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## Physics library

### Course: Physics library>Unit 11

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

# Electric potential energy

Explore the concept of electrical potential energy and its similarities to gravitational potential energy. Understand how work is required to move an object within a gravitational or electric field, and how this work translates into potential energy. Discover how potential energy changes can impact kinetic energy and velocity. 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. • why does the temperature of a solid conductor increase when the conductor is carrying current? • 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}). • The potential is constant throughout a given region of space . Is the electrical field zero or non zero • 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? • 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? • Are electric fields and magnetic fields related? Do they co-exist and if so, do they do so always? • They are just two different aspects of the same thing. That thing is called the electromagnetic interaction/force and is one of the 4 fundamental interactions of nature.
The reason, why the interaction is talked about as the combination of two different things is mostly historical as people didn't recognize the two things as one until Maxwell. (Could be wrong on the history :P)

The electric field is the irrotational (curl-free/gradient-field) aspect of the phenomenon. It's source is the scalar potential Φ (given by the distribution of electric charge). The magnetic field on the other hand is the solenoidal (source-free) part of the field. It is caused by some vector potential V.

Concerning the second part of your question: In case of a static potential Φ, the B aspect is zero. So you could say, they don't always coexist. On the other hand, "they" are in some sense one and the same thing.

-> http://en.wikipedia.org/wiki/Helmholtz_decomposition