# Perpetual Motion

#### danago

Gold Member
Hi. In class we are currently studying electromagnetism. The unit we are currently doing is on generators and motors using faradays law of induced emf.

If a generator is set up, such that its output current is directed to a motor, which creates rotation, which in turn is used to turn the generator coil, will the system continue to operate forever, assuming resistance free wires.

Ofcourse, the generator will have to be given an initial 'push' to get the current flowing.

Now, im sure there must be some flaw in what ive said, or some physical problem with the setup, since as far as i am aware, perpetual motion is impossible.

Ive just posted this really for the sake of curiosity, so if anybody is able to comment on what ive said, id be greatly appreciative :)

Thanks,
Dan.

Related Other Physics Topics News on Phys.org

#### Danger

Gold Member
To start with, there is no such thing as 'resistance-free' wires, unless you're going into superconductivity. If you can achieve that at room temperature, then you'll be so rich that nothing else matters.
More to the point, though, are things such as friction and air resistance.
No matter how you look at it, it ain't gonna work.

#### danago

Gold Member
Ok, thats fair enough. But im just asking this as a theoretical question; its not like im actually going to go and try it.

Lets say that it was possible to attain resistance free wires, and air resistance and friction etc. were negligiable. Would it work then?

#### olgranpappy

Homework Helper
You wouldn't get anything out of it. All the energy would have to be put back in to keep the thing running. Think about a frictionless ballbearing rolling back and forth in a dish. It can never get any higher than its original starting point.

The above are statements about ideal systems with no friction and may be summarized as: "You can never do any better than breaking even."

For the case of real systems (which you don't care about) there is an even worse rule which says: "You can't even break even!"

#### NoTime

Homework Helper
You might consider that if you eliminate all air friction and mechanical friction then you don't need the wires.
The rule is that an object in motion will stay in motion until an external force acts on it.

#### Danger

Gold Member
I gotta admit, No Time, that I never even thought of it that way. Great point!
Danago, let's review the 4 laws of thermodynamics for future reference.
1) No matter how hard you try, the best that you can do is break even.
2) You can only break even at 'absolute zero'.
3) 'Absolute zero' is impossible to attain.
4) No matter how much you shake it, the last drop always goes down your pants.
As Olgranpappy pointed out, the absolute maximum energy that you can attain under ideal conditions would be just enough to keep the system in motion. No useful work could be extracted from it. That applies to any perpetual motion device, regardless of what mechanism is involved.

#### russ_watters

Mentor
Well anyway, yes, if you have a perfectly efficient motor and generator and no other losses of any kind, your system will not lose any energy and will run forever according to Newton's first or the law of conservation of energy...

...but that isn't a terribly useful thought experiment.

#### ice109

HTS wire will carry a current forever? that's real...

#### Danger

Gold Member
'High Temperature' and 'Room Temperature' are worlds apart. I'm not sure what the cut-off point is, but I think that it's something like 50°K that is considered the 'HT' boundary.

#### ice109

'High Temperature' and 'Room Temperature' are worlds apart. I'm not sure what the cut-off point is, but I think that it's something like 50°K that is considered the 'HT' boundary.
thats not the point, its a real system that breaks even and btw the record for highest temperature while still SC is 135K

#### Danger

Gold Member
thats not the point
True. The actual point is that you can't extract anything useful from a PM device. How long would the current in your SC loop keep going if there was a light bulb in the circuit?
As to the temperature, I don't even try to keep track of what the record is. I was referring to the bottom limit of what is considered 'high temperature', and I really don't know what that is.

#### russ_watters

Mentor
thats not the point, its a real system that breaks even....
So is an orbit. The term "perpetual motion" isn't exactly the best term to describe the issue. People looking for perpetual motion aren't actually looking for an efficiency of 1, they are looking for an efficiency > 1.

#### Danger

Gold Member
Yeah. I never heard the term 'over unity' until I joined PF, but I like it.

#### hover

I gotta admit, No Time, that I never even thought of it that way. Great point!
Danago, let's review the 4 laws of thermodynamics for future reference.
1) No matter how hard you try, the best that you can do is break even.
2) You can only break even at 'absolute zero'.
3) 'Absolute zero' is impossible to attain.
4) No matter how much you shake it, the last drop always goes down your pants.
Hey Danger, I've heard all of those except the 4th one. I don't get what it is saying. What does it mean?

#### olgranpappy

Homework Helper
urea-related humor

Hey Danger, I've heard all of those except the 4th one. I don't get what it is saying. What does it mean?
I can't tell from your name--are you a guy or a girl? If the latter then it might help for you to think of this one from a guy's perspective.

#### hover

I can't tell from your name--are you a guy or a girl? If the latter then it might help for you to think of this one from a guy's perspective.
Is this suppose to be disgusting? If it is then don't tell me.

#### Danger

Gold Member
Thanks for the bail-out, Pappy.

#### Danger

Gold Member
Is this suppose to be disgusting?
Disgusting only because it's true. Why do you think that guys prefer dark trousers for formal wear?

#### hover

Never mind then. Forget i even asked.

#### olgranpappy

Homework Helper
I forget

Never mind then. Forget i even asked.
Consider it forgotten--consider what forgotten?

... Exactly.

#### hover

Consider it forgotten--consider what forgotten?

... Exactly.
:rofl::rofl:

Gold Member

#### hover

Yup... he must be a girl...
I just don't care to hear what it ment ok? Can you please leave it at that?

#### Danger

Gold Member
No worries; I left it on the last page.

#### NoTime

Homework Helper
I gotta admit, No Time, that I never even thought of it that way. Great point!
I got you on one? :surprised
Usually, it seems the other way around.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving