67
2-3
-
ses
m inute
si
ACTIVITY OVERVIEW
LAB
O
RY
50-
on
40
to
s
Hot Bulbs
ORA
T
Students investigate a specific energy transformation and explore the efficiency of
the transformation. They measure the efficiency of a flashlight bulb in producing
light, by measuring how much energy is wasted in producing heat. They also compare the “lifetime” costs for an incandescent, a compact fluorescent, and a halogen
bulb. They then consider the trade-offs involved when deciding which type of bulb to
purchase. This activity introduces the watt as a quantitative measure of the rate of
energy use.
KEY CONCEPTS AND PROCESS SKILLS
(with correlation to NSE 5–8 Content Standards)
1.
Energy is associated with heat, light, electricity, mechanical motion, sound,
nuclei, and the nature of a chemical. (PhysSci: 3)
2.
Electrical circuits are a means of transferring electrical energy. (PhysSci: 3)
3.
The total energy of the universe is constant. (NSE Grades 9–12 PhysSci: 4)
4.
Students use appropriate tools and techniques to gather, analyze, and interpret
data. (Inquiry: 1)
5.
Students apply evidence when developing descriptions, explanations, predictions, and models. (Inquiry: 1)
6.
Mathematics is important to all aspects of scientific inquiry. (Inquiry: 2)
7.
All technological solutions have trade-offs. (SciTech: 2)
KEY VOCABULARY
control
efficiency
incandescent
fluorescent
watt
D-125
Activity 67 • Hot Bulbs
MATERIALS AND ADVANCE PREPARATION
For the teacher
1
Scoring Guide: ANALYZING DATA (AD)
1
overhead transparency of Student Sheet 53.1, “Anticipation Guide:
Ideas About Energy” (optional)
*
1
overhead projector (optional)
*
1
incandescent bulb, 40-watt
*
1
incandescent bulb, 75-watt
*
1
standard screw-in socket or lamp fixture
*
1
incandescent bulb, either 75- or 40-watt, painted black (optional)
For each group of four students
1
battery harness and leads
1
foam cap with flashlight bulb and socket
1
foam cap
2
SEPUP hot bulb trays
1
graduated cylinder
2
metal-backed thermometers
*
1
9-volt battery
*
1
timer
For each student
1
Student Sheet 53.1, “Anticipation Guide: Ideas About Energy,” from
Activity 53, “Home Energy Use”
1
Scoring Guide: ANALYZING DATA (AD) (optional)
*Not supplied in kit
Obtain a 40-watt and a 75-watt incandescent lightbulb and a socket or desk lamp that
holds these bulbs. You might also obtain a third bulb, either 40- or 75-watt, and paint
it black for an optional demonstration.
Obtain eight 9-volt batteries for the activity. Assemble the bulb-socket-foam cap apparatus by inserting the base of the bulb through the hole in the foam cap and then
securely screwing it into the socket.
SAFETY NOTE
The flashlight bulb used in this activity is inserted into a plastic cover to prevent contact between the bulb’s metal base and water. This experiment avoids risks associated
with water and electricity by using a low-amperage, low-voltage system. Under no circumstances should students attempt to measure the heat production of 120-volt lightbulbs by immersing them in water. Emphasize the extreme danger of shock or
D-126
Hot Bulbs • Activity 67
electrocution associated with such experiments. Do not allow students to touch the
two alligator clips together when the battery is attached to the harness. This could
short-circuit the battery and lead to overheating and smoking of the battery.
TEACHING SUMMARY
Getting Started
1.
Conduct a demonstration about rates of energy transformation.
2.
Introduce watts as a unit of measurement.
Doing the Activity
3.
Students measure the heat production of a lightbulb.
4.
Students calculate the efficiency of the bulb.
Follow-Up
5.
Review students’ results and the concept of energy efficiency.
6.
(AD ASSESSMENT) Students compare types of lightbulbs.
7.
(LITERACY) Students revise their earlier ideas about electrical energy.if this works)
BACKGROUND INFORMATION
Measuring Power
Power is the concept of “energy per time,” a rate typically measured in watts. Watts is
a measure of how much energy, in joules, is used per second. For example, a 100-watt
bulb uses 2.5 times more energy than a 40-watt bulb over the same time period. In
other words, a 40-watt bulb lit for 2.5 minutes uses the same energy as a 100-watt bulb
lit for 1 minute.
Although the joule is the SI unit for measuring energy, this activity uses the calorie, a
more convenient, and possibly more familiar, unit for energy. One calorie is equal to
4.2 joules, which means there are 0.24 calories in a joule. In this activity, the bulb used
by students is rated at approximately 1.9 watts (joule/sec). Converting to
calories/minute, a 1.9-watt bulb uses 27.4 calories per minute.
Types of lightbulbs
Incandescent bulbs use a small metal filament that glows to transform electrical
energy into light and heat. When lit, the hot filament slowly evaporates until it
becomes so thin that it breaks, causing the light to “burn out.” To prolong the life of
the filament, it is contained within a bulb that has had most of the air removed. Halogen lights are special incandescent bulbs that contain a halogen gas within their bulb
to increase the brightness and extend the life of the filament. Fluorescent bulbs do not
have filaments and instead create visible light in a series of steps during which electrical energy causes a gas inside the bulb to emit light energy.
D-127
Activity 67 • Hot Bulbs
TEACHING SUGGESTIONS
GETTING STARTED
1.
Conduct a demonstration about rates of
energy transformation.
Have students read the introduction.
After they finish, reiterate the fact that
a lightbulb produces both light and
heat. Demonstrate the point made by the scenario
with an investigation of the transformations in a
lightbulb. Tell students they will now look at two
common lightbulbs to see what they can learn
about their energy efficiency. Do not mention the
power ratings of the bulbs, however. Using any standard desk lamp, table lamp, or socket, place the 40watt lightbulb into the socket. Now turn the bulb
on, and ask, What kinds of energy are being produced? Students’ responses will certainly mention
the light and probably the thermal energy produced. If not, bring up light and heat, and ask, How
could we compare the amount of each kind of energy
produced? After trying out students’ suggestions on
how to measure the light and thermal energy, have
a student hold a thermometer two to three centimeters from the illuminated 40-watt bulb for one
minute and relate observations of any temperature
change. The class should write the observations in
their science notebooks. Now, repeat the demonstration using the 75-watt bulb, and again measure
the temperature change after one minute. A temperature increase of approximately 5°C is typical
for the 40-watt bulb, while a 10°C increase is typical
for the 75-watt bulb. Have students also observe
and comment on the brightness of each bulb.
Reducing the room light and illuminating a piece of
paper or another object with the bulb-and-lamp
assembly will help students see the difference in the
amount of light produced by the two bulbs.
Some students might have the misconception that
the visible light coming from the lightbulb “heated
up” the thermometer. To demonstrate that this is
not the case, you might repeat this demonstration
using a bulb painted black. In this case, much less
light is visible, yet the temperature still goes up.
This demonstration suggests that the incandescent
lightbulb is a good source of thermal energy in
addition to light. Mention to students that archi-
D-128
tects use the fact that lights (both incandescent
and, to a much lesser degree, fluorescent lights)
also create thermal energy. They now design new
office buildings to get some of their space-heating
requirements from the lights in the building.
2.
Introduce watts as a unit of measurement.
Ask students to list factors that may explain why
one bulb produced more heat than the other bulb.
They may suggest that one bulb uses more electricity and produces more energy than the other. Tell
them that the watt is unit of measure for power,
which is a rate. Watts are the measurement of how
much energy, in the form of electricity, the bulb uses
over a certain amount of time. The 75-watt bulb
uses more energy than the 40-watt bulb, almost
twice as much, over the same time period.
Relate the calorie, a unit already introduced in this
unit, to the concepts presented here. Remind students that the calorie measures how much energy,
and a watt measures how much energy per unit of
time is used. You may want to share the table below
or review the ideas presented in it when comparing
the two common units. Students will not be using
joules in this activity, so they will need to know the
following equivalency to complete the activity:
860 calories per hour = 1 watt
Unit Comparison
Unit
Calorie
Watt
System
British
SI
Measures
How much
energy
How much
energy per unit
time (rate)
Equivalent to
°C x grams
Joules/second
DOING THE ACTIVIT Y
3.
Students measure the heat production of a
lightbulb.
Remind students that efficiency of a lightbulb is a
ratio of how much useful energy output we get from
a lightbulb compared to the energy put into it. This
shows how “good” the bulb is at being a light. Reinforce the idea of efficiency by asking, If the lightbulb
were perfect at transforming electrical energy to light
energy, what percentage of the energy input would
wind up as useful light energy? The answer, of
Hot Bulbs • Activity 67
course, is 100%, or all of it. Ask, If the lightbulb is
less than 100% efficient, what happens to the rest of
the energy? It is transformed into heat, which in
most instances, is wasted energy. Explain that the
goal of the investigation is for students to measure
the relative amount of useful energy produced by a
small flashlight bulb and to use this measurement
to determine the efficiency of the bulb. Make sure
students realize that if they know the amount of
energy lost, in this case heat, and the total amount of
electrical energy input, they can find the efficiency.
Ask, Which do you think will more accurately measure the energy lost—submerging the lightbulb in
water or holding a thermometer next to the bulb?
Point out that a thermometer held in the air will
only receive a small portion of the heat energy
being released from all portions of the bulb and
that the water will collect the energy that comes
from all sides of the bulb. Now tell students that it is
easier to measure the heat energy produced by a
lightbulb by using water to collect the energy (just
as the energy of the nut was collected in the
calorimetry experiments) than it is to directly measure the lightbulb’s light energy. Therefore, our
experiment will measure the heat energy produced
by the lightbulb by measuring the temperature
change of the water. If appropriate, review the differ-
ence between temperature and heat, and introduce
or review the calorie as a unit for measuring energy.
Have students read Procedure Steps 1–9, and then
ask, “What is the role of a control in an experiment?”
If needed, remind them of the purpose and value of
controls. Stress that a control should be set up so that
as many variables as possible—such as water volume—are the same as with the experimental cup,
except the variable being tested, which in this experiment is the energy released from the lightbulb.
Distribute the equipment, and have students work
in groups of four to conduct Part A of the experiment. Students will typically obtain a temperature
change of 4–5°C within three minutes, but there
can be a large range of results.
4.
Students calculate the efficiency of the
bulb.
Part B of the Procedure requires students to calculate the efficiency of the bulb using various equations. Depending on the mathematical proficiency
of your students, you may choose to model one or
more of the calculations before students do these
steps. Alternatively, you can review these calculations after students have finished. A sample set of
calculations for Cup A based on a temperature
change of 5°C is shown below:
11. Calculate the thermal energy released from the flashlight bulb using the equation:
Energy released (calories) = temperature change (°C) x mass of water (mL)
= 12 mL x 5°C
= 60 calories
12. If the flashlight bulb uses about 27 calories of electrical energy for each minute it is lit, calculate the electrical
energy input using the equation:
Electrical energy absorbed (calories) = time bulb is lit (minutes) x 27 calories/minute
Electrical energy input (calories) = 3 minutes x 27 calories/minute
= 81 calories
13. Calculate the percent of thermal energy produced by the bulb using the equation:
Thermal energy output (%) =
=
thermal energy released
x 100%
electrical energy absorbed
60 calories
81 calories
x 100%
= 74.1%
14. Calculate the light efficiency of the bulb using the equation:
Light efficiency (%) = 100% – thermal energy output (%)
= 100% – 74.1%
= 25.9%
D-129
Activity 67 • Hot Bulbs
■ Teacher’s Note: Because a significant amount of
heat energy is “lost” to the environment in this
experiment (through wires, air, plastic, etc.), the
calculated heat production of the bulb is less than
the actual heat production. This results in an overestimation of the efficiency of energy transformed
into useful light, which is known to be around 5%.
FOLLOW-UP
5.
Review students’ results and the concept of
energy efficiency.
Have each student group report its experimental
results, and write each group’s calculated efficiency
on the board. If you haven’t yet, review the calculations from Procedure Steps 9–12. Although most
students typically obtain a temperature change of
4–5°C, others’ results can vary widely.
Review students’ responses to Analysis Question 2,
which asks students to reflect on their result. Hold a
class discussion about the range of value obtained
by the groups. Comment on any results that significantly vary from the others, and discuss why groups
should have similar values. Discuss possible sources
of error that would make values not match the 5%
shown in the question. It could be caused by inconsistent heat production of the bulbs due to differences in the bulbs or batteries, not timing for
exactly 3 minutes, and heat loss to the surrounding
air and tray. Heat loss in the wires and other connections that carry the electricity to the bulb also
contributes to a percentage lower than ideal. Students should be able to conclude that the “waste
heat” measured by this experiment is underestimated because not all the heat was trapped by the
water. Some heat was lost to the environment.
When discussing the efficiency of the bulb, point
out that in this case the value represents a single
energy transformation of electrical energy to light
energy. In lighting a bulb, the overall efficiency of
getting light is lowered further when one considers
the generation of the electricity. Students may
recall that transforming electrical energy from fossil fuels has an efficiency of about 40%. Therefore,
from the complete transformation of fossil fuels to
D-130
the lightbulb, there is only 4% efficiency because of
the two major transformations. This quick calculation emphasizes the importance of efficiency in the
issue of energy use. If we want to use energy wisely,
we should try to make the intended energy output
as great as possible and the lost or wasted energy as
small as possible.
6.
(AD ASSESSMENT) Students compare types of
lightbulbs.
Before students go on to answer Analysis Question
5, review the kinds of lightbulbs. Bulbs that students
may be familiar with are incandescent, fluorescent, halogen, and LEDs. If students mention different wattages, sizes, shapes, bases (such as
screw-in or side-pin), or 3-way bulbs, explain that
these are varieties of a type of bulb, not an entirely
different kind of bulb. Halogen lights are special
incandescent bulbs that contain a halogen gas that
increases the brightness and extends the life of the
filament. Fluorescent bulbs do not have filaments
and instead create visible light in a series of steps
during which electrical energy causes a gas inside
the bulb to emit light energy.
Analysis Question 5 asks students to analyze data in
a table and choose a type of lightbulb. Since compact
fluorescent bulbs have the highest efficiency, use the
least energy per second, have the longest lifetimes,
and have the lowest total cost per hour, many students will select them. Some students, who forget to
take into account that over the long run you will
need to buy eight incandescent bulbs for each compact fluorescent, choose incandescent bulbs because
they have the lowest initial costs. Some students may
chose another bulb and explain how they like the
quality of the light they give off, their size or shape,
or that they prefer not to have the “up-front” cost of
the fluorescent or halogen bulbs.
7.
(LITERACY) Students revise their earlier ideas
about electrical energy.
✓
At the end of the activity, revisit
Student Sheet 53.1, “Anticipation
Guide: Energy Ideas.” Have students complete the
After column for Questions 8–11. Correct responses
are shown on the next page.
Hot Bulbs • Activity 67
Answers to Student Sheet 53.1, “Anticipation Guide: Ideas About Energy.”
+
8. Electricity generation means electricity is transformed from another energy type.
This is true and is a direct result of the conservation of energy. The term “generation” really implies a
transformation.
+
9. Chemical reactions can give off energy.
In a previous activity students explored an exothermic chemical reaction that gave off electricity.
+
10. Electrical energy can be transformed into light, sound, or thermal energy.
Students observed this in a previous activity where light, sound, and heat were given off in a circuit via one
of its components.
—
11. Solar energy is a nonrenewable energy source.
Solar energy is considered a renewable energy source because the supply is greater than the usage.
Review these main ideas from the activities in the
unit. Let students know that in the next activity,
they will explore the renewable energy source of
solar that was identified in the last statement.
SUGGESTED ANSWERS TO QUESTIONS
1.
Answer the following questions about the
control in Cup E:
a. Why should you use a control in an experiment?
Cup E was used to determine if the water
would change temperature without the bulb
running.
b. What did you place in the control cup? Explain
why.
Cup E contained 12 mL of water and had a
foam cap over it.
c. What measurements did you take? Explain why.
Students should have recorded the initial
and final temperature readings for the
water. This will show whether the observed
change in temperature of the water in Cup A
was due to the lighted flashlight bulb and
not any other factor.
d. What did the results of your control tell you?
Most students will report very little, if any,
temperature change in Cup E. This indicates
that the temperature change observed in
Cup A was due to the energy emitted by the
glowing bulb. However, if the water used was
different than room temperature, a change
of a degree or two may be observed in Cup E.
2.
A typical lightbulb is about 5% efficient at
producing light energy. Does your calculation agree with this? Explain why you think your calculation is or is not the same.
Students typically get light efficiencies higher
than 10%, and in the 25–35% range. This occurs
because some of the heat produced is “lost” to
the environment. This results in a measured
heat efficiency that is lower and a corresponding light efficiency that is higher than they
actually are. Most of the heat “loss” occurs in
two ways:
• Some parts of the lightbulb were not surrounded by water, and thus some of the heat
produced was not transferred to the water.
• Some of the heat that was absorbed by the
water was transferred to the plastic and air
surrounding the water.
Students often suggest using an insulated cup to
trap the heat better.
3. Are lightbulbs better at producing light or heat?
Explain, using results from this experiment.
Because the calculated heat efficiency should be
greater than 50% (and therefore the light efficiency should be less than 50%) students should
conclude that these bulbs are better at producing heat than light.
D-131
Activity 67 • Hot Bulbs
b. Why do you think people buy more incandescent
lightbulbs than any other bulb?
4. Do you think you would be more concerned about
inefficient bulbs in a home that is in a warm climate
or a colder one? Explain.
There are a lot of reasons why people like
incandescent bulbs. Perhaps consumers are
used to buying that type of product. But most
people buy such items based on initial price
alone. They are unwilling (or unable or
unaware) to spend the considerably greater
amount of “up-front” money initially
required even though they know they will
save money over the longer time period.
Some people prefer the warmer color given
off by incandescent bulbs in comparison to
the bluer hue of compact fluorescents. Additionally, compact florescent bulbs do not
instantly turn on, and some people are concerned that they contain mercury. Halogens
are sometimes not chosen because they get
very hot and can be a safety and fire hazard
if not handled properly. For example, some
school dorms have outlawed halogen lights.
Inefficient bulbs are more of a problem during
the summer, when it is hotter, because the
“waste” heat from a bulb adds heat to the house.
This makes it more uncomfortable, and if the
house is air-conditioned, the air conditioner(s)
would have to work harder. During the winter
the “waste” heat from the bulb would help heat
the house, and the heater would not have to
work as hard.
5.
(AD ASSESSMENT) The bulb used in this activity is
an incandescent lightbulb. Look at the table below
that compares an incandescent lightbulb to other
kinds of bulbs that are about the same brightness.
Answer the following questions:
a. Which is the best lightbulb? Using the table,
explain the evidence that helped you decide.
Level 3 Response:
Based on the energy efficiencies given in the
table, the compact florescent is between 2
and 4 times more efficient than other bulbs.
Although it also costs much more, it has a
longer lifetime, making it overall the most
economical. The total cost per hour for the
compact fluorescent is $0.35 per hour of use,
or about three times more economical than
incandescent and halogen bulbs. This makes
it the best bulb, based on energy efficiency
and economy.
Energy Comparisons for Equally Bright Lightbulbs
Characteristics
Incandescent
Compact florescent
Halogen
5%
20%
9%
100 watts
23 watts
60 watts
Average lifetime
1,000 hours
12,000 hours
2,000 hours
Cost of one bulb
$0.75
$8.50
$6.00
Cost of electricity over lifetime of bulb
$12.00
$33.50
$14.00
$1.28
$0.35
$1.00
Efficiency
Rate of energy use
Total cost per hour over lifetime of bulb
D-132