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ACTIVITY OVERVIEW

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Hot Bulbs

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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

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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

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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.

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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-

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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%

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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

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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.

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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

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