How Hot Is It?

Engineers must understand the thermal properties of materials to be able to predict the performance of any given material over its lifetime in a specific application. Engineers apply their understanding of the thermal properties of materials to the design of efficient heat transfer materials for better engines, spacecraft and electronic devices. They also examine the thermal properties of insulation to design more efficient buildings and homes. Engineers develop ways to minimize heat transfer from a motor to the surrounding environment. Often they find ways to insulate the motor to decrease the convective heat transfer from the motor. They design a refrigerator to keep heat out of the inside, as well as keep the refrigerator contents cool.

Introduction/Motivation

More on Conduction

When you cook on an electric stove, the pots and pans are heated by conduction. In this process, heat is transferred from the burner to the bottom of the pot. The particles (both the molecules and electrons) at the bottom of the pot move faster and have more collisions with neighboring particles, causing them to move faster. Eventually, the entire pot is hot because of transmission of thermal energy through these collisions.

Materials that are good conductors of electricity are also good conductors of heat. This is because the transmission of heat depends on the outermost electrons, just as the ability to conduct electricity does. Solids with weakly-bonded electrons are goodconductors of both heat and electricity. Materials that are poor conductors of heat and electricity are insulators. In general, both gases and liquids are good insulators.

Engineers use good insulator materials to maintain an object's temperature, whether it is hot or cold. For example, many types of materials are used to insulate buildings, including fiberglass and rigid foam. Much of the insulation value of these materials is provided by tiny air pockets in the material. Insulation works both ways; it slows the rate of heat transfer between a building and the environment. If it is warm in the building and cold outside, insulation slows the rate of heat transfer to the outside. If it is cold in the building and warm outside, insulation slows the rate of heat transfer to the inside.

More on Convection

Heat is transferred by convection in liquids and gases. When a fluid, either a liquid or gas, is in contact with a hot object, warmed fluid rises and cooler fluid sinks to take the place of the warmed fluid. This produces currents in the fluid. When a pan of water is heated on the stove, heat is transferred to the pan and then to molecules of water that are in contact with the pan by conduction. As a packet of water gains heat, and its molecules speed up, it expands, becoming less dense than the surrounding water. This less dense packet of water rises like a balloon through the surrounding liquid. Colder, denser water packets sink to take its place.

More on Radiation

Heat transfer can also occur between objects that are not touching and that are separated by empty space. This is exactly how energy from the sun reaches Earth across 150 million kilometers (93 million miles) of nearly empty space. Similarly, energy from a fire or stove burner reaches our bodies through empty space. In fact, all objects transfer energy in this way at all times. This phenomenon is known as radiation. Besides heat, radiation transmits other forms of energy, such as visible light, x-rays, radio, which are all forms of electromagnetic radiation. The energy in x-rays is greater than the energy in visible light, which is greater than the energy of heat (infrared radiation), which is greater than the energy of radio waves. In a vacuum, energy is transmitted at the speed of light by radiation. In an object that absorbs infrared radiation, the motion of its particles increases, thus increasing its temperature.

Everything is continually radiating energy and absorbing energy by radiation. An object's temperature increases if it absorbs more energy than it radiates and vice versa. If an object is warmer than its surrounding it transfers heat to its surroundings by radiation (and by conduction, if it is touching another object, and by convection if there is a fluid in the environment).

Black objects are good absorbers and radiators, while shiny or white objects are poor absorbers and radiators. For example, dark-colored cars get much hotter than white cars on hot days. White and shiny surfaces reflect radiation. This is why engineers, architects and builders sometimes paint roofs or houses white in hot climates to help reduce the need for air conditioning.

What is the difference between thermal energy and temperature?

The atoms and molecules in all matter (solids, liquids, gases and plasmas) are constantly vibrating. There is more movement of the particles in a warmer object and less movement of the particles in a cooler object. Temperature is a quantitative measurement of how warm or cool an object is. Measuring the temperature of an object gives the average kinetic energy — the energy due to motion — of the particles in the object.

Engineers use many types of thermometers to measure and monitor temperatures.

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Materials that are good conductors of heat feel hotter to the touch than materials than are insulators even if they are at the same temperature. Two objects at the same temperature that are identical in all respects but size have different amounts of thermal energy. The larger object has a greater amount of thermal energy than the smaller object due to its greater mass.

As the temperature of matter changes, its characteristic properties, such as electrical conductivity, may change. Almost all materials also change volume as their temperature changes. Most materials expand as their temperature increases and contract as their temperature decreases. Materials expand when their temperature increases because the particles are moving about a higher average speed and therefore have more energetic collisions. The energetic collisions force the particles to move farther apart.

The expansion and contraction of gases is dramatic. For example, the volume of a balloon decreases greatly if it is dunked in liquid nitrogen. The balloon returns to its original size as it warms back up to room temperature. The volume change of solids with a change in temperature is less noticeable. However, most people have experienced the result of the expansion of a metal lid on a glass jar (making it easier to open) when it is held under hot water. The metal lid expands more than the glass jar, so the lid can be removed. In general, expansion and contraction of liquids is greater than expansion and contraction of solids. Different materials expand and contract at different rates.

Why do some things heat up and cool down faster than others?

How long it takes a material to heat up or cool down depends on the amount of the material and its specific heat. Specific heat is the amount of heat required to raise the temperature of one unit mass of a material by one degree. Specific heat is unique for every type of material. Diamonds have high specific heat and thermal conductivity, which is why they are cold to the touch. The specific heat of water is much higher than for most materials. So, for example, it takes much longer to heat up or cool down a given mass of water than an equal mass of aluminum or iron. The high specific heat of water makes it useful for both cooling and warming. This characteristic of water explains why locations next to large bodies of water tend to have more moderate climates than those further inland. Most people have experienced the effects of varying specific heats when eating fruit pie right out of the oven. The fruit filling, which is made mostly of water, can still burn your mouth even though the crust is no longer hot.

A pie's fruit filling has a different specific heat than its crust.

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How is heat transfer important in engineering?

Understanding heat transfer, whether the goal is to maximize or minimize it, is important to engineers. Civil and architectural engineers use insulation and passive solar design to construct buildings that neither lose nor gain too much heat. Mechanical engineers design appliances, such as air conditioners, refrigerators, heaters and ventilation systems. Civil engineers design heating, cooling and ventilation systems for buildings. Aerospace engineers design thermal control systems for spacecraft and study how airplane designs affect aerodynamic heating.

Sometimes engineers need to maximize heat transfer of devices that tend to overheat or work less efficiently at higher temperatures. For example, the waste heat from an electric generator, a computer CPU, or a refrigerator compressor must be transferred away from the devices as much as possible. In other applications, engineers must minimize heat transfer to keep a device working efficiently. For instance, the storage space of a refrigerator must be well insulated to minimize the heat transfer from the outside to the inside.

Materials List

Each group needs:

• 2 thermometers
• 1 250–300 ml beaker or small jar
• 1 container to hold small beakers/jars (cake pans or larger jars work well)
• How Much Heat Will It Hold? Worksheet Download How Much Heat Will It Hold? Worksheet, one per student

For the entire class to share:

• Any of the following materials, a different one for each group: sand, water, shredded paper, shredded Styrofoam, cloth, or powdered materials such as Sakrete (concrete), plaster or finely crushed rock. Students can also bring from home specific materials the teacher approves for testing.
• Hot water (~85°C), enough for each group to fill large jars
• Ice water, enough for each group to fill large jars
• 2 large beakers, cans, dishpans or other containers (for cold and hot water control setup)
• Measuring cups (or jars with 200 ml level marked)
• Masking tape
• Markers

Procedure

Before the Activity

• Calculate the quantity of each material that you will need in order to provide each student group with ~4/5 cup (200 ml) of one of the materials.
• Gather materials and make copies of the How Much Heat Will It Hold? Worksheet Links to an external site..
• Set up stations for each test material: container of material, thermometers, jars, measuring cups or jars with 200 ml level marked and bucket (or other container for used materials).
• Prepare enough ice and hot water for the class. Wait until right before starting the activity to put hot water in one bucket.
• Put the ice and tap water in another bucket for the second half of the activity.
• As a control, the teacher measures the temperature of the hot water and ice water during the 10 minutes. This data may be used as comparison at the end of the activity, during a discussion of water temperature heat loss to the room air.
• The students evaluate the following variables: 1) The kind(s) of materials they are testing, and 2) Water temperature (testing in hot water or cold water or both).

With the Students

1. Divide the class into student teams of three or four students each.
2. Assign team "jobs." In each team, have one student in charge of reading the temperature on the thermometer in the material, another to read the temperature in the water and a third to be the recorder. If you have any groups of four, have the fourth student be in charge of watching the time between measurements.
3. Ask the students to predict which material will have the highest heat capacity (store thermal energy the best), and record this on their worksheets.
4. Have each team go to the appropriate station and look at their materials. Instruct them to measure 200 ml of their assigned material using a measuring cup or jar with the 200 ml level marked.
5. Have the student testing water pour the hot water in his/her container (large jar) and place a thermometer in the container. Have students testing the powdered material, sand or dirt, fill the beaker halfway (small jar), place a thermometer into the material and then finish filling their containers. The material must cover the thermometer bulb or bottom.
6. Using masking tape, label each container.
7. Measure and record the initial temperatures of the materials and water. Remind the recorders to document the starting temperatures on their worksheets.
8. Place the beakers/jars of material into the large container of hot water.
9. Have students measure and record the temperature of the material and the water bath every minute for 10 minutes (see Table 1 for a sample data table). Record the starting temperature of the 200 ml beaker of water, sand, and paper, etc., and the temperature of the hot water bath at "0" minutes. Record the temperature reading of each of the thermometers every 1 (or 2) minutes in the table. Record the temperature reading of each of the thermometers every 1 (or 2) minutes in the data table.

Table 1. Sample data table of temperature readings of different materials in an ice bath.

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10. Have each team member pour the contents of their beaker/jar into waste containers at the appropriate station.
11. Repeat the entire activity using ice water instead of hot water.
12. After all measurements have been made, direct students to make a bar graph showing their material and the highest temperature or the lowest temperature it reached.
13. Conclude with a class discussion:

• Compare the results of the different materials tested. Compare graphs. Which materials had the highest heat capacity (stored heat the longest)?
• Also compare the temperature gains (losses) to the teacher's measurement of the hot water and ice water, taken as a control. Discuss the water's heat loss (and gain) to (and from) the air in the room.
• Ask students to comment on their initial predictions, as recorded on the worksheets. What have they learned? Ask them to explain their understanding of which materials have a high-heat capacity and which have a low-heat capacity.
• How would an engineer use what the students have learned today? Which of the materials that the class measured would an engineer choose for insulating a home in the winter? To design a good food storage container for soup? For the design of a product that you want to heat up quickly?

 

Answer the questions to this assignment in a Word file.  Save the file in the appropriate place on the network.  Then, upload the completed file to this assignment link by clicking the Submit Assignment button in the upper right hand corner of this window.  Proofread your work carefully before you submit it.

Lesson copied from https://www.teachengineering.org/view_activity.php?url=collection/cub_/activities/cub_energy2/cub_energy2_lesson06_activity2.xml Links to an external site. (Links to an external site.)on December 12, 2014.