pendulum with magnetic mass will slow down the pendulum.

Applications to Everyday Life:

Principle of Lenz’s Law is used in magnetic breaks, or when there is a need to slow down moving conducting objects.

Metal detectors.

Questions and Answers:

a) What will happen if instead of copper pipe we use PVC pipe?

If we use PVC, or any pipe made of any non-conducting material, magnet will not slow down, because falling magnet will not create a current in the pipe.

b) What will happen if instead of copper we use an aluminum pipe?

If we use aluminum pipe the effect will be the same.

c) What effect will the diameter of the pipe play?

Wider the diameter of the pipe, faster the magnet will fall. If the pipe is wide than the distance between the magnet and the conductor (pipe) will be greater, changing B-field of the magnet in the pipe will be weaker therefor the opposing force will be weaker.

d) Does the magnet accelerate in the copper pipe?

No, magnet does not accelerate in the pipe; it quickly reaches the terminal velocity. When the magnet falls in terminal velocity the force of B-field balances gravitational force.

Photographs:

Eddy currents and their B-field in pipe.

Magnetic brakes of rollercoaster

Videos:

http://www.youtube.com/watch?v=kU6NSh7hr7Q

Jumping ring: This is another way to demonstrate Lenz’s Law

http://www.youtube.com/watch?v=V690VphqTwg

References:

http://en.wikipedia.org/wiki/Lenz's_law

http://www.launc.tased.edu.au/online/sciences/physics/Lenz's.html

http://www.ndt-ed.org/TeachingResources/NDT_Tips/LenzLaw.htm