## Electric Circuits Review

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### Part A: Multiple-Multiple Choice

1. Which of the following statements are true about an electric circuit? List all that apply.

1. Electrons are the mobile charge carriers in an electric circuit.
2. The path of charge flow from the + to the - terminal of the circuit can consist of nonconductive material.
3. In an electric circuit of an automobile, the 12-Volt car battery is sometimes referred to as the internal circuit because it is located inside of the hood of the car.
4. Charge is supplied with energy in the internal circuit and the energy is transformed into other forms in the external circuit.
5. Charge is consumed as it moves through the energy-transforming devices of the external circuit. For instance, the amount of charge entering a light bulb in a second is less than that which exits the light bulb in a second.
6. A battery is used to power a flashlight circuit. When the battery no longer works, it is because it has run out of charge.
7. The location on an electric circuit where the charge possesses the greatest amount of electric potential energy is the + terminal of the battery.
 Answer: ADG a. TRUE - Physicists often speak of conventional current as the direction that positive charge moves through a circuit. This is based on the convention that the direction of the electric field is the direction that a + test charge would be accelerated. In reality, the carrier of electric charge in an electric circuit is the electron. b. FALSE - For charge to flow through an external circuit, the pathway must consist of conductive materials. The presence of a nonconductor would inhibit charge flow and low the current to zero. c. FALSE - The internal circuit is the portion of the circuit inside the battery, located between the - terminal and the + terminal. Whether the battery is inside the hood or not has nothing to do with it. If you were to place your battery on top of your hood, it would still be the internal circuit. d. TRUE - The battery is the internal circuit; it is the place where charge acquires its energy. The external circuit consists of wires, light bulbs, resistors, etc. In the external circuit, charge has its electrical energy transformed into other forms as it passes through the elements of the external circuit. e. FALSE - Charge is neither created nor destroyed, and as such, the amount of charge which enters a light bulb in a second is equal to the amount of charge which exits the light bulb in a second. This quantity (charge per second) is the current and it is everywhere the same in a circuit. f. FALSE - Batteries produce energy through a exothermic chemical reaction. When a battery no longer works, the ability of its chemical reaction to produce products and energy has become limited by the depletion of reaction and the build up of products. It has not run out of charge; it has run of out chemical reactants. g. TRUE - Know this one! The + terminal is the location of high energy in an electric circuit.

 Useful Web Links What is an Electric Circuit? || Requirements of a Circuit [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

2. Which of the following statements are true about an electric potential or electric potential difference? List all that apply.

1. Electric potential can be expressed in units of volts.
2. Electric potential can also be expressed in units of Joules.
3. The electric potential of a charge at a given location provides a measure of the rate at which charge flows past that point.
4. Work must be done on a + charge to move it against (i.e., in the opposite direction of) an electric field.
5. As a + charge moves in the same direction as an electric field, it gains electric potential.
6. The electric potential difference between two points is simply the difference in potential energy possessed by charge between those two points.
7. If a unit of charge has a high electric potential at any given location, then that charge possesses a large amount of electric potential energy at that location.
8. The + terminal of a battery is a location with a higher electric potential than the - terminal of a battery.
9. Charge flowing through a battery will experience a gain in electric potential.
10. A 6-Volt battery would provide 12 Joules of energy to 2 Coulombs of charge as it is moved from the - to the + terminal.

 Answer: ADGHIJ a. TRUE - Electric potential, also known as voltage, is measured in units of volts. b. FALSE - The electric potential at a given location in a circuit is defined the electric potential energy per charge at the location. Its units would be energy units per charge units - a Joule per Coulomb (not simply a Joule). c. FALSE - The rate at which charge flows past a point is the current. Electric potential is the potential energy per charge at a given location. d. TRUE - By convention, an electric field is the direction which a positive test charge would be pushed or pulled. If a positive charge is moved against an electric field, then work must be done upon the charge to overcome the opposing effect of the field. e. FALSE - A positive charge moving in the direction of an electric field is moving without any external help. Such a charge is analogous to a bicycle moving downhill; the bicyclist would not have to pedal. In such instances, there is a loss in electric potential energy, and thus a loss in electric potential. f. FALSE - Close, but not correct. The difference in electric potential between any two points is simply the different in electric potential energy per charge between those two points. Using a gravitational analogy, gravitational potential is the difference in gravitational potential energy per mass between any two points. Gravitational potential energy is dependent upon the amount of mass present at a location (m•g•h); the more mass present at a location , the more gravitational potential energy. In contrast, gravitational potential reduces the importance of mass by defining itself as the potential energy per kilogram of mass (PE / m). By so doing, gravitational potential is now only location dependent - depending only upon the value of g at a given location and the height above some zero reference level. In the same manner, electric potential energy at a given location is charge-dependent. The more charge present at a location, the more repulsive (or attractive) effects and the greater the potential energy. But electric potential makes the amount of charge unimportant since it is the electric potential energy per charge. Now electric potential is location dependent - dependent for instance on the location in a circuit and not upon the amount of charge flowing past that location in a second. g. TRUE - Electric potential is the potential energy per charge at a given location. The more electric potential which a unit of charge has at a given location, the more electric potential energy that the charge has. h. TRUE - In the battery, the positive terminal is the location of high energy and high potential. The negative terminal is the location of low energy and low potential. i. TRUE - In the battery, a charge is moved from a location of low energy to a location of high energy. The charge would gain electric potential energy and as such, it would also gain electric potential. j. TRUE - A 6-Volt battery supplies every Coulomb of charge with 6 Joules of energy. The energy per charge ratio is 6 Joules per Coulomb. Thus, 2 Coulombs of charge would gain 12 Joules of energy.

 Useful Web Links Electric Field and the Movement of Charge || Electric Potential || Electric Potential Difference [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

3. Which of the following statements are true about electric current? List all that apply.

1. Electric current is measured in units of Amperes.
2. Electric current and drift speed are synonymous terms.
3. Electric current is defined as the number of Coulombs of charge which move past a point on a circuit.
4. Electric current is equal to the number of Coulombs of charge which move past a point on a circuit per unit of time.
5. Electric current provides a measure of how fast charge moves between two points on a circuit.
6. The electric current diminishes in value as charge progresses to locations further and further from the + terminal of the battery.
7. The electric current in a circuit will increase as the electric potential impressed across a circuit is increased.
8. The electric current in a circuit will triple in value as the electric potential impressed across a circuit is increased by a factor of three.
9. Suppose a miniature light bulb is connected to a battery in a circuit. A light bulb with a greater resistance will have a greater current.
10. Wider conducting wires are capable of carrying larger currents.
11. If one starts getting technical about the meaning of electric current, one could argue that electric current is not something that flows past a point on a circuit, but rather something that exists to a given measure at a point on a circuit.

 Answer: ADGHJK a. TRUE - Electric current is the rate at which charge flows past a point on a circuit. It is measured in Coulombs per second, also known as an Ampere or an "Amp." b. FALSE - No! Current refers to how many Coulombs of charge pass a cross-sectional area in a wire in a second of time. Drift speed refers to the distance that a mobile charge carrier travels in a unit of time. One quantity - current - has to do with how many? and the other quantity - drift speed - has to do with how far? c. FALSE - Current is a rate quantity much like velocity or power is a rate quantity. There is a per time basis to it. Current is defined as the Coulombs of charge which pass a point on a circuit per second of time. d. TRUE - Yes! This is the definition of current - a definition which ought to be committed to memory. e. FALSE - See the explanation to part b above. f. FALSE - Charge is marching around a circuit at the same rate everywhere within the circuit. The number of Coulombs which exit the battery at the + terminal in a second is equal to the number of Coulombs which enter the battery at the - terminal in a second. Charge is not created; charge is not destroyed; and charge does not accumulate at a location in a circuit. Charge simply marches together around the loop of a circuit. The quantity which is lost and gained is energy. Work is done on the charge as it passes through the battery, thus pumping the charge up to a high energy. And the charge subsequently does work upon the elements of the external circuit, transforming its electric potential energy to other useful forms (light, sound, mechanical, thermal, etc.). g. TRUE - A higher electric potential impressed across the two terminals of a battery will cause the rate of charge flow (i.e., current) to increase. This is consistent with the V = I•R equation. h. TRUE - Consistent with V = I•R, one could say that the current is directly proportional to the voltage. A tripling of the voltage (while R is held constant) would cause a tripling of the current. i. FALSE - A light bulb with a greater resistance will offer more hindrance to the flow of charge through it and as such lead to less current. Since I = V / R, the quantities current and resistance are inversely proportional. j. TRUE - A wider wire has a greater cross-sectional area and thus offers less resistance. As such, wider wires allow a greater current or flow rate. k. TRUE - Technically speaking, current does not flow or move. Current is a quantity which is measured at a location and is based upon the rate at which charge flows. Teachers and textbooks often get sloppy with their language and say that the current flows. In reality, it is the charge that does the moving; the current is simply measured at a location to reveal the rate at which charge flows past that location.

 Useful Web Links Electric Current || Common Misconceptions Regarding Electric Circuits || Journey of a Typical Electron [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

4. Which of the following statements are true about an electric resistance? List all that apply.

1. The resistance of an electric circuit is a measure of the overall amount of hindrance to the flow of charge through the circuit.
2. A large resistance value indicates that the charge is encountering a relatively large amount of difficulty in moving through the circuit.
3. The unit of resistance is the ampere.
4. A 5-ohm resistor would allow a current of 0.5 Amperes when 2.5 Volts is impressed across it.
5. A 10-ohm resistor would allow a current of 2 Amperes when 5 Volts is impressed across it.
6. The resistance of a conducting wire will increase as the length of the wire is increased.
7. The resistance of a conducting wire will increase as the cross-sectional area of the wire is increased.
8. Increasing the resistance of an electric circuit will cause the current in the circuit to increase.
9. A threefold increase in the resistance of an electric circuit will result in a threefold decrease in the electric current.
10. A miniature light bulb with a specific resistance is connected to a 1.5-Volt battery to form a circuit. If it were connected to a 6-Volt battery instead, its resistance would increase by a factor of 4.

 Answer: ABDFI a. TRUE - Yes! Remember this as the definition of resistance. b. TRUE - Resistance refers to the amount of hindrance (resistance) to the flow of charge. The more hindrance which charge encounters, the more resistance. c. FALSE - The unit of resistance is the ohm. Ampere or Amps is the unit of current. d. TRUE - The resistance of a resistor is the V/I ratio at that location. A 5-ohm resistor would have a 5:1 ratio of voltage to current. This is a true statement since (2.5 Volt) / (0.5 A) is equal to 5.0 Volt/Amp. e. FALSE - The resistance of a resistor is the V/I ratio at that location. A 10-ohm resistor would have a 10:1 ratio of voltage to current. This is a false statement since (5 Volt) / (2 A) is not equal to 10 Volt/Amp. f. TRUE - The resistance of a wire is mathematically related to the resistively of the material, the length of the wire and the cross-sectional area of the wire. Longer wires have a greater resistance. g. FALSE - The resistance of a wire is mathematically related to the resistively of the material, the length of the wire and the cross-sectional area of the wire. Wider wires have a lower resistance. h. FALSE - More resistance would cause less current. i. TRUE - In accordance with the equation I = V / R, current is inversely proportional to the resistance. Tripling the resistance will cause the current to decrease by a factor of three. j. FALSE - The resistance of a light bulb is an intrinsic property which depends upon the material and dimensions (length and diameter) of the light bulb filament. If the same light bulb is connected to a battery of greater potential, only the current at the bulb location will be modified.

 Useful Web Links Journey of a Typical Electron || Resistance || Ohm's Law [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

5. Which of the following statements are true about electric power and electric potential energy? List all that apply.

1. Power is the amount of energy used by an electrical appliance.
2. Power is the rate at which energy is used by an electric circuit.
3. A kiloWatt•hour is a unit of electric power.
4. A Joule/second is a unit of power.
5. A 60-Watt light bulb would consume 30 Joules of electrical energy in 2 seconds.
6. A 120-Watt light bulb would consume 1200 Joules of electrical energy in 6 seconds.
7. A charge moving through an electric circuit would gain electric potential energy in the internal circuit and lose electric potential energy in the external circuit.
8. Charge has the greatest amount of electric potential energy at the - terminal of the battery.
9. If a 60-Watt light bulb is placed in a 120-Volt circuit, then the current in the light bulb is 2 Amps.
10. Consider two light bulbs with different power ratings. A 60-Watt light bulb has a greater resistance than a 100-Watt light bulb.
 Answer: BDGJ a. FALSE - Power is a rate quantity; it is the rate at which energy is used by an electrical appliance. b. TRUE - Yes! Know this definition. c. FALSE - A kiloWatt is a unit of power. An hour is a unit of time. Power is energy per time. So when power is multiplied by time, the result is energy. A kiloWatt•hour is a unit of energy. d. TRUE - Power is the rate at which energy is used by an electrical appliance. As such, power is measure in energy per time units - Joules per second. e. FALSE - Power is the energy per time ratio. A 60-Watt bulb would consume 60 Joules of energy in 1 second, 120 Joules of energy in 2 seconds, and 30 Joules of energy in 0.5 seconds. f. FALSE - Power is the energy per time ratio. A 120-Watt bulb would consume 120 Joules of energy in 1 second, 720 Joules of energy in 6 seconds, and 1200 Joules of energy in 10 seconds. g. TRUE - This is how an electric circuit work. The internal circuit supplies electrical energy to the charge and the electrical energy is converted to other forms of energy in the external circuit. h. FALSE - The - terminal is the location of lowest energy in an electric circuit. the location of highest energy is the + terminal. i. FALSE - Power is related to voltage and current by the equation P = I•V. So a 60-Watt bulb would allow a current of 0.5 Amps if wired into a 120-Volt circuit. j. TRUE - A 60-Watt light bulb and a 100-Watt light bulb have different resistance values, which in turn effects the current at the light bulb location. The bulb with the greatest resistance has the lower current. So which bulb - high power or low power - has the greatest resistance? One must consider the expression for power which includes both the current (I) and the resistance (R) since the choice of bulb will effect both variables. The equation P = I2•R is of utmost importance here. In the equation, the I is of double the importance since it is the square term in the equation. So the bulb with the greatest I is the bulb with the greatest power. The 100-Watt bulb has the greatest current; and therefore it must have the lowest resistance. The 60-Watt bulb has the greatest resistance.

 Useful Web Links Power: Putting Charges to Work || Power Revisited [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

6. Which of the following statements are true about a series circuit? List all that apply.

1. Series circuits are characterized by the fact that there is a single pathway by which charge can travel from the + terminal to the - terminal.
2. During any single loop of charge around a series circuit, the charge will pass through each resistor in the circuit.
3. The overall resistance of a circuit increases as more and more resistors are placed in series in the circuit.
4. The total current in a circuit increases as more and more resistors are placed in series in the circuit.
5. Suppose that three identical resistors with a resistance of 3-ohms are placed in series with a 12-Volt battery. The current in the circuit is 4 Amps.
6. Suppose that three identical resistors with a resistance of 3-ohms are placed in series with a 12-Volt battery. The electric potential difference across each resistor is 12 Volts.
7. Suppose that two identical resistors are placed in series with a 12-Volt battery. The current in the circuit is 6 amps. The resistance of each resistor is 4 ohms.
 Answer: ABC a. TRUE - Series circuits are circuits which allow a single pathway for charge to follow; there is no no branching locations in a series circuit. b. TRUE - Since there are no branching points in a series circuit, charge is never left with a choice of passing through either Resistor A or Resistor B. It must pass through both resistors in consecutive fashion before returning to the - terminal of the battery. c. TRUE - Adding more resistors in series would mark the overall resistance of the circuit increase since each charge making a loop around the circuit will encounter each of these resistors. d. FALSE - If adding more resistors makes the overall resistance of the circuit increase, then it will make the overall current in the circuit decrease. For a constant battery voltage, current and resistance are inversely proportional. e. FALSE - Three resistors wit a resistance of 3 Ohms placed in series is equivalent to a single resistor with a resistance of 9 Ohms. The current through the circuit would be equal to the battery voltage divided by the overall or equivalent resistance. I = (VTot) / (RTot) = (12 Volts) / (9 Ohms) = 1.33 Amps. f. FALSE - In a series circuit, the voltage gained by the charge in passing through the battery - in this case, 12 Volts - is equal to the overall or cumulative voltage drop of the charge as it passes through the resistors. So the sum of the voltage drops in each of the three individual resistors must be 12 Volts. In the case of three resistors of equal resistance, the charge will lose 4 Volts in each resistors, thus accumulating a total voltage drop of 12 Volts. g. FALSE - The total or equivalent resistance of these two series resistors is simply the ratio of the battery voltage to the overall current. That is, RTot = (VTot) / (ITot) = (12 Volts) / (6 Amps) = 2 Ohms. For resistors in series, the overall or total resistance is the sum of the individual resistances. Thus, each resistor must have a resistance of 1 Ohm.

 Useful Web Links Two Types of Connections || Series Circuits [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

7. Which of the following statements are true about a parallel circuit? List all that apply.

1. Parallel circuits are characterized by the fact that there are multiple pathways by which charge can travel from the + terminal to the - terminal.
2. During any single loop of charge around a parallel circuit, the charge will pass through each resistor in the circuit.
3. The overall resistance of a circuit increases as more and more resistors are placed in parallel in the circuit.
4. The total current in a circuit increases as more and more resistors are placed in parallel in the circuit.
5. Suppose that three identical resistors with a resistance of 3-ohms are placed in parallel with a 12-Volt battery. The current in the circuit is 1 Amp.
6. Suppose that three identical resistors with a resistance of 3-ohms are placed in parallel with a 12-Volt battery. The electric potential difference across each resistor is 12 Volts.
7. Suppose that two identical resistors are placed in parallel with a 12-Volt battery. The overall current in the circuit is 6 amps. The resistance of each resistor is 4 ohms.
 Answer: ADFG a. TRUE - This is the definition of a parallel circuit. A parallel circuit contains a branching location. When a charge reaches the branching location, it may choose between a pathway which includes any one of at least two resistors. b. FALSE - This is a characteristic of a series circuit. In a parallel circuit, charge makes a either-or choice at the junction or branching locations, choosing to pass through either Resistor A or Resistor B. It does not pass through both of them in consecutive fashion, but only through one or the other resistor. c. FALSE - Increasing the number of resistors in parallel branches will have the effect of decreasing the overall resistance of the circuit. At first this seems rather counterintuitive. But consider that the new resistor actually opens up a new pathway through which the charge can pass. Not all the charge goes through all the resistors. By opening a new branch in the circuit, the fraction of charge which passes through any one resistor is reduced. So less charge actually encounters a single resistor. Now it makes sense to say that more resistors (i.e., more branches) leads to less overall resistance. d. TRUE - If increasing the number of resistors has the effect of decreasing the overall resistance of the circuit, then it must have the effect of increasing the overall current through the circuit. For a constant battery voltage, current and resistance are inversely proportional. e. FALSE - The overall resistance of a parallel circuit is expressed by the equation: 1/RTot = 1/R1 + 1/R2 + 1/R3 So for this circuit, substituting 3 Ohms for each of the resistors leads to a total resistance of 1 Ohm. The total current through the circuit is given by the equation I = (VTot) / (RTot) = (12 Volts) / (1 Ohm) = 12 Amps. f. TRUE - In any circuit - whether series or parallel, the voltage gained by the charge in passing through the battery - in this case, 12 Volts - is equal to the overall voltage drop of the charge as it passes through the external circuit. In the case of a parallel circuit, the charge makes an either-or choice between resistors. It passes through a single resistor instead of all three resistors. Thus, it must drop 12 Volts during the pass through that resistor. The voltage drop across each branch is 12 Volts. g. TRUE - The total or equivalent resistance of these two parallel resistors is simply the ratio of the battery voltage to the overall current. That is, RTot = (VTot) / (ITot) = (12 Volts) / (6 Amps) = 2 Ohms. For resistors in parallel, the overall or total resistance is expressed by the equation 1/RTot = 1/R1 + 1/R2 Substituting 4 Ohms for each of the individual resistances leads to the overall resistance of 2 Ohms. This is definitely a true statement.

 Useful Web Links Two Types of Connections || Parallel Circuits [ #1 | #2 | #3 | #4 | #5 | #6 | #7 ]

#### Navigate to:

Review Session Home - Topic Listing

Electric Circuits - Home || Printable Version || Questions with Links

Answers to Questions:  All || #1-7 || #8-51 || #52-59 || #60-72

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