1. Electric flux, Gauss’s law
Carl Friedrich Gauss
Physics 122
11/14/2018
Lecture IV

2. Concepts
Primary concepts:
Electric flux
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Lecture IV

3. Laws
Gauss’s law
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4. Definition of electric flux
Flux means flow
Nothing is actually flowing
Other than that thinking of flow really helps to understand the flux
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Lecture IV

5. Electric flux
If electric field is rain – electric flux is the amount of water in a bucket accumulated per unit of time:
Only component of the field perpendicular to the area A contributes to the flux
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Lecture IV

6. Electric flux
Alternatively we can define a vector A, which equals to the area A and is directed perpendicularly to the area
Flux is proportional to the number of field lines going through the surface
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Lecture IV

7. Compare fluxes Which flux is larger?2 1
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
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Lecture IV

8. Compare fluxes Which flux is larger?2 1
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
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Lecture IV

9. Compare fluxes Which flux is larger?1 2
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
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Lecture IV

10. Compare fluxes Which flux is larger?1 2
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
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Lecture IV

11. Compare fluxes Which flux is larger?1 2
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
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Lecture IV

12. Compare fluxes Which flux is larger?2 1
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
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Lecture IV

13. Non-uniform field, irregular surface
Closed surface
Influx <0
Outflux>0
Don’t worry, we’ll only deal with simple surfaces
Complex surfaces can be handled using numeric
integration by a computer.
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Lecture IV

14. Why do we need flux?
Gauss’s law:
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15. Gauss Coulomb Calculate E of point like (+) charge Q
Consider sphere radius r centered at the charge
Spherical symmetry: E is the same everywhere on the sphere, perpendicular to the sphere
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16. Sinks and sources What goes in – goes out unless
there is a source (positive charge)
or a sink (negative charge) + -
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17. What’s the flux?
F1=? F2=?
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18. Respect the symmetry
Find electric field for the following configurations:
Uniformly charged sphere. Charge Q, radius R.
Long uniform line of charge. Charge per unit length l.
Infinite plane of charge. Charge per unit of area s
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19. Field near conductor
Infinite plane of charge. Charge per unit of area s
Field inside conductor
is zero, outside perpendicular
to the surface.
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Lecture IV

20. Respect the symmetry
Find electric field for the following configurations:
Uniformly charged sphere. Charge Q, radius R.
Long uniform line of charge. Charge per unit length l.
Infinite plane of charge. Charge per unit of area s
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21. Two parallel plates + - Infinite plates One positive, one negative,
Same charge density s + -
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Lecture IV