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4. Electric potential

+ + + + + + + a. - - - - - - - b. E. 4. Electric potential. 4.1 Electric (electrostatic) potential energy. Compare electric force and gravitational force. Conservative forces. We can introduce potential energy:. Example:. 4.2 Electric potential and potential difference.

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4. Electric potential

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  1. +++++ + + a ----- - - b E 4. Electric potential 4.1 Electric (electrostatic) potential energy Compare electric force and gravitational force Conservative forces We can introduce potential energy: Example:

  2. 4.2 Electric potential and potential difference Definitions: potential – energy electric field – force Units (Volt): Example:

  3. 4.3 Electric potential and electric field Units: Example: +++++ ----- E

  4. 4.4 Potential due to a group of point charges a) One charge Usually we assume that b) Several charges (superposition)

  5. 4.5 Equipotential surfaces Definition:V = const Properties: • W = 0 for any motion along any equipotential surface • The electric field, E is always perpendicular to equipotential surfaces • The electric field, E points in the direction of decreasing potential • The surface of a conductor is always equipotential • All points of a conductor have the same potential

  6. 10 V V 20 V E 30 V y E x Examples: Topographic map of Mt. Fuji: 2D mapping of potential. Positive point charge.

  7. Example:What is the electric energy stored in a system of three charges q = 3.0 nC that form an equilateral triangle of side a = 1.0 cm? The stored energy is: A) Positive B) Negative C) Zero q Explanation: This question can be reformulated: • How much energy has been put in the system to built it? • How much work was done to built it? We have to push the charges to arrange them like this → Add energy Unless charges are somehow fixed, they will move to a situation with less energy a q q Solution:External work done to bring a charge from infinity: For the first charge: For the second charge: For the therd charge:

  8. Example:Three charges q = 3.0 nC are initially fixed at the corners of an equilateral triangle of side a = 1.0 cm. One of them is released. Find its kinetic energy when it has doubled the distance to each of the other two charges. q q 2a a q q q q

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