6B_Ch_20_GG - Chapter 20 Electric Potential and Capacitance...

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Chapter 20 Electric Potential and Capacitance
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Electric Potential Energy When a point charge, q o , is placed in an electric field, it experiences a force The work done by the electric field is As this work is done by the field, the potential energy of the charge-field system is changed by ± U
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Electric Potential The potential energy per unit charge, U/q o , is the electric potential – The potential is independent of the value of q o – The potential has a value at every point in an electric field • As a charged particle moves in an electric field, it will experience a change in potential We often take the value of the potential to be zero at some convenient point in the field Sometimes called ground
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Potential and Potential Energy • The potential is characteristic of the field only – It is independent of the charge placed in the field (very small charge that does not alter the filed!) – The difference in potential is proportional to the difference in potential energy • Potential energy is characteristic of the charge+ field system – Due to an interaction between the field and a charged particle placed in the field
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Work and Electric Potential The electric potential at an arbitrary point due to source charges equals the work required to bring a test charge from infinity to that point divided by the charge on the test particle – Assumes a charge moves slowly in an electric field without any change in its kinetic energy The work performed on the charge is W = ± U = q ± V
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Units 1 V = 1 J/C – V is a Volt – It takes one Joule of work to move a 1 Coulomb charge through a potential difference of 1 Volt In addition, 1 N/C = 1 V/m - electric field – This indicates we can interpret the electric field as a measure of the rate of change with position of the electric potential One electron-volt is defined as the energy a charge-field system gains or loses when a charge of magnitude e (an electron or a proton) is moved through a potential difference of 1 volt 1 eV = 1.60 x 10 -19 J
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Potential Difference in a Uniform Field The equations for electric potential can be simplified if the electric field is uniform: The negative sign indicates that the electric potential at B is lower than at point A
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Energy and the Direction of Electric Field When the electric field is directed downward, point B is at a lower potential than point A When a positive test charge moves from A to B, the charge-field system loses potential energy
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Equipotentials Point B is at a lower potential than point A Points B and C are at the same potential The name equipotential surface is given to any surface consisting of a continuous distribution of points having the same electric potential
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Charged Particle in a Uniform Field, Example A positive charge is released from rest and moves in the direction of the electric field The change in potential is negative The change in potential energy is negative The force and acceleration are in the direction of the field
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6B_Ch_20_GG - Chapter 20 Electric Potential and Capacitance...

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