Heat

TEACHERS’ GUIDE

SUBJECT: Physics

FORM: Senior one.

UNIT: 3

TOPIC: Heat

SUB-TOPICS: - Thermal expansion and application.

- Heat transfer.

INTRODUCTION:

Heat is a form of energy, which propagates by use of electromagnetic waves. It gives the sensation of warmth when absorbed by our bodies.

When incident on a body heat can be absorbed reflected, or transmitted. The amount of heat absorbed or reflected depends on the nature of the surface of the body and the surface area exposed. Dull black surfaces are good absorbers while shinny silvered surfaces are good reflectors.

Most materials expand (increase in size) when heated and contract when cooled.

It is not very easy to measure the absolute amount of heat energy a body contains, but the amount of energy gained or lost by a body can be quantified.

Temperature is a number that shows the degree of hotness or coldness of a body and changes in temperature of a body can be used to calculate the amount of heat lost or gained.

There are many different applications of heat transfer and expansion e.g.

• Car Radiators,
• Vacuum flask,
• Cooking utensils,
• Flat irons,
• Insulators,
• Rail lines Construction,
• Thermostats,
• Domestic hot water Supply system.

Heat is a form of energy. It is transferred from one place to another by electromagnetic waves where there is no material medium between the two points. When incident on object heat energy may be absorbed, reflected or transmitted through the object.

MAIN CONTENT AND CONCEPTS TO EMPHASISE

By the end of the lessons, students should be able to:

• Define and explain the different modes of heat transfer.

• Point out several applications of good conductors, insulators, good absorbers, reflectors and radiators.

• State the disadvantages and advantages of thermal expansion.

• Explain the use of different expansivities of metals.

• Explain the working of some home appliances such as, the vacuum flask, the domestic hot-water supply system and the thermostat.

• Explain the green house effect and how it causes global warming.

TEACHING /LEARNING MATERIALS, ACTIVITIES AND GUIDANCE

Materials required:

• Sources of heat e.g. Bunsen burners, electric radiators.
• Different materials of metal rods, glass, wood, water etc
• A vacuum flask
• Potassium permanganate crystals
• Shiny and Dull surfaces.

TEACHER ACTIVITIES

1. Rate of conduction of heat by different materials.

- Get rods a, b and c of different material and of the same length and thickness.

- Attach a small piece of wood to one end of each rod using candle wax.

- Heat the other end of all the rods at the same time and observe what happens.

The wax at the ends of the rods will melt at different times and the wooden pieces fall off one at a time.

The rod that loses its piece of wood first is the best conductor and the one that loses it last is the poorest conductor.

You should explain how the thickness and length of the rods would affect the experiment.

Explanation of conduction

When a solid is heated at one end, molecules near that end gain energy, they vibrate faster and with increased amplitude. They collide with neighbouring molecules transferring some energy to them. The neighbouring molecules start vibrating faster and also transfer heat to their neighbours. This process continues until heat is conducted to the other end of the conductor. Conduction does not involve the movement of mater as a whole.

2. Convection:

• Put some water in a round-bottomed flask or beaker.
• Gently drop some potassium permanganate crystals on to the bottom of the flask
• Apply a small flame at the bottom of the flask.

Observation:

Purple streaks can be seen rising with the water above the crystals. They are carried down the far side of the beaker away from the heat and eventually the whole body of water is circulating in the flask.

Give clear explanation of this occurrence. When water near potassium permanganate is heated it expands and becomes less dense (rarer) it then rise up and replaced by cooker liquid from up. This is shown by the arrows in the diagram and these fluid movements are called convection currents.

The process continues until the whole body of liquid is heated up.

Site other examples of application of convection e.g. Sea breezes and land breezes.

Sea breeze: During a hot day, the land is warmer than the sea. Air above the land is heated and rises up; it is then replaced by cooler air from the sea.

Land breeze: During the night, the sea is warmer than the land because the land loses its heat much faster than the sea. Air above the sea is warmer than that above the land so it rises up to be replaced by that above the land.

The working of the domestic hot water system

3. (a) Radiation.

- Students gather round a source of heat e.g. electric radiator or Bunsen burner.

- Let them feel the warmth from the source by placing their hands near the heat source (make sure students do not touch the source).

Heat from source to hands placed besides the source is transferred by means of radiation. Some students may argue that it is convection due to the presence of air. Note: heated air rises up above the radiator and doesn’t move side ways from source of heat.

(b) Effect of nature, surface of radiator on rate of radiation.

- Position a Leslie cube filled with hot water midway between two metal plates.

Arrange the cube in such a way that one of it faces painted dull black faces one metal plate and its shiny surface faces the other metal surface.

Observation

It will be observed that after sometime the cork B on the plate facing the black surface falls off before cork A.

Conclusion:

The black surface facing cork B radiates heat at a higher rate than the shiny surface facing cork A. This implies that dull black surfaces are better radiators of heat than shiny surfaces.

4. Absorption of heat.

- Position a radiator midway between two metal plates.

- Fix a cork on the reverse side of each metal plate using wax.

- Switch on the heater and observe. What happens?

- After sometime, the cork on the metal plate with a dull black surface falls off followed by the one on a shiny surface.

Conclusion

Dull black surfaces are good absorbers of heat radiation whereas shiny surfaces are poor absorbers of radiation.

5. Thermal expansion:

Required:

Metal bar that just fits into a gauge or metal sphere that just fits into a ring when both are at same temperature

• The bar or the ball is heated with a Bunsen burner.
• Test the fit after heating.
• Due to expansion, ball will not go through the ring or bar will not fit into gauge due to expansion.
• When both ball and bar cool down, they are able to fit again.

STUDENT ACTIVITIES

1. Poor conductors (insulators) do not allow heat to go through them easily.

• Mention any four examples of such materials.
• State three uses of poor conductors.

2. Brass expands at a higher rate when heated than irons. A bimetallic strip is made out of brass and iron as shown below.

1. Which of the two diagrams shows the most likely shape of the strip when heated?

Explain your answer.

2. State and explain one application of thermal expansion.

3. Explain the occurrence of land and sea breeze

4. What are the bases and handles of your cooking pans at home made of? Explain why you think these materials were chosen.

5. How does the vacuum flask keep cold drinks cold in hot weather?

6. Why is it a good idea to paint houses white in hot countries?

SOLUTIONS

1. - Rubber, wood, paper, cloth and glass.

• Poor conductors can be used to protect us from hot objects while handling them.

• Poor conductors are used to keep hot things such as food, hot for a long time

• Poor conductors keep us warm in cold weather

2. (a) Shape B

Since brass expands more than iron, it will be much longer than iron and so will be on our side of the curving.

(b) Thermostat.

A thermostat works as a switch to connect and disconnect on electric circuit to regulate temperature e.g. if a flat iron

4. Bases are mainly made of good conductors so that heat is conducted easily through pans. Handles are made of poor conductors such as wood or plastic to protect us from getting burnt.

5.

Heat will not go through the cork to the drink since cork is a poor conductor.

Heat will not pass through the vacuum by conductor or by convection.

Heat that may go through vacuum by radiation is reflected back by the silvered surfaces. The cold drink inside flask is kept cold for a long time.

6. Houses in hot countries are painted white because white is a good reflection of heat radiation and this will keep inside of house relatively cool on a hot day.

Teacher’s notes.

Conduction: when one end of a conductor is heated, molecules near that end gain energy and start vibrating faster. These molecules collide with neighbouring molecules; some of the energy gained is passed on to the neighbouring molecules, which in turn start vibrating faster. (Vibrating faster means that molecules have gained K.E).

The process continues as molecules pass on energy to their neighbours until the molecules on the other end of the conductor are moving faster than before. In this way heat is conducted from one end to the other.

Note: As molecules vibrate, they do not move from their fixed positions, so we say, in conduction of heat there is no movement of matter as a whole.

In good conductors such as metals, there are free electrons in the surfaces of these conductors. These can move from one point of the conductor to another.

Therefore when heated, free electrons at the hot end gain kinetic energy and drift to other cooler areas of the conductor hence transferring heat as they collide with molecules at that end. This makes the process of heat transfer faster.

Convection: this takes place in fluids (liquids and gases) and heat is transferred by movement of matter it self. When part of a fluid is heated, it expands and becomes less dense (rarer). It rises up to be replaced by cooler denser fluid. The cooler fluid is in turn heated and rises up convention currents are developed in the fluid and eventually the entire fluid is heated up.

Radiation: transfer of heat by radiation doesn’t require a material medium between the two points. Heat moves from one point to another by electromagnetic waves. These waves can go through vacuum.

Expansion: when matter is heated its molecules vibrate faster with bigger amplitudes. This greatly affects the size of matter. In most cases matter expands when heated (see support material for demonstration).

In day to day experiences., this expansion can be destructive and precautions must be taken to avoid accidents that might arise.

• Railway lines are constructed with gaps between them to allow for expansion on hot days.
• Bridges are constructed on rollers for the same reason.
• Power lines are fixed such a way as to allow for both expansion and construction during hot and cold weather respectively.