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5. Electric potential

5. Electric potential. 5.1 Electric (electrostatic) potential energy. + + + + + + + a. - - - - - - - b. E. Electric force. Gravitational force. Conservative forces. Example. 5.2 Electric (electrostatic) potential and potential difference. Definitions. Force – electric field.

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5. Electric potential

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  1. 5. Electric potential 5.1 Electric (electrostatic) potential energy +++++ + + a ----- - - b E Electric force Gravitational force Conservative forces Example

  2. 5.2 Electric (electrostatic) potential and potential difference Definitions Force – electric field Energy - potential Units: Example

  3. 5.3 Electric potential and electric field a) Uniform field b) General relations Units: Example +++++ ----- E

  4. 5.4 Potential due to a group of point charges a) One charge Usually we assume that b) Several charges (superposition) c) Example (electric dipole) r+ +Q r r- d -Q

  5. 5.5 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. • As you move away, the equipotentials look more and more like spheres because the conductor looks like a point charge. • The electric field inside a conductor and • in an empty cavity inside a conductor is zero.

  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? • 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? q The stored energy is: A) Positive B) Negative C) Zero a q q 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 (Uinfinity = 0). 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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