Preliminaries:
I intend to place the following lab in a sequence of labs dealing with electromagnetism. This will not be the first experiment dealing with Lenz's Law. Previous to this experiment, we will drop a "cow magnet" down a copper tube to demonstrate the slowing caused by energy lost through induced current in the copper. Self-induction and mutual-induction, and the Hall Effect will be topics of interest.
Descriptions of the theory and practical challenges to electromagnetic ballistics will include railguns and coilguns. We will study the concepts behind the "can crusher". We will look at coilguns in this brief conceptual study.
Prelab Questions:
1. Briefly explain what happens when we charge a capacitor.
(Misconception: During the "charging" process, energy is placed in the capacitor, not "charge". There is as much total electrical charge in a capacitor before charging as there is afterwards.)
2. Briefly describe what is meant by the following terms:
a. Capacitance
b. Inductance
c. Core Saturation
d. Self-Induction
e. Mutual-Induction
f. Faraday's Law of Induction
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Physics ### lab
Gauss Gun
(Coilgun)
Purpose: The purpose of this experiment is to investigate induction as a mechanism for accelerating a projectile. We will make a simple coilgun and investigate its operation.
Equipment: For this experiment, you will need:
* A disposable camera with flash
* A 3/4" D x 10" steel rod.
* An aluminum washer with hole bigger than 3/4"
* 24" of 30 AWG enamel magnet wire (from Radio Shack, p/n 278-1345B)
* a small flat bladed screwdriver
* Rubber gloves
* a 100 ohm resistor or a piece of pencil lead
* a voltmeter capable of reading 500 volts.
* 1-1/4" Alligator Clips jumpers (2)
* 22-gage Hook-Up Wire (from Radio Shack, p/n 278-1244)
* Soldering gun and solder
Background: Lenz's law was discovered by the German scientist H. F. E. Lenz in 1834. Qualitatively, it states that the electromotive force (emf) induced in a conductor moving perpendicular to a magnetic field tends to oppose that motion. Alternatively, a changing magnetic field will induce an emf (called "back emf") in a conductor, which causes a current perpendicular to the field. The induced field will oppose the changing field. An induction motor is a practical application of Lenz's law.
A coilgun is an example of simple linear motor. It demonstrates many basic concepts of magnetic machines. In the coilgun, stationary coils produced a quickly changing magnetic field and thereby accelerate a conductive projectile.
In theory, the speed of the projectile is not limited, as it is with conventional hot expanding gasses. Coils can be arranged in stages to provide successive energy kicks to the projectile. Superconductive versions of the coilgun, called quench guns, have been designed and tested. These are studied in sophisticated university laboratories. But practical limitations, such as their inherent inefficiency, and the high cost of power switches, presently rule out the coilgun as a candidate for space vehicle launchers.
Procedure:
1. Secure the steel rod vertically and drop the coil of magnet
wire around it. (Leave the wire on the spool.)
2. Place the
aluminum washer on top of the coil.
3. Remove the lacquer finish from the ends of the magnet wire
and connect alligator jumpers.
4. Put on the rubber gloves and then discharge the capacitor
through the 100-ohm resistor (or pencil lead). Remove the battery. Carefully
remove the circuit board from a disposable camera.
5. Use the alligator jumpers to connect the coilgun in parallel
to the flashbulb on the camera.
6. Install the
battery again.
7. Hold down the pushbutton, located near the center of the
circuit board, in order to charge the capacitor.
8. Use your screwdriver to jump the switch connections, which
protrude from the circuit board. You will see the flash and observe the
aluminum washer rise due to Lenz' Law.
Follow up Questions for Students:
1) How can a 1-1/2 volt battery charge the capacitor to 300 volts?
2) How can the 300 V capacitor produce the thousands of volts necessary to make the xenon bulb flash?
3) How is it that a non-ferromagnetic material (aluminum) is affected by the electromagnetic coilgun?
4) What would happen if you placed several capacitors in parallel with the one on the circuit board? Try it.
5) Coilguns are inefficient. Please list possible ways energy can be lost.
6) If you use a split ring washer in the experiment, how will your results change?
Try it.
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Comments
The coilgun is a good choice for a lab experiment because it appeals to student's interest in novelty, as it teaches properties of EM. The coilgun has no moving parts, and so seems magic with its utilization of invisible forces. And it requires no special construction techniques or unusual tools.
I believe that this lesson has value as an economics and
production manufacturing, too. We are handling a throwaway camera that is
obviously quite sophisticated. I would take time to ask students how someone
could make something that involved so cheaply.
The disposable camera is a convenient way to produce high voltages. I got four of them from a Meijers store at their one hour developing booth, for free! They recycle them. I explained that I'm a teacher, and they had no problems letting me have them. I ripped one apart and couldn't make it discharge. Then I discovered that there's a button you need to hold down to get the capacitor to charge (to 300 V). You can see the button below. It's a metallic disk shape in the center of the circuit board. Here, I'm wiring the coil in series with the flash unit. I'll keep the flash to work as a "trigatron". (An ordinary switch wouldn't last long.) In order to get it to flash, and send it's pulse to the coil, I short the two metal pieces that are sticking out to the bottom of the circuit board. One is right next to the cap. The other is to its left.
Below, you can see my stuff. I'm using a coil of magnet wire, just as it came from Radio Shack. I turned the core at ZHS and welded it to that base. There's one of the five big capacitors I bought at Dayton. I got a real good price: $20 for five. (A local supplier asked $32 each.) I never did hook more than one in parallel with the camera's capacitor.
I can get to fly off the core. Without the external cap, it flies only about an inch. I used a bigger battery to charge the capacitor. I discharged the big capacitor once with a screwdriver, and was amazed at the loud bang. You should use a resistor.
I couldn't get the CD to move. I thought it might since it looks metallic.
The
primary output of the camera circuit is a xenon-filled flash tube that produces
a flash of light. It takes over 10 kilovolts to fire this tube. The electronic
circuit produces this high voltage.
The
secondary output is a neon glow tube, a glass cylinder with two electrodes
inside. It glows when the circuit is charged.
Pressing
the charge button, a high pitch whine is produced by the oscillator driven
transformer as the circuit charges. When the capacitor is charged the neon lamp
glows.
A
second input is the shutter release button. Pressing this button triggers the
shutter to open. When the shutter is open fully, it closes a switch, which
fires the flash. If you rip the camera apart, like I did, you have to connect
the electrodes with your own external switch, or just jump them with a
screwdriver. (That's what I did.)
If you
keep the camera together, you must rotate the wheel inside the back of the
camera before the shutter can be fired. It has to think the film has advanced.
To
avoid electric shocks electrical engineers say, when working on high voltage,
over 24 volt, circuits, ”keep one hand in your pocket.” The most dangerous
shocks produce electric current through your heart, for example, when flowing
from one hand to the other. By using only one hand you reduce the risks from
electric shock. Since handling this circuit will require both hands, you should
wear rubber gloves.
If the
screwdriver blade is placed across the main capacitor leads, a large spark will
result. The vaporized metal from the screwdriver blade is not beneficial to
health. In addition, the ultraviolet light from watching the flash repeatedly
can sunburn your corneas leading to painful “sand-in-the-eyes syndrome.” This
is what the 100 ohm resistor or pencil lead is for. You can discharge the
capacitor through them.
Sources
Disposable camera flash electronics
http://www.exo.net/~pauld/activities/camera_electronics.html
Mini efficient coil launcher from disposable camera flash
http://www.electronics-lab.com/projects/science/017/
CAPACITOR COMPLAINTS
http://www.amasci.com/emotor/cap1.html
SELF-INDUCTANCE
http://www.tpub.com/neets/book2/2a.htm
Model Coilgun
http://marty.anstey.ca/projects/coilgun/
Barry's Coilgun Design Site
http://www.oz.net/~coilgun/home.htm