PHY 2049: Physics II
Coulomb’s law:
 Two kinds of charges,
positive and negative,
unlike attract, like repel.
 Electrons and Protons:
q = ± 1.602 x 10-19 C
 Current I = dq/dt
1 Amp. = 1 C/s
 Add forces as vectors

F k
q1 q 2
r12
2
rˆ
PHY 2049: Physics II
A rule
Try to calculate the force magnitude by ignoring
the signs of charges. Then assign the
direction by looking at the signs.

Never do both (using the charge signs in the
calculation of the force and then assign the
direction).
PHY 2049: Physics II



q1 = 2Q
q2 = - 2Q
q3 = -4Q
E at origin
[-0.344N, -0.218N],
212.38º
Phy 2049: Physics II trial run
In Fig. at right, the particles
have charges q1 = -q2 =
300 nC and q3 = -q4 = 200
nC, and distance a = 4.0
cm.
 What is the direction of
the force on particle 4?
A.
B.
C.
D.
3,1,2
PHY 2049: Physics II

Balls are touched and then separated. Rank
them by




Charge left
Magnitude of Force before
After
Charge transferred
TALP: Take a look Problem
PHY 2049: Physics II


d
Force on the center
charge = 6kq2/d2
Direction: -x, 180º,
PHY 2049: Physics II


q1 = q4 = Q; q2 = q3 = q
what is Q/q so that the
force on 1 is zero



All charges same sign.
There is no solution.
Q and q different sign.
There is a possible
solution.
Is the answer the same if
the force on 3 is to be
zero
PHY 2049: Physics II


What is the force on
the central chlorine
ion?
Suppose that we put
in an electron that
sits on a cesium site
and neutralizes it,
which way does the
Chlorine move?
PHY 2049: Physics II

Electric Field

Force F = qE

E = k ∑ rˆ qi/ri2
•Electric field lines denote direction
•Their density represents magnitude
•They do not cross
Physics
II
PHY 2049: Physics II

Ek
z
qz
2
R

2 3/ 2
zˆ
PHY 2049: Physics II
Induction and charge transfer on contact
Forces summed as vectors
Electric fields
PHY 2049: Physics II
Physics
II
• Potentials add like numbers