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ConcepTests
Chapter 21
Physics, 3rd Edition
James S. Walker
© 2007 Pearson Prentice Hall
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ConcepTest 21.1
Which is the correct way to
light the lightbulb with the
Connect the Battery
4) all are correct
5) none are correct
battery?
1)
2)
3)
ConcepTest 21.2
You double the voltage
across a certain conductor
and you observe the current
increases three times. What
can you conclude?
Ohm’s Law
1) Ohm’s law is obeyed since the
current still increases when V
increases
2) Ohm’s law is not obeyed
3) this has nothing to do with Ohm’s
law
ConcepTest 21.3
Wires I
Two wires, A and B, are made of the
1) dA = 4 dB
same metal and have equal length,
2) dA = 2 dB
but the resistance of wire A is four
times the resistance of wire B. How
do their diameters compare?
3) dA = dB
4) dA = 1/2 dB
5) dA = 1/4 dB
ConcepTest 21.3
Wires I
Two wires, A and B, are made of the
1) dA = 4 dB
same metal and have equal length,
2) dA = 2 dB
but the resistance of wire A is four
times the resistance of wire B. How
do their diameters compare?
3) dA = dB
4) dA = 1/2 dB
5) dA = 1/4 dB
The resistance of wire A is greater because its area is less than
wire B. Since area is related to radius (or diameter) squared,
the diameter of A must be two times less than B.
L
R
A
ConcepTest 21.4
Wires II
A wire of resistance R is
1) it decreases by a factor 4
stretched uniformly (keeping its
2) it decreases by a factor 2
volume constant) until it is twice
3) it stays the same
its original length. What happens
4) it increases by a factor 2
to the resistance?
5) it increases by a factor 4
ConcepTest 21.5
Series Resistors I
1) 12 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?
2) zero
3) 3 V
4) 4 V
5) you need to know the
actual value of R
9V
ConcepTest 21.6
Series Resistors II
1) 12 V
In the circuit below, what is the
2) zero
voltage across R1?
3) 6 V
4) 8 V
5) 4 V
R1= 4 W
R2= 2 W
12 V
ConcepTest 21.7
Parallel Resistors I
1) 10 A
In the circuit below, what is the
2) zero
current through R1?
3) 5 A
4) 2 A
5) 7 A
R2= 2 W
R1= 5 W
10 V
ConcepTest 21.8
Points P and Q are connected to a
Parallel Resistors II
1) increases
battery of fixed voltage. As more
2) remains the same
resistors R are added to the parallel
3) decreases
circuit, what happens to the total
4) drops to zero
current in the circuit?
ConcepTest 21.9
Current flows through a
Short Circuit
1) all the current continues to flow through
the bulb
connected across the
2) half the current flows through the wire,
the other half continues through the
bulb
bulb, what happens?
3) all the current flows through the wire
lightbulb. If a wire is now
4) none of the above
ConcepTest 21.10
Two lightbulbs A and B are
connected in series to a
constant voltage source.
When a wire is connected
across B, bulb A will:
Short Circuit II
1) glow brighter than before
2) glow just the same as before
3) glow dimmer than before
4) go out completely
5) explode
ConcepTest 21.11
Circuits I
The lightbulbs in the circuit below
1) circuit 1
are identical with the same
2) circuit 2
resistance R. Which circuit
produces more light? (brightness
 power)
3) both the same
4) it depends on R
ConcepTest 21.12
The three lightbulbs in the circuit all have
Circuits II
1) twice as much
the same resistance of 1 W . By how
2) the same
much is the brightness of bulb B greater
3) 1/2 as much
or smaller than the brightness of bulb A?
(brightness  power)
4) 1/4 as much
5) 4 times as much
A
C
B
10 V
ConcepTest 21.13
More Circuits I
What happens to the voltage
1) increase
across the resistor R1 when the
2) decrease
switch is closed? The voltage will:
3) stay the same
R1
S
R3
V
R2
ConcepTest 21.14
More Circuits II
1) increases
What happens to the voltage
across the resistor R4 when the
2) decreases
switch is closed?
3) stays the same
R1
S
R3
V
R2
R4
ConcepTest 21.15
Even More Circuits
1) R1
Which resistor has the
2) both R1 and R2 equally
greatest current going
through it? Assume that all
3) R3 and R4
the resistors are equal.
4) R5
5) all the same
V
ConcepTest 21.16
Dimmer
1) the power
When you rotate the knob of a
2) the current
light dimmer, what is being
3) the voltage
changed in the electric circuit?
4) both (1) and (2)
5) both (2) and (3)
ConcepTest 21.16
Dimmer
1) the power
When you rotate the knob of a
2) the current
light dimmer, what is being
3) the voltage
changed in the electric circuit?
4) both (1) and (2)
5) both (2) and (3)
The voltage is provided at 120 V from the
outside. The light dimmer increases the
resistance and therefore decreases the current
that flows through the lightbulb.
Follow-up: Why does the voltage not change?
ConcepTest 21.17
Lightbulbs
Two lightbulbs operate at 120 V, but
1) the 25 W bulb
one has a power rating of 25 W while
2) the 100 W bulb
the other has a power rating of 100 W.
3) both have the same
Which one has the greater
4) this has nothing to do
with resistance
resistance?
ConcepTest 21.18
Two space heaters in your living
room are operated at 120 V.
Space Heaters I
1) heater 1
Heater 1 has twice the resistance
2) heater 2
of heater 2. Which one will give
3) both equally
off more heat?
ConcepTest 21.19
Junction Rule
1) 2 A
What is the current in branch P?
2) 3 A
3) 5 A
4) 6 A
5) 10 A
5A
P
8A
2A
ConcepTest 21.20
Kirchhoff’s Rules
The lightbulbs in the
1) both bulbs go out
circuit are identical. When
2) intensity of both bulbs increases
the switch is closed, what
3) intensity of both bulbs decreases
happens?
4) A gets brighter and B gets dimmer
5) nothing changes
ConcepTest 21.20
Kirchhoff’s Rules
The lightbulbs in the
1) both bulbs go out
circuit are identical. When
2) intensity of both bulbs increases
the switch is closed, what
3) intensity of both bulbs decreases
happens?
4) A gets brighter and B gets dimmer
5) nothing changes
When the switch is open, the point
between the bulbs is at 12 V. But so is
the point between the batteries. If
there is no potential difference, then
no current will flow once the switch is
closed!! Thus, nothing changes.
Follow-up: What happens if the bottom
battery is replaced by a 24 V battery?
24 V
ConcepTest 21.21
Wheatstone Bridge
An ammeter A is connected
1) I
between points a and b in the
2) I/2
circuit below, in which the four
3) I/3
resistors are identical. The current
4) I/4
through the ammeter is:
5) zero
a
b
V
I
ConcepTest 21.21
Wheatstone Bridge
An ammeter A is connected
1) I
between points a and b in the
2) I/2
circuit below, in which the four
3) I/3
resistors are identical. The current
4) I/4
through the ammeter is:
5) zero
Since all resistors are identical,
a
the voltage drops are the same
across the upper branch and the
lower branch. Thus, the
potentials at points a and b are
b
also the same. Therefore, no
current flows.
V
I
ConcepTest 21.22
More Kirchhoff’s Rules
1) 2 – I1 – 2I2 = 0
Which of the equations is valid
2) 2 – 2I1 – 2I2 – 4I3 = 0
for the circuit below?
3) 2 – I1 – 4 – 2I2 = 0
4) I3 – 4 – 2I2 + 6 = 0
5) 2 – I1 – 3I3 – 6 = 0
1W
I2
2W
6V
22 VV
4V
I1
1W
I3
3W
ConcepTest 21.22
More Kirchhoff’s Rules
1) 2 – I1 – 2I2 = 0
Which of the equations is valid
2) 2 – 2I1 – 2I2 – 4I3 = 0
for the circuit below?
3) 2 – I1 – 4 – 2I2 = 0
4) I3 – 4 – 2I2 + 6 = 0
5) 2 – I1 – 3I3 – 6 = 0
Eqn. 3 is valid for the left loop:
The left battery gives +2V, then
there is a drop through a 1W
resistor with current I1 flowing.
Then we go through the middle
battery (but from + to – !), which
gives –4V. Finally, there is a
drop through a 2W resistor with
current I2.
1W
I2
2W
6V
22 VV
4V
I1
1W
I3
3W
ConcepTest 21.23
Capacitors I
1) Ceq = 3/2 C
What is the equivalent capacitance,
2) Ceq = 2/3 C
Ceq , of the combination below?
3) Ceq = 3 C
4) Ceq = 1/3 C
5) Ceq = 1/2 C
o
Ceq
o
C
C
C
ConcepTest 21.24
Capacitors II
How does the voltage V1 across
1) V1 = V2
the first capacitor (C1) compare
2) V1 > V2
to the voltage V2 across the
3) V1 < V2
second capacitor (C2)?
4) all voltages are zero
C2 = 1.0 mF
10 V
C1 = 1.0 mF
C3 = 1.0 mF
ConcepTest 21.25
How does the charge Q1 on the first
Capacitors III
1) Q1 = Q2
2) Q1 > Q2
capacitor (C1) compare to the
charge Q2 on the second capacitor
3) Q1 < Q2
(C2)?
4) all charges are zero
C2 = 1.0 mF
10 V
C1 = 1.0 mF
C3 = 1.0 mF