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HW15-F15(CH19)
12/23/15, 4:18 AM
WebAssign
Jessica Avina
PHYS272-FALL15, Fall 2015
Instructor: Virendra Saxena
HW15-F15(CH19) (Homework)
Current Score : 36 / 36
Due : Friday, November 6 2015 11:59 PM EST
The due date for this assignment is past. Your work can be viewed below, but no changes can be made.
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1. 1/1 points | Previous AnswersMI4 19.1.040.
(a) Suppose you charge a 2.0 F capacitor with two 1.5 volt batteries. How much charge was on each
plate?
6
C
(b) How many excess electrons were on the negative plate?
3.75e19
electrons
Additional Materials
Section 19.1
2. 1/1 points | Previous AnswersMI4 19.1.042.
Two circular plates of radius 0.13 m are separated by an air gap of 3.3 mm. The plates carry charge +Q
and −Q where Q = 2.5
✕
10−8 C.
(a) What is the magnitude of the electric field in the gap?
53206
V/m
(b) What is the potential difference across the gap? (Enter the magnitude.)
175.6
V
(c) What is the capacitance of this capacitor?
1.4e-10
F
Additional Materials
Section 19.1
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3. 1/1 points | Previous AnswersMI4 19.2.045.
(a) In zinc at room temperature, the mobility of mobile electrons is about 1.6
there are about 6.6
✕
✕
✕
✕
10-3 (m/s)/(V/m), and
1028 mobile electrons per m3. Calculate the conductivity σ. In actual practice, it is
usually easier to measure the conductivity σ and deduce the mobility u from this measurement.
1.6896e7
(b) What are the correct units for σ?
(A m)/V
(A/m2)/(V/m)
(A/m)/(V/m2)
(m/A)/(V/m3)
Additional Materials
Section 19.2
4. 1/1 points | Previous AnswersMI4 19.2.046.
Consider a tungsten wire (σ = 1.8
✕
107 Ω−1 · m−1) with a cross-sectional area of 1 mm2 (similar to
your connecting wires) and carrying 0.3 amperes of current, which is about what you get in a circuit with
a round bulb and two batteries in series. Calculate the strength of the very small electric field required
to drive this current through the wire.
1.67e-2
V/m
Additional Materials
Section 19.2
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5. 1/1 points | Previous AnswersMI4 19.2.049.
A carbon resistor is 4 mm long and has a constant cross section of 0.4 mm2. The conductivity of carbon
at room temperature is σ = 3
✕
✕
104 per ohm·m. In a circuit its potential at one end of the resistor is 14
volts relative to ground, and at the other end the potential is 17 volts. Calculate the resistance R and the
current I.
R = 0.333
I= 9
amperes
Additional Materials
Section 19.2
6. 1/1 points | Previous AnswersMI4 19.2.052.
In the circuit shown in the figure below, the emf of the battery is 8.1 V. Resistor R1 has a resistance of
20 Ω, and resistor R2 has a resistance of 47 Ω. A steady current flows through the circuit.
(a) What is the absolute value of the potential difference across R1?
2.42
V
(b) What is the conventional current through R2?
0.121
A
Additional Materials
Section 19.2
7. 7/7 points | Previous AnswersMI4 19.4.064.
Two resistors
In circuit 1 (see the figure), ohmic resistor R1 dissipates 2 watts; in circuit 2, ohmic resistor R2
dissipates 23 watts. The wires and batteries have negligible resistance. The circuits contain 10V
batteries.
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(a) What is the resistance of R1 and of R2?
R1= 50
R2= 4.35
(b) Resistor R1 is made of a very thin metal wire that is 5 mm long, with a diameter of 0.1
mm. What is the electric field inside this metal resistor?
2000
V/m
(c) The same resistors are used to construct circuit 3, using the same 10V battery as before.
Which of the following graphs best represents the potential as a function of position around
circuit 3?
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(d) In circuit 3, calculate the number of electrons entering R1 every second, and the number
of electrons entering R2 every second.
1.15e18
electrons/s (R1)
1.15e18
electrons/s (R2)
(e) What is the power output of the battery in circuit 3?
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1.84
W
Additional Materials
Section 19.4
8. 11/11 points | Previous AnswersMI4 19.4.066.
Three bulbs
A circuit is made of two 1.3 volt batteries and three light bulbs as shown in the figure. When the switch
is closed and the bulbs are glowing, bulb 1 has a resistance of 8 ohms, bulb 2 has a resistance of 44
ohms, bulb 3 has a resistance of 33 ohms, and the copper connecting wires have negligible resistance.
You can also neglect the internal resistance of the batteries.
(a) With the switch open, indicate the approximate surface charge on the circuit diagram.
(Do this on paper. Your instructor may ask you to turn in this work.) Refer to your diagram
to decide which of the following statements about the circuit (with the switch open) are
true:
The surface charge on the wire at location B is positive.
The electric field in the filament of bulb 3 is zero.
There is a large gradient of surface charge between locations M and L.
The electric field in the air between locations B and C is zero.
There is no excess charge on the surface of the wire at location C.
(b) With the switch open, find these potential differences:
VB - VC = 2.6
V
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VD - VK = 0
V
(c) After the switch is closed and the steady state is established, the currents through bulbs
1, 2, and 3 are I1, I2, and I3 respectively. Which of the following equations are correct loop
or node equations for this steady state circuit?
-I1*(8
)-I2*(44
+2.6V + -I1*(8
) + I3*(33
)=0
) + -I2*(44
)=0
I2 = I3
I1 = I2 + I3
+2.6V + -I1*(8
-I2*(44
) + I3*(8
) + I3*(33
)=0
)=0
(d) In the steady state (switch closed), which of these are correct?
VL - VA = -2.6V + I1*(8
)
VC - VF = 0
VC - VF = +I1*(8
)
VC - VF = +I3*(33
)
VC - VF = +I2*(44
)
(f) Now find the unknown currents, to the nearest milliampere. (I.e. enter your answer to
three decimal places.)
I1 = 0.0968
A
I2 = 0.0415
A
I3 = 0.0553
A
(g) How many electrons leave the battery at location N every second?
6.04e17
electrons/s
(i) What is the numerical value of the power delivered by the batteries?
P = 0.252
W
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(j) The tungsten filament in the 44 ohm bulb is 6 mm long and has a cross-sectional area of
2
✕
10-10 m2. What is the magnitude of the electric field inside this metal filament?
| | = 304.3
V/m
Additional Materials
Section 19.4
9. 6/6 points | Previous AnswersMI4 19.5.068.
A battery with internal resistance
(a) You short-circuit a 22 volt battery by connecting a short wire from one end of the battery to the
other end. If the current in the short circuit is measured to be 15 amperes, what is the internal
resistance of the battery?
1.47
(b) What is the power generated by the battery?
330
W
(c) How much energy is dissipated in the internal resistance every second? (Remember that one watt is
one joule per second.)
330
J
(d) This same battery is now connected to a 8
2.32
resistor. How much current flows through this resistor?
A
(e) How much power is dissipated in the 8
43.06
resistor?
W
(f) The leads to a voltmeter are placed at the two ends of the battery of this circuit containing the 8
resistor. What does the meter read?
18.56
V
Additional Materials
Section 19.5
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10.3/3 points | Previous AnswersMI4 19.5.069.
A 7 V battery is connected to a 220 Ω resistor, and a voltmeter shows the potential difference across the
battery to be 5.9 V.
(a) What is the internal resistance of the battery?
41.02
Ω
(b) If the resistor is replaced by a very low-resistance wire, what does the voltmeter read?
The voltmeter will read 12.9 V.
The voltmeter will read near zero.
The voltmeter will read 1.1 V.
The voltmeter will read 5.9 V.
The voltmeter will read 7 V.
(c) What is the current through this wire?
0.17
A
Additional Materials
Section 19.5
11.2/2 points | Previous AnswersMI4 19.7.074.
A 24 Ω resistor and a 3 F capacitor are in series with a 3 V battery. What is the initial current when the
circuit is first assembled?
0.125
A
What is the current after 51 s?
0.062
A
Additional Materials
Section 19.7
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12.1/1 points | Previous AnswersMI4 19.7.076.
Establishing a potential difference
The deflection plates in an oscilloscope are 10 cm by 2 cm with a gap distance of 1 mm. A 100 volt
potential difference is suddenly applied to the initially uncharged plates through a 975 ohm resistor in
series with the deflection plates. How long does it take for the potential difference between the
deflection plates to reach 55 volts?
13.78e-9
s
Additional Materials
Section 19.7
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