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Author Topic: Where does the energy in a magnet come from?  (Read 503 times)

Offline no1surfer

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Where does the energy in a magnet come from?
« on: 25/08/2016 11:09:25 »
If you can use magnets to induce an electrical current through a wire, where does the energy come from? how can a magnet move another object without obvious energy being added?
« Last Edit: 18/09/2016 09:44:51 by chris »


 

Offline chris

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Re: where does the energy in a magnet come from?
« Reply #1 on: 25/08/2016 18:05:31 »
When you (or some other driver) move the magnet (or the wire) then the electrical energy that is induced is actually being supplied by you, because you are doing work. The magnet supplies the magnetic field, not the energy. The magnetic field is a potential field that arises through the alignment of the electrical dipoles of the iron particles in the magnet.

Your question is a bit like saying "I jumped off a building and fell towards the ground. Where did the energy in the Earth come from to create the gravity that pulled me downwards?"

Again, it's you who supplied the energy - you did work against the gravitational field climbing to the top of the building in the first place. When you jumped, you converted that gravitational potential energy into kinetic energy on the way back down.
 
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Offline agyejy

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Re: where does the energy in a magnet come from?
« Reply #2 on: 25/08/2016 18:20:28 »
When I was in elementary school my teacher brought in a small table top hand cranked generator attached to some switches and light bulbs. If all the switches were open it was super easy to turn the handle of the generator. If you flipped a switch so that you were powering one of the light bulbs it immediately got harder. You could literally feel the increased electrical resistance working against you as you cranked. If you connected a bunch of bulbs in series the electrical resistance increased and it got harder and harder to spin the generator.

This demonstration made it very clear to young me that making electricity comes at the cost of some external energy input. The more electricity you want to make with a generator the harder you have to crank. It was a very visceral demonstration of conservation of energy and I think we'd have a lot less cranks if more people were able to experience the same thing.
 

Offline hamdani yusuf

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Re: where does the energy in a magnet come from?
« Reply #3 on: 26/08/2016 02:35:24 »
... If you connected a bunch of bulbs in series the electrical resistance increased and it got harder and harder to spin the generator.

It's true that connecting bulbs in series increases the electrical resistance, but actually this will reduce power consumption of the system, which in turn makes it easier to spin the generator, compared to single bulb.
P=Vē/R.
On the other hand if you connect them in parallel then the electrical resistance decreases and power consumption of the system increases, makes it harder to spin the generator.
 

Offline agyejy

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Re: where does the energy in a magnet come from?
« Reply #4 on: 26/08/2016 03:56:44 »
... If you connected a bunch of bulbs in series the electrical resistance increased and it got harder and harder to spin the generator.

It's true that connecting bulbs in series increases the electrical resistance, but actually this will reduce power consumption of the system, which in turn makes it easier to spin the generator, compared to single bulb.
P=Vē/R.
On the other hand if you connect them in parallel then the electrical resistance decreases and power consumption of the system increases, makes it harder to spin the generator.

You're right of course. It's been about 20 years and a flipped series and parallel. Shorting the generator with just a wire made it the hardest.
 

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Re: where does the energy in a magnet come from?
« Reply #4 on: 26/08/2016 03:56:44 »

 

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