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Chapter 8
8
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Electrostatic Energy
8.1
Review questions
• Review the argumentation that leads to (8.3)
• Why is there a minus sign in the definition of the electrostatic potential?
(9.1) is positive if both charges have the same sign. What does this mean
in terms of work?
• (9.1) is negative if the charges gave opposite sign. (8.35) is always positive.
What is the difference in these expressions?
• What is the energy of a capacity with a charge Q?
• What is the force between two oppositely charged capacitor plates?
• What is the significance of the minus sign in equation (8.15)
• Express the energy density in regions with an electric field in terms of E.
8.2
Examples
1. In equation (8.11) Feynman combines expression for the energy stored in
a capacitor and the previously calculated capacitance of a charged sphere
to find the energy of the charged sphere.
8.3
Grasps
1. Which system has the highest energy: two point charges +q1 and +q2 at
distance d, or charges −q1 and −q2 at the same distance?
2. Give an expression for the total electrostatic energy of a system of 3 point
charges q1 , q2 and q3 .
8.4
Problems
1. Express the capacity in terms of an integral over the electric field (hint:
use (8.35).
2. Using this result, derive the capacity of a parallel plate capacitor.
3. Find for the configuration of problem 6.4.1(a) the capacity per length
using (8.35) and (8.9) 38
4. Using (8.35), derive the work needed to charge a metallic sphere of radius
a.
5. Find the pressure between the shells in problem 5.4.4.
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