Electric Potential Energy
3.1 Electric Potential Energy
Learning Objectives
By the end of this section, you will be able to:
- Define the work done by an electric force
- Define electric potential energy
- Apply work and potential energy in systems with electric charges
Electric Potential Energy
Practice!
| Practice 3.1.1 |
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Two test charges are brought separately into the vicinity of a charge +Q. First, test charge +q is brought to a point a distance r from +Q. Then this charge is removed and test charge -q is brought to the same point. The electrostatic potential energy of which test charge is greater? |
| Practice 3.1.2 |
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 Which system of charges has more potential energy? |
| Practice 3.1.3 |
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 Which system of charges has more potential energy? |
| Practice 3.1.4 |
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 A proton and an electron are in a constant electric field created by oppositely charged plates. You release the proton from the positive side and the electron from the negative side. Which feels the larger electric force? |
| Practice 3.1.5 |
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A proton and an electron are in a constant electric field created by oppositely charged plates. You release the proton from the positive side and the electron from the negative side. Which has the larger acceleration? |
| Practice 3.1.6 |
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A proton and an electron are in a constant electric field created by oppositely charged plates. You release the proton from the positive side and the electron from the negative side. When it strikes the opposite plate, which one has more KE? |
Discuss!
Reflect on these questions and take notes on how you would answer them. Then we will share these thoughts together in a class discussion.
At the instant shown, a positively charged particle is moving at 5 m/s in the direction of a uniform electric field. Someone makes the following statement: “As the particle continues to move in the direction of the electric field, the particle will gain potential energy since it is moving in the direction of increasing electric potential energy.”
What, if anything, is wrong with the above statement? If something is wrong, explain the error and how to correct it. If the statement is valid, explain why.
Is the electrical potential energy of two point charges positive or negative if the charges are of the same sign? Opposite signs? How does this relate to the work necessary to bring the charges into proximity from infinity?
A proton (mass m, charge +e) and an alpha particle (mass 4m, charge +2e) approach one another with the same initial speed v from an initially large distance. How close will these two particles get to one another before turning around?
(Your final answer will be in terms of k, e, m, and v.)
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