If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Physics library

### Course: Physics library>Unit 11

Lesson 3: Electric potential energy, electric potential, and voltage

# Electric potential energy

Introduction to electric potential energy. Created by Sal Khan.

## Want to join the conversation?

• At Sal said that if we let go of the particle all of 30J would be converted to kinetic energy.
I get that if left, the particle would move away from the positively charged plate, but its still in the electric field so it should have some potential energy remaining, right?
• From the author:Yes, the 30J is really the increase in potential from it's starting point to the ending point 3 meters closer to the plate. If we let go at the ending point, the particle would start accelerating in the direction of the field (upward), and, in this case, when it passes its starting, all of that increase in potential energy will be converted to kinetic energy.

The particle, however, still has the field acting on it and will still be accelerated upwards (so you could say that it still has a positive potential relative to a point further away from the plate).
• around sal says that to get the charge moving downwards, we have to exert a force of 10N. But if we exert that force in the downward direction, seeing that the metal plate is ALSO exerting a force by the same amount, won't the charge just stay stationary over there (like suspended in the electric field)???
VERY IMPORTANT
• for one moment a force little greater than 10N is applied, so that the body gains a velocity. after that the force can be equal to 10N.(In that case net force will be zero and the charged body will continue to go with the velocity it has gained at first.
• I got a question about electric potential energy, though may not be related to Sal's video. When a positive charge is brought near a positive point charge. The work done will be changed to the electric potential energy and stored in the charge. However, when a negative charge is brought away from the positive charge, the negative charge gains electric potential energy.When r keeps increasing, the electric potential energy stored in the negative charge will be extremely large?? Hope you guys can understand my question....
• I understand now! for opposites charges, work is done to pull them away from each other. it changes into PE and stored inside the charge. But their separation is getting larger as well, the attractive force becomes smaller, then the work needed is smaller too. So the pulling force decreases. It means that the electric PE increases at a decreasing rate!
• At Sal says that a field of 5N/C is quite strong. According to Google a Newton is about 1/5 of a pound. So the field is something like 1 pound/coloumb.

Why is this so strong? I was under the impression that a coloumb was a fairly large amount of charge. 1 pound for a high amount of charge does not seem so strong.
• It's quite strong for an electrical field.
• why does the temperature of a solid conductor increase when the conductor is carrying current?
• Electricity gives off heat. The more electricity the more heat.
• im always confused at this fact .
if an object weighing mg newtons needs to be pulled up,it requires an opposite force EQUAL to mg newton.(should'nt the force be more than the downward force)(because if force to pull up would be equal it would be suspended in air {if applied from rest}).
• good question

OK...so

IF it is pulled up at a constant speed then it does not need extra force
but if it is pulled up accelrating then it does need extra force

(this is by Newtons first law...)
• The potential is constant throughout a given region of space . Is the electrical field zero or non zero
• two ways you might want to think about it...

1) if the potential is constant, what is the potential differnce like? and therefore what must the field be like?

2) what does the graph of potential look like? How is the graph of field related to the graph of potential?

Let us know how you get on
• at ,sal says that the work done is equal to force of gravity into h,but shouldn't the force be a little bit more if we want to lift it upward against the force of gravity?
• Yes. In later in the video he goes on to say that exact same thought. His equation shows 10n/m but the faster you wanted to go the more you would apply. (10n - SLIGHT SPEEED).
• i have 2 questions
1.what is the intuition behind a constant like G(gravitational constant)and K(coulombs constant),where do we get the values of them?
2.when sal said that we will apply a force of 10N DOWNWARDS wouldn't that force be balanced by the upward force since the field is also applying a force of 10N on the charge in the opposite direction that is UPWARDS?
• Constants like G and K simply arise from the fact that we need to relate on type of unit to another. For example, F = G(m1)(m2)/d^2. The G makes newtons out of mass^2/distance^2. K does something similar.
• Are electric fields and magnetic fields related? Do they co-exist and if so, do they do so always?