Electromagnetism

Location: Must be assembled: horseshoe magnet is on lower level on magnet rack;  wires hang below magnet rack, ringstands on lower level with clamp hardware; current supply is in DC voltage source cabinet on lower level.

Description: The “Force between current carrying wires” demonstration can be a bit subtle because the fields and therefore the force generated are small, leading to a small deflection of the wires. This demonstration uses a strong horseshoe magnet and some clever rigging to maximize the deflection of a single wire carrying a large current. The magnet is quite heavy, and needs a bit of support to stay in the correct orientation.

Using the current source shown, even at its lowest setting, produces a deflection of over 8cm. Turning it on can generate a substantial kick which should be visible even in the large lecture hall.

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: Cabinet 2, Shelf 2

Description: This demo comes as a kit of pendular blades, a pair of magnets in a C-shaped jig, and a black supporting rod with a pre-cut slot in it from which to suspend a blade such that it will swing between the poles of the magnets. The ring stand is in the lower level of the demo room with all of the stands and clamps.

When the solid blade swings between the poles of the magnets, it comes to an abrupt halt. Its motion induces a large eddy current that opposes the change in flux through the conductor and also dissipates the kinetic energy of the moving blade. However, when the comb-shaped blade swings between the poles, the geometry of the blade does not allow large current carrying loops, and the motion is not damped. Finally, when the slotted blade (the blade with long slotted holes) swings between the poles of the magnets its motion is damped. The continuous conducting path along the top and bottom of the slots allows for a large loop of current to be induced, again stopping the blade.

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: Cabinet 5, Shelf 4

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.

Location: Cabinet 5, Shelf 4

Description: A Wimshurst machine or generator uses the principle of electrostatic induction to generate a large charge imbalance and thus a voltage. It does not rely upon friction to create a separation of charge. Instead, it uses the initial, random, thermodynamic fluctuation of charge to initially charge one side of the system (say, positive). By clever connections of conductors this microscopic imbalance is used to induce a charge to transfer to the other side (in this case, negative). That induced charge then acts back on the original conductor to pull more positive charge onto its plates. After a few cranks a sufficient voltage is built up to cause a spark to jump from one of the spherical electrode to the other, if the gap is small. (See Wikipedia for a clever animation of this process.)

A second important feature are the cylindrical capacitors on each side, similar to Leyden jars. If they are connected to the generator (simply by flipping each of the conducting rods to make contact between the generator and the cylinders then charge can flow from the generator to the capacitors). Given the relationship for capacitors:

Q = CV

If the breakdown voltage for the air gap between the spherical electrodes is unchanged, a larger capacitance (C) allows a larger charge (Q) to be stored on the capacitors before the spark jumps. Operationally, this means that it will take many more turns of the generator handle before a much larger and louder spark jumps between the electrodes.

The demonstration works better in dry air, and thus works better late in the Fall semester and early in the Spring semester. When the capacitors are not connected and the spherical electrodes have a small separation, then at a moderate rate of cranking a soft “click-click-click” can be heard about once a second. The associated spark is dim. When the capacitors are connected, it can take twenty seconds or more to charge up to a sufficient voltage to generate a much louder and brighter spark.

The breakdown field for dry air is approximately 3 kV/mm (kilovolts per millimeter) or 3.0 x 10^6 V/m (volts per meter), so if the gap between the electrodes is N centimeters, the voltage at breakdown is N x 30 kV.

See also: Kelvin Water Drop Generator