Winter wk 2

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Winter wk 2 Tues.11.Jan.05

• Electrostatics and gravity
• Physics Ch.23 Gauss Law
• Physics Ch.24 Electric potential

Energy Systems, EJZ
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Causes and effects of E

• Charges attract or repel FkqQ/r2
• Charges can create electric fields FqE so E
• Field due to a point charge Q Epoint charge
• Lorentz force
• E fields can exert a
• force on charges
• FqEma

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Fields, gravitational electrostatic
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E field due to charge distributions

• Superposition add the E fields to to each
  charge
• For more complex charge distributions, find the
  Electric FLUX through a
• surface enclosing the charges
• Gauss E fields diverge from charges
• Practice Ch.23 Q1

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Gauss Law

• The electric flux through a surface is
  proportional to the charge enclosed.
• Practice Ch.23 Q 5, 10 P 4, 9, 17

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Charged plates
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Charged plates
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Ch.24 Electric potential
Potential energy U Fd can be due to mass or
charge Electrostatic potential energy of two
charges UEkqQ/r Potential Energy/unit mass
or charge Electrostatic potential due to point
charge VEU/q Work done in moving a charge q
through a potential V is WqV lower potential
energy preferred state Practice Ch.24 Q2,
P1, 2
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Equipotential surfaces and E fields
Equipotential constant voltage Conductors are
generally equipotentials Potential difference ?
Electric field dV/dx -E or, equivalently,
Practice Ch.24 Q5,8 (p.646), P3, 4, 6, 35
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Ch.24 4
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Ch.24 6
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Ch.24 35
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Ch.2 Electrostatics (d/dt0) charges ?
fields ? forces, energy

• Charges make E fields and forces
• charges make scalar potential differences dV
• E can be found from V
• Electric forces move charges
• Electric fields store energy (capacitance)

F q E m a
W qV, C q/V
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Why use electrostatic potential V?
Easy to measure V, physically with a
voltmeter Easy to find E from V,
mathematically Scalar V superpose more easily
than vector E