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In addition to the revision notes for Electric Potential Energy on this page, you can also access the following Electrostatics learning resources for Electric Potential Energy
| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 14.4 | Electric Potential Energy |
In these revision notes for Electric Potential Energy, we cover the following key points:
When a positive point charge +Q is placed inside a uniform electric field produced by two oppositely charged parallel plates of distance d from each other, the charge possesses its maximum value of potential energy when it is forced to stay near the positive plate. This is because the positive charge cannot stay naturally at that position as it is repelled by the left (positive) plate and attracted by the right (negative) plate. Hence, when the charge is released, it moves towards the negative plate. During this process, it loses potential energy and gains kinetic energy. The electric force does positive work W on the charge because the electric force F acts in the direction of electric field E, i.e.
This means the work done by the electric force to send the positive charge Q from the positive to the negative plate, contributres in the decrease of the potential energy of the charge. Thus, we have
where EPE stands for the electric potential energy.
Given that F = Q × E, we have
Therefore, the general formula for the electric potential energy is
where d is the distance of the charge Q from a point chosen as a reference (of zero potential), usually the negative plate.
The unit of electric potential energy is Joule [J], as all the other types of energy.
The formula EPE = Q × E × d is true only in some specific cases, when the electric field is uniform (for example between two oppositely charged parallel plates). For point charges, we use another formula to calculate the electric potential energy:
Electric potential energy can be defined in terms of work done by the electric forces. For example, the electric potential energy of a system composed by two like point charges is equal to the work done by the electric force to move one of the charges from the distance r to infinity.
On the other hand, when the charges are of opposite sign, the electric potential energy is equal to the negative value of the work done by the electric force to move the charge - Q from the distance r to infinity. This is because at infinity the potential energy is zero, as the value of any number divided by infinity is zero.
We can use the same approach even when there are more than two charges, especially when they are not collinear.
The potential energy of any charge at a given point is the algebraic sum of all individual potential energies due to each charge. No direction is involved, as potential energy is scalar.
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