Download Electricity Module 2

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QUESTIONS AND PROBLEMS
ELECTRICAL RESISTANCE, ENERGY, AND POWER
For all problems assume that the cost of electricity = $0.12 per kilowatt-hour
1.
What is the resistance of a resistor through which 8.0 x 104 C flow in one hour if the potential difference
across it is 12 V?
2.
How much current flows through the 240-Ω, tungsten filament of a light bulb when it is connected to a
120-volt outlet?
3.
Let’s say that you dry your hair for five minutes a day with a 1400 W hair dryer. How much energy do you
use per year drying your hair?
4.
The element in an electric frying pan has a resistance of 86 ohms. If the frying pan is hooked up to a
standard 120-volt source, how long will it take to generate 50,000 J of heat?
5.
In 3.0 minutes, an immersion heater delivers 36,000 J of energy to a coffee cup full of water. The
immersion heater is connected to a standard 120-volt source. What is the resistance of the immersion
heater?
1
6.
Ever heard of the cake circuit? That’s right, an electrical circuit with the resistance load being some cake
batter. It may sound odd, but the cake circuit is every bit as genuine as an electric lamp, a refrigerator, or a
toaster. The electric lamp circuit produces light (actually it’s mostly heat), the refrigerator circuit produces
motion (the motor), and the toaster produces heat. The cake circuit is most like the toaster because it
produces heat that results in the cake being cooked. When the cake batter is connected to a standard 120-V
potential it draws 1.5-A and the cake takes 12 minutes to cook.
a. Calculate the resistance of the cake.
b. Calculate the power being used to cook the cake.
c. Calculate the energy used to cook the cake.
d. Calculate the cost of cooking the cake.
7.
When you buy a box of 60-watt light bulbs, the assumption is that you will use the standard 120-volt
potential provided by electrical energy providers throughout the country. What is the resistance of the
filament in these light bulbs?
8.
How much current does a 1500-watt hair dryer draw when it is connected to a 120-volt source?
9.
How much does it cost to run a 5.0-watt alarm clock for an entire year?
GET CHECKED
BEFORE MOVING ON
2
LAB
LIGHTS IN CIRCUITS
PURPOSE
To be able to build and conceptually analyze series and parallel circuits.
PROCEDURE
1.
Use wires to connect the metal contact lips of two D-cell battery holders to the contact points of a single
light bulb socket as shown in the photograph below. Confirm that the schematic diagram below is a
“blueprint” for the circuit. Note the brightness of the light bulb in this circuit.
light bulb
Schematic Diagram
+
–
+ –
batteries
2.
Now design a series circuit that allows you to light
two bulbs. You may notice that sometimes
“identical” bulbs can look a little bit different in
brightness. Use both batteries in the same
configuration as above. Don’t hook any new wires
to either battery!
In the space to the right, draw a schematic diagram of
the connections you made using the standard symbols
shown above.
Schematic Diagram
a. What happens if you unscrew one of the bulbs? Does the other bulb go out or stay on? Why or why not?
b. Compare the brightness of the two bulbs now to the original single bulb earlier.
c. How does the resistance of this circuit compare to the first circuit that you built? Explain.
d. How does the current in this circuit compare to the first circuit that you built? Explain.
e. How do the voltage drops at each light bulb compare to the voltage drop at the light bulb in the first
circuit that you built? Explain.
f. Use your responses to “d” and “e” above to explain your observation in “b.”
3
2.
Now design a parallel circuit that allows you to light
two bulbs. Use both batteries in the same
configuration as before. Don’t hook any new wires
to either battery!
Schematic Diagram
In the space to the right, draw a schematic diagram of
the connections you made using standard symbols.
a. What happens if you unscrew one of the bulbs? Does the other bulb go out or stay on? Why or why not?
b. Compare the brightness of the two bulbs now to the original single bulb and to the bulbs in the second
series circuit.
• Original circuit:
• Second series circuit:
c. How does the current in this circuit compare to the original single bulb and to the bulbs in the second
series circuit? Explain.
• Original circuit:
• Second series circuit:
d. How does the resistance of this circuit compare to the original single bulb and to the bulbs in the second
series circuit? Explain.
• Original circuit:
• Second series circuit:
e. How do the voltage drops at each light bulb of this circuit compare to the original single bulb and to the
bulbs in the second series circuit? Explain.
• Original circuit:
• Second series circuit:
f. Use your responses to “d” and “e” above to explain your observation in “b.”
GET CHECKED
BEFORE MOVING ON
4
LAB
ELECTRICAL CIRCUITS
INTRODUCTION
Your goal in this lab is to find patterns and
characteristics that are uniquely series or parallel.
The lab is one of discovery – a little bit more difficult
than simply verifying a proposed rule. Take careful
measurements and think carefully about the reasons
for the patterns you find.
USING THE CIRCUIT BOARDS AND
ELECTRIC MULTIMETERS
In this lab, you will need to wire circuits on a
circuit board. Use Figure 9.13 to help in
understanding how connections are made between
wires. You will also need to measure resistance,
voltage, and current at various locations in the
circuits. One multi-meter will make all the
measurements, but you have to be careful to
connect the meter correctly. Incorrect connections
will give you bad readings and can cause damage to
the meter. Use the directions below as a guide for
making your measurements. Each of the
measurements is being made on a circuit like the
one below.
Connected beneath
circuit board
Battery
Access springs
to positive and
negative poles
of battery.
Batter
R1
R2
This is the proper placement of the resistors
on the circuit board. Note that no
connection is needed between pairs of
springs as they are connected underneath
the circuit board.
MEASURING RESISTANCE
Battery
When measuring the resistance of a resistor, the resistor
be connected to the circuit. To measure resistance the meter
out a current. If the resistor is connected to the circuit, the meter
measure the combined resistance of the entire circuit instead. So
the resistor first and then measure as shown.
R1
Batter
R
R2
Battery
MEASURING VOLTAGE
When measuring the voltage dropped across a resistor, the
resistor must be connected to the circuit. To measure the
voltage, the meter tests the voltage at one end of the resistor and
then at the other end. It gives you the difference between the
two measurements. Therefore, current must be flowing through
and losing voltage at the resistor in order for there to be a
voltage loss.
5
Batter
R1
R2
V
cannot
sends
will
remove
MEASURING CURRENT
When measuring the current passing through a resistor, the current
must fully flow through the meter. You have to dismantle the circuit and
place the meter within it.
Battery
R1
Batter
I
R2
Measuring voltage:
1.
Connect meter in parallel with circuit,
as shown on the previous page.
2.
Use the “DCV” portion of the meter in
the “2” volt setting (circled below).
3.
The measurement will be in Volts.
Measuring current:
Measuring resistance:
1.
Connect meter in series with circuit, as
shown on the previous page.
1. Measure before building the circuit, as
shown on the previous page.
2.
Use the “DCA” portion of the meter in
the “200m” ampere setting (circled
above).
2. Use the “Ω” portion of the meter in the
“2K” ohm setting (circled above).
3.
The measurement must be divided by
1,000 to be in Amperes.
3. The measurement must be multiplied
by 1,000 to be in Ohms.
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PART 1: SERIES CIRCUITS
PURPOSE
To be able to build, recognize, and analyze series circuits. The lab, if truly grappled with, will result in a solid
understanding of the concepts of and differences between current and voltage.
PROCEDURE
1.
Measure the resistance of each of the resistors on your circuit board using the multimeter.
2.
Build a series circuit using Electricity Lab equipment as shown in the schematic below.
3.
Measure the current “I” in the circuit using the multimeter.
4.
Measure the voltage “V” across the battery using the multimeter. Record this as a positive voltage.
5.
Measure the voltages across each of the resistors “V1”, “V2”, “V3” using the multimeter. These voltages are the
decreases in voltage or “energy per charge” that the charges experience as they move through each of the resistors.
Record these as negative voltages.
DATA
V
Resistance (Ω)
Voltage (V)
R1: __________
V1: __________
R2: __________
V2: __________
R3: __________
V3: __________
Current (A)
I: __________
I
Battery
R1
R2
R3
V: __________
QUESTIONS/CALCULATIONS (SHOW ALL WORK)
1.
Calculate the total resistance in the circuit by using Ohm’s Law with “V” as the voltage and “I” as the current. Look at
the individual resistances you measured and the total resistance you’ve just calculated and tell how they compare.
Show evidence. Carefully explain why this should be so.
2.
How does the voltage across the battery compare to the individual voltage drops across the resistors? Show evidence.
Carefully explain why this should be so.
7
PART 2: PARALLEL CIRCUITS
PURPOSE
To be able to build, recognize, and analyze parallel circuits.
PROCEDURE
1.
Use the same resistors you used in Part 1. Record the values you previously measured.
2.
Build a parallel circuit using Electricity Lab equipment as shown in the schematic below.
3.
Measure the total current “I” in the circuit using the multimeter.
4.
Measure the currents through each of the resistors “I1”, “I2”, “I3” using the multimeter.
5.
Measure the voltage “V” across the battery using the multimeter.
6.
Measure the voltages across each of the resistors “V1”, “V2”, “V3” using the multimeter.
DATA
V
Resistance (Ω)
Voltage (V)
Current (A)
R1: __________
V1: __________
I1: __________
R2: __________
V2: __________
I2: __________
R3: __________
V3: __________
I3: __________
V: __________
I: __________
QUESTIONS/CALCULATIONS (SHOW ALL WORK)
I
Battery
Batter
R1
I1
R2
I2
R3
I3
1.
How does the total current compare to the individual currents through
each of the resistors? Show evidence. Carefully explain why this
should be so.
2.
Show how the voltage across the battery compares to the individual voltage drops across the resistors. Carefully
explain why this should be so.
3.
Calculate the total resistance in the circuit by using Ohm’s Law with “V” as the voltage and “I” as the current. Show
how this compares to the individual resistances in the circuit. Carefully explain why this should be so.
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PART 3: SUMMARY
For each of the multiple choice questions, give clear and complete evidence for your choice in the space provided.
1. _____
A series circuit has two resistors. One has twice as much resistance as the other. The voltage drop at the larger
resistor is:
a. half as much as at the smaller resistor.
c. twice as much as the smaller resistor.
b. the same as the smaller resistor.
d. none of the above.
2. _____
A series circuit has two resistors. One has twice as much resistance as the other. The current through the larger
resistor is:
a. half as much as at the smaller resistor.
c. twice as much as the smaller resistor.
b. the same as the smaller resistor.
d. none of the above.
3. _____
A parallel circuit has two resistors. One has twice as much resistance as the other. The voltage drop at the larger
resistor is:
a. half as much as at the smaller resistor.
c. twice as much as the smaller resistor.
b. the same as the smaller resistor.
d. none of the above.
4. _____
A parallel circuit has two resistors. One has twice as much resistance as the other. The current the larger resistor is:
a. half as much as at the smaller resistor.
c. twice as much as the smaller resistor.
b. the same as the smaller resistor.
d. none of the above.
5. _____
A series and parallel circuit both have the same two identical resistors. The power produced by the parallel circuit is
a. greater than the series circuit .
c. less than the series circuit.
b. the same as the series circuit.
d. could be greater or less than the series circuit.
GET CHECKED
BEFORE MOVING ON
9
QUESTIONS AND PROBLEMS
ELECTRIC CIRCUITS
1.
Imagine you have a 12-volt battery and two resistors, one 24 Ω and the other 48 Ω. The components are organized in a
series circuit.
a. Draw the circuit.
b. Determine the total resistance of the circuit.
c. Determine the current flowing in the circuit.
d. Determine the voltage dropped at each of the resistors.
2.
Imagine you have a 12-volt battery and two resistors, one 24 Ω and the other 48 Ω. The components are organized in a
parallel circuit.
a. Draw the circuit.
b. Determine the total resistance in the circuit.
c. Determine the total current flowing through the circuit.
d. Determine the current flowing through each resistor.
10
3.
a. In Circuit 1, R1 = 6 Ω, R2 = 12 Ω, and R3 = 30 Ω. What is the current
measured by the ammeter?
24 V
I
Battery
V
R3
R1
R2
Circuit 1
b. What is the voltage drop measured by the voltmeter in Circuit 1?
c. How much total power is being produced by the first two resistors in Circuit 1?
4.
a. In Circuit 2, the ammeter reads 2.0 A. The three resistors all have the same resistance. If the power produced by the
R2 is 15 watts, what is its resistance?
I
Battery
V
b. What is the voltage supplied by the battery in Circuit 2?
R3
R1
R2
Circuit 2
c. If the resistance of R2 were tripled, what would be the total power output of the circuit?
11
5.
a. In Circuit 3, R1 = 24 Ω, R2 = 36 Ω, and R3 = 48 Ω. What is the current measured by the
ammeter?
24 V
Battery
Batter
I
R1
R2
R3
Circuit 3
b. What is the power being used by R3 in Circuit 3?
c. What is the current flowing through R2 in Circuit 3?
6.
a. In Circuit 4, the ammeter reads 9.0 A. The three resistors all have the same resistance. If
the power produced by the R3 is 12 watts, what is its resistance?
Battery
Batter
I
R1
R2
R3
Circuit 4
b. What is the voltage provided by the battery in Circuit 4?
c. Now imagine that R2 is doubled in resistance and R3 is tripled in resistance. What is the total power produced by
Circuit 4?
GET CHECKED
BEFORE MOVING ON
12
ACTIVITY
CONCEPTUALIZING CIRCUITS
PURPOSE
To use your knowledge of circuit principles to make predictions about what changes will occur in circuits after they have
been altered. We will test each question with an actual demonstration.
1.
If one of the light bulbs is removed from the circuit to the right, what will happen to the
brightness of the other two bulbs? Explain clearly.
120 V
360 Ω
A
360 Ω
360 Ω
B
2.
Now imagine all three bulbs are back in the circuit. If a wire is connected between A and B,
what will happen to the brightness of each of the three bulbs? Explain clearly.
3.
Does the power in the circuit increase, decrease, or remain the same when the wire is connected? Explain clearly.
4.
The light bulbs are arranged differently, like in the circuit to the right. Are the bulbs
brighter, dimmer, or the same brightness as those in the original series circuit? Explain
clearly.
120 V
360 Ω
Batter
360 Ω
360 Ω
5.
If one of the light bulbs is removed from this new circuit, what will happen to the brightness of the other two bulbs?
Explain clearly.
6.
Now imagine that the bulb that was removed is replaced with a 240-Ω bulb. What happens to the brightness of the
other two light bulbs? And how does the brightness of the 240-Ω bulb compare to the brightness of the 360-Ω bulbs?
Explain clearly.
13
COMBINATION CIRCUITS
1.
In Circuit 5, R1 = 24 Ω, R2 = 48 Ω, and R3 = 36 Ω. What is the total
resistance of the circuit?
R1
I2
104 V
R3
2. What is the total current in the circuit?
Circuit 5
3. What is the total power produced by the circuit?
4. What is the current, I2?
5. What is the voltage drop across R1?
6. What is the power being produced in R1?
7. How much energy is produced in R1 in 4.0 minutes?
8. If R1 were doubled what would be the power produced by the entire circuit?
14
I1
R2
CONCEPTUALIZING CIRCUITS (CONTINUED)
1.
The light bulbs on page 13 are again arranged differently, like in the
circuit to the right. How does brightness in each bulb of this new circuit
compare with the brightness of the bulbs in the first series circuit (be
specific about how many times brighter or dimmer)? Explain clearly.
360 Ω
A
B
360 Ω
120 V
360 Ω
2.
How does brightness in each bulb of this new circuit compare with the brightness of the bulbs in the second
parallel circuit? Explain clearly.
3.
If the bottom bulb in this circuit is removed, what happens to the brightness in the other two bulbs?
Explain clearly.
4.
If the top bulb in this circuit is removed, what happens to the brightness in the other two bulbs? Explain
clearly.
5.
Now imagine all three bulbs are back in the circuit. If a wire is connected between A and B, what will
happen to the brightness of each of the three bulbs? Explain clearly.
GET CHECKED
BEFORE MOVING ON
15
LABETTE
COMBINATION CIRCUITS
PURPOSE
To be able to build and analyze combination circuits.
PROCEDURE
1.
Choose three resistors and measure each of their resistances using a multimeter.
2.
You and your partner will build the circuit to the right, but first you
will make the calculations below. Make sure that R1 and R2 are not
the same resistance. Show all work neatly, completely, and
carefully.
CALCULATIONS
1.
Total resistance in the circuit.
2.
Current through R3.
3.
Voltage drop on R3.
4.
Current through R2
5.
R1
R2
1.5 V
R3
When you have finished,
build your circuit and then
call me over to measure the
voltages and currents.
Power dissipated in R1.
GET CHECKED
BEFORE MOVING ON
16
LAB
MYSTERY CIRCUITS
INTRODUCTION
You can buy a simple “bath bar” at any home
improvement store. It is a bathroom light fixture with
a number of light bulbs that are all wired in parallel. I
bought several of these four-bulb circuits and wired
them in some of the other ways they could possibly
be wired. Your job is to:
• Test the bath bars by unscrewing various bulbs
and recording how this affects the
brightness of the other bulbs in the bath
bar. (Note that when all the bulbs are screwed
in, all bulbs are on, even if too dim to notice.)
• Draw a predicted schematic for the mystery
circuit, giving adequate explanation for your
decision (making one statement will not be
sufficient for most circuits).
The wiring schematic for the unaltered four-bulb bathroom bar is shown in Box 0. It is wired so that all four bulbs
are in parallel with each other. For each circuit, record what happens when you experiment with it (as I have
illustrated below) and give rationale for the schematic that you suggest (also illustrated below).
Observations: In this circuit, if any of the light bulbs were
removed, all the others would stay lit at the same level of
brightness
0
1
2
3
Rationale: Each bulb in this parallel circuit is independent of all
the others, and is therefore like its own series circuit, with one
bulb. Removing any other bulb neither changes the voltage drop
across the other bulbs nor the current flowing through them. It is
therefore, a simple parallel circuit.
4
1
17
2
Observations:
Rationale:
3
Observations:
Rationale:
4
Observations:
Rationale:
5
Observations:
Rationale:
18
6
Observations:
Rationale:
7
Observations:
Rationale:
8
Observations:
Rationale:
9
Observations:
Rationale:
19