Activity/Demo overview:
The eddy current demonstration provides an introduction to some fundamental physics notions, such as the Lorentz force, gravity, magnetic field, eddy current and kinetic energy. In this demo, we essentially observe how the magnetic ball falls differently than a non magnetic ball when it passes through a conducting pipe. The interesting physics behind this difference is that the free electrons in the copper tube are compelled to flow in circular patterns known as eddies when a magnet passes through a conductor which is a hollow copper tube in our case. This circular path generates an opposing magnetic field to the magnet’s own magnetic field, which prevents the magnet from moving freely. Another way to think about this demonstration is that a non-magnetic ball falling through a copper pipe will fall much faster because it gains kinetic energy from gravity, but a magnetic ball falling through a copper pipe will fall slower because some of the energy is lost in creating eddy currents.
Activity/Demo Instructor Notes:
- Smaller groups would make this demonstration more impactful because the equipment and materials used are portable and easy to use, giving viewers a clearer understanding of what is happening with a closer view and hands-on experience.
- Also make sure to let students guess the motion of objects passing through the copper tube to engage them.
- This demo is appropriate for all the age groups because it uses simple yet important laws of physics.
- The magnetic balls are heavy so while giving it to the children for hands make sure they don’t hurt themselves.
Materials needed:
- A hollow copper or aluminum tube
- A strong magnetic ball
- A non magnetic ball
Steps to run demonstration:
- Hold the copper tube vertically in your hand.
- First, hold the non magnetic ball at the top of the copper tube.
- At this point, pause for a while and ask the audience to predict the motion of the non magnetic ball through the copper tube.
- It will fall down normally with no effects.
- Next, use a magnetic ball and carry out the same procedure. Ask them once again if the ball’s motion will change in any way. Make them explain the reasoning behind the change in motion as well.
- After giving them some time to respond, gently release the magnetic ball. Now, it can be clearly seen that the magnetic ball will descend to the ground significantly more slower than the non-magnetic ball or any other item in the air that is falling freely.
Explanation:
- When the ball falls through the tube, it creates a changing magnetic field inside the tube.
- According to Faraday’s law of electromagnetic induction, a changing magnetic field induces a current in a conductor (in this case, the copper tube).
- These induced currents, known as eddy currents, generate their own magnetic fields, which oppose the motion of the magnetic ball. This resistance to motion causes the ball to fall slowly.
Real life application:
- An interesting application of eddy currents can be observed in coin-operated vending machines to identify fake coins.
- Eddy current also serves as a brake. The motion of the train can be stopped by the reaction between the eddy current and the conductive metals without the use of physical brakes.
- Eddy currents are also used often in induction cooktops. When metal-base utensils are positioned on copper-rod cooktops, magnetic field that is produced in the coil as a result of ac current, causes eddy current in the utensils making the heating and cooking feasible.