Register to reply

Do all electric motors follow these two laws?

by Dash-IQ
Tags: electric, laws, motors
Share this thread:
DaleSpam
#19
Mar2-14, 09:38 PM
Mentor
P: 17,301
Quote Quote by cabraham View Post
energy involves force. How do you study in terms of one w/o the other?
You can have energy in fields without forces.
Simon Bridge
#20
Mar2-14, 09:47 PM
Homework
Sci Advisor
HW Helper
Thanks
Simon Bridge's Avatar
P: 12,873
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.
cabraham
#21
Mar2-14, 09:55 PM
P: 1,041
Quote Quote by DaleSpam View Post
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
Simon Bridge
#22
Mar2-14, 10:40 PM
Homework
Sci Advisor
HW Helper
Thanks
Simon Bridge's Avatar
P: 12,873
Quote Quote by cabraham View Post
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.
DaleSpam
#23
Mar3-14, 08:56 AM
Mentor
P: 17,301
Quote Quote by cabraham View Post
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.
Dash-IQ
#24
Mar4-14, 03:03 AM
P: 107
This maybe a bit off topic but, I struggle to define energy without relating it to force.

So by stating this:
Quote Quote by DaleSpam View Post
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?
Simon Bridge
#25
Mar5-14, 01:30 AM
Homework
Sci Advisor
HW Helper
Thanks
Simon Bridge's Avatar
P: 12,873
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.
Simon Bridge
#26
Mar5-14, 01:31 AM
Homework
Sci Advisor
HW Helper
Thanks
Simon Bridge's Avatar
P: 12,873
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.
DaleSpam
#27
Mar5-14, 07:42 AM
Mentor
P: 17,301
Quote Quote by Dash-IQ View Post
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.


Register to reply

Related Discussions
Does robotics follow the laws of physics? General Physics 3
Electric motors General Engineering 9
Just curious how electric motors work, specifically DC motors. General Physics 17
Why do both Coulomb and Gravity follow 1/r^2 laws? Classical Physics 8
Electric Motors Electrical Engineering 3