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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
Causes and effects of E
Charges attract or repel: F=kqQ/r2
Charges can create electric fields: F=qE so E=
Field due to a point charge Q: Epoint charge =
Lorentz force:
E fields can exert a
force on charges:
F=qE=ma
Fields, gravitational & electrostatic
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
Flux  field * area 
Practice Ch.23 Q1
 E  dA
Gauss’ Law
The electric flux through a surface is proportional
to the charge enclosed.


q
 E  dA  
Practice: Ch.23 Q 5, 10; P# 4, 9, 17
0
Charged plates
Charged plates
Ch.24: Electric potential
Potential energy U= F*d can be due to mass or charge
Electrostatic potential energy of two charges
UE=kqQ/r
Potential = Energy/unit mass or charge
Electrostatic potential due to point charge
VE=U/q
Work done in moving a charge q through a potential V
is W=qV: lower potential energy = preferred state
Practice: Ch.24 Q2, P#1, 2
Equipotential surfaces and E fields
Equipotential = constant voltage
Conductors are generally equipotentials
Potential difference  Electric field
dV/dx = -E or, equivalently, V    E  dr
Practice: Ch.24 Q5,8 (p.646), P#3, 4, 6, 35
Ch.24 #4
Ch.24 #6
Ch.24 #35
Ch.2: Electrostatics (d/dt=0):
charges  fields  forces, energy
 E.dA = q/0=, E = F/q
V (r ) 
1
 (r ' )
4  
 
d '    E  dl

E   V
F=qE=ma
W = qV, C = q/V
• 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)
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