chapter16_lecture - Electric Energy and Capacitance The...

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Electric Energy and Capacitance
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The electrostatic force is a conservative force just like gravitational force One can define a potential energy function associated with this force Work done by a conservative force is equal to the negative of the change in potential energy A charge always gains kinetic energy and loses potential energy if released from rest in an electric field E i f W PE PE PE E W KE 0 : on conservati Energy PE KE
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+q d + + + + + + + + - - - - - - - - E - - - - - - - Example: Releasing a ball of charge q a distance d in a uniform electric field Two large parallel plates with opposite charges produce a uniform electrical field (except near edges) y downward) ( | | : Force E q F E d E q d E q y y F W i f E E | | ) |)( | ( ) ( force electrical by the done Work d E q W PE E E | | ) ( : decreases energy potential Electric d E q W KE E | | ) ( increases energy Kinetic 0 : conserved is Energy PE KE
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If an electron and a proton are released from rest in an electric field, which of the following statements is true (1) The potential energy of both electron and proton decreases (2) Electron gains potential energy, proton loses potential energy (3) Electron loses potential energy, proton gains potential energy
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If a charge is released from rest in an electric field, the potential energy always decreases .
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The Electric Potential Difference A B How much is the potential energy? Depends on how heavy he is The height of the building Can we define a quantity that is the characteristic of the field independent of test objects? The potential difference between points A and B is defined as the change in the potential energy ( final value minus initial value ) of a charge q moved from A to B divided by the size of the charge charge unit per energy Potential : Meaning V or J/C : Unit SI ) ( q PE V V V E A B
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The electric potential difference Potential difference is not the same as potential energy For a positive charge, the higher the potential, the higher the potential energy For a negative charge, the opposite is true. charge) unit per energy (Potential V or J/C : Unit SI ) ( q PE V V V E A B Gravitation field: Always the higher the potential, the higher the potential energy
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Relation between the electric field and the potential difference: Uniform electric field +q d + + + + + + + + - - - - - - - - E - - - - - - - y qEd W PE E E ) ( changes energy Potential Ed q qEd q PE V E ) ( difference Potential Negative sign means electric field points from high potential to low potential (in this case, downward) Another unit for electric field: V/m=N/C d V E or
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Electric potential is characteristic of the field only Independent of any test charge that may be placed in the field Electric potential energy is characteristic of the charge-field system Due to an interaction between the field and the charge placed in the field
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