PHYS 2212 Module 2.2

Explaining Gauss’s Law

Recommended Reading

2.2 Explaining Gauss’s Law

Learning Objectives

By the end of this section, you will be able to:

  • State Gauss’s law
  • Explain the conditions under which Gauss’s law may be used
  • Apply Gauss’s law in appropriate systems

Gauss’s Law

Gauss’s Law is

Practice!

Practice 2.2.1

A positive point charge q is surrounded by an imaginary sphere of radius r, centered on the charge, as shown in the figure. What is the electric flux through the sphere?
A.
B.
C.
D.
Check your answer: A
Practice 2.2.2

A positive point charge q is surrounded by an imaginary sphere of radius r, centered on the charge, as shown in the figure. Suppose the positive point charge is replaced with a negative point charge, –q. What is the electric flux through the sphere?
A.
B.
C.
D.
E.
Check your answer: B.
Practice 2.2.3
A Gaussian surface which is not a sphere has a single charge (q) inside it, not at the center. There are more charges outside.

What can we say about total electric flux through this surface?
Check your answer: A. It is q/ε0
Practice 2.2.4
The space in and around a cubical box (edge length L) is filled with a constant uniform electric field.

What is the TOTAL electric flux through this closed surface?
A. 0
B.
C.
D.
E. Cannot answer without knowing the enclosed charge
Check your answer: A. 0

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.

Five point charges q and four Gaussian surfaces S are represented in the figure shown.

(a) Through which of the Gaussian surfaces are the total electric flux zero?

(b) What is the total electric flux through each surface?