15
LESSON
JEFF MCADAMS, PHOTOGRAPHER, COURTESY OF CAROLINA BIOLOGICAL SUPPLY COMPANY
Resistors in Electrical Circuits
INTRODUCTION
This lesson is the first in a series in which you
will explore how different electrical components
function in circuits. You have already used some
components in circuits. What components can
you name in the circuits that you built in Part 1?
In Lesson 14, you looked inside electrical
devices. You identified many new components
used in the circuits of the devices. Among the
components was something called a resistor. Your
challenge in this lesson will be to determine what
resistors do in circuits and how they affect the
operation of other components.
A set of resistors. What do resistors do in circuits?
OBJECTIVES FOR THIS LESSON
Investigate how resistors affect
the operation of a fan.
Observe and describe energy
transformations in resistors.
Calculate the value of a
resistor’s resistance.
Design a circuit to control the
brightness of a lightbulb.
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MATERIALS FOR
LESSON 15
Getting Started
the operation of the dual-speed fan
1. Review
that you looked inside in Lesson 14.
For you
1 copy of Student
Sheet 15.1: Circuit
Component Sheet
15.1 shows a schematic of the fan
2. Figure
circuit. There is a single-pole/double-throw
switch in this circuit. Now examine the
actual fan. Discuss the following questions
with the class:
For your group
1 circuit systems kit
1 switch
2 resistors
1 thermometer
1 student timer
What is the purpose of having the singlepole/double-throw switch in the fan circuit?
What happens when the switch is put in
different positions? Suggest an explanation for what you observe.
A
Figure 15.1
B
Fan-circuit schematic
the single-pole/double-throw
3. Examine
switch at your inquiry station and discuss
its design with your group.
fan-circuit schematic in Figure 15.1
4. The
contains a new symbol. Identify it by
referring to your electrical symbols chart
in Lesson 5.
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LESSON 15
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Inquiry 15.1
Putting Resistors in Circuits
Inquiry 15.2
Investigating Energy
Changes in Resistors
PROCEDURE
PROCEDURE
Student Sheet 15.1: Circuit
1. Review
Component Sheet. You will use copies of
this sheet throughout the lessons in Part
2, completing a circuit component sheet
for each component you investigate.
the following questions with
1. Discuss
your group:
How does having a resistor in the fan
circuit affect the speed of the fan?
the resistors at your inquiry
2. Examine
station. Draw a picture of a resistor on
Does the resistor use up any electrical
energy? How could you find out?
your circuit component sheet. Draw the
symbol for a resistor.
3.
Design and set up a circuit that will make
the fan run as fast as possible, using three
batteries, a switch, and the fan.
2. Set up the circuit shown in Figure 15.2.
you watch the fan run, discuss the
4. While
following question with your group:
What do you think will happen if you
add a resistor in line with the fan?
a procedure to determine the
5. Design
effect of having a resistor in the circuit.
Figure 15.2
out your procedure and describe
6. Carry
what you observe.
a thermometer on the resistor
3. Place
as shown in Figure 15.3. Record the
initial reading on the thermometer
with the switch open.
the following question in
7. Answer
your science notebook:
What is your definition of a resistor
based on the observations you have made?
Figure 15.3
Place the thermometer
bulb on top of the resistor.
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Setup for Inquiry 15.2
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LESSON 15
RESISTORS
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ELECTRICAL CIRCUITS
the switch. Observe the thermome4. Close
ter for 2 minutes. What happens to the
temperature? After 2 minutes, open the
switch and record the temperature on
the thermometer.
Steps 3 and 4 using a different
5. Repeat
resistor.
Figure 15.4
6.
Setup for Inquiry 15.3
Propose an explanation for your observations of the temperature.
the following question in your
7. Answer
science notebook:
How would you describe what a resistor
does in a circuit?
Inquiry 15.3
Measuring Resistance
PROCEDURE
1. Read “Measuring Resistance.”
up the circuit shown in Figure 15.4.
2. Set
Now use the voltmeter to measure the
voltage drop across the resistor when the
switch is closed. Record the current
through the resistor.
the value of the resistance of
3. Calculate
the resistor in the circuit. Remember to
use the following equation:
Resistance =
Voltage
Current
the resistor in the circuit with your
4. Replace
other resistor and calculate its resistance.
the following questions in your
5. Answer
science notebook and then discuss them
with the class:
A. What can you conclude about the resistance of the resistors?
B. How is the amount of resistance related
to the amount of current in the circuit?
C. How would you now describe the function of a resistor in a circuit?
SAFETY TIP
The resistor may
get quite hot. Allow
it to cool for a
minute before you
replace or remove
the resistor in
the circuit.
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LESSON 15
RESISTORS
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ELECTRICAL CIRCUITS
MEASURING RESISTANCE
You saw in Lesson 8 that if you increased the number of batteries in
series in an electrical circuit, the current in the circuit increased. The
current in the circuit was directly proportional to the number of batteries.
What do you find if you measure the voltage across a device and the
current through it?
In the 1820s, German scientist Georg Ohm investigated the relationship of voltage and current for conductors. To do that, he measured the
voltage across a conducting material and the current through it at the
same time. He found that as the voltage across the material increased,
the current through it also increased. The graph below shows what
Ohm found.
The graph shows a direct proportion between voltage and current.
What that means is that when you increase the voltage across the circuit, the current in the circuit increases in the same way.
This relationship between voltage, current, and resistance is called
Ohm’s law. Ohm’s law is one of the basic laws of electrical circuits. To
honor Ohm’s work, the unit of resistance is named the ohm. The symbol
for the ohm is Ω.
Current and Voltage for Constant Resistance
Current (in amps)
15
10
5
0
2
4
Voltage (V)
What Ohm discovered about voltage and current
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LESSON 15
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Ohm recognized that the relationship he observed in the graph could
be described by an equation. Ohm’s equation had voltage, current, and
something Ohm called “resistance.” Ohm was able to write his law as
the following equation:
Voltage = current × resistance
Ohm could measure voltage with a voltmeter and current with an
ammeter. But he could not measure resistance directly. Ohm realized
he could use his equation to calculate the resistance of conducting
materials. He rearranged the equation:
Resistance =
Voltage
Current
In other words, the resistance of a material is calculated by dividing
the voltage across it by the current through it.
Like Ohm, you can measure the voltage with a voltmeter and the current with an ammeter. And, by making these two measurements, you can
calculate the resistance of a material or device in an electrical circuit.
Here’s an example of how to use Ohm’s law to find the resistance of
a lightbulb:
A lightbulb in an electrical circuit has a voltage of 3.0 volts across
it and a current of 0.5 amperes through it. What is the resistance of the
lightbulb?
Resistance =
Voltage
Current
Resistance =
3.0 V
0.5 A
Resistance = 6.0 Ω
You can also use Ohm’s law to predict the current through a device if
you know the voltage across it and its resistance:
Current =
Voltage
Resistance
Electricians use this equation to provide just the right amount of
current to electrical components in circuits.
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REFLECTING ON WHAT YOU’VE DONE
the remainder of the circuit
1. Complete
component sheet for resistors. Cite evidence to support your conclusions.
the fan circuit you examined at
2. Revisit
the beginning of this lesson. Write an
explanation of why the fan operates as it
does based on what you have learned in
this lesson.
a circuit that has a single lightbulb
3. Design
in it so that it can be brightly or dimly lit
with the throw of a switch.
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THE LIGHTBULB:
A BRIGHT IDEA
Standard Incandescent
An incandescent lightbulb lights when an electrical current flows through a filament inside
and the filament heats up to incandescence—
that is, it gets so hot that it glows with light. The
filament is a thin wire made of the metal tungsten. Heat generated by the current through the
tungsten causes the tungsten gradually to evaporate, leaving a dark spot on the inside of the
glass. Because tungsten is evaporating, the
filament gets thinner and thinner.
The evaporated tungsten condenses on the inside surface
of the lightbulb, creating a
black deposit, which you
can see. Eventually the
lightbulb “burns out”
because the filament
at some spots gets so
thin that it breaks.
Other parts of a standard incandescent lightbulb include a glass bulb and a metal base. The
glass bulb encloses the filament. The bulb is
filled with nitrogen and argon gases, which
reduce the Tungsten’s evaporation rate. A slower
evaporation rate makes the filament last longer.
Screw-type bases hold lightbulbs in lighting fixtures and provide the means to connect the
lightbulb to an electrical power source. The base
completes a circuit and allows current to flow
through the filament.
Standard incandescent lightbulbs are inexpensive and come in many shapes and colors. They
produce different amounts of light, and require
40–150 watts of power. A 100-watt bulb will last
about 1,000 hours. Standard incandescent bulbs,
however, are not very energy efficient.
Only about 8 percent of the electrical energy produces light. The
rest is lost as heat.
(continued)
COURTESY OF PHILIPS LIGHTING COMPANY
The sun is setting. Daylight is fading. It’s getting
hard to see the book you’re reading. What do
you do? You turn on the lights! Many kinds of
lightbulbs light our homes. The most common
ones are standard incandescent, halogen, and
fluorescent.
Incandescent lightbulb
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LESSON 15
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COURTESY OF PHILIPS
LIGHTING COMPANY
Halogen
A halogen lightbulb is a modified incandescent
lightbulb. Halogen lightbulbs use a tungsten filament, too, but they contain a small amount of a
chemical element—usually iodine or bromine.
Some of the tungsten combines with the iodine
or bromine, causing the tungsten to deposit
back on the filament instead of the lightbulb.
The filament does not thin as quickly as in an
incandescent lightbulb. Therefore, a halogen
lightbulb lasts longer. Because halogen
lightbulbs operate at very
high temperatures,
ordinary glass cannot
be used for the bulb.
Most halogen bulbs
are made of quartz
to withstand the
extreme heat.
Because halogen
lightbulbs burn at
very high temperatures, they produce
Halogen lightbulb
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Tips for Changing Lightbulbs
• Always turn off or unplug the lamp or
device before you replace its lightbulb.
• Make sure that the lightbulb has cooled
sufficiently before you touch it to avoid
burning your fingers.
• Do not touch the glass part of the lightbulb with your fingers. Put a cloth
around the glass part of the lightbulb
when removing or putting in the new
lightbulb. The oils from your fingers
can reduce the life of some lightbulbs.
a brilliant white light. They are more expensive
than standard incandescent lightbulbs, but they
can last twice as long. In addition, halogen lightbulbs are more energy efficient. Depending on the
number of watts, they may use 20–40 percent less
electrical energy to produce the same amount of
light as standard incandescent lightbulbs.
LESSON 15
RESISTORS
IN
ELECTRICAL CIRCUITS
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Fluorescent lamp
Fluorescent
Fluorescent lamps come in different shapes.
Some are long glass tubes. Others, such as compact fluorescent lightbulbs, look more like standard incandescent lightbulbs. Whatever their
shape, they work the same way. The inside surface of the tube or bulb is coated with powders
called phosphors. The tube or bulb contains
small amounts of mercury vapor. When the
lamp is switched on, electrical current flows
through the mercury vapor, which emits ultraviolet light. The ultraviolet light is absorbed by
the phosphors, which makes the phosphors
glow, or fluoresce.
Fluorescent tubes spread light over a large
area. This kind of fluorescent lighting is commonly used in large rooms, such as in schools
and stores. In homes, fluorescent tubes are
sometimes used in kitchens and large playrooms. Compact fluorescent lightbulbs usually
fit in standard incandescent lightbulb sockets
and can be used throughout a home.
On average, fluorescent tubes last about
15,000 hours, fluorescent lightbulbs about
13,000 hours. That’s a lot longer than incandescent lightbulbs last. For example, a 100-watt
standard incandescent lightbulb burning for 8
hours every day will last about 4 months. A
compact fluorescent lightbulb of equivalent
wattage would last more than 4 years! In addition, fluorescent lamps use 60–80 percent less
energy than incandescent lightbulbs, and they
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LESSON 15
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Georg Simon Ohm:
LIBRARY OF CONGRESS, PRINTS & PHOTOGRAPHS DIVISION, LC-USZ62-40943
Taking a
Different
Approach
Trying to change the way people think
about things can be quite difficult.
Georg Ohm, a mathematician and
physicist, certainly found that to be
true. In the early 19th century, Ohm
conducted important research in electricity. However, he did not present
his findings in the way most scientists
of his day reported their scientific
research. Ohm analyzed his data
mathematically and demonstrated
mathematical relationships between
the data variables. At that time, most
scientists did not use this kind of
Georg Simon Ohm (1787–1854) is famous for his investigations
of electrical resistance.
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LESSON 15
mathematical approach. As a result, Ohm
found it difficult for other scientists to accept
his findings.
Ohm grew up and attended school in Erlangen,
Bavaria (now Germany), but most of Ohm’s early
education came from his father, Johann. He gave
his son a strong background in science and
mathematics.
Ohm Gets to Work
In 1811 Ohm received a doctorate in mathematics from the University of Erlangen. He
stayed on there as a lecturer, but he was frustrated with the low wages and lack of prestige
of the job. He really wanted to be a professor at
one of the Germany’s great universities—the
University of Munich, for example.
So Ohm left Erlangen in 1812. During the
next few years, he dedicated most of his time
to studying the works of other scientists and
mathematicians. Ohm was especially intrigued
with the work of Oersted, the discoverer of
electromagnetism. Inspired by Oersted’s work,
Ohm began conducting his own experiments in
electricity. In 1827, Ohm published a book on
his theory of electricity.
RESISTORS
IN
ELECTRICAL CIRCUITS
Ohm’s Law
One of the Ohm’s experiments was an investigation of the relationship of voltage and current in
electrical elements. He analyzed his data and
used mathematics to show how voltage, current,
and something called “electrical resistance” are
related in an electrical circuit. That relationship—voltage equals current times resistance, or
V = IR—is now known as Ohm’s Law. Today it is
recognized as a significant discovery and a very
useful formula, but at the time, Ohm’s German
colleagues were not impressed. They did not
believe in his mathematical approach to physics.
However, the value of Ohm’s work slowly
gained recognition. He received the Copley
Medal from the Royal Society in 1841. In 1845
he became a member of the Bavarian Academy,
a prestigious group of scientists. In 1852, Ohm
realized his lifelong dream. He became a professor at the University of Munich.
Ohm’s contributions are still recognized
today. If you visit his hometown of Erlangen,
you’ll see a school and a square named in his
honor. And students everywhere say his name
when they learn about the ohm—the unit of
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