General Physics 2
Gauss’s Law
Chapter 2
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Marvin T. Tejano
Charge & Electric Flux
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→ In Order to determine the
content of the box, we only need
to measure 𝐸 in the surface.
Charge & Electric Flux
Electric Flux
→ Amount of electric field that
passes (“flows”) through the
surface.
→ The word “flux” comes from the
latin word means “flow”.
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Charge & Electric Flux
➢The direction of the flux depends on the sign of the charge enclosed
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➢ Charges outside the surface do not give flux through the surface
➢ The net electric flux is proportional to the amount of charge enclosed within the
surface but independent of the size of the surface
Calculating Electric Flux
-> fluid flow analogy
dV  
=vA
dt
 
E = E  A
 E = EA cos 
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 
 E =  E  dA
electric flux for
uniform E
magnitude
Non-uniform
electric field
Calculating Electric Flux
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-> the vector A is directed perpendicular to the surface (always)

A = Anˆ
-> the direction of the area vector must be specified if the surface is not closed
Calculating Electric Flux
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Calculating Electric Flux
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Calculating Electric Flux
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Calculating Electric Flux
F =k
q1q2
r
2
𝑘=
1
,
4𝜋ϵ0
𝑘 = 9.0 x 109 N•m2 / C2
ϵ0 – permittivity of free space
ϵ0 =
1
4π𝑘
= 8.85 x 10-12 C2 /N•m2
Charge on one electron: e = - 1.602 x 10-19 C
Charge on a proton: +e
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Quiz (1/2 paper)
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Quiz (1/2 paper)
A point charge 𝑞 = +5.0 𝑛𝐶 is
surrounded by an imaginary
sphere of radius 𝑟 = 0.50 𝑚
centered on the charge (Fig.
22.9). Find the resulting
electric flux through the
sphere.
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