Is uniform on a conducting sphere physics

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Unformatted text preview: charges Compute electric field and potential from charge distributions Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 18   Work is done by forces moving things.   When a conservative force from the field is applied, all of this work is converted from potential energy.   Likewise, when work is done by some external force, that work is converted INTO potential energy. b W = ∫ a F ⋅ ds b ΔU = − ∫ Physics 2B - Summer Session 2 - C. Palmer a August 12, 2013 F ⋅ ds 19   Potential energy is in general a measure of the ability to do work.   Often potential energy is converted to kinetic energy in our problems.   Energy is a scalar. It has no spatial direction   If the force is conservative, then work done on an object will convert fully to potential energy.   Gravity is conservative   Electromagnetic forces are conservative Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 20 UGrav, Point m1m2 = −G r   If I lift a boulder over my head, I have done work to increase the potential energy of the boulder.   My work AGAINST the force of gravity was converted to gravitational potential energy.   Gravity is a conservative force.   The boulder moves in the OPPOSITE direction of gravitational force.   When I release the rock, the potential energy is converted to kinetic energy as the force of gravity pulls the boulder and the earth together.   The boulder moves in the direction of the force. Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 21   Think about if the U Elec, Point charge moves with or against the force. ΔU Elec   When the charge moves against the force, work is being done by something external.   And that work is converted to potential energy. q1q2 = kE r = U Elec,Final − U Elec,Initial q1q2 q1q2 = kE − kE r12,Final r12,Initial ⎛1 1⎞ = kE q1q2 ⎜ − ⎟ ⎝ r12,Final r12,Initial ⎠ Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 22 ΔU Elec = U Elec,Final − U Elec,Initial +y +100Q B q1q2 q1q2 = kE − kE r12,Final r12,Initial A +Q +Q +x If I move a point charge, +Q, from A to B, which of the following is true? A)  The difference in potential energy is positive. B)  The difference in potential energy is negative. C)  The difference in potential energy would be the same regardless of the sign of the test charge. D)  A) and C) are true E)  B) and C) are true Physics 2B - Summer Session 2 - C. Palmer August 12, 2013 23 ΔU Elec = U Elec,Final − U Elec,Initial +y +100Q B q1q2 q1q2 = kE − kE r12,Final r12,Initial A +Q +Q +x If I move a point charge, +Q, from A to B, which of the following is true? A)  The difference in potential energy is positive. B)  The difference in potential energy is negative. C)  The difference in potential energy would be the same regardless of the sign of...
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