Electric Flux
2.1 Electric Flux
Learning Objectives
By the end of this section, you will be able to:
- Define the concept of flux
- Describe electric flux
- Calculate electric flux for a given situation
Electric Flux
The electric flux is defined as
where 
is the electric field and 
is the area vector (
).
If the electric field is constant and uniform everywhere on the surface, then the flux can be expressed as
Practice!
| Practice 2.1.1 |
|---|
 A disk of radius 0.10 m is in a uniform electric field E and the disk’s normal vector is oriented at 30° to the electric field, as shown in the figure. If the disk is rotated counterclockwise such that its normal vector is perpendicular to the electric field, what will be the electric flux through the disk? |
| Practice 2.1.2 |
|---|
A disk of radius 0.10 m is in a uniform electric field E and the disk’s normal vector is oriented at 30° to the electric field, as shown in the figure. If the disk is rotated clockwise such that its normal vector is parallel to the electric field, what will be the electric flux through the disk? |
| Practice 2.1.3 |
|---|
A disk of radius 0.10 m is in a uniform electric field E and the disk’s normal vector is oriented at 30° to the electric field, as shown in the figure. If the disk is rotated counterclockwise such that its normal vector points opposite to the electric field, what will be the electric flux through the disk? |
| Practice 2.1.4 |
|---|
A disk of radius 0.10 m is in a uniform electric field E and the disk’s normal vector is oriented at 30° to the electric field, as shown in the figure. If the disk is replaced with one that has a radius of 0.20 m, how will the electric flux through the larger disk compare to the electric flux through the original disk? |
| Practice 2.1.5 |
|---|
 A positive point charge q is surrounded by an imaginary sphere of radius r, centered on the charge, as shown in the figure. Compare the electric flux through this sphere to the electric flux through an imaginary sphere of radius 2r, if it enclosed the same charge q. |
| Practice 2.1.6 |
|---|
A positive point charge q is surrounded by an imaginary sphere of radius r, centered on the charge, as shown in the figure. If you replaced the positive point charge with an equal but negative point charge, what is true about the electric flux through the sphere? |
Discuss!
Reflect on this question and take notes on how you would answer it. Then we will share these thoughts together in a class discussion.
A cylindrical piece of insulating material is placed in an external electric field as shown.
Is the net electric flux passing through the surface of the cylinder positive, negative, or zero? Explain.
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