Lecture%2020B0 - Physics 1B Lecture 20B Equipotentials An...

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Physics 1B Lecture 20B
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Equipotentials An equipotential surface is a surface on which all points are the same potential. Normally, the potential differences between nearby equipotential lines are the same, say 1V, 1kV… It takes no work to move a particle along an equipotential surface or line (assume speed is constant). The electric field at every point on an equipotential surface is perpendicular to the surface. Equipotential surfaces are normally thought of as being imaginary; but they may correspond to real surfaces (like the surface of a conductor).
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Equipotentials Let’s construct an equipotential surface for a lone negative charge. First, draw the field lines for the charge. If I move 1m away would it matter if it was up or down or left or right if I were to calculate potential. So our equipotential surfaces would be spheres. Also, since V goes as (1/r), the spacing would increase between equipotential surfaces. r q k V e =
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Equipotentials For a lone positive charge, the equipotential surfaces are all spheres centered on the charge. We represent these spheres with equipotential lines. Equipotential lines are shown in blue, electric field lines are shown in red. Note that the field lines are perpendicular to the equipotential lines at every crossing. s E V v r Δ = Δ
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Equipotentials As you increase the number of charges in the distribution the equipotential lines get more complicated. Take the electric dipole that we introduced earlier. The equipotential lines bunch up between the two charges. If you ever have trouble drawing equipotential lines, start by making the electric field lines and make the equipotential lines perpendicular at each crossing. Ed V = Δ | / | E V d Δ = /E d V 1 V 1 = Δ
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Parallel Plates If we had a very large plane of positive source charge, what would the electric field look like? The electric field would
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Lecture%2020B0 - Physics 1B Lecture 20B Equipotentials An...

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