Summary

• Electric charge
• Electrostatics

Illustrative applets:


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• Electrical forces
• charging by rubbing (activity sheet)
• action at a distance
• Coulomb's law PhET applet
• 1-d Coulomb's law applet
• 2-d Coulomb's law applet
• induced charges
• neutral objects
• compare with gravity

Example #1

Example #2

Example #3


• microscopic vs. galactic forces


• Electric field

• definition
• point charges applet

Example #4


• lines of force applet
1. Lines point in direction of force
2. Number of lines proportional to the charge
3. Line density indicates field strength


• Electric field video






• Lecture learning outcomes
  A student who masters the topics in this lecture will be able to:
• describe the basic features of the electrostatic force between two charges
• use algebra to find the numerical value of force F, charge q₁, charge q₂, or separation distance r when any three of these quantities are given
• describe the difference between electric force and electric field and distinguish their units
• draw or interpret an electric field diagram in terms of either vectors or lines of force




• Practice:
  Try these additional examples

Example #5

Example #6



• Prepare:
  Read textbook section 20-1 before the next lecture

kw4
Two small spheres separated by 2 m contain negative charges of 2 C and 3 C. What is the electric force between them?
A. +1.35×10¹⁰ N
B. −1.35×10¹⁰ N
C. +2.70×10¹⁰ N
D. −2.70×10¹⁰ N
Answer















gc6 16.2
How many electrons make up a charge of −30.0 µC?
A. 3.00×10⁻⁶
B. 3.33×10⁵
C. 1.87×10¹⁴
D. 6.25×10¹⁸
Answer















sb5 23.5
Suppose 1.0 g of hydrogen is split with the protons on the north pole and electrons on the south. What is the compression force on the earth?
A. 5.74×10⁻⁴² N
B. 1.73×10² N
C. 6.55×10⁴ N
D. 5.14×10⁵ N
Answer















gc6 16.32
The electric field midway between two equal but opposite point charges is E = 745 N/C, and the distance between the charges is d = 16.0 cm. What is the magnitude of the charge on each?
A. 3.31×10⁻⁹ C
B. 2.65×10⁻¹⁰ C
C. 4.14×10⁻⁶ C
D. 3.02×10⁸ C
Answer















APB 1998.16


The figure above shows two particles, each with a charge of +Q, that are located at the opposite corners of a square of side d. What is the direction of the net electric field at point P?
A. ↖       B. ↗       C. ↙       D. ↘       E. ↓
Answer















APB 1998.13
Which of the following is true about the net force on an uncharged conducting sphere in a uniform electric field?
A. It is zero.
B. It is in the direction of the field.
C. It is in the direction opposite to the field.
D. It produces a torque on the sphere about the direction of the field.
E. It causes the sphere to oscillate about an equilibrium position.
Answer















 



A. +1.35×10¹⁰ N





















 



C. 1.87×10¹⁴





















 



D. 5.14×10⁵ N





















 



B. 2.65×10⁻¹⁰ C





















 



C. ↙
The electric field points away from positive charges, thus the contribution from the upper left Q at point P is downward and the contribution from the lower right Q at point P is toward the left. The vector sum of these two contributions is down and to the left.




















 



A. It is zero.
The electrons on the conducting sphere will move around in response to the electric field, which represents force per charge. Positive charges on the conducting sphere will be pushed in the direction of the field and negative charges will be pulled in the opposite direction. Because the field is uniform these two forces are equal in magnitude and opposite in direction, resulting in a zero net force on the sphere. If the field were nonuniform, such as the field due to a nearby charged object, the attractive force on the near side of the sphere would be stronger than the repulsive force on the far side, and the neutral sphere would be attracted to the charged object. This is the idea behind induced polarization, such as when a charged balloon sticks to a neutral wall.