# Electricity from a Magnet

• I
Can you charge a capacitor with magnet and wire? Can a wire attached to a capacitor be given amperage by a stationary magnet? Can the wire be made to a short length so that the magnetic field creating the current causes the current to reach the capacitor to charge it? I imagine this doesn't work, but would be interested to know why.

OmCheeto
Gold Member
Can you charge a capacitor with magnet and wire? Can a wire attached to a capacitor be given amperage by a stationary magnet? Can the wire be made to a short length so that the magnetic field creating the current causes the current to reach the capacitor to charge it? I imagine this doesn't work, but would be interested to know why.
It will work, but the battery magnet has to move in relation to the wire.

Example: Mechanically powered flashlight, Shake type design
The linear generator consists of a sliding rare earth magnet which moves back and forth through the center of a solenoid (a coil of copper wire) when it is shaken. A current is induced in the loops of wire by Faraday's law of induction each time the magnet slides through, which charges the capacitor through a rectifier and other circuitry.

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CWatters
Homework Helper
Gold Member
Magnet ha to move in relation to the wire.

sophiecentaur
Gold Member
2020 Award
Can you charge a capacitor with magnet and wire? Can a wire attached to a capacitor be given amperage by a stationary magnet?
Charging a Capacitor involves a source of energy. That energy has to come, in this case, from a change in the magnetic field around the wire. There will be energy available as you first bring the magnet and wire in place but it will just be a 'one off' - same as you supply gravitational potential energy to a book when you lift it up onto a shelf; that's another one-off situation.

Dale
Mentor
2020 Award
Can you charge a capacitor with magnet and wire?
Yes. This is called a generator

CWatters
Homework Helper
Gold Member
The "pulse" of electricity produced by moving a single length of wire through the magnetic field produced by a hand held magnet will be quite short duration. When the wire isn't moving through the field the wire will discharge the capacitor rapidly so you would also need a diode to prevent that.

Thought I would run some numbers for a wire moving past a 50mm (2") diameter magnet. If the wire is moving past it (eg perpendicularly to the magnetic field) the voltage produced will be roughly = B*L*V where B is the magnetic field, L is the length of the wire in the field, V is the velocity.

Typical value of B might be 1 Tesla. If the magnet is 50mm in diameter the length of wire in the field is approximately the same so L=0.05m. How fast can you move the wire? Lets say you wave/move it with your hand at 1 meter per second. The voltage would therefore be about..

B*L*V = 1 * 0.05 * 1 = 0.05 Volts = 50mV

That would only exist while the wire is moving through the field. At 1m/s it moves through the magnetic field and out the other side in 0.05/1 = 50mS. So you would generate a 5mV 50mS pulse (your mileage may vary). That's not enough to overcome the forward voltage of a diode (0.7V) and charge a capacitor but enough that you might see it twitch the needle on a meter.

If you used 100 turns of wire you might be able to generate a higher voltage, say 100 * 0.05 = 5V pulse.

If you want the generate 5V continuously you have to keep it moving within the magnetic field continuously. Typically by rotating the coil as per a dynamo or generator.

PS: You might also be interested in reading up on "space tether" which involves generating electricity by moving a long wire hanging from a space craft through the earths magnetic field.

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ZapperZ
Staff Emeritus
Can you charge a capacitor with magnet and wire? Can a wire attached to a capacitor be given amperage by a stationary magnet? Can the wire be made to a short length so that the magnetic field creating the current causes the current to reach the capacitor to charge it? I imagine this doesn't work, but would be interested to know why.

First and foremost: are you aware of Faraday's Law?

Zz.

DaveC426913
Gold Member
When the wire isn't moving through the field the wire will discharge the capacitor rapidly so you would also need a diode to prevent that.
If he uses a translational motion (like a shake flashlight) rather than a rotational motion, it will only charge when moving in one direction, so he'll want to add a bridge rectifier in there, so it charges twice as fast.