Entries:

• Conservation of energy and rotational inertia(LC)
• Current-carrying loop in a magnetic field(LC)
• Observing magnetic fields(LC)
• Ring shooter revisited(LC)
• Optics tank(LectDemo)
• Monkey and hunter revisited(LectDemo)

Damon McCausland
Colorado State University Physics Department
Fort Collins, Colorado 80521
(970) 491-3309
dmccausl@holly.colostate.edu
dmccausl@lamar.colostate.edu

Abstract

This aluminum cylinder allows its moment of inertia to remain fixed while changing the diameter of the axle around which it rotates. This simple device demonstrates how energy is conserved and what fraction of the total gravitational potential energy is converted to translational kinetic energy versus rotational kinetic energy.

Support required A table top a minimum of 2 meters long. This device does not require electrical power. It is not intended for use with an overhead projector.

I will be present to set up the apparatus on the morning the room is available.

Equipment required to construct apparatus:
Bulk aluminum cylinder ~10 lbs $3.00/lb your local metal recycling store
Short length of threaded rod $1.50/ft Ace hardware
2 lab stand rods or ramp $0.00 (on hand)
total cost .....................................................................................................$31.50

Sketch

Description

This cylinder allows the axles to be threaded in and out to change the effective diameter of the axle without changing the object’s mass or moment of inertia. Although a small error is introduced because the threaded rod is not aluminum it is far smaller than any measurement errors that you will encounter. The cylindrical geometry allows your students to do all relevant calculations with you in class.

Construction: This requires a metal lathe and moderate experience to safely build it right.

• 1. Make two identical parts that are square right circular cylinders that have two dramatically different diameters on each end.
• 2. Square up your larger cylinder and then cut away the material so the two parts you just made will fit inside the large cylinder.
• 3. Drill the center of each of the pieces and tap threads through them.
• 4. Cut the threaded rod to the correct length and then epoxy it into the large cylinder.
• 5. The other two pieces should screw onto the rod and into the large cylinder. To avoid over tightening the adjustable axles it is a good idea to put a rubber washer on the rod prior to tightening the axles.

Abstract

A homemade Helmholtz coil produces a uniform magnetic field along its axis. A smaller coil is connected to a 6V lantern battery is hung at the center of the Helmholtz coil. The magnetic field produced by the smaller coil causes the coil to rotate until it is aligned with the field of the Helmholtz coil.

Support required: This demonstration requires power from a standard wall outlet.

I will be present to set up the apparatus on the morning the room is available.

Equipment required to construct apparatus

1 12" length of 16" diameter PVC of sonotube $10.00 local plumbing supply
1 500 ft spool of 18 gauge stranded wire $10.00 local electrical supply
1 12V 1.2 amp power supply (from old halogen lamp) $5.00 (garage sale item)
1 8" wooden embroidery ring $5.00 (local fabric store)
1 6V lantern battery $3.99 Ace hardware
1 coat hanger or welding rod $0.00 (on hand)
2 fuse-able link wires $2.50 local auto parts store
Total   ....................................................................................................$36.49

Sketch

Description

All first year physics students are asked to calculate the torque on a current carrying loop in a uniform magnetic field. This demonstration clearly shows the motion of such a loop. Hooking up the Helmholtz coil to the power supply will produce a uniform field and the battery produces the necessary field for the pivoting loop. This is suitable for auditorium use.

Construction:

• 1. Cut the PVC to the length you desire. (~12")
• 2. Cut large windows in the front and back of the PVC (be careful not to weaken the PVC too much and allow 2" on the ends for the wire to wrap around). This will become your Helmholtz coil.
• 3. Wrap a smaller coil around the wooden ring.
• 4. You will need to attach a piece of wire to the wooden loop so it can freely pivot from above.
• 5. The pivot wire should rest on something with low friction. I found that if you cut a 2" hole in the top of the Helmholtz coil you can use a small piece of spring steal with a dimple as a pivot.
• 6. Run the leads from the wooden loop up the pivot wire and attach the fuse-able link wire to the ends. Then run a loose arch from the pivot wire to a fixed point on the Helmholtz coil. The more flexable the wire the smaller the effect the wire has on the movement of the inner ring.
• 7. Hookup the power supply to the Helmholtz coil. The battery should be connected simply by holding two leads against the battery. This will preserve the battery life.

Abstract:

These acrylic sheets are ideal for demonstrating what the magnetic field produced by a current carrying wire, solenoid, and toroid look like. It is easy to observe that the field circles a vertical wire and is completely contained within a toroid and diverges when it leaves the ends of the solenoid.

Support required: This demonstration requires an overhead projector and an additional power source both of which I will provide.

I will be present to set up the apparatus on the morning the room is available.

Equipment required to construct apparatus

3 1’x1’x1/8" acrylic sheets $1.85/sqr ft Fort Collins Plastics
20 ft 14 gauge stranded household wire $18.86/500 ft
#6X794 THA10bk Grainger
48 compasses with clear glass on both sides $5.00/12
# 1 6V lantern battery or 2 amp 6V power supply $3.99 Ace hardware
1 pinball button $.50 Surplus
Total ....................................................................................................................$48.90

Sketch:

Description:

This demonstration is most commonly seen using iron filings and dead shorting a car battery to move the filings. If you replace the iron filings with see-through compasses you can replace the car battery with a 6V lantern battery while dramatically improving the visual effect. You can also demonstrate the validity of the "right hand rule" using the various components. This demonstration has been used with classes ranging from 30 to 320 students.

Construction:

• 1. With an erasable pen mark out the patterns you would like the wire to follow.
• 2. Slowly drill the holes out that form your pattern. Take care not to crack the acrylic.
• 3. Thread the wire through the holes.
• 4. Glue short legs to the corners of the acrylic sheets and construction is complete.
• 5. Hook the wire leads to a switched battery.
• 6. If you desire you could glue the compasses to the sheet but it is not necessary. Just make sure they are spaced out enough to prevent interference from adjacent compasses.

Abstract

This ring shooter is powered straight from a standard 20 amp wall outlet and has reached heights of 30 ft. It is useful as a tool when discussing electromagnetic propulsion as well as transformers.

Support required This demonstration requires a standard wall outlet.

I will be present to set up the apparatus on the morning the room is available.

Equipment required to construct apparatus

300 ft 14 gauge stranded household wire $18.86/500 ft
#6X794 THA10bk Grainger Plywood for the base 2’x4’x ¾" $3.50 Southerlands
1 old power strip with a thermal 10 amp surge $0.00 On hand
1 small length of aluminum pipe 2" id $3.00/lb Local metal supply
1 4’ length of 1 ¾" thin wall PVC $.39/ft Ace hardware
1 20 lb sleeve of 1/16" welding rod $10.00 Local welding supply
1 pinball button $.50 Surplus
Total $37.42

Sketch:

Description:

Ring shooters have been around for a long time and most physics departments have one for demonstrations. It turns out that most of them are far more complicated than they need to be. This ring shooter is simple to the core and powerful. The trick is in tuning it for optimum performance by manipulating the placement and geometry of the core. There are various different rings used with it which allow different heights to be reached. This is helpful when there are low ceilings. Also a light bulb array can be used to show the relative field strengths.

Construction:

• 1. Make a coil using a 13 ½" length of 1 ¾" thin walled PVC and wrap ~300 ft of 14 gauge household stranded wire tightly around it.
• 2. Build a base for the coil out of plywood.
• 3. Take apart a power strip and connect the coil leads through the thermal switch and the pin ball button and to the power cord.
• 4. Take the 36" welding rod and cut 15 pieces to 6"/30" long and cut 15 pieces to 7"/29" long and so forth. This will yield a core with a slightly larger divergence in the field which will allow for a stronger shot.
• 5. Cut an aluminum ring off the pipe that is approximately ½" tall. Use this as your standard ring.
• 6. Place the core inside the coil and using an inductive style ammeter, measure the current through the coil. It will probably be around 8 amp. I have found the best performance can be achieved by raising the core up inside the coil until you draw 60 amps through it. You will have to make a spacer that is non metallic for this purpose. The thermal fuse will blow faster than the wall circuit so you can use the ring shooter virtually anywhere. At 60 amps the thermal switch will only last ~1 second then you will have to reset it.
• 7. If you want only certain heights to be available then you can cut rings of different masses.

Abstract

This demonstration allows Snells law and total internal reflection to be observed by everyone in a moderately dim auditorium (it works with safety lights on). A special optics tank has been built for this purpose. This tank is filled with "Lemon Fresh Pinesol" in order to see the laser.

Support required: The demonstration requires a 4’x4’ table and power. The lights can be on for close viewing. I may provide a screen to block some light.

I will be present to set up the apparatus on the morning the room is available.

Equipment required to construct apparatus:

1 Vinyl tank 36"x12"x1 ½" (10" radius at one end) $180.00 Fort Collins Plastic
(!!!DO NOT USE ACRYLIC, IT WILL CRAZE!!! The vinyl used for this tank is rigid 1/8". )
1 HeNe laser $0.00 On hand
1 small jack stand $0.00 On hand
4 Large bottles of "Lemon Fresh Pinesol" $5.00 each Local grocery store
Total ....................................................................................................................$200.00

Sketch:

Description:

This optic tank is useful for several different demonstrations, three of them are as follows:

• 1. Fill the tank to just above the top of the radius with "Lemon Fresh Pinesol" and rotate a HeNe laser about the center of the radius. This will allow the students observe exactly when total internal reflection is reached.
• 2. Place a HeNe laser on a small jack stand and point it along the axis of the tank entering the curved end. Slowly raise the jack and observe Snells law in action.
• 3. If you lower the fluid level to approximately 1½", a steady hand can simulate a fiber optic cable with a beam that is internally reflected multiple times down the length of the cavity.

Abstract

This demonstration is a variation on the classic monkey and hunter projectile problem. A standard paint ball CO2 gun is the launch mechanism while a laser sight allows the students to see where the gun is aimed. The trigger is a photo gate mounted at the end of the barrel which controls an IR transmitter. This signal is picked up by the receiver and controls a retractable linear solenoid thus allowing the monkey to drop.

Support required: This demonstration does not require any special support. It is battery powered and self contained. If the experiment is actually going to be used a large room with a tall ceiling is required. Otherwise a power source to run a VCR would be necessary.

I will be present to set up the apparatus on the morning the room is available.

Equipment required to construct apparatus:
1 Air Concepts Industries "Hornet" paint ball gun $110.00 Jax Mercantile
1 12 oz CO2 bottle (included) $0.00 Jax Mercantile 1 stuffed animal $5.00 Toys-R-Us
1 laser sight $80.00 Jax Mercantile
1 linear solenoid G5628 $3.50 Electronic Goldmine
1 IR switch kit C6748 $12.95 Electronic Goldmine
1 IR remote C6506 $2.00 Electronic Goldmine
1 photo gate (use parts available locally) $ 15.00 Local electronics supply
1 Plastic cylinder 1" (machine to barrel diameter) $1.00 Fort Collins Plastic
Misc. parts $40.00 N/A
Total ...............................................................................................................$269.45

There will be a packet on the table with the experiment that contains a complete parts list, construction methods, circuit schematic, cost with all part numbers, and possibly a video clip for viewing it in action.

Sketch: A sketch is not available at this time.

Description:

This monkey and hunter demonstration allows the instructor to eliminate the student’s natural tendency to think that the experiment only works for one particular configuration. Other skeptical students might think that the device was rigged to give the illusion of success. The mobility of this demonstration allows the professor to wander around the auditorium and have the students decide where to shoot from. The laser sight allows the students to see exactly where the gun is aimed when it is fired. The gun has been tuned down from a muzzle velocity of ~300ft/sec to a more acceptable ~40ft/sec. This speed will allow a substantial drop for the target and the projectile is visible to the naked eye. Although the speeds are low caution must be observed. The full details including all part numbers, cost, and schematics will be provided at the conference.