Announcements

Homework for tomorrow…
(Ch. 25, CQ 12, Probs. 18 & 40)

Office hours…
MWF 12:30-1:30 pm
TR 9-10 am

Tutorial Learning Center (TLC) hours:
??
Chapter 25
Electric Forces & Charges
(Coulomb’s Law)
Newton’s Law of Gravitation, revisited..
where G  6.67  10
11
Nm
kg


r
2

Notice:

m2
2
Mass is always positive.
Gravity is always attractive.
m1
Coulomb’s Law..
where
N ×m
K = 8.99 ´10
C2
q2
2
r
9
q1
Notice:
 Charges can be positive or negative.
 Force can be attractive or repulsive.

Coulomb’s Law - restrictions
charges must be small compared to their
separation (“point-like”)
OK
Not OK
Quiz Question 1
Two uniformly charged spheres are firmly fastened to and electrically
insulated from frictionless pucks on an air table. The charge on
sphere 2 is three times the charge of sphere 1.
Which force diagram correctly shows the magnitude and direction of
the electrostatic force on each object?
Coulomb’s Law and Superposition

Q: What is the
?
q1
q2
q3
Coulomb’s Law and Superposition

Q: What is the

A:
?
q1
q2
q3
Quiz Question 2
Two protons (p1 and p2) and an electron (e) lie on a
straight line as shown. The direction of the net
force on p1 , p2 , and e, respectively, are:
p1
1.
2.
3.
4.
5.
left, right, left
left, right, right
right, left, left
right, left, right
right, right, left
p2
e
Prob. 25.16
What is the net electric force on charge A in Figure EX25.16?
Prob. 25.37
What is the force F on the 5.0 nC charge in Figure P25.37? Give your
answer as a magnitude and an angle measured cw or ccw (specify
which) from the + x-axis.