Heat Transfer

TEACHERS’ GUIDE

SUBJECT : PHYSICS

TOPIC : HEAT

SUB-TOPIC : Heat transfer

CLASS : Senior One

CLASS SIZE : 60 Students

TIME REQUIRED : Minimum: 240-360 minutes

Brief description of the unit

This unit discusses the types of heat transfer, the factors that affect the rate of heat transfer as well as applications of heat transfer. Heat is a form of energy that produces a sensation of warmth when absorbed by our bodies. Heat energy is the commonest form of energy in applications by humans and other living things. Heat energy is used for preparing food, keeping us warm, drying clothes, etc. Almost all forms of energy finally degrade to heat.

Unlike electricity, heat energy leaks easily. There is therefore need to study the ways heat energy is transferred with the aim of conserving it, using it economically so as to reduce global warming.

Main Content to emphasise

Heat transfer by conduction, convection and radiation.

Objectives

By the end of the topic, the learners should be able to:

1. Define thermal conduction, convection and radiation.
2. Arrange Iron, copper, Aluminium and glass in order of their thermal conductivities.
3. Describe the mechanism of thermal conduction in terms of the simple kinetic theory.
4. Explain the applications of good and poor conductors of heat.
5. Illustrate the process of thermal radiation.
6. Discuss the factors that determine the rate of thermal radiation.
7. Demonstrate practical applications of good and poor thermal radiators.
8. Explain the process of thermal convection.
9. Describe practical applications of thermal convection.
10. Identify situations where energy can be conserved through good usage of factors affecting thermal transfer.
11. Compare the efficiency of different types of charcoal stoves with reference to heat transfer.

Methods

Group work

The class should be divided into groups of 5 – 10 with each group having a secretary and a chairperson.

Teaching / learning aids

Source of heat and tripod stand; Iron, copper, aluminium and glass rods of the same length and diameter; Wax; Stop clock

Job related life skills

In forming the groups and during the activities, ensure that the following job-related life skills are deliberately achieved.

• Personal attributes: - behave appropriately, punctuality, reliability, self confidence, seek advice, show tenacity and motivation, be analytical and imaginative.
• Communication: - ability to read, write, listen, and speak in appropriate ways for different audiences. Know and apply general and specialized vocabulary.
• Team work: - ability to cooperate and share tasks with colleagues.
• Problem solving: - goal focused, seek out relevant information, identify constraints, evaluate alternatives and make decisions/ choices.
• Implementation and application: - the ability to carry out complex operations and follow instructions to achieve accurate results.
• Application of number: - ability to work with and present numerical data, using appropriate intermediate calculations.
• Information skills: - ability to present evidence to meet the needs of different audiences using graphs, reports and images.

Activity one

Comparing the rates of heat transfer by conduction in different materials.

Ask the learners should do the following:

• Place one end of each rod in the flame of a candle or a Bunsen burner, while holding the other end.
• Describe the mechanism by which heat travels along the rod to their hands.
• Measure and record the length of time it takes the heat to reach their hands through the rod.
• Describe a better way of comparing the rates of thermal conduction of the different materials, using the apparatus given or any other that is available.
• Carry out the experiment and arrange the materials in order of their conductivities by attaching three pieces of wax at equal intervals on each rod and then simultaneously heating the ends of the rods in a flame.

	Time taken for the 1st piece of wax to fall off	Time taken for the 2nd piece of wax to fall off	Time taken for the 3rd piece of wax to fall off	Ranking in order conductivity(best No. 1)
Copper
Iron
Aluminium
Glass

NB

The rods should be long enough to get measurable time differences.

Ask the learners to discuss the applications of good conductors and poor conductors.

Activity Two

Heat transfer by convection.

Round bottom flask, heat source, potassium permanganate, glass tube.

Ask the learners to do the following;

• Fill the flask with water and carefully drop grains of potassium permanganate at the bottom of the flask using the glass tube.
• Apply a gentle flame at the bottom of the flask and observe.
• Describe and explain the observations.
• Discuss how convection takes place in gases.
• Discuss how convection is applied in house ventilation. Explain the value of good ventilation to comfortable living.

Activity Three

Heat transfer by radiation.

Teaching / learning aids

Hot water, cubic metal container, or Leslie cube, thermometers, kitchen foil, source of heat (radiant coil, filament bulb, or candle).

Ask the learners to do the following;

• Place their hands at a safe distance above, below and at the side of a source of radiant heat and describe the mechanism by which heat reaches their hands in each of the positions.
• Fill the metal cube with hot water and feel the heat at a distance from each side of the cube.
• Describe and explain the difference in the amount of heat felt on each side of the cube.
• Suggest better ways of comparing the rates of thermal radiation from the different surfaces.
• Carry out an experiment to compare the rate of radiation of heat by different surfaces.

One possible experiment is by holding a thermometer with a blackened bulb (placed centrally) at distance from a surface of the Leslie cube containing hot water and recording the temperature readings at regular time intervals. This is repeated for the different types of surfaces of the cube.

	Temperature in oC recorded by thermometer at 30 second intervals
	30	60	90	120	150	180	210	240	270	300
Dull surface side of the cube
Shiny surface side of the cube
White surface side of the cube
Black surface side of the cube

Plot the graph of temperature against time for the different surfaces on the same axes. Use computer spreadsheets where possible.

Comment on the graphs.

1. Discuss the type of finishing you would recommend for each of the following:

a. The inside walls of a bedroom of a house in a hot area.

b. The outside wall surface of a house in a hot area.

c. The inside walls of a bedroom of a house in a cold area.

d. The wall surface of a house in a cold area.

2. Discuss the economic implications of the choice of colours when finishing a house.

Experiment on rates of absorption of heat.

Learners are given a shiny polished pan with a cover that has a hole for a thermometer and a similar pan that is all blackened. The two pans are put out in the hot sun. Temperatures of the water are taken every minute for ten minutes.

The results are recorded in a table.

	Temperature in oC recorded by thermometer at time intervals in minutes
	1	2	3	4	5	6	7	8	9	10
Dull surface pan
Shiny surface pan

Plot the graph of temperature against time for the two surfaces on the same axes. Use computer spreadsheets where possible.

Comment on the graphs.

The learners should discuss the applications of good / poor radiators and absorbers in the home, in dressing, in vehicles, etc.

With reference to conservation of energy, the learners describe one situation where it is necessary to reduce the rate of heat transfer so that energy is conserved (economised).

With reference to conservation of energy the learners describe a situation where it is necessary to enhance the rate of heat transfer so as to conserve (economise) energy.

Activity Four

1. Groups should carry out a project on one of the following

• Keeping cooked food warm for a long time using local materials.
• Firing bricks more effectively.
• An energy saving charcoal stove.

2. The groups present their essays to the class.

Links

http://en.wikipedia.org/wiki/Thermal_radiation

http://en.wikipedia.org/wiki/Solar_hot_water

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm

Requests

Pictures of a vacuum flask, green house, domestic solar water heater.

Exercise

1. Describe an experiment to show that black surfaces are better absorbers of heat than shiny surfaces.

2. The diagram below shows two cubes of the same dimensions A is black while B is shiny. The cubes are both filled with cold water. A source of heat is placed mid-way between the cubes.

Explain what is observed on the thermometers.

3. Explain why the sea remains cooler during day and warmer during night.

4. State any two factors on which the rate of heat transfer along the length of a metal bar depends.

5. Distinguish between conduction and convection.

6. Distinguish between convection and radiation.

7. What factors affect the rate of heat radiation from a surface?

8. Explain how convection determines ventilation of a house.

9. Use the kinetic theory of matter to distinguish between poor conductors and good conductors of heat.

Scheme of work

Sub-topic	Periods	Specific objectives	Content	Teaching and learning strategies	Notes
Conduction	6	Define conduction. List the factors affecting the rate of conduction. Carry out experiments to compare good and bad conductors. Carry out experiments to show that water is a poor conductor of heat. Explain applications of conduction.	Concept of conduction. Factors affecting the rate of conduction in solids. Compare conductors and insulators. Water as a poor conductor of heat. Applications of conduction.	Learners carry out experiments to compare good and bad conductors. Discuss applications of good and bad conductors	Application treated qualitatively eg vacuum flask, flat iron and insulation. Rate of conduction treated qualitatively using simple experiments.
Convection	6	Define convection. Describe how convection current is formed. Experimentally demonstrate convection current. Describe applications of convection.	Concept of convection. The convection current. Applications of convection	Experiments to show convection.	Applications eg in the domestic hot water system, land and sea breezes, ventilation and car radiators.
Radiation	7	Define radiation. Carry out experiments to compare radiators and absorbers. Explain the application of radiation of heat. Describe how solar energy can be trapped and used in a water heating system.	Concept of radiation. Factors affecting radiation (or absorption). Comparing radiators(emitters) and absorbers. Application of radiation. Green house effect. Solar heating system.	Experiments with good and bad absorber (or radiators). Discuss how conduction, convection and radiation considerations are applied in the vacuum flask.	Effect of nature of surface on absorption and emission of radiation.

Sample lesson plan

Class: Senior one.

Subject: Physics

Topic: heat.

Sub topic: transfer by conduction.

Time: 80min

Number of learners: 40 - 80

Objectives: to define conduction, carry out experiments to compare good and poor conductors.

Method:

Group Work and experiments

Teaching aids:

Source of heat and tripod stand; Iron, copper, Aluminium and glass rods of the same length and diameter; Wax; Stop clock.

Time	Theme	Teachers activity	Learners’ activity
5min	Review	Review work done on temperature and thermometers	Answer question on temperature and thermometers.
15min	Rates of thermal conduction.	Provide apparatus and give instructions for experiment to compare rates of conduction	Carry out experiment to compare rates of conduction.
20min	“	Guide learners to present results and discuss them.	Present their results to class and class discusses the results
15min	Applications of thermal conduction	Gives instructions and supervises.	Discuss applications of conduction in groups
20min	“	Guides the discussions	Present results of their discussions of applications of conduction to the class.
5min	“	Give assignment on domestic applications of conduction.	Take assignment.

Exercises:

1. Describe an experiment to show that black surfaces are better absorbers of heat than shiny surfaces.

2. The diagram below shows two cubes of the same dimensions A is black while B is shiny. The cubes are both filled with cold water. A source of heat is placed mid-way between the cubes.

Describe and explain what is observed on the thermometers.

3. Explain why the sea remains cooler during day and warmer during night.

4. State any two factors on which the rate of heat transfer along the length of a metal bar depends.

5. Distinguish between conduction and convection.

6. Distinguish between convection and radiation.

7. What factors affect the rate of heat radiation from a surface?

8. Explain how convection helps in ventilation of a house.

9. Use the kinetic theory of matter to distinguish between poor conductors and good conductors of heat.