Electric Charge and Electric field
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Electric Charge and Electric field
ConcepTest 16.1a Electric Charge I
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
repelling each other as
they hang from the ceiling.
What can you say about
their charges?
ConcepTest 16.1a Electric Charge I
The fact that the balls repel each
other only can tell you that they
have the same 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
repelling each other as
they hang from the ceiling.
What can you say about
their charges?
Follow-up: What does the picture look like if the two balls are oppositely
charged? What about if both balls are neutral?
Q QF1 = 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
of the force F2?
ConcepTest 16.3a Coulomb’s Law I
The force F2 must have the same 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 reverse.
Note that this sounds suspiciously like Newton’s 3rd Law!!
Q QF1 = 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
of the force F2?
ConcepTest 16.3a Coulomb’s Law I
Which of the arrows best
represents the direction
of the net force on charge
+Q due to the other two
charges?
+2Q
+4Q
+Q
1 2
3
4
5d
d
ConcepTest 16.6 Forces in 2D
The charge +2Q repels +Q towards
the right. The charge +4Q repels +Q
upwards, but with a stronger force.
Therefore, the net force is up and to
the right, but mostly up.
+2Q
+4Q
+Q
1 2
3
4
5d
d
+2Q
+4Q
ConcepTest 16.6 Forces in 2D
Which of the arrows best
represents the direction
of the net force on charge
+Q due to the other two
charges?
Follow-up: What happens if the
yellow charge would be +3Q?
BA
1. “A” Negative, “B” Positive
2. Both Negative
3. Both Positive
4. Cannot tell
Electric field lines is as shown below.
What type of charges are present at “A” and “B” ?
BA
1. “A” Negative, “B” Positive
2. Both Negative
3. Both Positive
4. Cannot tell
Electric field lines is as shown below.
What type of charges are present at “A” and “B” ?
Electric Potential and Capacitors
Which two points have
the same potential?
1) A and C
2) B and E
3) B and D
4) C and E
5) no pair
A
C
B DEQ
ConcepTest 17.6 Equipotential of Point Charge
Since the potential of a point charge is:
only points that are at the same distance
from charge Q are at the same potential.
This is true for points C and E.
They lie on an Equipotential Surface.
Which two points have
the same potential?
1) A and C
2) B and E
3) B and D
4) C and E
5) no pair
A
C
B DEQ
r
QkV
ConcepTest 17.6 Equipotential of Point Charge
Follow-up: Which point has the smallest potential?
electron
proton
electron
proton+
-
1) proton
2) electron
3) both acquire the same KE
4) neither – there is no change of KE
5) they both acquire the same KE but with opposite signs
ConcepTest 17.1c Electric Potential Energy III
A proton and an electron are in
a constant electric field created
by oppositely charged plates.
You release the proton from
the positive side and the
electron from the negative side.
When it strikes the opposite
plate, which one has more KE?
electron
proton
electron
proton+
-
1) proton
2) electron
3) both acquire the same KE
4) neither – there is no change of KE
5) they both acquire the same KE but with opposite signs
Since PE = qV and the proton and electron
have the same charge in magnitude, they
both have the same electric potential energy
initially. Because energy is conserved, they
both must have the same kinetic energy after
they reach the opposite plate.
ConcepTest 17.1c Electric Potential Energy III
A proton and an electron are in
a constant electric field created
by oppositely charged plates.
You release the proton from
the positive side and the
electron from the negative side.
When it strikes the opposite
plate, which one has more KE?
1) increase the area of the plates
2) decrease separation between the plates
3) decrease the area of the plates
4) either (1) or (2)
5) either (2) or (3)
What must be done to
a capacitor in order to
increase the amount of
charge it can hold (for
a constant voltage)?
+Q –Q
ConcepTest 17.9a Varying Capacitance I
Since Q = C V, in order to increase the charge
that a capacitor can hold at constant voltage,
one has to increase its capacitance. Since the
capacitance is given by , that can be
done by either increasing A or decreasing d.
1) increase the area of the plates
2) decrease separation between the plates
3) decrease the area of the plates
4) either (1) or (2)
5) either (2) or (3)
dA
C 0
What must be done to
a capacitor in order to
increase the amount of
charge it can hold (for
a constant voltage)?
+Q –Q
ConcepTest 17.9a Varying Capacitance I
Electric Current and Circuits
ConcepTest 18.1 Connect the Battery
Which is the correct way to
light the lightbulb with the
battery?
4) all are correct
5) none are correct
1) 3)2)
Current can only flow if there is a continuous connection from
the negative terminal through the bulb to the positive terminal.
This is only the case for Fig. (3).
ConcepTest 18.1 Connect the Battery
Which is the correct way to
light the lightbulb with the
battery?
4) all are correct
5) none are correct
1) 3)2)
ConcepTest 19.2b Parallel Resistors II
1) increases
2) remains the same
3) decreases
4) drops to zero
Points P and Q are connected to a
battery of fixed voltage. As more
resistors R are added to the parallel
circuit, what happens to the total
current in the circuit?
ConcepTest 19.2b Parallel Resistors II
1) increases
2) remains the same
3) decreases
4) drops to zero
As we add parallel resistors, the overall
resistance of the circuit drops. Since V =
IR, and V is held constant by the battery,
when resistance decreases, the current
must increase.
Points P and Q are connected to a
battery of fixed voltage. As more
resistors R are added to the parallel
circuit, what happens to the total
current in the circuit?
Follow-up: What happens to the current through each resistor?
ConcepTest 19.1a Series Resistors I
9 V
Assume that the voltage of the battery
is 9 V and that the three resistors are
identical. What is the potential
difference across each resistor?
1) 12 V
2) zero
3) 3 V
4) 4 V
5) you need to know the
actual value of R
Since the resistors are all equal,
the voltage will drop evenly
across the 3 resistors, with 1/3 of
9 V across each one. So we get a
3 V drop across each.
ConcepTest 19.1a Series Resistors I
9 V
Assume that the voltage of the battery
is 9 V and that the three resistors are
identical. What is the potential
difference across each resistor?
1) 12 V
2) zero
3) 3 V
4) 4 V
5) you need to know the
actual value of R
Follow-up: What would be the potential difference if
R= 1 W, 2 W, 3 W
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