# Lecture 2

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Experiment 1: Equipotential Lines
Week of January 18
Tutorial and Test 1 on chapter 18
Chemistry 1310 Midterm
Is NOT at the same time as the PHYS 1030 midterm!
Aurora: CHEM1310, Thursday, March 4, 7-9 pm
Friday, January 8, 2010
43
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If you bought a NEW copy of Cutnell &amp; Johnson
(not second-hand) without a WileyPLUS code for
edition 8,
you can claim a WileyPLUS code...
...see me.
Friday, January 8, 2010
44
The story so far...
Electric charge is carried by electrons, protons. Because they
each have a fixed amount of charge, charge is conserved.
The electrostatic (Coulomb) force is exerted between charges.
Like charges repel, unlike charges attract.
The force between charges is given by Coulomb’s law:
F = kqq0/r2,
Electric field due to q is E = F/q0
E
q, q0 in Coulombs (C),
k = Coulomb constant = 9x109 N.m2/C2.
Friday, January 8, 2010
45
The story so far...
Electric charge is carried by electrons, protons. Because they
each have a fixed amount of charge, charge is conserved.
The electrostatic (Coulomb) force is exerted between charges.
Like charges repel, unlike charges attract.
The force between charges is given by Coulomb’s law:
F = kqq0/r2,
Electric field due to q is E = F/q0
E
q, q0 in Coulombs (C),
k = Coulomb constant = 9x109 N.m2/C2.
Friday, January 8, 2010
46
Prob. 18.66/27: A ball (mass = 0.012 kg) carries a charge of –18 !C.
What electric field is needed to cause the ball to float above the
ground?
�F = q�E
q = –18 !C
m = 0.012 kg
mg
What is the weight of the ball?
What magnitude and direction of electric field supplies an equal
upward force?
Friday, January 8, 2010
47
iClicker Question
A positive charge +q is fixed at the centre of a square. A second
charge is fixed to either corner B, corner C or corner D. The net
electric field at corner A is zero.
At which corner is the second charge located?
At B, C or D ?
Answer: C
Friday, January 8, 2010
Q
48
iClicker Question
A positive charge +q is fixed at the centre of a square. A second
charge is fixed to either corner B, corner C or corner D. The net
electric field at corner A is zero.
Is the second charge
(A) positive, or
(B) negative ?
Answer: B
Q
Friday, January 8, 2010
49
iClicker Question
A positive charge +q is fixed at the centre of a square. A second
charge is fixed to either corner B, corner C or corner D. The net
electric field at corner A is zero.
Does the second charge have
(A) greater magnitude,
(B) smaller magnitude, or
(C) the same magnitude
as the charge at the centre?
Answer: A
Q
Friday, January 8, 2010
50
On a thin, nonconducting rod, positive charges are spread evenly, so
that there is the same amount of charge per unit length at every
point.
Deduce the direction of the electric field at point P that is located
directly above the midpoint of the rod.
E
C
P
B
A
D
+++++++++++++++++++++++++++++++++++++
Pair off charges on equal sections of the rod equidistant from the
midpoint. Work out the direction of the field from the paired-off
charges...
Friday, January 8, 2010
51
iClicker Question
On another identical rod, positive charges are spread evenly over
only the left half, and the same amount of negative charge is spread
evenly over the right half.
Deduce the direction of the electric field at point P that is located
directly above the midpoint of the rod.
E
C
P
Answer A)
B
A
D
+++++++++++++++++++–––––––––––––––––––
Hint: pair off charges on equal sections of the rod equidistant from
the midpoint. Work out the direction of the field from the pairedoff charges...
Friday, January 8, 2010
52
Where should q1 to placed so that the net force on it is zero?
Assume all charges are positive.
q1
q2
q3
L
Draw in the forces on q1 due to q2 and q3
Find where the forces are equal in magnitude and opposite in
direction
When F = 0, the electric field due to q2 and q3 is also zero
Friday, January 8, 2010
53
Prob. 18.35/29: Where is the electric field equal to zero?
Let point P be a guess for where E = 0, a distance x from the charge at
the left.
E=0?
P
x
q1 = –16 !C
Friday, January 8, 2010
L=3m
3–x
q2 = +4 !C
54
18.-/65: A proton is moving parallel to a uniform electric field. The
electric field accelerates the proton and thereby increases its
linear momentum to 5x10-23 kg.m/s from 1.5x10-23 kg.m/s in a time of
6.3 !s. What is the magnitude of the electric field?
Friday, January 8, 2010
55
Prob. 18.28/68: Two small, identical objects are 0.2 m apart. Each
carries a charge and they attract each other with a force of 1.2 N.
The objects are brought into contact so that the net charge is
shared equally, and then they are returned to their initial positions.
There is now a repulsive force of 1.2 N between the objects.
What is the initial charge on each object?
Write down Coulomb’s law for charges q1 and q2
Apply conservation of charge to find the charge q on each object
after the charges have equalized
Write down Coulomb’s law again
Friday, January 8, 2010
56
18.51/45: Two point charges of the same magnitude but opposite
signs are fixed to either end of the base of an isosceles triangle. The
electric field at the midpoint M between the charges has a magnitude
EM. The field at point P has magnitude EP.
The ratio of these two field magnitudes is EM/EP = 9.0. Find the angle
αin the diagram.
L
L
cos α =
d
Friday, January 8, 2010
d
L
d
57
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