Do all electric motors follow these two laws?

by Dash-IQ
Tags: electric, laws, motors
Mentor
P: 16,298
 Quote by cabraham energy involves force. How do you study in terms of one w/o the other?
You can have energy in fields without forces.
 Homework Sci Advisor HW Helper Thanks PF Gold P: 10,267 Where you do have forces: the force is the negative gradient of the potential energy. If you know how PE varies in space, you don't need to know the forces. A lot of complicated systems are easier to handle that way - and you can end up, as Dalespam says, with situations where you have energy and no forces: so you cannot use "forces" to analyse them. It's OK to use forces where they make the math simpler - but you should get used to using energy directly, as it's much more useful in the long run.
P: 991
 Quote by DaleSpam You can have energy in fields without forces.
Of course you can but the OP question was specifically about motors. To spin the rotor force/torque is needed. Motors are well described using force, torque, current, and magnetic fields. OP asked about 2 laws and I mentioned a 3rd, Ampere's, as being relevant. Ampere, Faraday, and Lenz pretty much describe motors. Lorentz is also very applicable since it relates to induced currents needed to sustain magnetic field.

Claude
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PF Gold
P: 10,267
 Quote by cabraham Of course you can but the OP question was specifically about motors. To spin the rotor force/torque is needed
Naturally - but OP has digressed a slightly since then - having obtained an answer to the original question. See post #12.

I was just concerned about the emphasis on forces in OPs thinking.
I suggest further discussion about force vs energy descriptions to be continued in another thread.
Mentor
P: 16,298
 Quote by cabraham Of course you can but the OP question was specifically about motors.
Agreed. In the context of motors you have to consider the work performed on matter, which involves forces. Your statement was just a little overly broad in general, but I agree that it is reasonable in this context.
P: 45
This maybe a bit off topic but, I struggle to define energy without relating it to force.

So by stating this:
 Quote by DaleSpam You can have energy in fields without forces.
It makes its more complicated... please do explain.

How can Force = 0, while Energy = x. I'm troubled here... how can a field have energy in general, like when a magnetic field or electric field store energy, while forces are zero?
 Homework Sci Advisor HW Helper Thanks PF Gold P: 10,267 I'm thinking of Gravity in GR and the Fields of QFT. In both cases - the "force" is an emergent phenomenon. In GR the "force" of gravity is a pseudoforce - a product of geometry rather than a Newtonian inertial force. In QFT the appearance of "force" is the result of lots of interactions via gauge bosons. Have a look at: http://www.hep.manchester.ac.uk/u/da...tro-to-QFT.pdf http://math.ucr.edu/home/baez/physic...particles.html But DaleSpam may have other things in mind or a better illustration.
 Homework Sci Advisor HW Helper Thanks PF Gold P: 10,267 I'm thinking of Gravity in GR and the Fields of QFT. In both cases - the "force" is an emergent phenomenon. In GR the "force" of gravity is a pseudoforce - a product of geometry rather than a Newtonian inertial force. In QFT the appearance of "force" is the result of lots of interactions via gauge bosons. Have a look at: http://www.hep.manchester.ac.uk/u/da...tro-to-QFT.pdf http://math.ucr.edu/home/baez/physic...particles.html But DaleSpam may have other things in mind or a better illustration.
Mentor
P: 16,298
 Quote by Dash-IQ How can Force = 0, while Energy = x. I'm troubled here... how can a field have energy in general, like when a magnetic field or electric field store energy, while forces are zero?
Here is a good page on the topic: http://farside.ph.utexas.edu/teachin...es/node89.html

In short, the electric field has an energy density proportional to EČ, and the magnetic field has an energy density proportional to BČ. So, if you consider a closed region of vacuum there is, by definition, no matter in there and so nothing on which to exert a force.

Nevertheless, despite there being no forces, if there is a magnetic or electric field inside the region then there is energy there, as given above. Energy can be transferred to or from the region by increasing or decreasing the fields inside the region.

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