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ConcepTest 16.1ConcepTest 16.1 Electric Charge IElectric Charge I
The fact that the balls repel each
other only can tell you that they
have the same chargesame charge, but you do
not know the sign. So they can
be either both positive or both
negative.
1) one is positive, the other
is negative
2) both are positive
3) both are negative
4) both are positive or both
are negative
Two charged balls are Two charged balls are
repelling each other as repelling each other as
they hang from the ceiling. they hang from the ceiling.
What can you say about What can you say about
their charges?their charges?
Follow-up:Follow-up: What does the picture look like if the two balls are oppositely What does the picture look like if the two balls are oppositely charged? What about if both balls are neutral?charged? What about if both balls are neutral?
Clearly, the ball will be attracted if its
charge is negativenegative. However, even if
the ball is neutralneutral, the charges in the
ball can be separated by inductioninduction
(polarization), leading to a net
attraction.
1) positive
2) negative
3) neutral
4) positive or neutral
5) negative or neutral
A metal ball hangs from the ceiling
by an insulating thread. The ball is
attracted to a positive-charged rod
held near the ball. The charge of
the ball must be:
remember the ball is a conductor!
ConcepTest 16.2ConcepTest 16.2 Conductors IConductors I
Follow-up:Follow-up: What happens if the What happens if the metal ballmetal ball is replaced by a is replaced by a plastic ballplastic ball??
While the conductors are connected, positive positive
charge will flow from the blue to the green charge will flow from the blue to the green
ball due to polarizationball due to polarization. Once disconnected,
the charges will remain on the separate charges will remain on the separate
conductorsconductors even when the rod is removed.
Two neutral conductors are connected
by a wire and a charged rod is brought
near, but does not touch. The wire is
taken away, and then the charged rod
is removed. What are the charges on
the conductors?
ConcepTest 16.3ConcepTest 16.3 Conductors IIConductors II1) 0 0
2) + –
3) – +
4) + +
5) – –
0 0
? ?Follow-up:Follow-up: What will happen when the What will happen when the
conductors are reconnected with a wire?conductors are reconnected with a wire?
The force F2 must have the same magnitudesame magnitude as F1. This is
due to the fact that the form of Coulomb’s Law is totally
symmetric with respect to the two charges involved. The
force of one on the other of a pair is the same as the reverseforce of one on the other of a pair is the same as the reverse.
Note that this sounds suspiciously like Newton’s 3rd Law!!Note that this sounds suspiciously like Newton’s 3rd Law!!
QQ QQF1 = 3N F2 = ?
1) 1.0 N
2) 1.5 N
3) 2.0 N
4) 3.0 N
5) 6.0 N
What is the magnitude What is the magnitude
of the force of the force FF22??
ConcepTest 16.4ConcepTest 16.4 Coulomb’s Law ICoulomb’s Law I
Originally we had:
Fbefore = k(Q)(Q)/d2 = F
Now we have:
Fafter = k(Q)(Q)/(3d)2 = 1/9 F
1) 9 F
2) 3 F
3) F
4) 1/3 F
5) 1/9 F
The force between two charges
separated by a distance d is F. If
the charges are pulled apart to a
distance 3d, what is the force on
each charge?
QQFF
QFF
dd
QQ??
QQ??
33dd
ConcepTest 16.5ConcepTest 16.5 Coulomb’s Law IIICoulomb’s Law III
Follow-up:Follow-up: What is the force if the original distance is halved? What is the force if the original distance is halved?
APHY20204/21/23 6
APHY20204/21/23 7
17.1 Electric Potential Energy and Potential Difference
Review of work and energy
The electrostatic force isconservative.
W = Fd = qEd
PEb – PEa = -W
APHY20204/21/23 8
17.1 Electric Potential Energy and Potential Difference The electric potential is the electric
potential energyper unit charge.
Voltage
q
PEPEV abba
q
PEV aa
APHY20204/21/23 9
17.2 Electric Potential and Electric Field
If we have uniform electric field, then W = -qVba and W = qEd
Therefore Vba = -Ed or
In general, the electric field is the rate at which theelectric potential decreases over a distance.
d
VE ba
APHY20204/21/23 10
17.3 Equipotential Lines
APHY20204/21/23 11
17.3 Equipotential Lines No work is required to move along an
equipotentialline and E is perpendicular to the line.
APHY20204/21/23 12
17.4 The Electron Volt
The energy acquired by a charge (± e) after passing through a potential difference of 1 volt.
1 eV = 1.6 x 10-19 J
Lichtenberg Figuresand
Lightning
APHY20204/21/23 13
17.5 Electric Potential due to Point Charges
If V = 0 at r = ∞ then
The principle of superpositionallows us to determine the potential due to multiple
charges
r
QkV
APHY20204/21/23 14
17.5 Electric Potential due to Point Charges