Exploring the Relationship Between Energy, Potential, and Work

[Dale]

but have the potential to give potential energy
so how does electron gets this potential?

Again, not by itself, only as part of a system. Then it is the configuration of the whole system that determines the potential energy.

 

[nouveau_riche]

Again, not by itself, only as part of a system. Then it is the configuration of the whole system that determines the potential energy.

well if that is true then the charge of an electron should be the property of system not some constant value like 1.6x10^-19

 

[Dale]

Why is that? I don't get your logic here. Potential energy is not a property of a particle, but why would that lead you to believe that charge is not a property of a particle?

 

[MrNerd]

I think he means that an electron has an electric field around it. This is what the potential is. Inserting another charge into the field gives the particle potential energy, at the expense of another form. You can't just pop energy into existence(That would make the existence of energy impossible, though. But unless you're doing some uber-crazy mad scientist on meth type of stuff that hasn't been thought of yet, energy will not change. That's not the point though). I don't think work is the ability to use the energy. I believe it means that energy is transferred from one form/place to another, high to low.

Back to the second charge. It's now in the electric field. But it took work to move it there, as given by the equation W = Fd. Since work is a change in energy between two differing sources, it therefore requires a transfer of energy.

Think of it like a scifi forcefield that you can penetrate if you push hard enough. If you don't push, you don't get through, or maybe even pushed back. But if you push hard enough, and perform work, you can get closer to the source.

If you're wondering about the definition of energy, I don't think there is a perfect answer to it. I just consider it "stuff" that is existence. Without energy, there is no existence. So perhaps one could define it as existence.

 

[galtsgulchb]

Defining energy looks like it should be simple but is not. The best way to understand the concept “energy” is to get familiar with it in many different contexts (often by solving homework problems or doing labs). Once you are familiar with the concept of energy you can then appreciate a nice tidy definition of energy. That said, I will take a whack at defining energy.

The first clue to the answer is to realize that energy is not a separate entity; it is property of an entity. (Therefore, you should not imagine little blobs of energy.) In order to avoid the circular definitions pointed out above, you might define energy this way:

energy: a property of entities that makes things happen.

Of course, if you were not already familiar with the concept energy in the real world you would ask, “Wait! Make what happen?” By "happen" I mean to move a thing or change the state of a thing. This is where you need real world experience so you can imagine examples, “Ah, yes, I can move a block and compress a piston.”

In summary, do not worry too much about a tidy definition of “energy” at the beginning; that is putting the cart before the horse. It is better to get familiar with the concept of energy in real world examples in the beginning. Leave the tidy definition to the end.

 

[Dale]

Defining energy looks like it should be simple but is not.

It is simple. In Newtonian mechanics, "Energy is the capacity to do work." Easy, done.

I don't know why people think that energy is so difficult. See:

https://www.physicsforums.com/showthread.php?t=507345
https://www.physicsforums.com/showthread.php?t=498497

Both threads were locked, and both resulted in the ban of one or more of the participants. By going through the existing threads you can probably find the information you need without risking getting banned yourself.

 

[galtsgulchb]

It is simple. In Newtonian mechanics, "Energy is the capacity to do work." Easy, done.

I don't know why people think that energy is so difficult.

I agree this is a good start. The difficulty comes when the new student is told, "Work is the amount of energy transferred to a body."

 

[MrNerd]

I agree this is a good start. The difficulty comes when the new student is told, "Work is the amount of energy transferred to a body."

That doesn't seems too bad. Energy is the ability to do something. Work is the amount of something done.

Think of it like this:

A job is an ability to earn money. A wage/salary is merely a measure of how much one earns.

Change it to:


Energy is the ability to change something. Work is merely a measure of the amount of change.

 

[nouveau_riche]

It is simple. In Newtonian mechanics, "Energy is the capacity to do work." Easy, done.

I don't know why people think that energy is so difficult. See:

https://www.physicsforums.com/showthread.php?t=507345
https://www.physicsforums.com/showthread.php?t=498497

Both threads were locked, and both resulted in the ban of one or more of the participants. By going through the existing threads you can probably find the information you need without risking getting banned yourself.

it is not about an easy or difficult definition,it's about the unclear context

suppose there is wall i am trying to push,it requires 20 N of force to displace it,i push it with 10 N for the first time,according to you there will be no energy in this case(it will be better if you for a moment will put aside the perception of seeing me,u can only perceive the wall)
so at the end you will not take me in existence(considering you cannot see me)

but 2 seconds later you find that the wall is moving(this time i applied the force of 30N)
so you count my existence

 

[MrNerd]

it is not about an easy or difficult definition,it's about the unclear context

suppose there is wall i am trying to push,it requires 20 N of force to displace it,i push it with 10 N for the first time,according to you there will be no energy in this case(it will be better if you for a moment will put aside the perception of seeing me,u can only perceive the wall)
so at the end you will not take me in existence(considering you cannot see me)

but 2 seconds later you find that the wall is moving(this time i applied the force of 30N)
so you count my existence

There should still be energy at the 10 Newton mark. There just won't be any work done.

Work = Force x distance. In order to perform work, you need energy. Admittedly, there is no work done in the example. However, if the wall were to stop exerting the force on you(I mean not move as a result,it will always exert an equal and opposite reactionary force), the wall would then move. The impeding forces are generally from things like friction and being bolted down. So if the impeding force is eliminated, then the wall would move, and thus work would be done, meaning energy has been exchanged.

Energy must be present if a force is to be applied. I don't know about the things like the strong and weak forces, though, but just in general, a force requires energy.

 

[nouveau_riche]

There should still be energy at the 10 Newton mark. There just won't be any work done.

Work = Force x distance. In order to perform work, you need energy. Admittedly, there is no work done in the example. However, if the wall were to stop exerting the force on you(I mean not move as a result,it will always exert an equal and opposite reactionary force), the wall would then move. The impeding forces are generally from things like friction and being bolted down. So if the impeding force is eliminated, then the wall would move, and thus work would be done, meaning energy has been exchanged.

Energy must be present if a force is to be applied. I don't know about the things like the strong and weak forces, though, but just in general, a force requires energy.

i guess you didn't read what i was saying,i told you to forget about the perception of seeing me,how will you say there is energy?

 

[galtsgulchb]

it is not about an easy or difficult definition,it's about the unclear context

I agree. Often, when solving homework problems or understanding what awriter means by 'energy', the context is not well spelled out. Do you think you understand what energy is now or do you still have any unresolved questions?

FYI- I found a wonderful discussion of energy here:
http://www.ftexploring.com/energy/energy.html

 

[russ_watters]

Energy must be present if a force is to be applied.

No!

...

 

[russ_watters]

i guess you didn't read what i was saying,i told you to forget about the perception of seeing me,how will you say there is energy?

What you wrote - and what your point in this thread is- is very unclear.

 

[MrNerd]

i guess you didn't read what i was saying,i told you to forget about the perception of seeing me,how will you say there is energy?

I can ignore see you, but you still exert a force. IF you weren't there at all, there wouldn't be a force, anyway, aside from the obvious like air pressure and gravitational. I suppose since I am only a freshman at college, I'm not an expert to back my ideas up. If a comparison can be made where a compressed gas cylinder is pushed against the wall instead of you, it may be easier to explain.

The compressed gas cylinder is held up against the wall. The cylinder has a piston on the wall side that will enable it to push out when it has higher than outside pressure inside it(kind of like those things under chairs than enable the height to be changed). The piston, because the pressure is higher inside the cylinder, pushes on the wall with 10 N of force. This is the same situation as before, except the compressed gas cylinder has replaced you. The cylinder does indeed have energy(potential energy) stored in the compressed gas. Allowing the piston to pull out is the same as doing work. W = p$\Delta$v, where W is work, p is pressure, and v is volume. The +/- change in the volume matters, too.

If the piston is to push the wall out, it must have energy stored in the compressed gas. Say we compress it to the point it pushes out at 30 N. It will make the wall move, work happens, and energy is transferred. Now say we only compress it to the point it exerts 10 N. The gas doesn't provide enough force to push the piston to make the wall move. BUT it does still have potential energy. I believe this is what you were trying to say it doesn't have.

Forces result when an object with high energy has the ability to transfer it to one with low energy if the conditions are met. Your arm has the high energy in the arm/wall example above. However, it is not high enough to overcome other forces resulting from high/low imbalances. Friction is an electromagnetic force. The electric part repels the atoms if they get too close, unless they're bonding. But because the surface is all uneven, they sometimes have to get closer to other atoms, or go around them entirely by going backwards.This takes energy to overcome. If an insufficient amount of energy is supplied by the object, it can't overcome the friction, and doesn't move.

If we still disagree, I'm going to assume that either one of us is wrong but just can't figure it out, or we're having a communication difficulty. We should probably get a more experienced person to help decide this, then.

 

[MrNerd]

No!

...

Hmmmm... I was sure it was. Is it necessary for general forces, like pushing or pulling physically? I suppose it could be possible in some higher up thing, maybe even one or two of the fundamental forces. I'm thinking electromagnetism would be a good bet, since it repels by existing. But I don't really understand why the charges repel/attract, either. Could you explain which forces don't need energy to operate, please? I haven't reached higher up classes in my college yet, so I wouldn't have done it in class.

 

[BruceW]

If a force is applied to a particle that is perpendicular to the particle's motion, then the particle doesn't gain any kinetic energy.

For example, an electron in a constant magnetic field will go round in a circle, but won't get an increase in energy.

Edit: P.S. I'm talking about the electron as a classical particle here.

 

[Dale]

I agree this is a good start. The difficulty comes when the new student is told, "Work is the amount of energy transferred to a body."

I wouldn't tell a student that. Work is f.d

 

[Dale]

it is not about an easy or difficult definition,it's about the unclear context

suppose there is wall i am trying to push,it requires 20 N of force to displace it,i push it with 10 N for the first time,according to you there will be no energy in this case(it will be better if you for a moment will put aside the perception of seeing me,u can only perceive the wall)
so at the end you will not take me in existence(considering you cannot see me)

but 2 seconds later you find that the wall is moving(this time i applied the force of 30N)
so you count my existence

That is a weird example, but sure. If you exert 10 N over a distance of 0 m then the work is 0 J, and if you exert 30 N over a distance of x m then the work is 30x J. What in any of that is at all unclear?

 

[nouveau_riche]

No!

...

i knew someone will pick this line
okay give me an example justifying your "no"

 

[nouveau_riche]

That is a weird example, but sure. If you exert 10 N over a distance of 0 m then the work is 0 J, and if you exert 30 N over a distance of x m then the work is 30x J. What in any of that is at all unclear?

i don't know why you are finding this unclear
it is simple in every aspect
in the example i gave you ,just give the way by which you can count my presence considering the fact that you can only do the same by seeing the motion in wall?

 

[nouveau_riche]

What you wrote - and what your point in this thread is- is very unclear.

it's just that you are finding it hard to connect it

 

[russ_watters]

Hmmmm... I was sure it was. Is it necessary for general forces, like pushing or pulling physically?

Could you explain which forces don't need energy to operate, please?

If there is no motion, there is no energy requirement, as per w=fd. If d=0, w=0, regardless of what f is.

 

[Dale]

give the way by which you can count my presence considering the fact that you can only do the same by seeing the motion in wall?

Obviously we cannot "count" your presence if we are only allowed to infer your presence due to motion of the wall and you do not move the wall. So what? That is entirely due to the restriction that we are only allowed to infer your presence from the motion of the wall, and has nothing directly to do with energy.

Why do you consider that at all relevant or important to a discussion on energy?

 

[russ_watters]

it's just that you are finding it hard to connect it

You should be self-aware enough to accept that when multiple people have trouble with your use of a pretty basic concept, it's probably not everyone else who has the problem, but you.

 

[MrNerd]

If there is no motion, there is no energy requirement, as per w=fd. If d=0, w=0, regardless of what f is.

Alright then. Just to be clear, though, when you say no energy requirement, do you mean no net difference between energy levels? I'm thinking of the second law of thermodynamics here. Would a high relative energy cause a force on a low relative energy only if there is motion/change in position(using W = Fd, not things like nuclear energy)?

 

[Dale]

The second law of thermo doesn't say anything about force.

 

[MrNerd]

Ok, then. I'll think I'll just wait until the upper classes in college for those kinds of things.

 

[galtsgulchb]

Hmmmm... I was sure it was. ...Could you explain which forces don't need energy to operate, please?

It would be easier to answer a more specific question since I don't quite follow what you mean by a force that needs energy or operate. Let me try to clarify the relationship between force, energy, work, and distance.

Imagine a cylinder filled with gas at pressure 'p' pushing on a piston. This means that there is potential energy stored in the gas so it is capable of doing work. The force applied to the piston is, F=p*A. Now let's consider two cases: [1] the piston does not move and [2] the piston moves.

[1] the piston does not move. If I am holding the piston from the other side so that it can't move then no work is done since W=Fd. This means that none of the potential energy from the gas is transferred to the piston. However, it is worth noting that there is a force applied due to the potential energy in the gas. So in this sense you are correct, the force was due to the energy stored in the gas. However, what physicist are usually interested in is whether energy is transferred (work done) and here it was not so W=0.

[2] the piston moves. This is the more interesting case. If I let go of the piston it now moves some distance and work is done. This means that the energy is transferred from the gas to the piston and the work done is W=Fd.

 

[nouveau_riche]

You should be self-aware enough to accept that when multiple people have trouble with your use of a pretty basic concept, it's probably not everyone else who has the problem, but you.

probability not certainity

 

[nouveau_riche]

Obviously we cannot "count" your presence if we are only allowed to infer your presence due to motion of the wall and you do not move the wall. So what? That is entirely due to the restriction that we are only allowed to infer your presence from the motion of the wall, and has nothing directly to do with energy.

Why do you consider that at all relevant or important to a discussion on energy?

well think more and you will get a change in perception to your "so what"

 

[Dale]

So you don't have any actual logical reason why your scenario is either relevant or important to a discussion on energy.

 

[BruceW]

nouveau - does galtsgulchb's post at the end of the last page answer your question?