1. Electric flux, Gauss’s law
Carl Friedrich Gauss
Physics 114
11/14/2018
Lecture III

2. Concepts
Primary concepts:
Electric flux
11/14/2018
Lecture III

3. Laws
Gauss’s law
11/14/2018
Lecture III

4. Definition of electric flux
Flux means flow
Nothing is actually flowing
Other than that thinking of flow really helps to understand the flux
11/14/2018
Lecture III

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
11/14/2018
Lecture III

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
11/14/2018
Lecture III

7. Compare fluxes Which flux is larger?2 1
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
11/14/2018
Lecture III

8. Compare fluxes Which flux is larger?2 1
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
11/14/2018
Lecture III

9. Compare fluxes Which flux is larger?1 2
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
11/14/2018
Lecture III

10. Compare fluxes Which flux is larger?1 2
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
11/14/2018
Lecture III

11. Compare fluxes Which flux is larger?1 2
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
11/14/2018
Lecture III

12. Compare fluxes Which flux is larger?2 1
Which flux is larger?
1). F1>F2 2). F1<F2 3). F1=F2
11/14/2018
Lecture III

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.
11/14/2018
Lecture III

14. Why do we need flux?
Gauss’s law:
11/14/2018
Lecture III

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
11/14/2018
Lecture III

16. Sinks and sources What goes in – goes out unless
there is a source (positive charge)
or a sink (negative charge) + -
11/14/2018
Lecture III

17. What’s the flux?
F1=? F2=?
11/14/2018
Lecture III

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
11/14/2018
Lecture III

19. Field near conductor
Infinite plane of charge. Charge per unit of area s
Field inside conductor
is zero, outside perpendicular
to the surface.
11/14/2018
Lecture III

20. Two parallel plates + - Infinite plates One positive, one negative,
Same charge density s + -
11/14/2018
Lecture III