Electromagnetism

Location: TBD

Description: A high voltage transformer and two copper rods that can be inserted into sockets and fastened in place.

Arrange the copper rods so that there is a small air gap between them at the base. The very large voltage will ionize the air between the two electrodes, creating a conducting path, allowing current to flow. The air will be heated by the current and rise, until the length of the electric arc is so long that the voltage is insufficient to maintain ionization. This process will repeat.

The arc is hot enough to set paper on fire if it is inserted between the electrodes.

Dielectric breakdown at electrical substation video: https://www.youtube.com/watch?v=PXiOQCRiSp0

Location: In front of Cabinet 4 in box on the floor

Description: There are several pipes in the demo room that can be used with/instead of the copper pipe shown. Taking a spherical magnet, drop it down a metal pipe, and the changing magnetic flux through each horizontal slice of the pipe will induce a current that in turn generates a magnetic field that opposes the change in flux. This will slow the magnet’s fall. For the purpose of showmanship, an unmagnetized ball bearing is also available which you can use to switch out with the magnetized one to make it seem that when the student drops the ball bearing it moves slowly, but when the instructor drops it, it moves normally.

There are two copper pipes of differing thickness as well as an aluminum one and a plastic one. Since the changing magnetic field induces a voltage, the dissipation (V²/R) is largest when the resistance is smallest. There is no current generated with the plastic pipe so the magnet will fall normally. Teachable moment: a “perfect conductor” will generate an opposing field causing the dropped magnet to levitate in place.

Caution: The magnets are strong, and if not handled with care, can easily be shattered when they are collide rapidly with a ferromagnetic material.

Location: Cabinet 6, Shelf 2

Description: A carefully balanced lever has a closed metallic loop on one end and a metallic “C” forming a similar shape but not closed on the other. The metal is non-magnetic. When a strong magnet is moved near the closed loop, it will oppose the change in magnetic flux, moving away from a magnet that approaches the loops and towards a magnet that is pulled away from the loop. This occurs because the induced current in the closed loop produces a magnetic field that opposes the change in flux.

Ideally there is no effect on the “C” end of the balance because there is not a closed loop. However, an extremely strong magnet will still produce a weaker effect. To see why, treat the gap in the “C” as if it were closed by a capacitor. For large changes in flux there will still be a current induced, with a displacement current across the gap.

Be sure to demonstrate that the balance itself is completely non-magnetic!

Location: Cabinet 6, Shelf 2

Description: This demo requires no setup. It is just three magnets that ‘levitate’.

Location: Cabinet 6, Shelf 3

Description: This demo requires no setup. Check to ensure the bulb filament is not burned out and turn the handle. This will cause the bulb to light up.

Location: TBD

Description: This apparatus demonstrates the force between two parallel current-carrying wires.

Warning: Because the currents required to produce a noticeable force between two wires, the power supply is specific for this demonstration and should not be used for other applications. Evidence for the size of the currents involved is found in the melting of the plastic sheet separating the wires when the wires were drawn towards each other by their magnetic interaction. There is a knife switch on the top of the demonstration box that allows the currents to be switched between parallel and anti-parallel. The deflection of the wires is small, and so this demonstration is more suited for classes where students can move in close for an observation.

By flipping the switch repeatedly, it is possible to push the wires in resonance with their natural oscillation frequency in order to get a larger amplitude displacement.

Location: Cabinet 6, Shelf 4

Description: These are two pasco electrical boards. They come with manuals on how to use them and ideas for labs/demos. 

Location: (pending permanent location)

Description: Thread the long conducting cylinder through the solid ring (upper ring, in figure shown) and let the ring rest on the disk above the base. When the power is switched on, a large AC current will flow through the coil, creating a large magnetic field collimated by the permeable core. This changing flux induces a current in the ring. The current in the ring will itself create a magnetic field that opposes the changing magnetic flux, causing the ring to launch very rapidly into the air.

A second ring, which has a split in it, (lower ring in figure) is also provided. This slit or gap in the ring prevents current from flowing around the ring and so no opposing magnetic field will be generated. This second ring will not launch into the air, even though the gap is quite small.

Location: Main generator is in the cabinet under the upper level platform, rear of area. Additional generator in location TBD.

Description: A rubber belt is driven between a set of electrified contacts (bottom) and a metal globe. The belt mechanically pushes the charges against the electric potential and deposits them on a contact in the field free region inside the globe. This allows a substantial voltage to be developed between the sphere and ground. A grounded metal wand is attached to the unit allowing the voltage to be discharged. The dielectric breakdown field of air is ~3MV/m, so if a spark jumps from the globe to the wand when they are separated by ~ 10cm, the potential difference must be on the order of 100,000V. However, the total current in a spark is tiny.

Possible demonstrations include:

• Charging by conduction and induction.

• Levitating pie tins (charged by conduction, then the electrostatic force causes them to fly off).

• Flying styrofoam (packing peanuts).

• Ionic propulsion  (A rotor with sharp endpoints will ionize the air near the tips of the rotor, and by repelling them cause the rotor to spin).  

• Fly-away hair (subject must have long hair.  There is an insulated stand available in the cabinet.   After demonstration, if the generator is turned off and the subject removes their hand from the generator, they will slowly discharge to the air without generating any (mildly) painful shock.