MadSci Network: Engineering
Query:

Re: Magnets and how they work with a battery operated on/off switch.

Date: Sat May 20 23:55:55 2000
Posted By: Jim Stana, Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Engineering
ID: 956928794.Eg
Message:

An electro-magnet is a type of magnet that can be switched on and off.  
This is formed by winding a coil of wire around a piece of magnetic steel. 
 When you apply a voltage to the wire, the wire forms a magnetic field.  A 
switch can be used to connect the voltage to the wire.  If you wind wire 
around a nail and put a voltage through it, you can pick up small washers 
and other pieces of steel just like with a regular permanent magnet.

Unfortunately, the drag strip you describe is not so simple to use this 
effect.  Magnets can attract a magnetic car which has some steel in it, but 
it is very difficult to attract steel with a magnet for a very long 
distance such as the 3 feet you describe.  So unless you can pull the 
magnet down the track ahead of the car, your car won't go very far.

If the car contains a magnet itself, you could use the opposite polarity of 
the magnet to push the car down the track by repulsion of like poles.  But 
even this only works for short distances, probably only a fraction of an 
inch or so.

To really get your car up to speed so that it reaches the end, you have to 
know a little bit more about the physics of the problem.

To accelerate an object like a toy car which weighs M lb up to a speed, V, 
you have to push it with a force,F for a time, T.  

The car will accelerate with an acceleration of A = F/M

For every second you apply the force, the speed will increase by A*T or 
F*T/M

But as soon as you stop pushing on the car, it will travel at the final 
speed until friction of the tires, etc slows it down.

Since you will be able to repel the car by your electromagnet for a short 
distance, as soon as the car leaves the field of the magnet, it will begin 
to slow down.

If you want to make your car go fast, you have to have a really powerful 
magnet that will kick the car with a large force so that it gets up to 
speed very quickly before it leaves the magnet.  This is just like having a 
large spring that would kick the car just like a pinball plunger kicks a 
pinball in a pinball machine.  Such a magnet will require probably more 
than a small battery to power it.  In fact, a magnet such as this will want 
to draw a lot of current which will run the battery down with only a few 
tries.

To make the force last along the length of the track, you would need to 
build a linear motor which is used to power some monorails.  This is rather 
complicated to do.

You might try looking up electromagnet in a science encyclopedia and see 
how you might build one to try pushing your car.  You may need more than a 
battery to power it, however, so get an adult to supervise the construction 
and use.

I found the following website which shows the complicated formulas used to 
determine the strength of a magnetic field of an electromagnet.  I couldn't 
find much else in the way of web sites that explain their construction.
 http://www.geocities.com/CapeCanaveral/Hall/6153/





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