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2. Circuits Solutions
1. The graph represents the relationship between the voltage across a metal conductor and the current through the conductor at a constant temperature.
What is the resistance of the conductor?
1. 1 ohm 2. 0.01 ohm 3. 0.1 ohm 4. 10 ohms Ohm's law expresses the resistance of a conductor as the ratio of the voltage to the current: R = V/I. Reason Even good conductors offer some resistance to electrical current. The resistance of a conductor is the ratio of the voltage to the current: R = V/I or V = RI. The resistance depends on the temperature, which in this case is constant. Look at any point on the graph; resistance = slope = volts/
amps, which is 10 ohms.
2. Conductivity in metallic solids is due to the presence of free
1. nuclei 2. protons 3. neutrons 4. electrons
3. A light bulb operating at 120 volts draws a current of 0.50 ampere for 240 seconds. The power rating of the light bulb is
1. 30. W The amount of electrical power used depends on the voltage and the current.
2. 60. W The power rating of an electrical appliance is the product of the voltage at which it operates and the current that it draws: P = VI . Given that V = 120 volts and I = 0.50 A = amps, the power rating of the bulb is:
3. 75 W
P = VI = (120 volts)(0.50 amps) = 60. watts
4. 120 W
The power rating has nothing to do with the amount of time the light bulb is lit.
4. A metallic conductor obeys Ohm's law. Which graph best represents the relationship between the voltage (V) across the conductor and the resulting current (I) through the conductor?
1. 2.
3.
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2. Circuits Solutions
5. An operating 75­watt lamp is connected to a 120­volt outlet. How much electrical energy is used by the lamp in 60 minutes (3,600 seconds)?
1. 4,500 joules 2. 270,000 joules 3. 540,000 joules 4. 32,000,000 joules
The amount of electrical energy used by the lamp is the product of the power it expends and the time it is operating: W = Pt. The power rating of the lamp is P = 75 watts, and it is operating for t = 60. min = 3,600 seconds. Calculate the amount of energy used as:
W = Pt = (75 watts)(3,600 sec) = 270,000 joules [Remember that 1 watt = 1 joule per second]
6. An electric motor draws 150 amperes of current while operating at 240 volts. What is the power rating of this motor?
1. 1.6 W 2. 380 W 3. 36,000 W The watt, W, is the SI unit of electrical power. The motor draws a current of I = 150 amps from a source of V = 240 volts. That means the power it develops is:
P = VI = (240 volts)(150 amps) = 36,000 watts
4. 54,000 W
7. What is the voltage across a 2.0­ohm resistor that draws 2.0 coulombs of charge per second?
1. 1.0 V 2. 2.0 V 3. 3.0 V 4. 4.0 V
A flow of 2.0 coulombs of charge per second represents a current of 2.0 amperes.
Resistance is related to voltage and current: R = V/I. Given R = 2.0 ohms and I = 2.0 coulombs/sec = 2.0 amps, calculate the voltage, V, in the circuit segment as: V = RI = (2.0 ohms)(2.0 amps) = 4.0 volts
[Recall that 1 ampere = 1 coulomb per second]
8. When a lamp is lit, it draws a current of 0.50 ampere. The amount of charge passing through the lamp in 10. seconds is
1. 0.050 C 2. 2.0 C 3. 5.0 C 4. 20. C
Current is a measure of the flow of charge: current = charge / time Current is the amount of charge passing a given point per unit time. The lamp draws a current of 0.50 A, and so the amount of charge flowing in 10. sec is 0.50 A × 10. sec = 5.0 C.
[A note on units: 1 ampere = 1 coulomb per second]
9. If the voltage across an operating 300.­watt floodlight is 120 volts, what is the current through the floodlight?
1. 0.40 A 2. 2.5 A 3. 7.5 A 4. 4.8 A
Electrical power is measured in watts.
The power developed in an electrical circuit is the product of the voltage and the current: P = VI. The floodlight draws P = 300 watts of power from a source of V = 120 volts. Calculate the electrical current in the circuit:
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2. Circuits Solutions
10. The graph shows the relationship between current and voltage for four resistors, A, B, C, and D .
Which resistor has the greatest resistance?
1. A 2. B 3. C 4. D
11. While operating at 120 volts, an electric toaster has a resistance of 15 ohms. The power used by the toaster is
1. 8.0 W The power drawn by the toaster can be calculated from the voltage and the resistance. Substitute V = 120 volts and R = 15 ohms to calculate the power, P, in watts:
2. 120 W 3. 960 W P = V2/R = (120 volts)2/(15 ohms) = 960 watts
4. 1,800 W 12. An electric iron draws a current of 5 amperes and has a resistance of 20 ohms. The amount of energy used by the iron in 40 seconds is
1. 100 J 2. 500 J 3. 4,000 J 4. 20,000 J
First determine the power developed in the circuit from the current and the resistance. Then calculate the amount of energy used by iron in 40 seconds.
Electric power is the rate at which energy is supplied to an electric circuit. The iron draws a current of 5 amps and has a resistance of 20 ohms, so the power, P, supplied to the circuit can be calculated as: P = I2R = (5 amps)2 × 20 ohms = 500 watts. The work done in 40 s is the amount of electric power used in that time: W = Pt = 500 watts × 40 s = 20,000 J.
13. The total amount of electrical energy used by a 315­watt television during 30.0 minutes of operation is
1. 5.67 × 105 J
2. 9.45 × 103 J
3. 1.05 × 101 J
4. 1.75 × 10­1 J
14. Which combination of units can be used to express electrical energy?
Electrical energy = volt•Amp * sec 1.
=volt *(coulomb/sec) *sec
2.
3.
4.
= volt * coulomb
volt•coulomb
volt•coulomb•second
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2. Circuits Solutions
15. An electric dryer consumes 6.0 × 106 joules of electrical energy when operating at 220 volts for 1.8 × 103 seconds. During operation, the dryer draws a current of
1. 10. A
2. 15 A
3. 9.0 × 102 A
4. 3.3 × 103 A
16. The diagram below shows currents in a segment of an electric circuit. What is the reading of ammeter A?
1. 1 A
2. 5 A
3. 9 A
4. 15 A
Ask Kirchhoff about conservation of charge and the current entering the central point of the circuit!
Kirchhoff’s second law for electric circuits says that the sum of the current flowing into the junction is equal to the sum of the current flowing out of the junction. Therefore, since 12 A are entering the junction in the diagram and 3 A are leaving one branch of the circuit, the other branch must carry 9 A.
17. Moving 4.0 coulombs of charge through a circuit requires 48 joules of electric energy. What is the potential difference across this circuit?
1. 190 V
2. 48 V
3. 12 V
The potential difference or voltage is the work that is done per unit of charge as the charge is moved through the circuit.
4. 4.0 V
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2. Circuits Solutions
18. The diagram represents a circuit consisting of two resistors connected to a source of potential difference.
What is the current through the 20.­ohm resistor?
1. 0.25 A
2. 6.0 A
3. 12 A
4. 4.0 A
19. How much total energy is dissipated in 10. seconds in a 4.0­ohm resistor with a current of 0.50 ampere?
1. 2.5 J
2. 5.0 J
3. 10. J
4. 20. J
20. The circuit diagram below represents four resistors connected to a 12­volt source.
What is the total current in the circuit?
1. 0.50 A
2. 2.0 A
3. 8.6 A
4. 24 A
21. Which electrical unit is equivalent to one joule?
1. volt per meter
2. ampere•volt
3. volt per coulomb
A coulomb, “C,” is an electrical unit equal to the transfer of one ampere per second. A volt is equal to one joule per coulomb. We can express 1 J in terms of 1 V and 1 C to 1 J by using this relationship.
4. coulomb•volt
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2. Circuits Solutions
22. Which circuit has the smallest equivalent resistance?
1.
2.
3.
4. 23. What is the magnitude of the electrostatic force between two electrons separated by a distance of 1.00 × 10–8 meter?
1. 2.56 × 10–22 N
2. 2.30 × 10–20 N
3. 2.30 × 10–12 N
4. 1.44 × 10–1 N
24. Which diagram below correctly illustrates an operating circuit that includes:
• a battery as a source of potential difference
• two resistors in parallel with each other
• an ammeter that reads the total current in the circuit
1.
2.
3.
4. 25. What is the minimum equipment needed to determine the power dissipated in a resistor of unknown value?
1. a voltmeter, only
2. an ammeter, only
3. a voltmeter and an ammeter, only
Use P = IV
4. a voltmeter, an ammeter, and a stopwatch
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2. Circuits Solutions
26. In the electric circuit diagram below, possible locations of an ammeter and a voltmeter are indicated by circles 1, 2, 3, and 4. Where should an ammeter be located to correctly measure the total current and where should a voltmeter be located to correctly measure the total voltage?
1. ammeter at 1 and voltmeter at 4
2. ammeter at 2 and voltmeter at 3
3. ammeter at 3 and voltmeter at 4
4. ammeter at 1 and voltmeter at 2
27. In which circuit would current flow through resistor R1, but not through resistor R2 while switch S is open?
1.
2.
3.
4. The only diagram that shows a complete circuit in which electricity would flow through R1 and not R2 is choice 1; the open switch does not affect the circuit for R1.
28. An electric series circuit contains a source of potential difference and 5­ohm resistors that combine to give the circuit an equivalent resistance of 15 ohms. Which diagram of this circuit correctly uses circuit symbols given in the Reference Tables for Physical Setting/Physics? [Assume the availability of any number of 5­ohm resistors and wires of negligible resistance.]
1.
2.
3.
4. 29. An electrical appliance draws 9.0 amperes of current when connected to a 120­volt source of potential difference. What is the total amount of power dissipated by this appliance?
1. 13 W
2. 110 W
3. 130 W
4. 1100 W
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2. Circuits Solutions
30. In the circuit diagram, two 4.0­ohm resistors are connected to a 16­volt battery as shown.
The rate at which electrical energy is expended in this circuit is
1. 8.0 W
2. 16 W
3. 32 W
4. 64 W
31. Base your answer to the question on the diagram, which represents an electric circuit consisting of four resistors and a 12­volt battery.
What is the current measured by ammeter A?
1. 0.50 A
2. 2.0 A
3. 72 A
4. 4.0 A
32. The diagram represents a simple circuit consisting of a variable resistor, a battery, an ammeter, and a voltmeter.
What is the effect of increasing the resistance of the variable resistor from 1000 Ω to 10000 Ω? [Assume constant temperature.]
1. The ammeter reading decreases.
2. The ammeter reading increases.
3. The voltmeter reading decreases.
4. The voltmeter reading increases.
33. In which circuit represented below are meters properly connected to measure the current through resistor R1 and the potential difference across resistor R2?
1.
2.
3.
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2. Circuits Solutions
34. Base your answer to question on the information below.
An 18­ohm resistor and a 36­ohm resistor are connected in parallel with a 24­
volt battery. A single ammeter is placed in the circuit to read its total current.
Which diagram of this circuit is drawn correctly using symbols from the Reference Tables for Physical Setting/Physics?
1.
2.
3.
4. 35. The diagram below represents part of an electric circuit containing three resistors.
What is the equivalent resistance of this part of the circuit?
1. 0.67 Ω
2. 1.5 Ω
3. 6.3 Ω
4. 19 Ω
36. In the circuit diagram shown, ammeter A1 reads 10. amperes.
What is the reading of ammeter A2?
1. 6.0 A 2. 10. A 3. 20. A 4. 4.0 A
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2. Circuits Solutions
37. Base your answer to the question on the information and diagram below.
A 20.­ohm resistor and a 30.­ohm resistor are connected in parallel to a 12­volt battery as shown. An ammeter is connected as shown.
What is the current reading of the ammeter?
1. 1.0 A
2. 0.60 A
3. 0.40 A
4. 0.20 A
38. Base your answer to the question on the information and diagram below.
A 20.­ohm resistor and a 30.­ohm resistor are connected in parallel to a 12­volt battery as shown. An ammeter is connected as shown.
What is the equivalent resistance of the circuit?
1. 10. Ω
2. 12 Ω
3. 25 Ω
4. 50. Ω
39. Which circuit diagram shows voltmeter V and ammeter A correctly positioned to measure the total potential difference of the circuit and the current through each resistor?
1.
2.
3.
4. An ammeter is placed in series in the circuit, and a voltmeter is placed in parallel across a resistor. Choice (1) shows a series circuit with the ammeter also placed in series to measure the total current. The voltmeter in choice (1) is placed in parallel across both resistors.
40. A 30.­ohm resistor and a 60.­ohm resistor are connected in an electric circuit as shown below.
Compared to the electric current through the 30.­ohm resistor, the electric current through the 60.­ohm resistor is
1. smaller
2. larger
3. the same
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Attachments
Conceptual Physics_ Demo­ Electric Current.3gp