1. Chapter 21, Electric Charge, and electric Field
HW3:Chapter 21, Pb.1, Pb.12, Pb.13, Pb.25,
Pb.31, Pb.36,Due Friday, Feb.2

2. 21-5 Coulomb’s Law
The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same.
Figure The direction of the static electric force one point charge exerts on another is always along the line joining the two charges, and depends on whether the charges have the same sign as in (a) and (b), or opposite signs (c).

3. 21-5 Coulomb’s Law Unit of charge: coulomb, C.
The proportionality constant in Coulomb’s law is then:
k = 8.99 x 109 N·m2/C2.
Charges produced by rubbing are typically around a microcoulomb:
1 μC = 10-6 C.

4. Question 21-5 Coulomb’s Law a) b) c) d)
What is the correct Forces Diagram for charge 3 where F31 is the force on Q3 due to Q1 and F32 is the force on Q3 due to Q2?
Question
Q Q Q3 + - +
F31
F31
a) b)
c) d) 3 3
F32
F32
F31
F31 3 3
F32
F32

5. Question A) F= Fp B) 2F= Fp C) 4F= Fp D) F= 2Fp E) F= 4Fp
An  particle with a charge +2e and a mass of 4mp is on a collision course with a proton with a charge +e and mass mp. The only force acting is the electrostatic force. Which of the following statements is true about the magnitudes of the forces on the two particles?
Question
A) F= Fp
B) 2F= Fp
C) 4F= Fp
D) F= 2Fp
E) F= 4Fp  p
Charge
+2e +e
Mass
4mp mp

6. Question A) a= ap B) 2a= ap C) 4a= ap D) a= 2ap E) a= 4ap
An  particle with a charge +2e and a mass of 4mp is on a collision course with a proton with a charge +e and mass mp. The only force acting is the electrostatic force. Which of the following statements is true about the accelerations of the two particles?
Question
A) a= ap
B) 2a= ap
C) 4a= ap
D) a= 2ap
E) a= 4ap  p
Charge
+2e +e
Mass
4mp mp

7. Problem 10
10. (II) Compare the electric force holding the electron in orbit (𝑟=0.53× 10 −10 𝑚) around the proton nucleus of the hydrogen atom, with the gravitational force between the same electron and proton. What is the ratio of these two forces?

8. Problem 11
11. (II) Two positive point charges are a fixed distance apart. The sum of their charges is QT .What charge must each have in order to (a) maximize the electric force between them, and (b) minimize it?

9. Problem 12
12.(II) Particles of charge +75 μC, +48μC, and -85μC are placed in a line (Fig. 21–52). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two.

10. 21-5 Coulomb’s Law
Example 21-3: Electric force using vector components.
Calculate the net electrostatic force on charge Q3 shown in the figure due to the charges Q1 and Q2.
Figure Determining the forces for Example 21–3. (a) The directions of the individual forces are as shown because F32 is repulsive (the force on Q3 is in the direction away from Q2 because Q3 and Q2 are both positive) whereas F31 is attractive (Q3 and Q1 have opposite signs), so F31 points toward Q1. (b) Adding F32 to F31 to obtain the net force.
Solution: The forces, components, and signs are as shown in the figure. Result: The magnitude of the force is 290 N, at an angle of 65° to the x axis.