2 Gauss Law Electric Flux and its applications.ppt

2 Gauss Law Electric Flux and its applications

1. 10/02/25 Lecture III 1
Physics 114
Electric flux,
Gauss’s law
Carl Friedrich Gauss

2. 10/02/25
Lecture III
2
Concepts
•Primary concepts:
–Electric flux

3. 10/02/25
Lecture III
3
Laws
•Gauss’s law

4. 10/02/25
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

5. 10/02/25
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
cosEAAE


6. 10/02/25
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
cosEAAE

Flux is proportional to the number of field lines going through the surface

7. 10/02/25
Lecture III
7
Compare fluxes
Which flux is larger?
1). 

 2). 





1
2

8. 10/02/25
Lecture III
8
Compare fluxes
Which flux is larger?
1). 

 2). 





1
2

9. 10/02/25
Lecture III
9
Compare fluxes
Which flux is larger?
1). 

 2). 





1 2

10. 10/02/25
Lecture III
10
Compare fluxes
Which flux is larger?
1). 

 2). 





1 2

11. 10/02/25
Lecture III
11
Compare fluxes
Which flux is larger?
1). 

 2). 





1 2

12. 10/02/25
Lecture III
12
Compare fluxes
Which flux is larger?
1). 

 2). 





1
2

13. 10/02/25
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.

14. 10/02/25
Lecture III
14
Why do we need flux?
•Gauss’s law:

15. 10/02/25
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

16. 10/02/25
Lecture III
16
Sinks and sources
•What goes in – goes out
•unless
•there is a source (positive charge)
•or a sink (negative charge)
+
-

17. 10/02/25
Lecture III
17
What’s the flux?






18. 10/02/25
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 
•Infinite plane of charge. Charge per unit of area 

19. 10/02/25
Lecture III
19
Field near conductor
•Infinite plane of charge. Charge per unit of area 
•Field inside conductor
is zero, outside perpendicular
to the surface.

20. 10/02/25
Lecture III
20
Two parallel plates
•Infinite plates
•One positive, one negative,
•Same charge density 
02



E

+ -
0
2



E

0



EEE
