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Heat from the light bulb

Heat from the light bulb

Introduction: (Initial Observation)

Light bulbs are available in many different sizes, wattage and styles. Almost all light bulbs get hot, some more and some less. Heat of light bulb may be high enough to cause fire. A piece of paper or cotton cloth next to a hot light bulb is a recipe for disaster.
Heat of light bulbs are often high enough to be used as heat source. I have seen an egg incubator warming up by a few light bulbs.

Egg incubators are machines for hatching eggs artificially.

Do all light bulbs produce the same amount of heat? Does the size of light bulb affect the heat it produces? Does the wattage of a light bulb affect the heat it produces? These are all possible questions that may be studied.

In this project you will study the effect of wattage on the heat a light bulb produce.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about light bulbs and how they work. Read books, magazines or ask professionals who might know in order to learn about the effect of wattage on producing heat. Keep track of where you got your information from.

Following are samples of information you may find:

Types of light bulbs

There are three different basic types of light bulbs, incandescent, fluorescent, and high-intensity discharge. Each type works a little differently. There are also differences within each of these groups. Bulbs come in different shapes, sizes, wattages and voltages. There are also a number of different bases on these
different bulbs.


An incandescent light bulb is a glass bulb with a metal wire called a filament inside a vacuum. As electricity flows through the filament it heats up and glows, thereby producing light. Much of the energy used to create the light is lost in heat which is why incandescent are not the most efficient type of light.

Incandescent bulbs come in a variety of shapes and sizes. Some common shapes are a round globe, like we use in a bathroom bar type fixture. The standard pear shape used in common lighting fixtures. Long tube shape for use in fish tank lights and wall painting accent lights. Reflector bulbs are those found in recessed lights. PAR or parabolic aluminized reflector bulbs can be used to direct light and with a thick glass exterior, can be used outdoors.
Halogen bulbs are a recent improvement on the incandescent idea. The filament is encased in a small quartz envelope and because it burns at a high temperature a chemical reaction occurs causing the filament to last much longer than an ordinary incandescent.


Fluorescent light bulbs are comprised of a thin glass tube that is coated on the inside with a white powder called phosphor. A ballast produces a high current which passes through the bulb and with the gas inside creates ultraviolet energy which hits the phosphor and gets converted to light.
The fluorescent light has traditionally been used mostly in retail, commercial and educational buildings. In recent years, with the improvement in the quality of light from fluorescents and the introduction of compact fluorescents, there has been a large move into the residential market.

The advantages of fluorescent is they last much longer than incandescent and are much more energy efficient.


High intensity discharge lamps work in a similar way to
fluorescent light bulbs. They work by having electricity create an arc which makes a gas inside a quartz envelope burn very hot thereby producing light. This type of light is very efficient and is used mostly in outdoor applications. Most street lighting you see and stadium lighting is HID.

There are advantages and disadvantages with different types of bulbs. Halogens, for instance, give a close to sunlight type of light but the trade off is they burn very hot. Fluorescent lighting is both economical to run and cool but generally don’t come in a lot of attractive fixtures.


Incandescent lights give off heat as well as light energy. The higher the wattage of the light bulb the higher the temperature. A compact fluorescent bulb gives off very little heat energy because they do not use resistance and cause a light to glow hot.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to study the heat production by different sizes of light bulbs. The question is:

Does the wattage of a light bulb affect the amount of heat it produces?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable (also known as manipulated variable) is the wattage of the light bulb.

Dependent variable (also known as responding variable) is the heat each light bulb produces.

Constant is the type of light bulb (incandescent), experiment procedure and method.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

This is a sample hypothesis:

All same type light bulbs produce the same amount of heat. The wattage only affects the amount of light each light bulb produces. My hypothesis is based on my observation of incandescent light bulbs that are all hot and fluorescent light bulbs that are just warm.

This is another sample hypothesis:

In same type light bulbs, as the wattage increases, the amount of heat will increase as well. My hypothesis is based on my observation of my desk lamp that gets very hot and my night light that does not get very hot.

Please note that your experiment results may or may not support your hypothesis. In other words your hypothesis may be proven wrong.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: Compare the radiated heat energy of different wattage light bulbs


  1. A goose-neck style lamp.
  2. An extension cord.
  3. Different wattage of incandescent light bulbs – 25 watt, 40 watt, 60 watt, 75 watt, 100 watt, 150 watt.
  4. Compact Fluorescent light bulbs – 7 watt, 23 watt – They are expensive; so shop around for ones that don’t cost so much.
  5. Thermometer.
  6. A ruler or yard stick to measure distance from the thermometer to the light bulb.
  7. A white towel.
  8. A watch or stop watch to measure the time.
  9. A piece of paper and pencil to record your observations.


Put the towel on a flat table.

Put the goose neck lamp on the end of the towel on the table
Put the thermometer under the light of the lamp and measure the distance from the bulb.
Make sure the lamp is unplugged and screw in the smallest wattage light bulb

5. Measure the temperature and write down the start temperature
Angle lamp over thermometer and turn on lamp.
Leave lamp shining on the thermometer for at least five minutes
Start watch and at the end of five minutes read the temperature and mark down what the final temperature is.

Repeat the steps above with each different light bulb.

  • Allow the lamp and desk to cool for half an hour between each bulb.
  • Do not unscrew the light bulb right after turning off the lamp as the bulb may be hot and can burn you.
  • Unplug the lamp before changing the bulb.
  • Make sure the distance between the thermometer and the light bulb is the same for each different bulb. The thermometer should be in the same spot.
  • The starting temperature for thermometer should be about the same for each light bulb.

Hints for better results:

  1. Push the lamp down so it will be very close to the thermometer bulb before you turn on the lamp. Heat tends to move up; so, if your lamp is far from the thermometer you may not see any temperature increase. With a few trials you will learn what is the best, but safe distance of the bulb for your experiment.
  2. Wrap a black paper or cloth to the bulb of your thermometer so it will absorb the radiated heat.

Experiment 2: Compare the total heat energy of different wattage light bulbs


Light bulbs may come in different sizes. In larger light bulbs the heat will distribute over a large surface, so it may feel cooler. In some light bulbs part of heat may be produced by their transformers or other external components. Low efficiency light bulbs may get very hot without radiating much heat and light. The previous experiment that only measures the radiated heat does not provide a reliable result for such light bulbs. In this experiment we collect the entire heat in a box and record the temperature increase in the box.


  1. Cliplight, small lamp or a surface mount receptacle.
  2. Table
  3. Carton box with one open side
  4. 2 thermometers (one used as control)
  5. 3 different light bulbs
  6. Clock or stopwatch


Secure a surface mount receptacle on the center of a table so you can screw different light bulbs on that for your experiment. The receptacle may be connected to an outlet near the table.

An inexpensive cliplight may substitute the receptacle. Cliplights are available in hardware stores and home improvement stores. They come with a long electric cord and they have a switch used to turn off and on the light.

Prepare a box that is open from one side. This box must be large enough to cover the light bulbs that you test and have at least 15 centimeters (6 inches) space around the bulb. If the light bulb is very close to the sides of the box it may cause overheat and smoke.

Insert a thermometer in one corner of the box. Partial immersion thermometers have a marking that show how deep you may insert the thermometer.


  1. Read the startup temperature and record it.
  2. Mount (screw) the first light bulb, turn it on and immediately place the cover and record the time.
  3. Wait for 5 minutes, record the temperature again, remove the box and then turn off the light bulb.
  4. Wait until everything cools off to the room temperature and then repeat the steps 1 to 3 with every other light bulb that you have prepared to test.
  5. Record your results in a table like this:

Temperature increase caused by three incandescent light bulbs with different wattages.

Light Bulb Description Wattage Initial temperature Final temperature Temperature increase
Incandescent 40
Incandescent 60
Incandescent 100

Need a control experiment?

Place another thermometer away from your experiment setup and do nothing with that. Just look at it and record the ambient temperature before and after each test. The purpose of having control is to show that temperature increase in the box is not a weather phenomena or is not caused by an unknown condition, but it is caused by the light bulb inside the box.

Materials and Equipment:

List of material for experiment 1:

  1. A goose-neck style lamp.
  2. An extension cord.
  3. Different wattage of incandescent light bulbs – 25 watt, 40 watt, 60 watt, 75 watt, 100 watt, 150 watt.
  4. Compact Fluorescent light bulbs – 7 watt, 23 watt – They are expensive; so shop around for ones that don’t cost so much.
  5. Thermometer. (where to buy?)
  6. A ruler or yard stick to measure distance from the thermometer to the light bulb.
  7. A white towel.
  8. A watch or stop watch to measure the time.
  9. A piece of paper and pencil to record your observations.

List of material for experiment 2:

  1. Cliplight, small lamp or a surface mount receptacle.
  2. Table
  3. Carton box with one open side
  4. 2 thermometers (one used as control) ( where to buy? )
  5. Different wattage of incandescent light bulbs – 40 watt, 60 watt, 100 watt.
  6. Clock or stopwatch to measure the time
  7. Paper and pencil to record your data

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


If you do any calculations for your your project, you must write your calculations in this part of your report.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and find books about electricity, heat, light bulbs and physics. Name such books as your references in your report. You may also name ScienceProject.com and other related websites as your online references.