# Do all electric motors follow these two laws?

1. Feb 13, 2014

### Dash-IQ

Do all electric motor's follow Faraday's law and Lorentz? Is it safe to say generalize that statement?

2. Feb 13, 2014

### Simon Bridge

Faraday and Lenz laws are general Laws of Nature - so everything has to follow them.
"Lorentz" usually refers to the transformations of special relativity - which is also a general Law of Nature - everything is subject to the transformations though they are usually thought of as applying to space-time rather than the objects in it.

Faraday's Law has to be modified for relativity though.

Further: a machine which exploits Faraday's and Lenz's laws to generate work via electric currents and magnets would pretty much be the definition of an electric motor.

Thus the answer to your question appears to be "yes" - pretty much by definition.
So I suspect that is not what you meant to ask.

What statement?
You have only posed questions.

3. Feb 13, 2014

### Dash-IQ

Way above my level of understanding and my basic question... could you simplify and explain this a bit more if possible? I got interested!

My statement was do all electric motors apply those two law's of nature, and you've already answered it above.

4. Feb 13, 2014

### Simon Bridge

Um OK. The central task in physics, and the whole point of science, is to discover general laws of nature - these are the rules that do not depend on your point of view. That kind of rule will apply to everyone.

There arn't very many physical rules we know that fit that description well - the laws of electromagnetism are amongst those that fit very well.

The full set are described in Maxwell's equations - when you get to those, later, a whole lot suddenly gets simple. Faraday and Lenz laws are part of Maxwell's equations.

The laws apply to pretty much everything you can see and touch - so they are very general.

They are not quite complete though - they have to be modified for the case of very small objects or very high speeds. The modifications are called Quantum Mechanics and Special Relativity. These are really big subjects in their own right...

But I have to go - it's St Valentines down here already :)

5. Feb 14, 2014

### mrspeedybob

I thought the Maxwell equations foreshadowed special relativity by allowing the speed of light to be calculated without specifying what that speed was relative to, thus making them incompatible with non-relativistic physics. What modifications does SR require?

6. Feb 14, 2014

### Simon Bridge

Maxwell's equations did foreshadow relativity in that way. I'm being a bit glib to cope with the education level I have to deliver the answers at. None of the above answers should be considered 100% accurate.

The modification from Maxwel to the 4-vector form and the introduction of the Faraday tensor is beyond the scope of this thread. The interested student may benefit from considering:
http://physics.ucsd.edu/students/courses/fall2009/physics130b/Spec_Rel.pdf
http://rsta.royalsocietypublishing.org/content/366/1871/1849.full
... I suspect that any difference of opinion we may have on this point is philosophical in nature.

7. Feb 16, 2014

### Dash-IQ

Wait, aren't they only applied with systems that have a conductor, moving in a changing magnetic field? Or vice versa?
How are they applied in everything? See here... is my limitation of understanding.

8. Feb 16, 2014

### sophiecentaur

If you want to jump in with both feet, you could look at this Hyperphysics link or, for a bit more description , this wiki link.
Description of magnetic phenomena like the motor effect is only a part of what Maxwell tell us.

9. Feb 16, 2014

### Dash-IQ

@Simon Bridge, the motion produced in all electric motors is due to Lorentz force = IL x B ?

10. Feb 16, 2014

### Dash-IQ

That's the only part I understood from the equations, and that is my problem.

11. Feb 16, 2014

### Simon Bridge

Maxwells equations cover everything that is electromagnetic in nature - this includes light.
Everything solid is made of electrons and protons - moving electric charges.
When you touch something, what you feel as solid is the electric charges in the object repelling the charges in your hand. This is why you cannot walk through a wall even though the wall and your body are "mostly empty space".

Again: define "electric motor".

12. Feb 17, 2014

### Dash-IQ

Um... the system that converts electrical input into mechanical output?
What I meant to say, to find the force of any kind of electrical motor Lorent'z formula of force( F = IL x B ) would always be useful, since I know no other method to find F.

13. Feb 17, 2014

### Dash-IQ

@ "Further: a machine which exploits Faraday's and Lenz's laws to generate work via electric currents and magnets would pretty much be the definition of an electric motor." Forgot this point, never mind then....

14. Feb 18, 2014

### Simon Bridge

Well done.
Lorentz force - more generally given as: $\vec F=q(\vec E+\vec v\times \vec B)$ - is pretty much a core relationship for the force picture.

You can also use an energy/potential picture but it boils down to the same thing.

Using your description: "the system that converts electrical input into mechanical output" allows for other methods though ... i.e. you can use the electricity to run the heat source for (say) a Stirling engine, power for laser-propulsion or the ignition system for a rocket. All those would be systems that turn electrical input to mechanical output that does not rely on the Lorentz force; but I don't think those're what you had in mind.

15. Mar 2, 2014

### Dash-IQ

Yet very interesting. It would be nice to study how work is done for those systems.
And understanding the force related in converting one form of energy to another.

16. Mar 2, 2014

### cabraham

All electric motors also follow Ampere's Law as well. AL is 1 of the 4 Maxwell equations. Let's not forget about AL.

Claude

17. Mar 2, 2014

### Simon Bridge

These are well studied examples.
However, it is unhelpful to study them in terms of forces.
Energy systems are best studied in terms of energy rather than force.

You have to be very careful about the force methods - it is easy to get mislead.

18. Mar 2, 2014

### cabraham

??? But energy involves force. How do you study in terms of one w/o the other?

Claude

19. Mar 2, 2014

### Staff: Mentor

You can have energy in fields without forces.

Last edited: Mar 3, 2014
20. Mar 2, 2014

### Simon Bridge

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.