Can a Magnet Effectively Charge a Capacitor?

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Discussion Overview

The discussion revolves around the feasibility of charging a capacitor using a magnet and wire, exploring the principles of electromagnetic induction and the conditions necessary for such a process to occur. Participants examine various scenarios, including stationary versus moving magnets, and the implications of Faraday's Law.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that a stationary magnet cannot charge a capacitor, as energy must come from a change in the magnetic field, which requires movement.
  • Others argue that charging a capacitor can occur through a moving magnet, referencing practical examples such as mechanically powered flashlights that utilize a sliding magnet through a coil to induce current.
  • A participant mentions that the pulse of electricity generated by moving a wire through a magnetic field is short-lived and would require additional components, like a diode, to prevent rapid discharge of the capacitor.
  • There is a suggestion that using multiple turns of wire could increase the voltage generated, potentially allowing for more effective charging of the capacitor.
  • One participant questions the awareness of Faraday's Law, implying its relevance to the discussion of charging a capacitor with a magnet.
  • Concerns are raised about the need for a bridge rectifier when using translational motion to ensure consistent charging of the capacitor.

Areas of Agreement / Disagreement

Participants generally agree that movement is necessary for charging a capacitor with a magnet, but there are differing views on the specifics of how this can be achieved and the effectiveness of various methods. The discussion remains unresolved regarding the optimal approach and the conditions under which charging can occur.

Contextual Notes

Participants highlight limitations related to the duration of the induced current, the necessity of additional components like diodes or rectifiers, and the dependence on the configuration of the wire and magnet. There are also unresolved mathematical considerations regarding the voltage generated under different conditions.

BobbaD
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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.
 
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BobbaD said:
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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Magnet ha to move in relation to the wire.
 
BobbaD said:
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.
 
BobbaD said:
Can you charge a capacitor with magnet and wire?
Yes. This is called a generator
 
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? Let's 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 spacecraft through the Earth's magnetic field.
 
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BobbaD said:
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.
 
CWatters said:
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.
 

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