Download Packet 13: Electric Circuits

Unit V: Electromagnetism
Subunit A: Electric Circuits
Equation
When to use it
Unit V: Electromagnetism
Subunit A: Electric Circuits
Objectives
By the time we finish the labs and related materials in this unit, you should be able to:
1. Define electric potential difference (voltage), electric current, and electrical resistance
and the relationship between them.
2. Differentiate between AC and DC and describe how electrons move in a DC and in an
AC circuit.
3. Distinguish between series and parallel circuits and list the properties of each.
4. State Ohm’s Law and use it to solve circuit problems.
5. Find the equivalent resistance of resistors in series, parallel and combination circuits.
6. Connect voltage sources so that voltages add, and correctly connect and read a
voltmeter.
7. Connect series and parallel circuits and correctly connect and read an ammeter.
8. Use an ohmmeter to measure electrical resistance.
Unit V-A: Electric Circuits
Worksheet 1
1. For each diagram, indicate whether work must be done upon the charge to move it from
point A to point B. Finally, indicate the point (A or B) with the greatest electric potential
energy and the greatest electric potential (PE/charge).
A)
B)
Work done on test charge?
Potential energy is greatest at:
Electric potential is greatest at:
Yes
A
A
No
B
B
C)
Work done on test charge?
Potential energy is greatest at:
Electric potential is greatest at:
Yes
A
A
No
B
B
D)
Work done on test charge?
Yes
No
Potential energy is greatest at:
A
B
Electric potential is greatest at:
A
B
Work done on test charge?
Yes
No
Potential energy is greatest at:
A
B
Electric potential is greatest at:
A
B
2. A) When work is done on a positive test charge to move it from one location to another,
potential energy _________ (increases, decreases) and electric potential _________
(increases, decreases).
B) When a positive test charge naturally moves from one location to another (without the
exertion of a non-conservative force), potential energy _________ (increases, decreases)
and electric potential _________ (increases, decreases).
Unit V-A: Electric Circuits
Worksheet 2
1. To maintain a charge flow in an electric circuit, at least two requirements must be met:
#1: An external energy supply (e.g., battery, wall outlet, generator, etc.) to pump the charge
through the internal circuit and establish a potential difference across the circuit.
#2: The external circuit must make up a "closed conducting loop" between the + and terminal.
a) Charge Flow: Yes or No?
Explanation:
c) Charge Flow: Yes or No?
Explanation:
b) Charge Flow: Yes or No?
Explanation:
d) Charge Flow: Yes or No?
Explanation:
2. If an electric circuit could be compared to a water circuit at a water park, then the
A. water pressure
D. bottom of the slide
B. gallons of water flowing down slide per minute
E. water pump
C. water
F. top of the slide
... battery would be analogous to the ________.
... positive terminal of the battery would be analogous to the ________.
... current would be analogous to the ________.
... charge would be analogous to the ________.
... electric potential difference would be analogous to the ________.
3. Mark True or False.
A) ______ When a battery no longer works, it is out of charge and must be re-charged before it
can be used again.
B) ______ A battery can be a source of charge in a circuit. The charge that flows through the
circuit originates in the battery.
C) ______ Charge becomes used up as it flows through a circuit. The amount of charge that
exits a light bulb is less than the amount that enters the light bulb.
D) ______ Charge flows through circuits at very high speeds. This explains why the light bulb
turns on immediately after the wall switch is flipped.
E) ______ Electric companies supplies millions of electrons to our homes everyday.
4. The net speed of electrons in a wire is about
A) the speed of light
B) 10 m/s
C) a few million kilometers per hour
D) less than 1 cm/s
5. The frequency of AC current in North America is
A) 120V
B) 60Hz
C) 50Hz
D) 30V
6. In alternating current, the motion of the charges
A) continuously changes directions
C) is equal to the speed of light
B) is greater than the speed of light
D) is always in the direction of the field
7. When electrons move through a metal conductor,
A) they move in a straight line through the conductor.
B) they move in zigzag patterns because of collisions with the vibrating metal atoms
C) the temperature of the conductor decreases
D) they move at the speed of light in a vacuum
8. How much charge flows through an electric iron in 10 seconds if the iron is rated at 6 A?
9. Calculate the current flow in a wire through which 1.0 x 1018 electrons pass per second.
10. The batteries in your flashlight draw 0.5 A of current, allowing a total of 5400 C of charge to
flow through the circuit until the batteries die. How long do you have to use the flashlight until you
need new batteries?
11. Calculate the current in a lightning stroke that lasts 0.05 s and transfers 100 C.
12. Household current in a circuit cannot generally exceed 15 A for safety reasons. What is the
maximum amount of charge that could flow through this circuit in a house during the course of a
24-hour day?
Unit V-A: Electric Circuits
Worksheet 3
1. Which of the following will cause the current through an electrical circuit to decrease?
Choose all that apply.
A) Decrease the voltage
C) Decrease the resistance
B) Increase the voltage
D) Increase the resistance
2. You have likely been warned to avoid contact with electrical appliances and electrical outlets
when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry)
because wet hands cause
A) the voltage of the circuit to be higher
C) the voltage of the circuit to be lower
B) your resistance to be higher
D) your resistance to be lower
3. If the resistance of a circuit is tripled, then the current through the circuit would be
A) 1/3 as much
B) 3 times as much
C) unchanged
4. When charge moves through a resistor, this is reduced.
A) current
B) potential difference
C) resistance
D) charge
5. Which has more resistance:
A) a long wire or a short one?
B) a thin wire or a thick one?
C) copper wire or aluminum wire?
6. A circuit is set up such that it has a current of 8.0 amps. What would be the new current if
A) … the resistance (R) is doubled?
B) … the resistance (R) is increased by a factor of 4?
C) … the resistance (R) is decreased by a factor of 3?
D) … the battery voltage (∆V) is increased by a factor of 3?
E) … the battery voltage (∆V) is cut in half?
F) … the resistance (R) is doubled and the battery voltage (∆V) is cut in half?
G) … the resistance (R) is decreased by a factor of 4 and the battery voltage (∆V) is increased
by a factor of 3?
7. Label the components of the schematic diagram shown.
8. Use circuit symbols to construct schematic diagrams for the following circuits:
A) A single cell, light bulb and switch are placed together in a circuit such that the switch can be
opened and closed to turn the light bulb on.
B) A generator, a lamp, two resistors, an ammeter, and a switch.
C) A three-cell battery, a fuse, 2 resistors, and an ammeter.
Unit V-A: Electric Circuits
Worksheet 4
1. An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 A to flow through it.
What is the voltage of the source?
2. A hair dryer operates on 110 V and draws 1100 mA. What is the resistance of the hair dryer?
3. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 Ω. What is the
current in the bulb?
4. A 4.5 kΩ electric heater has 5 mA of current passing through it. What is the voltage across the
heater?
5. A fan motor is rated a 600 mA and uses 120 V. What is the resistance of the fan motor?
6. The windings of a 120 V band saw motor have a resistance of 9.23 Ω. How much current flows
through the motor windings when the band saw is in operation?
7. 200 mA is flowing through a coil with a resistance of 8.1 MΩ. What is the voltage across the
coil?
8. A speaker is rated at 8 Ω. How much voltage is there across the speaker's input terminals
when it is connected to a 125 mA current supply?
9. The stepper motor in the printing head of a computer printer requires 24 V and has an internal
resistance of 100 Ω. How much current passes through the stepper's motor coils as it operates?
10. An LED in a digital clock has a 2-V supply. If 25 mA passes through the LED, how much is
the resistance of the LED?
Unit V-A: Electric Circuits
Worksheet 5
Matching.
A. Ammeter
B. Ampere
C. Battery
D. Conventional Current
E. Electric Current
G. Parallel Circuit
H. Series Circuit
I. Short Circuit
J. Voltmeter
1. _______ Occurs when a circuit forms that has a very low resistance.
2. _______ A connection that provides more than one current path.
3. _______ The flow of positive charge.
4. _______ A flow of charged particles.
5. _______ The unit of measurement of current, a flow of 1 Coulomb per second.
6. _______ A device that measures current.
7. _______ A connection that provides only one path for current.
8. _______ A device that measures the potential difference of a circuit.
9. _______ Several voltaic cells connected together to provide a potential difference in a circuit.
10. The current flowing in an electric circuit can be increased by
A) increasing the voltage or decreasing the number of paths
B) decreasing the voltage or increasing the number of paths
C) increasing the voltage and increasing the number of paths
D) decreasing the voltage and decreasing the number of paths
11. In a series circuit, the equivalent resistance is any single resistance.
A) larger than
B) equal to
C) determined by
D) smaller than 3
12. If the battery voltage does not change, adding more devices in series the current
A) sometimes decreases
B) sometimes increases
C) always decreases
D) always increases
13. Given the voltage of a series circuit, you first calculate to find the current through the circuit.
A) the voltage
B) the power
C) the equivalent resistance
D) the equivalent voltage
14. The overall or equivalent resistance of three resistors placed in series will be
A) greater than the resistance of the biggest R value of the three.
B) less than the resistance of the smallest R value of the three.
C) somewhere in between the smallest R and the biggest R value of the three.
15. The overall or equivalent resistance of three resistors placed in parallel will be
A) greater than the resistance of the biggest R value of the three.
B) less than the resistance of the smallest R value of the three.
C) somewhere in between the smallest R and the biggest R value of the three.
16. The current is a series circuit.
A) higher at the beginning of
C) the same everywhere in
B) lower at the beginning of
D) variable in
17. Three resistors connected in series carry currents labeled I1, I2, and I3. Which of the
following expresses the total current, IT in the combined system?
A) IT = I1 + I2 + I3
B) IT = I1 = I2 = I3
18. Three resistors connected in parallel carry currents labeled I1, I2, and I3. Which of the
following expresses the total current, IT in the combined system?
A) IT = I1 + I2 + I3
B) IT = I1 = I2 = I3
19. Two electric circuits are diagrammed below. For each circuit, indicate which two devices
are connected in series and which two devices are connected in parallel.
In series? ___________________
In series? ___________________
In parallel? _________________
In parallel? _________________
20. Comparing Series vs. Parallel Circuits
Fill in the table below to indicate the manner in which series and parallel circuits differ.
Series Circuit
a. Definition: The pathway by which
charge loops around the circuit is
characterized by __________ pathway.
b. Observation: If one light bulb goes
out, the other light bulbs _________.
c. Observation: As the number of
resistors is increased, the overall
current ______.
d. Observation: As the number of
resistors is increased, the overall
resistance ______.
Parallel Circuit
Unit V-A: Electric Circuits
Worksheet 6
1. A) What is the potential difference between points
i) A and B
ii) B and C
iii) C and D
iv) D and A
B) Calculate the equivalent resistance of the circuit.
C) Calculate the current through each resistor in the circuit.
2. A) What is the potential difference between points
i) A and B
ii) C and D
B) Calculate the equivalent resistance of the circuit.
C) Calculate the current through each resistor in the circuit.
D) What is the current in the wire leading from the battery to point a?
True or False.
3. _______ To measure the current through a resistor, an ammeter should be connected in
series with the resistor.
4. _______ The equivalent resistance of a parallel circuit is always less than the resistance
of any resistor in the circuit.
5. _______ A voltmeter should have a very low resistance so that it causes the largest
possible changes in currents and voltages in the circuits.
6. _______ The resistance of an ammeter should be as low as possible.
7. ______ To measure the current across a resistor, connect a voltmeter in parallel with the
resistor.
8. Household circuits are connected in parallel so that _____. Select all that apply.
A) houses get the same effect with less current and thus save on energy costs.
B) the turning off of one appliance does not result in the shut down of others.
C) the hazard of electrocution and overheating of circuits is avoided.
9. What is the equivalent resistance of three 12  resistors connected in series?
9. What is the equivalent resistance of a 6  and a 9  resistor connected in parallel?
10. Two resistors of 3.0  and 8.0  axe connected in series across a 9.0-V battery.
A) Draw a schematic diagram of this circuit.
B) What is the equivalent resistance of the circuit?
C) What is the current through the 3.0- resistor?
D) What is the current through the 8.0- resistor?
E) What is the voltage drop across each resistor?
11. A 15.0- resistor and an 8.0- lamp are connected in parallel and placed across a
difference in potential of 42 V.
A) Draw a schematic diagram of this circuit. Include an ammeter in the circuit and a
voltmeter across the lamp.
B) What is the equivalent resistance of the circuit?
C) What is the current in the circuit?
D) What is the current through each resistor?
E) What is the voltage drop across each resistor?
Unit V-A: Electric Circuits
Worksheet 7
1. Determine the equivalent resistance for the combination of three resistors, the total
current (i.e., at the battery), and the current at each of the three indicated locations.
2. Fill in the blanks.
RTot= ____________ ITot = ____________
∆V1= ____________ I1 = ____________
∆V2= ____________
I2 = ____________
∆V3= ____________ I3 = ____________
3. Fill in the blanks.
RTot= ____________ ITot = ____________
∆V1= ____________ I1 = ____________
∆V2= ____________ I2 = ____________
∆V3= ____________ I3 = ____________
4. Fill in the blanks.
RTot= ____________ ITot = ____________
∆V1= ____________ I1 = ____________
∆V2= ____________
I2 = ____________
∆V3= ____________ I3 = ____________
5. Fill in the blanks.
Unit V-A: Electric Circuits
Worksheet 8
Use the following choices to answer questions 1-5.
A) current
B) energy
C) resistance
D) voltage
E) power
1. _______ To reduce the loss of energy during transmittal of power over long distances,
either the current or the __________ must be reduced.
2. _______ Current can be reduced without reducing power by increasing the ______.
3. _______ The rate at which energy is delivered to a circuit by the energy source or the
rate at which energy is consumed by an electrical device is known as the electric ______.
4. _______ The electric companies measure their ______ sales in kiloWatt-hours.
5. _______ The most dangerous element for humans in an electric circuit is the ______.
6. People often claim that an electrical appliance "uses up electricity." Explain what is
actually being "used up" and what becomes of this thing that is being used up.
7. A) Determine the amount of energy used when listening to your 5.0-Watt IPod Nano for
10 hours.
B) Determine the resistance of the IPod if it uses a 5.2 Volt battery.
C) What quantity of charge passes through the IPod battery during this 10-hr time period?
8. Calculate the energy used and current drawn by a 200-W window fan when plugged into
a 110-V outlet and left on for an 8-hr time period.
9. Calculate the resistance of the 1000 W microwave oven that gets plugged into to a 110-V
outlet.
10. The TI-84 calculator uses four 1.5-V batteries and has a power of 0.0008 W. What is
the current?
11. Calculate the energy used and the current drawn by a 3400-W air conditioner that is
wired to a 220-V circuit and left on for an 8-hr time period.
12. A 2000-W hand dryer in a public bathroom at a zoo runs for 30 seconds per cycle. At a
cost of 13 cents per kW•hr, determine the cost of running the dryer 200 times in a day.
Unit V-A: Electric Circuits
Review Worksheet
EQUATIONS
PE = qV
Potential Energy
I=q/t
Current
V = IR
Ohm’s Law
P = IV
Watt’s Law
VARIABLES & UNITS
PE = potential energy in Joules, q = charge in
Coulombs, V= potential difference in Volts
I = current in Amps, q = charge in Coulombs, t = time
in seconds
V = potential difference in Volts, I = current in Amps,
R = resistance in Ohms (Ω)
P = power in Watts, I = current in Amps, V = potential
difference in Volts
Review Questions
1. Describe the energy conversions that take place in a circuit that includes a battery and a
lamp.
2. How is resistance related to:
A) length of a wire?
B) cross-sectional area of a wire?
C) conductivity of the wire?
3. How are current and total resistance affected when additional pathways (containing
resistors) are added to a circuit?
4. How are current and total resistance affected when more resistors are added to existing
pathways in a circuit?
5. Draw a schematic diagram including a 6.0-V battery, an ammeter measuring the current
out of the battery, two resistors in parallel, and a lamp in series with all the rest.
6. Use + or = signs to show how to find the total voltage, current, and equivalent resistance
in series and parallel circuits.
SERIES
VOLTAGE
VTOT
CURRENT
ITOT
I1
RESISTANCE
REq
R1
V1
V2
I2
R2
V3
I3
R3
PARALLEL
VTOT
V1
ITOT
I1
REq
R1
V2
I2
V3
I3
R2
R3
8. A) Find the reading of each ammeter and each voltmeter in the diagram below.
B) The equivalent resistance of the resistors in parallel is:
C) For the entire circuit, the equivalent resistance is:
D) The voltage drop across each 8.0-Ωresistor is:
E) What is the power in watts used by each resistance in the diagram above?