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Q & A: variable electromagnet

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Most recent answer: 10/22/2007
Q:
How do u make a variable electromagnet??
- yasmin (age 13)
A:
One way is to include a variable resistor in series with the electromagnet and battery. The power dissipated in a resistor is V*V/R, where V is the voltage across it in volts and R is the resistance in Ohms. In the steady state, the voltage across the electromagnet's terminals should be very small (although this depends on how many turns of wire it has and how thick the wire is). The power dissipated in the resistor should be maybe a half a watt or so for a reasonable flashlight battery in order to make it have a reasonable lifetime (less power dissipated will make your battery last longer).


Tom


One drawback with that simple circuit is that most of the power gets thrown away in the resistor. Another method would use a variable transformer and an ac source to get a variable ac voltage. That can then be converted to dc with a bridge made from four diodes, and this variable dc can drive the electromagnet. The advantage is that there are only minor power losses in the transformer and the diodes. However, if you're starting with line voltages, you have to be very careful not to get electrocuted. Low voltage batteries are very safe.

Mike

(published on 10/22/2007)

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