Download Unit_8_AP_Review_Problems---Current_Electricity_and_RC_Circuits

Current Electricity (Series, Parallel, and Combination) REVIEW problems
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Do resistors destroy the electrons as the electrons move through a resistor? Explain.
How many electrons flow past a point in a wire each second if the wire has a current of 1 A?
A service station charges a battery using a current of 5.7A for 6 hours. How much charge (in Coulombs) passes
through the battery in this time?
A lighting bolt delivers a charge of 35 C to the ground in a time of 1.0 x 10 -3 s. What is the current?
A simple circuit consists of a battery, a resistor, an ammeter, and some connecting wires.
a. Draw a circuit diagram of this circuit.
b. Show the polarity (+ and – ends) of the battery and the direction of the conventional current.
c. How must an ammeter be connected in the circuit to correctly read the current?
d. How must a voltmeter be connected in the circuit to read the voltage drop of the resistor?
e. Does the ammeter reading depend on if the ammeter is placed before the resistor or after the resistor?
Explain.
Why must an ammeter have a very low resistance?
Why must a voltmeter have a very high resistance?
Describe what would happen to the current in a series circuit if a voltmeter, inadvertently mistaken for an ammeter,
were inserted into the circuit.
Why does a wire (in real life) become warmer as current flows through it?
Thinking of Ohm’s Law, describe two ways of increasing a circuit’s current.
If the voltage across a circuit is kept constant and the resistance is doubled, what effect does this have on the circuit’s
current?
What is the effect on the current in a circuit if both the voltage and the resistance are doubled?
Why is it frustrating when one bulb burns out on a string on Christmas tree lights connected in series?
Several resistors with different values are connected in parallel. The total circuit resistance will be less than which
resistor?
Compare the amount of current entering a junction in a parallel circuit with that leaving the junction. Note: A junction
is a point where the circuit branches off.
Why is household wiring done in parallel instead of in series?
Two bulbs of resistance R1 and R2 (where R2 > R1) are connected in series. Which is brighter? What if they are
connected in parallel? Explain in detail.
What is the total resistance of three 40-ohm bulbs and three 80-ohm bulbs if they’re
a. connected to each other in series?
b. connected to each other in parallel (one bulb per branch)?
Explain the function of a fuse in an electric circuit.
Why is it dangerous to replace a 15A fuse in a circuit with a 30A fuse?
What happens to the total circuit current if one lamp in a three-lamp series circuit burns out?
What happens to the total circuit current if one lamp in a three-lamp parallel circuit burns out?
If all you had were 6V bulbs (bulbs that will break if they have more than 6V across them), would it be possible to use
a 120V line without burning them out? How?
For each part of this question, write the form of circuit that applies: Series or Parallel (for the parallel, you can assume
there is only one resistor per branch).
a. The current is the same at every point in the circuit.
b. The total resistance is equal to the sum of the individual resistances.
c. The voltage drop is the same across each resistor, regardless of its resistance.
d. The voltage drop across each resistor is proportional to the value of the resistor.
e. Adding a resistor decreases the total resistance.
f. Adding a resistor increases the total resistance.
g. If the current through one resistor goes to zero, there is no current in the entire circuit.
h. If the current through one resistor goes to zero, the current through all other resistors remains the same.
i. This is the form that we have in our house wiring.
A portable compact disc player is designed to play for 2.0 h on a fully charged battery pack. If the battery pack
provides a total of 180 C of charge, how much current (in amps) does the player use in operating? (0.025A)
A defibrillator is used during a heart attack to restore the heart to its normal beating pattern. A defibrillator passes 18
A of current through the torso of a person in 2.0 ms (milliseconds).
a. How much total charge moves during this time? (0.036C)
b. How many electrons pass through the wires connected to the patient? (2.25 E17 electrons)
Unit 8 AP Review Problems
Current Electricity and RC Circuits
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27. A beam of protons is moving toward a target in a particle accelerator. This beam constitutes a current whose value is
0.50 A (microamperes). How many protons strike the target in 15 seconds? (4.69 E13 protons)
28. The filament of a light bulb has a resistance of 580 . A voltage of 120 V is connected across the filament. How
much current is in the filament? (0.21 A)
29. What is the power output of a 6V portable cd player that draws 400mA of current? (2.40W)
30. An electric blanket is connected to a 120-V outlet and consumes 140 W of power. What is the current in the wire in
the blanket? (1.17 A)
31. The element of an electric oven is designed to produce 3.3kW of heat when connected to a 240V source. Assuming
100% efficiency, what is the resistance of the element? (17.45 )
32. What is the resistance of and current through a 60W bulb if it’s connected to a 120V source? What about a 100W
bulb? (240, 0.5A) (144 , 0.83A)
33. A battery charger is connected to a dead battery and delivers a current of 6.0 A for 5.0 hours, keeping the voltage
across the battery terminals at 12 V in the process. How much total energy is delivered to the battery in this amount
of time? (1.30 E6 J)
34. The resistance of a bagel toaster is 14 . To prepare a bagel, the toaster is operated for one minute from a 120-V
outlet. How much energy is delivered to the toaster? (6.17 E4 J)
35. In doing a load of clothes, a clothes dryer uses 16 A of current at 240 V for 45 min. A personal computer, in contrast,
uses 2.7 A of current at 120 V. With the energy used by the clothes drier, how long (in hours) could you use this
computer to surf the internet? (8.89 hr)
36. The resistivity of a silver wire is 1.59  108 m. The radius of the wire is 5.04  104 m. If the length of the wire
is 3.00 m, what is the resistance of the wire? (5.98 E-2 )
Series
37. Eight identical bulbs are connected in series across a 110V line.
a. What is the voltage across each bulb? (13.75V/bulb)
b. If the current is 0.5A, what is the resistance of each bulb and the power dissipated by each? (27.5, 6.88W)
38. Three resistors, 25, 45, and 75 , are connected in series, and a 0.51-A current passes through them. What is
a. the equivalent resistance? (145)
b. the combined voltage drop across the three resistors? (73.95 V)
39. Three resistors, 9.0, 5.0, and 1.0 , are connected in series across a 24-V battery. Find
a. the current in, (1.6A)
b. the voltage across, (14.4V, 8V, 1.6V)
c. the power dissipated in each resistor. (23.04W, 12.80W, 2.56W)
40. The current in a 47  resistor is 0.12 A. This resistor is in series with a 28- resistor, and the series combination is
connected across a battery. What is the battery voltage? (9V)
41. Eight 7.0W identical bulbs are connected in series to a 110V source. What is the resistance of each bulb? (27)
42. A 60.0-W lamp is placed in series with a resistor and a 120.0-V source. If the voltage across the lamp is 25V, what is
the resistance R of the resistor? (39.58)
43. The current in a series circuit is 15.0 A. When an additional 8.00- resistor is inserted in series, the current drops to
12.0 A. What is the resistance in the original circuit? (32)
44. A 47- resistor can dissipate up to 0.25 W of power without burning up. What is the smallest number of such resistors
that can be connected in series across a 9.0-V battery without any one of them burning up? (3 resistors)
Parallel
45. A number of light bulbs are to be connected to a single electrical outlet. Will the bulbs provide more brightness if
they are connected in series or in parallel? Why?
46. A 16- loudspeaker and an 8.0- loudspeaker are connected in parallel across the terminals of an amplifier.
Determine the total resistance of the two speakers. (5.33)
47. What resistance must be placed in parallel with a 155- resistor to make the total resistance 115 ? (445.63)
48. An electric heater (R = 9.60 ) and a toaster (R = 17.0 ) are connected in parallel across 120V.
a. What is the voltage across the heater? (120V)
b. Determine the current in the toaster. (7.06A)
c. What is the total power supplied to the heater and the toaster? (2.35 E3 W)
49. Eight identical bulbs are connected in parallel to a 110V source. If 0.24A flows through each, what is the resistance
of each bulb? (458.33)
50. Two resistors, 42.0 and 64.0 , are connected in parallel. The current through the 64.0- resistor is 3.00 A.
a. Determine the current in the other resistor. (4.57 A)
b. What is the total power consumed by the two resistors? (1.45 E3 W)
Unit 8 AP Review Problems
Current Electricity and RC Circuits
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51. A coffee cup heater and lamp are connected in parallel to the same 120-V outlet. Together, they use a total of 84 W of
power. The resistance of the heater is 6.0 x 10 2 . Find the resistance of the lamp. (240)
52. Circuit breakers are re-settable automatic switches that protect against a dangerously large total current by “opening”
to stop the current at a specified safe value. A 1650-W toaster, a 1090-W iron, and a 1250-W microwave oven are
connected in parallel across 120V and simultaneously turned on in a kitchen. There is a 20-A circuit breaker
connected in the main part of the circuit.
a. Find the equivalent resistance of the three devices. (3.61)
b. Obtain the total current delivered by the source and determine whether the breaker will “open” to prevent an
accident. (33.24 A, yes)
Combo
Use the following diagram to answer the following questions:
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2
3
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53. A 14- coffee maker and a 16- frying pan are connected in series with each other (in Branch 2) across a 120-V
source of voltage. A 23- bread maker is also connected across the 120-V source and is in parallel with the series
combination (in Branch 3). Find
a. The total resistance of the circuit. (13.02)
b. the total current supplied by the source of voltage. (9.22A)
c. The power of each device. (224 W, 256 W, 626 W)
54. A 60.0- resistor (on Branch 2) is connected in parallel with a 120.0- resistor (on Branch 3). This parallel group is connected in
series with a 20.0- resistor (Location 1). The total combination is connected across a 15.0-V battery. Find the current through,
the voltage drop of, and the power of all three resistors. (20: 0.25 A, 5 V, 1.25 W) (60: 0.17 A, 10V, 1.7 W) (120:
0.08A, 10V, 0.80 W)
55. A 5 and a 2 resistor are in series with each other (in Branch 2). These two resistors are in parallel with two other
resistors in series with each other (a 4  and a 7, in Branch 3). This four-resistor parallel group is in series with 2
more resistors that are in series with each other (a 3 at Location 1, and an 8 at Location 7). The whole circuit is
across 120V. Determine the power dissipated in each of the resistors. (3: 184.87 W, 8: 492.98 W, 5: 115.2 W,
2: 46.08 W, 4: 37.21 W, 7: 65.12 W)
Capacitors
56. In the diagram below, a 3 uF and a 4 uF capacitor are connected in series across a 40 V battery. A 10 uF capacitor is
also connected directly across the battery terminals. Find
a. the total charge that the battery delivers to the capacitors when they are fully charged. (4.68 E -4 C)
b. the total electrical potential energy the capacitors possess when they are fully charged. (9.36 E -3 J)
3uF
10 uF
40V
4 uF
57. A 3 uF and a 5 uF capacitor are connected in series across a 30 V battery. A 7 uF capacitor is then connected in
parallel across the 3 uF capacitor. Determine the voltage across the 7 uF capacitor when they are fully charged. (10
V)
 In addition, if you want to do well on the free-response in this unit, you should go back and re-do the
capacitor practice problems until you fully understand capacitors. They’re not difficult---they’re just
different.
Unit 8 AP Review Problems
Current Electricity and RC Circuits
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