Education

Coulomb’s Law (1785)

Coulomb’s law says there will be forces between charges. Like charges repel each other and unlike charges attract each other.

In this demo,

• opposite charges were induced on the aluminum can near the PVC pipe, so we were able to use the PVC pipe to guide the motion of the can
• like charges were deposited on both PVC and the plastic loop, so a repulsive force was resulted on the plastic loop to counteract gravitational force

Homopolar Motor (1821)

This was the first motor (of magnetic type) ever being invented! It involves the three basic elements of an electric motor: magnetic field, current carrying conductor, and current.

It is called a homopolar motor because both the magnetic field and the current are unidirectional, which is not common in most electric machines out there–usually at least one of them is AC. In fact, a close examination of the current path reveals that the magnet portion of the current is AC (hinting that the AC or DC depends on the reference frame the observer uses).

Michael Faraday invented this motor in one day! Here is a nice presentation of this piece of history: https://youtu.be/z1uOsg2-LTA?si=Xszd8IQWT1G7FIGN.

Ampere’s Law (1820)

With current we can produce magnetic field. Hans Ørsted discovered the phenomenon and Ampere formulated the governing equation.

In the presented setup,

• a coil had 750 turns
• a 3A current was flowing through the coil
• a magnetic field was produced
• the south pole pointed upwards
• the north pole pointed downwards

• the south pole of the test magnet pointed downwards
• the two south poles were opposing each other
• a magnetic force was resulted and counteracted the gravitational force of the test magnet
• once the current was turned off, the magnetic field of the coil went away
• without the opposing force, the test magnet falled through

This demonstration was different from the original setup of Hans Ørsted’s, which involved a single wire and a compass.

Faraday’s Law (1831)

With changing magnetic flux, we could produce electricity. The keyword here is “changing”. Constant magnetic flux, such as that linked by a stationary coil around a stationary permanent magnet, cannot induce electricity.

To see the induced electricity, an LED (in series with a resistor) was connected with the coil. As the magnet traveled through the coil, the LED flashed once. (LED is a diode. Depending on the magnet orientation and coil-LED connection, the LED might flash when entering or leaving the coil.)

The induced electricity was a secondary effect due to ohm’s law. What was more fundamental here was the induced back-EMF (or voltage). The flux meter clearly showed that positive and negative voltages were produced when the magnet entered and left the coil respectively. Again, the positive voltage could be associated with the departure of the magnet (instead of approaching), if the terminals were reversely connected.

Faraday’s law is fundamental to the majority motors and generators out there. You will see the application of this law in several other demos of this page, including the homopolar motor one.

DC Machine with Gramme Ring (1871)

Tesla’s Egg of Columbus (1893)

We can use multiphase coil and multiphase current to create a rotating magnetic field!

Each coil produced a standing wave, because the coil was stationary. The summation of all standing waves had a net rotating component. This field induced eddy current on the aluminum top and the eddy current in turn interacted with the rotating field and generated force/torque on the top. Due to precession, the top was flipped from a horizontal orientation to a vertical orientation.

Using this effect, the levitation globe toy could have featured one more functionality–rotation!

We can also create such a field by spinning a permanent magnet, as in the magnetic stirrer.

Eddy Current and Laminations

Faraday’s law is useful for the operation of motors and generators, but it could also be detrimental. If it was not for the electrical steel laminations, motors and generators would not be very efficient.

As demonstrated in the video, the relative motion between the magnet and the conductive sheet created an eddy current which sank energy from potential and kinetic energy and converted it into heat. This action quickly damped out the motion of the conductive sheet.

If the continuous sheet was sliced such that eddy current was prohibited, the motion of the conductive sheet was sustained as if the magnet was not present.

Electrical steel laminations can be thought as the sliced version of a solid chunk of electrical steel. Instead of having air windows as in the video, insulation materials were used on the electrical steel to prevent eddy current and increase the packing factor.

Thomson’s Coil

A fun toy using the principles behind Faraday’s law. After connecting the spool wire to 110V at 60Hz, the flux of the coil changed continuously. The aluminum ring next to it experienced such a changing magnetic flux. According to Faraday’s law, a back-emf was induced and a current was resulted on the closed ring. This current in turn interacted with the magnetic field produced by the coil and generated a force pushing the ring away. This can be thought as the translational version of the Tesla’s Egg of Columbus–only this time the motion was a one-time thing.

This Thomson’s coil can be used to demonstrate levitation and wireless power transfer with some variation and addition.

Railgun (1917)

Besides Ampere’s law and Faraday’s law, Lorentz force is the last missing piece in the force/torque production of motors and generators.

In this demostration,

• the magnets were oriented such that their north poles were facing up
• the power supply provided a 5A current to the moving object (“gun”), pointing from the positive rail to the negative rail
• a magnetic force to the left was resulted and sustained the motion of the gun

A more impressive demo could use a multi-turn coil, such as the one used in the Ampere’s Law demo. However, the Lorentz force would be not easily presented.

Magnetohydrodynamic Pump (1950s)

Lorentz force can also be demonstrated on fluidic current-carrying object.

In this demonstration,

• the magnet (encapsulated inside the plastic block at the bottom) was oriented such that its north pole was facing up
• the power supply applied a 10V voltage across the two electrodes
• the salt water provided a path for current to flow
• a magnetic force towards the audience was resulted and sustained the motion of salt water
• at the same time hydrolysis occurred

This is the fundamental principle behind magnetohydrodynamic drives (MHDs). The first full-scale working MHD ship is Yamato 1 prototyped in Japan.

Wisconsin Whirlpool (2017)

Electrostatic field can be used to generate traveling wave as well! This is the basis for AC electrostatic motors, just like the traveling wave demonstrated by Nicolas Tesla for AC electromagnetic motors.

In this demonstration,

• a PCB board was submerged in a dielectric liquid
• a donut shape toy was floating on the surface of the liquid
• there were three groups (A, B, C) of electrodes arranged circumferentially and they went like A, B, C, A, B, C, etc.
• these electrodes were excited with 3-phase sinusoidal voltages
• an electric potential wave was created due to the 3-phase voltages (temporal) and 3-phase electrodes (spatial)
• this wave induced charges on the toy
• these charges in turn interacted with the electric potential wave and experienced electric force
• the wave kept revolving, and the force kept spinning the toy

This demo was first carried out in 2017 at the University of Wisconsin-Madison by Aditya Ghule, Graham Reitz, Justin Reed, Dan Ludois, and Baoyun Ge.

Below is an animation of the potential on electrodes.