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-   -   Im so confused about electromagnetic induction. Need help please? (http://www.physicsforums.com/showthread.php?t=655360)

elishamei9000 Nov28-12 03:13 AM

Im so confused about electromagnetic induction. Need help please?
 
I first thought that electromagnetic induction disobeyed the law of conservation of energy since there is another current created by another current by changing it (AC). but then there is Lenz law that says when you have yourself an current and also a changing magnetic flux, there will be an induced current that will make another magnetic field opposing the magnetic field of the other so it does not change the system.


Question is if Lenz law is not obeyed, so there would be no induced current or anything, would there be a created energy out of nothing? How would that be possible? Im so confused! Why doesnt Lenz law or electromagnetic induction does not violate the law of conservation of energy? I mean why have to create another current that would oppose the original current if there would be no energy created from the original current?

grzz Nov28-12 03:25 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
When an emf is induced some work MUST always be done by some external agent since energy must be conserved.

grzz Nov28-12 03:26 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
That is the reason why Lenz's law states that this external agent must meet with some opposition. The work done against this opposition results in an induced emf.

elishamei9000 Nov28-12 03:41 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
so basically the lenz law says in order not to have change the system, it has to generate or induce a voltage, then creates a current and that current generates magnetic field to cancel out the magnetic field of the original. i got that but..

so this means that if LENZ LAW IS NOT obeyed, there would be energy generated out of nothing? that's what i want to know..and in order not to have an energy created out of nothing, there must be an induced voltage? Please answer thanks very much. appreciated your reply. please go easy on me, im really confused.

grzz Nov28-12 03:46 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
Quote:

Quote by elishamei9000 (Post 4176272)
so basically the lenz law says in order not to have change the system, it has to generate or induce a voltage, then creates a current and that current generates magnetic field to cancel out the magnetic field of the original. i got that but..

I would replace ' not to have change the system' by ' not to have energy created out of nothing' and replace 'cancel out' by 'decrease'

grzz Nov28-12 03:48 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
Quote:

Quote by elishamei9000 (Post 4176272)
... so this means that if LENZ LAW IS NOT obeyed, there would be energy generated out of nothing?

Yes!

grzz Nov28-12 03:51 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
Quote:

Quote by elishamei9000 (Post 4176272)
..and in order not to have an energy created out of nothing, there must be an induced voltage? ...

I would rather see it as ...
in order not to have energy created out of nothing there must be some external agent doing some work. And this work provides the induced voltage.

elishamei9000 Nov28-12 04:13 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
okay now i see. if lenz law is not obeyed, then the induced voltage, thus also induced magnetic field would not oppose the original magnetic field, it would instead increase the magnet's velocity and supply more induced voltage and create more kinetic energy on magnet? (im imagining a magnet plunged into coil). so you have to do more work to induce a greater voltage so magnet is being repelled?

Drakkith Nov28-12 04:14 AM

Re: Im so confused about electromagnetic induction. Need help please?
 

An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux.


As it says here, the EMF causes a current whose magnetic field opposes your original circuit and you have to do work against that magnetic field. But even more basic is the fact that you have to accelerate electrons in the conductor, which requires that you perform work on them, which requires energy to do so, always. The EM field is simply the means by which you accelerate them.

Drakkith Nov28-12 04:24 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
Lenz's law is a result of the basic interaction between electrically charged particles and EM fields. Instead of asking about what would happen if it isn't obeyed, understand that Lenz's law is obeyed BECAUSE of the basic laws governing these interactions. For Lenz's law to be violated, other more fundamental laws would also have to be violated. In such a case we cannot even guess as to what would happen. For example, for the induced current not to oppose the EMF via a magnetic field Faraday's law of Induction would have to be violated. But if it's violated, can you even induce a current in the first place? You see what I mean?

elishamei9000 Nov28-12 04:38 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
really confused about how energy can be created if a magnet in a coil attracts instead of repel? please go easy on me. how can you produce energy when your magnet attracts the induced magnetic field so it increases the velocity of the magnet so how could that produce an energy out of nothing?

elishamei9000 Nov28-12 05:01 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
so basically if there's a bicycle dynamo, the work or energy you the cyclist gave is just the same as the energy that lit up the light? so the energy won't be an issue? please tell me if im correct

Drakkith Nov28-12 05:26 AM

Re: Im so confused about electromagnetic induction. Need help please?
 
Quote:

Quote by elishamei9000 (Post 4176324)
really confused about how energy can be created if a magnet in a coil attracts instead of repel? please go easy on me. how can you produce energy when your magnet attracts the induced magnetic field so it increases the velocity of the magnet so how could that produce an energy out of nothing?

Energy isn't created and magnets don't attract the magnetic field.

Quote:

Quote by elishamei9000 (Post 4176339)
so basically if there's a bicycle dynamo, the work or energy you the cyclist gave is just the same as the energy that lit up the light? so the energy won't be an issue? please tell me if im correct

Exactly. Any generator uses external energy, typically in the form of mechanical energy, that is input into it to generate the electrical energy. You expend energy to rotate the shaft, which generates electrical current in the circuit. So this is merely energy transfer. Due to inefficiencies you cannot transfer your mechanical energy into electrical energy with 100% efficiency. Some will always be lost in the form of heat and friction.


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