# Do all electric motors follow these two laws?

• Dash-IQ
In summary: I don't know what you mean by 'again'.I am assuming we are talking about a basic electric motor that uses electric currents and magnets to generate motion. In that case, yes, the motion is due to the Lorentz force, as you stated. This force is produced when the current-carrying wire in the motor interacts with the magnetic field. The wire experiences a force perpendicular to both the direction of the current and the direction of the magnetic field, causing it to move. This motion can then be transferred to other parts of the motor to produce mechanical work.
Dash-IQ
Do all electric motor's follow Faraday's law and Lorentz? Is it safe to say generalize that statement?

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.

Is it safe to say generalize that statement?
What statement?
You have only posed questions.

1 person
Simon Bridge said:
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.

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

Simon Bridge said:
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.

Simon Bridge said:
What statement?
You have only posed questions.

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

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

Simon Bridge said:
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...

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?

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.

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

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.

Dash-IQ said:
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.

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

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

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

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

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Dash-IQ said:
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.
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".

Dash-IQ said:
@Simon Bridge, the motion produced in all electric motors is due to Lorentz force = IL x B ?
Again: define "electric motor".

Simon Bridge said:
Again: define "electric motor".

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.

@ "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...

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.

1 person
Simon Bridge said:
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.

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.

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

Dash-IQ said:
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.
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.

Simon Bridge said:
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.

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

Claude

cabraham said:
energy involves force. How do you study in terms of one w/o the other?
You can have energy in fields without forces.

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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.

DaleSpam said:
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

cabraham said:
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.

cabraham said:
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.

This maybe a bit off topic but, I struggle to define energy without relating it to force.

So by stating this:
DaleSpam said:
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?

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/dasgupta/teaching/Dasgupta-08-Intro-to-QFT.pdf
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

But DaleSpam may have other things in mind or a better illustration.

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/dasgupta/teaching/Dasgupta-08-Intro-to-QFT.pdf
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

But DaleSpam may have other things in mind or a better illustration.

Dash-IQ said:
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/teaching/em/lectures/node89.html

In short, the electric field has an energy density proportional to E², and the magnetic field has an energy density proportional to B². 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.

## 1. Do all electric motors follow these two laws?

Yes, all electric motors follow the two fundamental laws of electricity: Ohm's law and Faraday's law. Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Faraday's law states that the induced electromotive force (EMF) in a closed loop is directly proportional to the rate of change of the magnetic flux through the loop.

## 2. What is Ohm's law and how does it relate to electric motors?

Ohm's law relates the current, voltage, and resistance in an electrical circuit. It is often used to calculate the amount of current flowing through an electric motor, as well as the amount of resistance in the motor's components.

## 3. How does Faraday's law apply to electric motors?

Faraday's law is essential for understanding the operation of electric motors. It explains how the changing magnetic field created by the motor's electromagnets induces a current in the motor's conductors, which then produces the motor's rotational motion.

## 4. Are there any exceptions to these two laws in electric motors?

While these two laws apply to the majority of electric motors, there can be exceptions in certain cases. For example, some specialized motors, such as brushless DC motors, operate using different principles and may not follow Ohm's law or Faraday's law in the same way as traditional electric motors.

## 5. How do these two laws impact the performance and efficiency of electric motors?

The two laws play a crucial role in the design and operation of electric motors. By understanding and applying these laws, engineers can optimize the performance and efficiency of electric motors, resulting in more efficient and reliable machines.

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