A 10 minute railgun
A railgun is a device for accelerating an object by running electric current
through it along a pair of rails. When large amounts of power are used, the
railgun becomes a potent weapon. But the principles can be demonstrated
safely by using a smaller amount of power, in our case, from a 9 volt battery.
When a small amount of power is used, the rails can be re-used many times.
If larger amounts of power are used, the railgun becomes a one-shot device,
as the rails are destroyed in the process of shooting the gun, due to
arcing and flexing of the rails.
There are several types of rail gun, each with a different method of
accelerating the object. This version is called a linear homopolar motor.
Click on image for an animated picture
What you need
-
A piece of cardboard or wood for a base. It can be any length.
Ours is about 18 inches long, and about 6 inches wide.
-
Two strips of aluminum foil, 2 inches wide, and 2 inches longer
than the base.
-
A 2 inch wide length of steel wire, such as from a coathanger.
-
2 disc magnets, plated in a good conductor, such as gold.
The super-powerful magnets in our
catalog
work great, and were the ones used in this project.
-
Some white glue
-
A 9 volt battery
-
Two aligator clip leads
Click on image for a larger picture
We start by spreading a thin layer of glue on the base, to hold down
the foil strips.
Click on image for a larger picture
The foil strips are then placed on the base, about a half inch apart,
and smoothed down with fingers to remove wrinkles. The extra 2 inches
is left hanging over one side, to make it easy to clip on the leads
for the battery.
Click on image for a larger picture
Next we attach the battery with the clip leads. We won't worry about
battery polarity at this time -- if the gun shoots the wrong way, we
will reverse the battery.
Click on image for a larger picture
The nest step is to file the ends of the coathanger wire flat. This will
allow the magnets to stick flat to the ends of the wire axle. If the
filing is done carefully to make the flat ends perpendicular to the wire,
then there will be less wobbling as the magnets travel down the rails.
Click on image for a larger picture
Now the magnets are placed on either end of the wire axle. The magnets should
have their poles facing in opposite directions. The magnets repel one another
when aligned this way, but will still stick firmly to the steel axle.
No glue is necessary, as the
magnets are very strong.
Firing the railgun
To fire the railgun, just drop the wheels on the rails. They will start to
accelerate immediately. If they don't move at all, the magnets are probably
not pointing the same way. Lift the wheels, and flip one magnet, and try again.
If it still doesn't work, check the battery and the connections.
If the wheels move in the wrong direction, you can either start them at the other
end, or reverse the battery.
Click on the photo below to see the railgun in action.
Click on image for an animated picture
How does it do that?
The homopolar motor was one of the first motors ever built. Michael Faraday
built one.
There are many different designs. The one shown below is only one of them.
Click on the picture to see it run.
Click on image for an animated picture
A magnet is placed north pole facing up in a bowl of vinegar.
The magnet is one of our super-powerful ones from our
catalog.
Suspended over it is a heavy piece of copper tube or wire, hanging
from a flexible stranded wire alligator clip. The vinegar covers the
tube to a depth of about a half of an inch. The tube does not quite
touch the magnet, but is free to swing around.
A piece of aluminum foil also rests in the vinegar (this can be a copper
wire instead, it is not critical).
When we connect a source of about 30 to 50 volts (some 9 volt batteries
connected in series) to the foil and the tube, the tube starts to revolve
around the magnet. We also get lots of bubbles of hydrogen and oxygen,
but for this project that is just a side effect.
We have created a homopolar motor. Unlike our previous motors, this one
does not change the poles of an electromagnet from north to south and back.
As the current flows through the copper wire (or tube in our case),
a magnetic field
is created around it. This magnetic field interacts with the magnetic
field of the magnet at the bottom of the bowl. The arrangement is set up
so that the magnetic field in the wire exerts its force at a right angle
to the magnetic field of the bottom magnet. This makes the wire circle that
magnet.
Another simple homopolar motor is shown below.
Click on image for a larger picture
The photo above shows the parts before assembly. There are three magnets --
two small disks, and one cube whose north pole we have marked with an "N".
The magnets are from our
catalog.
There is a D cell battery, and a bare copper wire we have formed into a shape
whose form will make sense shortly.
Click on image for an animated picture
The two small disk magnets are set on top of the North pole of the cube
magnet. The loop in the bottom of the wire form is then placed in top of
the two disks. The loop is just barely larger than the disks. The positive
pole of the battery is then carefully placed on top of the disks, and the
pointed end of the wire form is placed in the dimple at the top of the
battery.
When the whole structure is balanced on the cube magnet, the wire begins
spinning around the battery.
What is going on here is similar to what is going on in our vinegar version.
The current from the battery is flowing through the wire on both sides of
the wire form. This creates a magnetic field around the wire. This field
interacts with the field from the cube magnet and the disks. The wire's
field creates a force at right angles to the field from the magnets. This
causes the right side of the wire to be pushed towards you, and the left
side to be pushed away from you. This action continues until the battery
runs down.
Suppose we held onto the wire, and let the magnet move? We can do that
by simply setting the battery on the table, on its side. Click on the
photo to see an animation of what happens.
Click on image for an animated picture
The wire rotates until it hits the table. The force continues after the
wire can no longer rotate, and so instead of the wire rotating, the battery
and the magnets rotate. This makes the battery roll along the table like
a steam roller.
You can now see that we are very close to our railgun design. The railgun
is really just two of these battery rollers connected end-to-end. The battery
is now external, and the wires have been replaced by the rails. The magnetic
field in the rails creates a force at right angles to the magnetic field of
the railgun trolley's wheels, and causes them to roll.
Next:
A 30 second homopolar motor
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Simon Quellen Field
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