Physics 110A & B: Electricity, Magnetism, and Optics (Parts I & II)

Text: Introduction to Electrodynamics by David Griffiths


110A


Chapter 2: Electrostatics

Electric Field Apparatus This demonstration shows how electric field lines curve from one charged point to another. You can see the field lines between two point charges, between two concentric rings, and from two parallel plates.

Electric Fields Inside a Sphere This demo shows students definitively that there is no electric field or charge inside of a charged sphere, showing Gauss’ law to be true. Some students may find it hard to believe that there should not be a field inside a sphere, and this demonstration can serve to convince them of this fact.

Electrostatic Pendulum This demonstration shows charging by induction of a pendulum which causes it to oscillate. The pendulum is metal and placed next to a Van der Graaf generator that induces a positive charge on one side of the sphere. This positive charge is attracted to the generator and swings closer to the generator. At some point the force of gravity on the sphere will overcome the attractive force between the sphere and the generator and the sphere will swing back. This process continues as the pendulum gets closer and closer to the generator.


Chapter 3: Potentials

Thermoelectric Converter; Thermocouple This demonstration shows that a change in temperature of two conducting materials can create a potential difference that causes a current to flow.


Chapter 5: Magnetostatics

Helmholtz Coil This demonstration can be used to show how charged particles (in this case, an electron beam) with an initial velocity move in a spiral in the direction of the magnetic field.


Chapter 7: Electrodynamics

Eddy Current Pendulums The eddy current pendulum is a great demonstration to show how eddy currents affect the path of motion of two pendulums. In the solid aluminum pendulum, eddy currents are created in the bulk of the material due to the relative motion of the pendulum with respect to the stationary magnet. This leads to the pendulum quickly losing momentum, seemingly on its own accord. The pendulum with small slits in it does not stop when swinging through a magnetic field because the small slits discourage eddy currents from forming.

Faraday’s Laws This demo shows Faraday’s Law, a basic principle of electrmagnetism predicting how an electromotive force can be generated by passing a bar magnet through a loop of wire. The result of this is a pulse of current that travels through the circuit. This pulse can be shown either using an oscilloscope, which will show the pulse in voltage, or using a simple ammeter that shows the real time voltage passing through the loop.

Lamp in Series with Inductance (Lenz’s Law) This demo shows Lenz’s law, which states that when a current is induced in a conductor due to Faraday’s law, its direction will be such that the magnetic field it produces will oppose the change that produced it. In this demo, we show that you can slow a current moving through a wire by putting a solenoid in series with a lightbulb. The induced current due to the changing magnetic field will oppose the current created by the power supply, the result of which is that the lightbulb in series with an inductor will light more slowly than one without an inductor.

Magnetically Coupled Harmonic Oscillators: This demonstration shows the effects of Lenz’s law through two coupled solenoids by oscillating two bar magnets on springs through two large coils to produce an oscillating current.

Wonder Tubes (Lenz’s Law) This demonstrates Lenz’s Law by showing a magnetic cylinder falling more slowly through an aluminum tube than an aluminum cylinder. The eddy currents created in the aluminum tube by the falling magnet induce a magnetic field that causes a force to oppose gravitational force pulling the magnet downward.

Jumping Ring (Lenz’s Law) This demo shows Lenz’s law by showing a solid aluminum ring “jumping” off of a solenoid fitted with an iron core. However, when a slit is cut in the ring it does not jump off. This is because the slit in the aluminum ring discourages the formation of the eddy currents that create a magnetic field to oppose the change in magnetic field due to the solenoid.

Induction Coils This demo shows the properties of induction by connecting an inductor in parallel with a lightbulb and switch. The lightbulb is first illuminated when there is no iron core in the solenoid. While the lightbulb remains lit, the iron core is placed in the solenoid. Now, when the lightbulb is turned off, it will briefly become much brighter than it was before extinguishing entirely.

Rail Gun Demo This demonstration showcases one of the practical applications of the Lorentz force with a small steel rod being accelerated along a track using a large current. It is also a classic case when studying changing magnetic flux due to the constant magnetic field, increasing enclosed area, and resulting \mathcal{E}_{mf} it produces according to Lenz’s law.

Driven EM Oscillator: Amass on a spring oscillator driven by electromagnetic forces. These forces are provided by magnets on the bottom of the mass, an induction coil surrounding the mass, and an AC voltage applied to the coil.


110B


Chapter 9: Electromagnetic waves

Light Pipe This demonstration shows how the angle of incidence of a laser on a plexiglass light guide can change how the light is reflected and refracted along the guide. You can show a variety of scenarios from total internal reflection to total transmission and accompanying refraction through the guide.