Physics Department Homepage
Table of Contents Attraction/Repulsion -- bar magnets on pivoting stands show attraction and repulsion.
Field Around Magnets (Project) -- iron filings on a plastic shield are placed on the overhead projector and used to show the shape of the field around bar and horeshoe magnets. Small projection compasses are also available to show the sense of the field.
Broken Magnets -- a broken bar magnet held together by its magnetism acts as a single magnet when whole. It can be pulled apart into two or more pieces and their fields traced with compasses and/or iron filings to show that each piece is also a complete magnet.
Compass -- large arrow compass to demonstrate Earth's field, field around magnets, solenoids.
Dip Needle -- the compass above may be oriented vertically along a North-South line to serve as a large dip needle to show the angle of declination of the Earth's field.
Lodestone -- a rock formed of magnetite, a naturally-occurring magnetic mineral. Color-coded with North and South poles.
Levitron -- a spinning magnetic top that levitates stably (sort of) above a large permanent magnet. It takes some adjustment to get it working right, so please give us lots of notice and be prepared to practice beforehand.
Ampere's Frame -- a square frame of aluminum rod, free to rotate, through which a large DC current flows.
A magnet brought near one side of the frame causes rotation (in a direction predicted by the right hand rule).
NOTE: This demo is no longer in service, due to concerns over the exposed mercury involved in its operation.
It is however, available on our laser disk collection.
Barlow's Wheel -- a flat disk of aluminum on bearings whose bottom half passes between the poles of a powerful magnet.
A large DC current runs from the center to the bottom point of the disk, and the force on the electrons flowing through
the magnetic field causes the disk to rotate.
NOTE: This demo is no longer in service, due to concerns over the exposed mercury involved in its operation.
It is however, available on our laser disk collection.
D'Arsonval Meter (Model Galvanometer) -- large open model of an galvanometer. A large coil on spring-mounted bearings twists in the magnetic field of a magnet when current flows in the coil.
Deflect Electron Beam -- an electron beam (open oscilloscope or simple electron tube) is deflected by the field from a bar magnet or electromagnet.
Jumping Wire -- a single thick wire passes between the poles of a powerful magnet. When a heavy DC current flows, the wire jumps out of the field.
Faraday Disk Dynamo -- an aluminum disk spinning between magnet poles produces a current between the center and the edge of the disc as shown on a large galvanometer.
Ion Motor (Force on Ions) -- a dish contains copper sulfate solution and a transparent rectangular-shaped ion motor with electrodes and a magnet. When current flows between the electrodes, the charges are moving at right angles to the magnetic field, causing the solution to move through the chamber. This is seen through overhead projection.
Hall Effect Probe -- shows the voltage developed at right angles to a current in a conductor in a magnetic field; as used in Gaussmeters.
Fine Beam Tube -- an electron beam in an evacuated glass sphere is bent into a circle by the magnetic field from a pair of large open coils. The beam is faintly visible in a dark room because of collisions with a tiny amount of helium in the tube.
Software:
EM FIELD -- A good program for both electric fields and magnetic fields. Drag and drop charges or currents, then compute field strength or draw field lines around them.
HELIX -- displays the 2-D trajectory of a charged particle in crossed electric and magnetic fields. Field strengths and initial particle velocity can be chosen by the user.
LORENTZ -- similar to Helix, but displays a three-dimensional trajectory.
MIRROR -- displays the trajectory of a charged particle in a “magnetic mirror” formed from three coaxial solenoids. Initial particle velocity and solenoid conditions are user-selctable. Motion of the particle is viewed from two directions simultaneously, and maps of B along both the axis and radius of the solenoids can also be displayed.
Oersted's Needle -- a wire carrying a large current is brought near a bar magnet on a rotating stand. No matter how the wire is oriented, the bar magnet will turn so that it is perpendicular to the wire
Magnetic Fields Around Conductor -- bare wires in various configurations on transparent boards carry a large current; board is placed on overhead and sprinkled with iron filings to show shape of the field. Small transparent compasses can be used to show the sense of the field.
Ampere's Frame with Wire -- same as Ampere's Frame (see last section), but uses a hand-held current-carrying wire instead of a permanent magnet to rotate the frame. WARNING: This demonstration involves mercury. Students must not be allowed to operate or interact with it.
Pinch Wires -- parallel hanging wires are either attracted or repelled by one another, depending on the direction of currents in the wires. Three different configurations.
Electromagnet with 1.5 Volt Battery -- a small coil that can be slipped on and off an iron core is powered by a 1.5 V battery. With the core in, the magnet will hold up kilogram weights, but without the core it won't even hold a few grams.
Big Electromagnet -- huge coil carries up to 25A; very strong field will attract nails, etc. that are thrown near. Nail on a string allows the shape of the field to be probed, and the removeable iron core concentrates the magnetic flux.
Solenoid Bar Magnet -- free-hanging solenoid acts as a bar magnet when current flows, otherwise not.
DC Motor -- a large open coil powered by DC current through a split-ring commutator spins between the poles of a set of permanent magnets.
Circuit Breaker -- a standard household magnetic breaker which will shut off current to a load if the current exceeds approximately 3 amps.
Wire and Magnet -- single loop of wire is passed between poles of a magnet, causing current to flow (shown by the projection galvanometer). The faster the wire is moved, or the greater the number of loops, the larger the current.
10/20/40 Turn Coils w/Magnet -- coils of 10, 20, and 40 turns wired in series on a common stand, through which a permanent magnet is moved to produce small currents as shown on a large galvanometer.
AC and DC Generator -- large model generator with a coil spinning between permanent magnets. Can produce DC (split-ring commutator) or AC (solid ring) current. Good visibility. Can also be run as a motor.
Army Surplus Generator -- hand-cranked generator will provide up to 80 watts of output power for lamps, etc. A board of individually switchable lamps allows electrical load to be increased, which increases cranking difficulty.
Hand Crank Generator w/Lamp -- small crank generator lights lamp - easier to turn with the bulb out of the socket.
Earth Coil -- hand-held coil is moved in the Earth's magnetic field and produces a current (must be shown with projection galvanometer).
Faraday Disk Dynamo -- an aluminum disk spinning between magnet poles produces a current between the center and the edge of the disc.
Two Coils (Mutual Induction) -- two coils slide on a track so that the distance between them can be varied. Current is pulsed into one coil with a switch, which induces current in the second coil. Meters show the currents in both coils, and show that there must be a changing current in the first coil to induce current in the second. Intensity of induced current changes with separation, and various metal cores can be inserted to determine their effect on the magnetic flux.
Vertical Primary Coil and Various Secondary Coils -- a tall primary coil that carries AC current has an iron core which extends out of the top, on which various secondary coils can be placed (light a small or large light bulb, boil water with the waste heat from hysteresis, etc).
Transformers -- two coils sit on a common iron core; AC current is fed into one coil, and the magnitude of the voltage in the second coil is shown by the brightness of a lamp. Turn ratios can be 1:1, 2:1, or 1:2.
Induction Coil -- standard induction coil with mechanical “make and break” oscillator produces approximately 100,000 V.
Tesla Coil -- Tesla air-core transformer produces 1/2 million volts at 350 kHz
Arago's Disk -- aluminum disk spins beneath a magnet on bearings, causing the magnet to rotate due to eddy currents in the plate. The magnet may be restrained by a rubber band to make a model of a speedometer linkage.
Magnetic Damping Pendulum -- a flat plate of aluminum on the end of a pendulum swings between the poles of a magnet. Eddy currents in the plate damp out the swing. Both a plate and a ring are available, split and unsplit. The split limits the size of the eddy currents and greatly decreases the damping in both plate and ring.
Motor with Disk -- a motorized spinning disk can be slowed down by slipping the poles of a magnet over the edge.
Thompson's Flying Ring -- AC current in large solenoid creates eddy currents in an aluminum ring; ring goes flying. Split ring does not.
Spinning Can -- a large horseshoe magnet magnet is spun over a Pepsi can sitting on a bearing under a glass beaker. The can spins in the same direction as the magnet due to eddy currents from the rotating magnetic field.
Hysteresis Waste Heat -- a thimblefull of water sits atop the secondary coil of a transformer. Waste heat from eddy currents and magnetic hysteresis boils the water.
Transformer Laminations -- a transformer core that has been pulled apart to show the individual laminations.
Superconductor -- an LN2 cooled ceramic superconductor (Yttrium Barium Copper Oxide, YBa2Cu3Ox) will levitate a small rare-earth magnet due to induced eddy currents in the superconductor (Meissner Effect).
Inductors (Show and Tell) -- various commercial and homemade coils.
Lamp in Parallel with Solenoid -- large DC current introduced suddenly to this large inductor cannot pass through the coil at first, so an incandescent lamp in parallel with the coil lights brightly. After the current becomes steady, the coil draws more current and bulb dims. When the current is switched off suddenly, the induced voltage in the coil (back EMF) again lights the lamp. A separate neon lamp in parallel with the coil shows that the direction of the second voltage surge is the opposite of the first.
LR Circuit -- a large inductor in series with a resistor, a battery and a switch, When the switch is closed the the current rises slowly from zero to a steady-state value as shown by the voltage across the resistor.
Broken Bar Magnet -- a broken bar magnet held together by its magnetism acts as a single bar magnet when whole. It can be pulled apart into two or more pieces and their fields traced with compasses and/or iron filings to show that each piece is also a complete magnet.
Domain Model -- plastic plate holds small bar magnets on bearings which simulate domains - they line up with one another, flip in the presence of an external field, etc. (OH proj.).
Barkhausen Effect -- powdered iron core is surrounded by a pickup coil. Bringing a permanent magnet near causes domains in the iron to flip, which is picked up by the coil and amplified into a crackling, rushing noise.
Curie Temperature (Magnetic Wheel) -- spinning wheel made of a material with a Curie point somewhat above room temperature passes through the poles of a magnet. A spot on the wheel directly above the magnet is heated with focused light and loses its magnetic properties. The spot directly below the magnet is then drawn upwards and the wheel begins to revolve. By the time the first hotspot makes a complete circle, it has cooled enough that the spinning is continuous.
Curie Temperature (Heated Canadian Nickel) -- nickel is magnetic at room temperature, non-magnetic when heated in a Bunsen flame. The nickel hangs from a wire and is initially “levitated” by a strong magnet. After heating, the nickel falls away from the magnet
Dysprosium with Liquid Nitrogen -- dysprosium is magnetic when cooled to liquid nitrogen temperatures, but loses magenetism as it warms up, and swings away from the magnet which held it up when it was cold.
Para- and Diamagnetism -- test tubes filled with manganese chloride, copper sulfate and bismuth are balanced at opposite ends of hanging bars which are free to rotate. Bring a powerful horseshoe magnet near the manganese chloride (paramagnetic) and it will be pulled slowly into the magnet. The copper sulfate (also paramagnetic) will be less strongly attracted to the magnet. Bismuth (diamagnetic) will be slightly repelled.
Superconductor -- modern ceramic superconductor cooled in LN2 levitates a small neodymium magnet (viewed on projection TV).
Ferromagnetism (Rowland Ring) -- show and tell simple transformer.
Magnetic Shielding -- various metals, etc. are inserted between an electromagnet and some nails the magnet is holding up. Materials that are magnetic (iron) absorb the magnetic flux, allowing the nails to fall - nonmagnetic materials don't. A double thickness of iron causes more nails to fall. Warning: We're not sure how reliable this demo is.
Nuclear Magnetic Resonance -- nuclei of atoms in an intense magnetic field absorb radio-frequency energy at their resonant frequency; shows up as a “blip” in the voltage in a small pickup coil surrounding the sample of copper sulphate.
Permalloy (High Permeability) -- a permalloy rod is not itself magnetic, but if the rod is aligned with a pre-existing magnetic field such as the Earth's or a magnet's, it becomes magnetic enough to pick up small pieces of iron.
Huesler Alloy -- an alloy of manganese, aluminum, and copper is attracted to a magnet even though none of its constituent metals is magnetic by itself.
Stainless Steel -- stainless steel is non-magnetic even though it is mostly iron.
University of Washington Homepage
Physics Department Homepage
Table of Contents