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Q & A: magnetic projectile launcher

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Most recent answer: 10/22/2007
Q:
I am doing a physics experiment, and I have a question about magnets. My project is to launch a projectile into the air, and I am only going to use magnets. How much electricity would I need in order to launch something based on the magnetic force only? (what size battery)
- Kelly Wilson (age 17)
Kent, Washington
A:
Hi Kelly,

One warning right away: making projectile lauchers is DANGEROUS. You can easily get hurt by making something which shoots a metallic object at high speeds. Do not point your experiment at anyone or anything. Projectiles that go up also come down, and can injure people or damage objects even if this is not intentional. You may want to test your projectile launcher only in a padded, enclosed area. There is also a problem with heat and possibly sparks -- make sure your padding is non-flammable!

A common way that magnets are used to make things move is to put an iron rod inside an electromagnet coil. Have a look at a doorbell mechanism (not one of the newer ones with an electronic synthesizer and a speaker, but an older one with metal chimes). The rod is arranged so that it is drawn into the solenoid when current flows in the electromagnet, and a spring pushes it back out when the current stops. It's often set up so that both swings of the rod end up hitting chimes on opposite sides -- hence the "ding-dong" of a doorbell.

You can set up something similar, but instead start with a bar magnet inside of a cylindrical coil of wire. Flowing current one way around the wire will tend to hold the magnet firmly inside, while flowing current the other way will tend to expel the magnet.

But how much current? It turns out that if the resistance of your wires (and the rest of your circuit, including your battery) is low enough, you can get quite a lot of current to flow through the coil with even a 1.5 volt battery. It may take some time to build up the current though. And typical batteries are not designed to supply large amounts of current -- their voltages drop if the current gets too high (hooking an electromagnet coil up to a battery isn't much different from making a short circuit across the battery. And there's a limit to how much current goes through a short-circuiting wire across an ordinary 1.5-volt battery).

The current is limited by the resistance of the whole circuit, whicl likely is mostly in the battery. You can get around this by hooking up lots of batteries in parallel to supply more current to the electromagnet. Electromagnets that are used for real-world applications have power supplies that supply fixed, known amounts of current.

You may want to restrain your projectile inside the coil while the current builds up, and then release it when the electromagnet is at its strongest. Keep one end of the electromagnet closed off so that you know which end your permanent bar magnet is going to go out.

If you open a switch to stop the current in your electromagnet after you've fired your projectile, be aware that there will be a spark at the switch. Electromagnets do not easily change the amount of current flowing through them. That's why it takes some time to get the current flowing, and to get the current to stop quickly usually involves dumping the energy stored in the electromagnet into the switch. This can be reduced somewhat by putting a capacitor across your switch (pick one that can take hundreds, if not thousands, of volts).

Having more turns of wire in your electromagnet strengthens its magnetic field for the same amount of electrical current, but increases its resistance -- it also makes the sparking problem worse.

Tom

(published on 10/22/2007)

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