Physics - Potential energy

[tima]

The Earth has a negative charge, so taking an electron away from it is easy. We are transporting an electron from the Earth's surface to the positive charge on the ionosphere. Thus, the change in potential energy is negative. In doing so it will lose potential energy and gain kinetic energy.

How does work = negative Potential energy?

How am I doing positive work if an electron is automatically attracted to the positive clouds.


W = - qV or PE = qv.

PE = - Work

Thanks.

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[loveoforganic]

Potential energy due to electrostatic forces. You've gone over that in physics, I assume?

 

[sv3]

The Earth has a negative charge, so taking an electron away from it is easy. We are transporting an electron from the Earth's surface to the positive charge on the ionosphere. Thus, the change in potential energy is negative. In doing so it will lose potential energy and gain kinetic energy.

How does work = negative Potential energy?

How am I doing positive work if an electron is automatically attracted to the positive clouds.


W = - qV or PE = qv.

PE = - Work

Thanks.

I'll take a stab: Your not doing any work, the force of the system is. Work is positive because the system is doing "its job" as the negative charge goes where its supposed to go, to the positive happy land. When things work like they would "naturally", the system does work (gets credit as I see it) as potential energy goes down.

On the other hand, if the negative charge were pushed to the negative earth, where it does not want to go "naturally", there is an increase in PE, and work is negative. So why is work negative? Because this scenario fights the way the system wants to work (which is to seperate like charges). Remember though that work did come from somewhere to make this happen - an outside source acted on the system to make this happen.

not sure if this helped, its just the way it works in my head.

 

[phillips101]

Wow I am utterly confused by the question AND the explanation. But I'll try my best to explain it the way I see it.
Sv3: if the electron is pushed back to the earth, wouldn't that be POSITIVE work, since you have to supply a force because the electron would "naturally" repel the negative Earth?

Here's my stab:
Think about it as gravity. When you're picking a ball up from the ground to displace it x meters above the ground, you gain PE. However, let's say that you drop the ball from a cliff. You are losing PE, and gaining KE, BUT work is still there. Work = force supplied to cause a displacement of x. So in this case, when the ball is dropped, gravity (the force) is doing work to move the ball downwards. So even though delta PE is negative, work is still positive.

So essentially, in the electron case, it is losing PE because electrons are attracted to positive charges, but the electrostatic force (F=kqq/r^2) is still doing work to move the electron the distance r. Thus, work is positive, but delta PE is negative.
That's how I see it intuitively.

 

[sv3]

Wow I am utterly confused by the question AND the explanation. But I'll try my best to explain it the way I see it.
Sv3: if the electron is pushed back to the earth, wouldn't that be POSITIVE work, since you have to supply a force because the electron would "naturally" repel the negative Earth?

Here's my stab:
Think about it as gravity. When you're picking a ball up from the ground to displace it x meters above the ground, you gain PE. However, let's say that you drop the ball from a cliff. You are losing PE, and gaining KE, BUT work is still there. Work = force supplied to cause a displacement of x. So in this case, when the ball is dropped, gravity (the force) is doing work to move the ball downwards. So even though delta PE is negative, work is still positive.

So essentially, in the electron case, it is losing PE because electrons are attracted to positive charges, but the electrostatic force (F=kqq/r^2) is still doing work to move the electron the distance r. Thus, work is positive, but delta PE is negative.
That's how I see it intuitively.

i don't see anything wrong with your gravity explanation which is what also happens when a negative charge moves away from a positive. However, the statement in your response i bolded is inaccurate i believe.

The work the system does is negative (the PE would be positive, and you know they have to be opposite). The work done on the system is positve. Think that's what i said im my explanation.

otherwise i don't see what's confusing. you practically said the same thing i did, in your own words.

 

[phillips101]

Well it's a good thing that we practically gave the same explanation, right? It's just that things come intuitively in a different way to different people. It's not whether we have the same explanation or not, it's the one that the OP finds easier to picture that will help him/her the most.

 

[sv3]

Well it's a good thing that we practically gave the same explanation, right? It's just that things come intuitively in a different way to different people. It's not whether we have the same explanation or not, it's the one that the OP finds easier to picture that will help him/her the most.

Yup. I like the gravity one - makes sense. Our difference was mainly how we looked at what did the work. I think regardless if you can remember work and PE will have opposite signs, its helpful.

cheers

(and yes im relieved it was the same explanation - i finished physics a month ago and hopefully my review in 2 weeks won't be soo painful!)

 

[tima]

Thanks for the responses.

If the charges are the same sign, then the PE increases as the charges approach each other. Because an external force must do positive work in forcing the charges closer together against their repulsion.

When the positive charges are left alone, they will try to move to regions of lower PE. Now, the repulsive electrical force now does + work by moving the charges further apart, thus a decrease in PE. The electrical field is point outward.

"When an outside non-electric force is exerted on a charge in an electric field, then the positive (negative) work done by that force in moving the charge from 1 location to another will equal the increase (decrease) in the electric potential energy of the charge."

W = -PE only applies in the direction of the electric field. I bolded the terms.


This is verbatim of what you probably said.

 

[matth87]

Work = Force x Displacement

Change In PE = -W
If you look at the equation you can see that if you orient your system the Force and the Displacement will be in opposite directions. If force and displacement are in opposite directions in the case with increaseing PE then you will see DeltaPE = -W = -( -F) x displacement, so you end up increasing PE. Thats why it has to = -W.