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South Pasadena  A.P. Physics 1
ANSWERS
Practice Test
Name____________________ Period___
18  Electric Currents
& 19  DC Circuits
Terms and Symbols
Match the following units or symbols with their appropriate names:
Match the following units or symbols with their appropriate names:
1. Ohms
c
a) Current
b&d
a
c
a
c
b&d
2. Volts
3. Amperes
4. Ω
5. I
6. VVVV
7. |‫ا‬
b) Electric Potential
c) Resistance
d) Battery
**********************************************************
8. What is the definition (or formula) for
a) watt
1 Watt
=
b) volt
1 Volt
= 1 J/Coulomb
c) ampere 1 Ampere =
d) ohm
1 Ohm
9. If 10 coulombs of charge pass through a resistor in 5
seconds, what is the current? 10C/5seconds = 2 amps
1 J/sec
1 C/sec
= 1 V/A
Match the following Terms with their correct definitions
A) Resistor
B) Battery
C) Fuse
D) Voltmeter
E) Short-Circuit
C
10. This protects an electrical circuit from an overload or surge of current.
A
11. This device can influence the amount of current that passes through a circuit. A tolerance value is
usually stated on this device.
E
12. This causes electrical current to take an alternate pathway through a circuit.
B
13. This can come in different sizes and is analogous to water pressure in a garden hose.
D
14. This is placed in parallel in a circuit, to make measurements of a circuit.
Write the names listed below in the correct box:
Voltmeter
Resistor
Light Bulb
Battery
Battery
Voltmeter
V
Switch
Resistor
Light Bulb

Switch
Ohm’s Law
Solve the following problems:
1. A simple circuit consists of a 12-volt battery and a resistor of 100 ohms.
What is the current passing through the circuit?
12V/100 Ω =0.12 Amps
2. A certain toaster has a heating element with a resistance of 14 ohms. We connect
it to a 120 Volt outlet.
a) What is the current that is drawn? 120 V/14 Ω = 8.6 A
b) What is the power dissipated?
8.6 A x 120 V = 1032 W or 1030 Watts
3. The resistance from hand to hand across the human body, if it is dry, is 100 k Ω.
If you apply a 120 volt difference across your hands, what current
goes through your body?
I = 120 V / 100,000 Ω = 0.0012 A or 1.2 mA
4. For a human to have a “heart attack”, the current through the body must be
100 to 200 mA. If the body is affected by a 120-V potential of alternating current,
what resistance would be required for a current of 100 mA?
R = 120 V / 0.1 A = 1200 Ω
How might this resistance be achieved? By having moist or sweaty fingers which decreases resistance
5. What potential difference or voltage is required to cause 2.0 A to flow through a resistance
of 8.0 Ω?
8 Ω x 2.0 A = 16 Volts
6. If the voltage impressed across a circuit is constant but the resistance doubles, what
change occurs in the current?
Current halves or decreases to one-half
7. a) How much current flows through your fingers if you touch them to the terminals
of a 12-volt lawn garden battery, if the resistance between your fingers is 1200 ohms?
I = 12 V/1200 Ω = 0.01 A
Electric Power
1. How many joules/coulomb are given to charges that flow in a 120-volt circuit? 120 J/C
2. Recalling that Work = force x distance and is measured in joules,
a) what is the unit of power?
Rate
b) Power is the _______ at which work is done.
3. To calculate electric power, we use the formula;
Watts
Power = I x V or I2 R
4. A 60-V cell is connected to a 200 Ω resistor and a switch. The switch is closed for 90 minutes.
a) What is the current through the resistor while the switch is closed?
b) What is the power dissipated while the switch is closed?
I = 60 V / 200 Ω = 0.3 A
P = 0.3 A x 60 V = 18 Watts
c) What is the heat dissipated (in Joules) while the switch is closed?
Heat = 18 W x 90 min x 60 sec/min = 9.72 x 104 Joules
5. How many watts of power would be used by a calculator, that operates on 8.0 volts
and 0.1 amperes of current?
P = 0.1 A x 8.0 V = 0.80 Watts
6. If the calculator is used for one hour, how much energy in
kilowatt-hours does it use? Energy = 0.80 Watts x 1 kW/1,000 W x 1 hour = 0.00080 kW-hours
D.C./A.C. & Power
1.
If you made a graph of current vs. time with time on the horizontal axis,
a) what shape would the graph have for direct current from a battery?
b) what shape would the graph have for alternating current from an outlet?
|
|
I |
___________________
|
Straight line
|
|__________________________
time
(DC)
2.
|
|
I |
|
Sine Wave
|
|__________________________
time (AC)
If you use a 1200-watt hair dryer operating on an A.C. line of 120 volts,
will the dryer work if the current is limited to 15 amperes by a safety fuse?
YES
P = IV I = P/V = 1200 Watts/120 Volts = 10 amps
3.
If you used two identical hair dryers on the same A.C. line above, would
both be able to operate?
NO
10 Amps + 10 Amps = 20 Amps, which is greater than 15 amps
4.
What is the function of the third prong in a household electric plug? Acts as a ground
This prong is on top of the outlet
5.
How many amperes flow through a 60 watt bulb when 120 volts
are impressed across it?
60 W/ 120 V = 0.5 amps
Light Bulbs
1.
Draw a picture using a battery, bulb and wire that shows a successful way to make the light bulb light
up and an unsuccessful way to make the light bulb light up.
The successful way: one wire should touch the bottom of the bulb and the other wire touches the
side of the bulb. There are several unsuccessful ways to light the bulb. I will rely on your
memory from the activity, for which ways did not work.
2.
If you have a three-way bulb (50 W, 200 W & 250 W), explain how only two filaments would be
needed to provide these three values and draw a picture of how these two filaments might be different?
One filament would be thinner and be for the 50-Watt bulb.
The other filament would be thicker and be for the 200-W bulb.
When both filaments are working, that makes it 250-Watts (50 Watts + 200 Watts)
3.
Draw a circuit diagram showing 2 light bulbs connected in series with a 3.0 Volt battery.
Light
Bulb
3.0
Volts
4.
Light
Bulb
a) Which of the substances in the table below would be the best material to be used for a light bulb
filament? D would be the best material, since it has the highest melting point
Material
A
B
C
D
Melting Point º C
1959
2200
2893
3422
b) What is most likely the name of the material that you selected?
Tungsten Metal (Elemental Symbol is W)
5.
You have three wires of copper, A, B & C, which have diameters of 0.24 mm, 0.53 mm and 0.39 mm,
respectively. Place them in order of least resistance to greatest resistance.
_____ _____ _____
First would be .53 mm, then .39 mm and finally .24 mm
or B < C < A
Resistors in Series
1.
What is the є or voltage of the battery?
4V
2.
What is the voltage drop across the 10 ohm resistor?
2V
3.
What is the current passing through the 2 ohm resistor?
4.
What is the sum of the voltage drops across the 2 Ω and 8 Ω resistors?
5.
A resistor is listed as 100 ohms with a tolerance of ± 5%. What is the range of
values of resistance that you would expect with this resistor?
95 Ω
105 Ω
(100 Ω ± 5Ω)
6.
Three resistors of 5 Ω, 15 Ω and 25 Ω are connected in series. What is the total value of their
resistance?
45 Ω
7.
Three identical resistors of 100 Ω are connected in series to a battery. A current of 12 Amps
flows from the battery. How much current flows through any one of the resistors? 12 Amps
8.
What is the voltage of the battery from the previous problem?
9.
If one of the light bulbs goes out (breaks) what will happen to the other two light bulbs?
0.2 A (same everywhere)
2V
12 A x 300 Ω = 3600 V
They will go out!
Resistors in Parallel
Refer to the Diagram
for Questions 1-3
(circle the correct responses)
Five identical light bulbs are connected to a 120 V power supply. Each light bulb has a resistance of 15 Ω.
The switch is closed.
1. What is the net resistance of the circuit?
(A) 3 Ω
(B) 30.1 Ω
(C) 40 Ω
2. What is the current in the light bulb L1?
(A) 1.6 A
(B) 3 A
(C) 8 A
(D) 75 Ω
(D) 40 A
3. Which light bulb or bulbs could burn out without causing any others to go out?
(A) Only L1
(B) Only L2
(C) Only L4
(D) Only L5
4.
Three resistors of 5 Ω, 15 Ω and 25 Ω are connected in parallel. What is the total
value of their resistance?
3.3 Ω
5.
Three identical resistors of 100 Ω are connected in parallel to a battery. A current of 12 A
flows from the battery. How much current flows through any one of the resistors?
4A
6.
What is the voltage of the battery from the previous problem?
7.
If one of the light bulbs goes out (breaks) what will happen to the other two light bulbs?
400 V
Nothing
8.
Draw a circuit diagram of the
circuit in question #5 above:
9.
A 6.0 Ω and a 12 Ω resistor are connected in parallel to a 36-volt battery.
a) What is the current traveling through each resistor?
I1 = 36 V/6 Ω = 6 amps
I2 = 36 V/12 Ω = 3 amps
(I T = 36 Volts/4 Ω = 9 amps, and this is the sum of 6 amps and 3 amps)
b) What is the power dissipated by the 6.0 Ω resistor?
P = IV = 6 A x 36 V = 216 watts
Miscellaneous
1.
If you connected two 1.5 V batteries in series, what would be the total voltage?
3.0 Volts
2.
Draw a diagram with the electrodes of each battery labeled + and ─
and show how the two batteries should be connected.
3.
If two 4.0 Ω resistors are connected in parallel and then this combination is connected in series with a
3.0 Ω resistor, what is the total resistance of the three resistors? R parallel = 2 Ω
+ ─ + ─
R series = 3 Ω
R Total = 5 Ω
4.
Two resistors of 5.0 Ω and 9.0 Ω are connected in parallel. A 4.0 Ω resistor is then connected in
series with the other two. A 6.0 V battery is then connected to the series-parallel combination. What
is the current through the 5.0 Ω resistor? Tough Question 0.536 amps
5.
Which of the following factors will affect the resistance in a wire?
(1) the length of the wire
(2) the thickness of the wire
(3) the composition or type of material in the wire
(4) the temperature of the wire
6.
a) 1 and 2
c) 1, 2, and 3
b) 2 and 3
d) all of these
A light bulb filament is usually made up of which of the following metals?
a) silver
b) aluminum
7.
d) tungsten
A light filament should have a low or high melting point?
a) low
8.
c) iron
b) high
c) it depends on what the light bulb is used for
As the length of a wire increases, its resistance does what?
a) it also increases
b) it decreases
9.
A light bulb has a current of 0.5 amps running through it when it is connected to a 120-volt electrical
outlet. If the voltage source were changed from 120 volts to 10 volts, then the current reduces to only
0.2 amps. The new resistance of the light bulb . . .
a) would increase
b) would decrease
c) would stay the same
10.
What is the resistance of the circuit above, when the current flowing is 0.5 amps?
240 Ω
R = V/I = 120 Volts/ 0.5 A = 240 Ω
11.
What is the resistance of the circuit above, when the current flowing is 0.2 amps?
10 V/0.2 A = 50 Ω
Concepts
Circle the best answer:
SERIES CIRCUITS
1. In a Series Circuit, how does the voltage across the various resistors compare to the battery voltage?
a) They are the same values as the battery voltage.
b) The voltages have different values compared to the battery voltage.
2. In a Series Circuit, how does the current through the various resistors compare to the source current?
a) It is the same everywhere in the circuit.
b) The current varies through each resistor.
PARALLEL CIRCUITS
3. In a parallel circuit, how does the voltage across the various resistors compare to the battery voltage?
a) They are the same values as the battery voltage.
b) The voltages have different values compared to the battery voltage.
4. In a parallel circuit, how does the current through the various resistors compare to the source current?
a) It is the same everywhere in the circuit.
b) The current may vary through of the branches where there is a resistor.
Fill in the Chart Below for a 3 Volt Battery Source
Resistance
Current
x Voltage
= Power
2Ω
1.5 A
3V
4.5 W
2Ω
1.5 A
3V
4.5 W
1Ω
3.0 A
3V
9.0 W
Websites
This website has good simulations of electrical concepts:
http://micro.magnet.fsu.edu/electromag/java/index.html
This website has good interactive simulations where you get to construct a circuit yourself and then insert
meters to measure various values.
http://phet.colorado.edu/new/index.php
Of course, another good website to help with your review is the website for the Conceptual Physics textbook:
http://www.phschool.com/science/cpsurf/
Mind Trap Puzzle:
You are in a house with a first floor and a second floor. There are no windows in the house.
There are three wall switches on the first floor that operate three lights on the second floor. You are on the
1st floor and you want to know which switch operates which light. You may go upstairs only one time to
figure out which lights are operated by which switches. WHAT DO YOU DO?