Demonstrations

Lenz's Law Demonstration

Materials Required:

Copper tube up to a meter in length
Copper plate roughly an inch thick
Neodymium pemanent sphere magnet of diamter 1.5cm (grade N48)
Neodymium block magnet 5x2x2 cm (grade N48)

Assembly:

No assembly required for this demonstration.

Demonstration:

I. Hold the copper tube vertically and drop the neodymium sphere magnet into it. The tube should be big enough such that the magnet does not experience a lot of friction while falling through the tube. As the magnet falls through the copper tube, the copper experiences a changing external magnetic field and eddy currents will develop on the surface of the copper tube. These eddy currents produce a magnetic field that will oppose the change and will push the falling magnet in the opposite direction, slowing its fall.

II. Place the copper plate on a desk and move the neodymium block magnet rapidly just above the plate's surface. Once again, the eddy currents within the copper will produce a magnetic field that opposes the changes in the external magnetic field created by the block magnet as it moves about. As the copper is not fixed to the desk and is also experiencing forces due to the two interacting magnetic fields it will move about slightly on top of the desk.

Lenz's Law Demonstration

Materials Required:

Two 1 cm screws
Two pieces of plastic board ~5mm thick
Four 5cm bolts and four nuts for the corresponding bolts
9V battery
Neodymium pemanent sphere magnet of diamter 1.5cm (grade N48)
3-Pronged switch
Insulated copper wire (multi fiber)

Assembly:

Cut the plastic board pieces so that they are both approximately the same size (roughly 12x12cm)
Near the four corners of each board, drill a hole to fit one of the bolts through. Then drill a smaller hole in the dead-centre of each board for the screws.
Screw one screw through the centre of each of the plastic boards. For one of the boards, strip the wire of its insulation and wrap a bit of the exposed fibers around the screw before screwing it in all the way down. This will be the bottom board.
Insert the bolts into the bottom board and place it so that the bolts are facing up the same way as the screw. Attach all four nuts to the bolts and screw the nuts about 3/4's of the way down.
Keep one of the wires from the battery terminal free.
Wire the switch as shown below:

Demonstration:

I. Place the sphere magnet up to the screw in the bottom board then place the top lid on top of the nuts. Adjust the nuts so that the top plate is level.

II. Lift the sphere magnet up so that it is suspended between both screws,

lower the top plate using the nuts so that the top screw is just barely touching the surface of the magnet.

III. Flip the switch and arefully touch the free wire from the battery to the surface of the magnet.

IV. The current flowing across the metallic surface coating of the magnet will have a velocity component perpendicular to the magnetic field at each point along its path. This causes the electrons to experience a Lorentz force in the direction of v x B and thus applies a net torque to the sphere magnet causing it to rotate.

V. Do not keep the magnet rotate for too long or let it rotate too fast as this drains the battery and may damage the surface coating of the magnet.

Energy Well Demonstration

Materials Required:

A Diamagnetic Levitator Kit
- (purchasable here: https://www.kjmagnetics.com/proddetail.asp?prod=lev2)

Assembly:

Follow instructions on kit

Demonstration:

I. Place the small neodymium magnet between the two diamagnetic plates

II. Adjust the top magnet knob so that it just cancels out the force of gravity for the small magnet. Once the force of gravity is cancelled out, the magnet will be equally repelled by both diamagnetic plates. Since moving towards either plate will increase its potential energy, the magnet will happily sit suspended between the plates where its potential energy is at a minimum.

Flux Pinning and Meissner Effect

Materials Required:

Type II disc Superconductor, dime sized and thickness 0.3cm
Type II disc Superconductor, loonie size and thickness 0.3cm
Nine rectangular 1x5x0.3 cm neodymium magnets
One small 0.5x0.5x0.5 cube neodymium magnet
Liquid Nitrogen
Safety goggles
Safety gloves
Two styrofoam cups
plastic tweezers

Assembly:

On a desk, connect the rectangular magnets lengthwise in groups of three, then attach the 3x1 arrays together at the magnet tips to form one 3x3 array. (Make sure to be doing this on a flat surface, the magnets can snap together and pinch!)
Cut the bottom out of one of the styrofoam cups to form a makeshift dish. Make sure it is deep enough so that the superconductors can be immersed in liquid nitrogen while sitting in the dish.

Demonstration:

I. Using tweezers place the superconductors into the styrofoam dish.

II. While wearing safety goggles and gloves, decant some liquid nitrogen into the styrofoam cup.

III. Pour enough liquid nitrogen from the cup to the dish to almost completely submerse the two superconductors, leaving just the top exposed. Wait until the liquid nitrogen stops boiling.

IV. Place the small square magnet above the larger superconductor. When a superconductor enters its superconducting state, it excludes all magentic fields (Meissner Effect) due to eddy currents on the surface of the superconductor and has strong diamagnetic qualities. Since the superconductor has basically zero resistance in its superconducting state, the eddy currents are free to shift in response to any changes in the magnetic field of the small square magnet due to shifts in its position. The result is that the magnet will be suspended above the superconductor.

V. Take the smaller superconductor out of the dish with tweezers and gently place it just above the very centre magnet in the 3x3 array. The superconductor will stay locked in place due to a combination of the Meissner effect and Flux pinning. Flux pinning occurs when imperfections withing the superconductor allow small amounts of flux to penetrate it. These magnetic vortices lock the superconductor in place and stabilize it above the magnets. A change in magnetic field will result in an increase in potential energy and a strong resistance to movement from the superconductor; it sits in an energy well. Since the magnetic field down the length of the centre magnet is uniform, the superconductor is free to move lengthwise across the magnet as can be demonstrated by giving the magnet a slight push.