Core Unit IV: Heat

A. Heat and Temperature

Key Concepts

Some theories are based on supporting postulates. A postulate is a statement which is agreed on by consensus among scientists.

The following are important postulates of the kinetic molecular theory:

• All matter consists of atoms.

• Atoms may join together to form molecules.

• Solids usually maintain both their shape and their volume.

• Liquids maintain their volume, but not their shape.

• Gases do not maintain shape or volume. They will expand to fill a container of any size.

• Molecular motion is random.

• Molecular motion is greatest in gases, less in liquids, and least in solids.

• Collisions between atoms and molecules transfers energy between them.

• Molecules in motion possess kinetic energy.

• Molecules in gases do not exert large forces on one another, unless they are colliding.

As information is acquired in science, new theories can develop, or existing theories can be further supported, modified, or rejected.

Many observable phenomena give support to the kinetic molecular theory.

A theory is a system of ideas or a sphere of abstract knowledge which attempts to explain why certain phenomena occur, whereas a law is a statement of specific conditions or relationships that exist in nature.

Models are useful in science to illustrate abstract or complicated concepts.

Thermal energy is the average of the potential and kinetic energies possessed by atoms and molecules experiencing random motion.

Heat is transferred by convection, conduction, or radiation.

Heat is the thermal energy transferred from one object to another due to differences in temperature. (Some texts do not make a distinction between heat and thermal energy. The distinction between heat and thermal energy need not be emphasized to the same extent as that between heat and temperature.)

Heat energy is measured in joules.

There is no direct method used to measure heat. Indirect methods must be used.

Temperature is a measure of the average kinetic energy of the molecules of a substance.

Temperature can be measured with a thermometer.

One way a thermometer can be calibrated is by the amount of thermal expansion and contraction that occurs within a given type of substance.

Thermometers are limited by the physical properties of the substance from which they are made. (i.e., An alcohol thermometer is of little use above the boiling point of alcohol, and a mercury thermometer will not be of any use below the freezing point of mercury.)

The Celsius scale is commonly used to measure temperature. Its scale has been calibrated to the physical properties of pure water. The normal freezing point of water was arbitrarily set as 0 °C and the normal boiling point of water was arbitrarily set at 100 °C. Arithmetic gradations represent uniform temperature changes on the scale.

The Kelvin scale, also called the Absolute scale, sets 0 K as absolute zero. (-273.15 °C) Temperature increases on the scale are the same as on the Celsius scale (1 K = 1 C°).

To convert from Celsius to Kelvin:
K = °C + 273

Substances vary in their amount of thermal expansion.

The linear expansion of a solid depends on its initial length, temperature change, and the type of material it is made from.

For most solids, their linear expansion is directly proportional to the change in temperature T
The change in length L is also proportional to the original length (L_o):

is called the coefficient of linear expansion, measured in °C^-1 or K^-1.

The coefficient of linear expansion is different for different materials. (For a given material, the values of à for different temperature ranges vary so little that they can be considered constant, unless extreme precision is required.)

The thermal expansion of materials must be considered in the design of certain kinds of structures.

Volume expansion is extremely important in gases. (It is extremely important to recognize any potentially hazardous situations which could result in an increase in pressure in closed containers.)

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: thermal energy, heat, temperature, convection, conduction, radiation, thermal expansion, linear expansion, coefficient of linear expansion.

2. Identify some important postulates of the kinetic molecular theory.

3. State what is meant by a theory.

4. Explain that, as new information accumulates, a theory could be supported, modified, or rejected in favour of new theories which better help to explain the evidence.

5. Describe the difference between a theory and a law.

6. Give an example of an observable phenomenon which lends support to the kinetic molecular theory.

7. Explain the difference between heat and temperature.

8. State the correct units used to measure heat energy and temperature.

9. Explain that heat can not be measured directly whereas temperature can.

10. State that a thermometer, like any other measuring instrument, must be calibrated in some way.

11. Recognize the limitations of certain materials that are used in making thermometers.

12. Explain the reference points that were used to calibrate the Celsius temperature scale.

13. Compare the Celsius and Kelvin temperature scales.

14. Convert a temperature reading from degrees Celsius to Kelvin and vice versa.

15. State that substances vary in their amount of thermal expansion.

16. State three important factors which determine the linear expansion of a material.

17. State the correct units for the coefficient of linear expansion.

18. Recognize that the coefficient of linear expansion is based on the unique physical properties of different substances.

19. Suggest some applications in which an understanding of thermal expansion would be extremely useful.

20. Recognize any potentially hazardous situations that could arise from the thermal expansion of materials, especially those involving an increase in pressure from the expansion of gases in closed containers.

21. Solve problems involving heat and temperature, and thermal expansion.

Teaching Suggestions, Activities and Demonstrations

1. Qualitatively examine the mechanical equivalent of heat. Place a measured quantity of water into a plastic ice cream bucket, or some similar large container. Using an electric beater, beat the water for some given period of time. Record the final temperature of the water.

   This activity can also be used in Physics 30 in the section on energy transformations. Calculate the energy input to the beater. Calculate the energy transferred to the water. Determine the efficiency of this system for heating water. Try using a milk shake beater in a calorimeter to see if there is any difference in the efficiency of the system. The students may also find a "boat race" challenge project of interest, in which teams try to design an apparatus to raise the temperature of a specified amount of water from room temperature to some predetermined final temperature in the shortest amount of time, by supplying only mechanical energy to the system.

2. Place some steel shot into a cardboard cylinder. Seal off both ends. Shake the tube vigorously for several minutes. Remove the shot and pour it into water. Measure the change in water temperature. Energy transformations, specific heat capacity, and the laws of thermodynamics can all be developed through this activity.

3. Investigate various designs for active and passive solar heated homes.

4. Perform an activity using a model (or models) to investigate several postulates of the kinetic molecular theory.

5. Explain that a thermometer takes advantage of some important physical properties of the substance from which it is made.

6. Give some examples of how models are used to illustrate abstract ideas in a concrete form.

7. Investigate the insulation values and costs of different types of commercially available insulation materials. Compare the cost of insulating a given area to some desired RSI value using different insulating materials.

8. Students could research the importance of thermal expansion in specific applications.

9. Place a thin capillary tube and a thermometer into a two-holed stopper. Seal the stopper. Place it on an Erlenmeyer flask which is filled with coloured water. Gently heat the liquid. (Caution: Do not bring it to a boil. Pressure will build up inside the flask.) Record the height of the column of liquid as a function of temperature. Calculate the coefficient of volume expansion of the liquid.

10. Design an amplification device which would allow a small expansion of solids or liquids with a change in temperature to be measured. (One way to do this is to support an iron rod at one end, and have the other end supported on a roller. Along the axle of the roller is a long pointer, or a mirror on which a light beam reflects to a distant scale, as in a mirror galvanometer.)

11. A Crookes radiometer is a very inexpensive device which illustrates the transformation of light into heat. The radiant energy causes the vanes within the vacuum of the tube to rotate.

12. Examine some ways in which roads, buildings, and other structures are designed to take into account volume expansion and contraction that occurs in the application for which they have been designed.

13. Take two similar metal cans with screw-on lids. Drill a hole on top of each one, so that a thermometer will fit inside through a rubber stopper. Spray paint one of the cans black, and leave the other one shiny. Place both containers in a sunny place and record temperature changes over time. As a variation, place a given amount of water in each container, making sure that they are both at the same starting temperature. Then place them in a sunny place and record temperature changes over time. Alternatively, fill each with hot water and compare the cooling of the shiny can to that of the "black body radiator."

14. Compare the heats of combustion of various types of oils using "uncandles" -- small plastic rings and a wick which float on oil. Heat a pre-weighed sample of water using a known amount of oil. Based on the temperature change of the water, determine the heat of combustion of the oil.

15. Probe a Bunsen burner flame with a small thermocouple connected to a sensitive ammeter. Interpret the results. The temperature of the flame varies at different places within the flame.

    The flame can be probed with wooden splints placed horizontally in the flame at different heights. The scorch patterns on the splints give an indication of the different temperature zones present in the flame. Put a safety match in the flame. At some places in the flame, the match will begin to burn before the head of the match ignites, indicating that some regions of the flame are hotter than others.