Chapter 6: Heat Transfer

By the end of this chapter, you will be able to:

a) understand how heat energy is transferred and the rate at which transfer takes place.

b) understand what is happening at particle level when conduction, convection, and radiation take place and their application.

c) understand that greenhouse effect and global warming are aspects related to heat
transfer on the Earth’s surface.

Keywords

1. conduction
2. convection
3. global
4. warming
5. greenhouse
6. radiation

Introduction

Have you ever thought of why:

• you feel warm while near a heat source?

a metallic spoon feels hot while stirring tea?

a thermos flask keeps something hot or cold? Heat Transfer

In this chapter, you will learn and be able to explain the modes of heat transfer and their application to daily life.

6.2: Heat Transfer

Heat energy can be transferred from one body to another or from one location in a body to another. To facilitate heat transfer between two bodies, there needs to be a temperature difference between them. This means that these bodies must be at different temperatures with one higher than the other, to allow heat to flow from one body to the other.

This means that no heat transfer occurs between two bodies which are at the same temperature. At the same time, it is very important to note that heat only flows from a body at higher temperature to a body at a lower temperature. Heat may be transferred from one place to another in three ways:

Activity 6.1 Modes of heat transfer

In groups, using the internet and other textbooks, research on the modes of heat transfer and define each mode and where it is applied in day-to-day life.

Discuss with the rest of the class.

Activity 6.2 Demonstrating conduction through solids
Key Question: How can you demonstrate conduction in solids?
What you need
A metallic cup or mug, saucepan, spoon and hot water.
What to do
a) In groups, pour hot water into a metallic cup or saucepan and then hold a spoon in the hot water for some time.

Figure 6.1: A spoon in hot water

b) Note and record all your observations.

c) Explain your observations in (b).
d) Using your observations in (b) above, define the term conduction.
Conduction is most obvious in solids. Conduction occurs when heat energy travels through a solid, passing from particle to particle as they vibrate against each other. A good conductor must have particles which are close enough together to collide with enough force for energy to be transferred. Metals are all good conductors of heat, especially copper, aluminum and silver, because they have “free” electrons which are easily able to transfer heat energy.

Activity 6.3 Demonstrating that different solids have different
Key Question: How can you show that different materials have different conduction rates?

What you need
Lead rod, copper rod, iron rod, aluminium rod, and wooden rod each of the same length and diameter, four match sticks, molten wax, source of heat such as a Bunsen burner and a tripod stand.
What to do

Pass rods of different materials but of the same dimensions through corks inserted in holes in the side of a metallic vessel.

Using a little molten wax, attach a match stick to one end of each of the rods.

Pour boiling water into the vessel to heat the ends of rods equally.
What do you observe?
What conclusions can you draw from the activity about metals?
Which metal does the above set-up show to be the best conductor?
Share your findings with the whole class.

When the temperatures of the rods at the ends where the match sticks are attached reach the melting point of wax, the match sticks drop off.

The experiment shows that different solids have different rates of heat conduction.

Question: Why are metallic solids better conductors of heat than non-metallic solids?

In metals, two processes take place. Firstly, metals have many free mobile electrons moving randomly in them. When one part of a metal is heated, electrons move faster and as a result, they collide with electrons or atoms in cooler parts, and so pass on their energy raising the temperature of these parts. This is a major reason for the high rate of heat conduction in metallic solids. This process is the predominant way of heat conduction. In non-metallic solids, the motion of particles is, however, slow, explaining the low rate of heat conduction in non- solids.

Activity 6.4 Performing an activity to show that water is a poor conductor of heat

Key Question: How can you prove that water is a bad conductor of heat?
What you need
Boiling tube, ice cubes, heat source, wire gauze, metal gauze,water

What to do
a) Wrap ice in a wire gauze and place it at the bottom of a boiling tube containing water.
b) Incline the boiling tube at an angle and heat the water near the top as shown in the diagram above.

In your groups:
c) Note down your observations and finally discuss with the whole class.

Good and bad conductors of heat

Thus, materials that allow heat to pass through them easily are called good conductors of heat and materials that do not conduct heat as easily and quickly as metals do are bad conductors.

Most metals are good conductors of heat; but silver and copper are exceptionally good. On the other hand, substances such as cork, wood, cotton and wool are bad conductors.

Good and poor conductors and their applications

Activity 6.5 Applications of good and poor conductors of heat in daily life
a) In your groups, categorise good and poor conductors of heat.
b) From each category, suggest possible useful applications of each conductor in the daily interactions of life.
c) Discuss your information with the rest of the class members.

A material of low conductivity used for preventing heat transfer is called lagging material.
Handles of heating appliances are made of poor heat conductors. Air is one of the worst heat conductors. This is why materials which trap air, such as wool, are also very bad conductors of heat.

6.3: Convection

Have you ever heated clean water in a saucepan? What did you observe? Well, you may have observed the formation of bubbles at the bottom. The bubbles rise to the surface of the liquid as the cold water flows below to fill the space left by the bubble. The process continues until the liquid boils. The process is known as convection!

Convection is the transfer of heat by the movement of the heated particles themselves. This can only take place in liquids and gases because in solids, the particles are not able to move from their fixed positions. When a liquid or gas is heated, it expands and becomes less dense. The lighter liquid or gas rises allowing a flow of cooler material to take its place. This, in turn, becomes heated and so a current is set up. Heat will continue to be transferred through the available space in
this way until it is evenly distributed.

Activity 6.6 Demonstrating convection currents in water
Key Question: How can you demonstrate convection currents in water?
What you need

a) Fill a glass flask with clear water up to the neck. Using a glass tube, place a few crystals of potassium permanganate at the bottom of the flask.

b) Heat the water gently at the bottom.

c) Note down your observations and discuss with the whole class.

d) Explain your observation.
e) Share your findings with the whole class.

6.4: Convection Current

A convection current is a process which involves the movement of energy from one place to another. It is also called convection heat transfer. What makes you feel hotter when placing your hands on a fireplace or when sitting next to it? Or, why is the movement of liquid so rapid when water is boiled in a pot? These things happen as a result of the convection currents.

Convection currents transfer heat from one place to another by mass motion of a fluid such as water or air. The heat transfer function of convection currents drives the Earth’s ocean currents, atmospheric
weather and geology.

A simple example of convection currents is warm air rising towards the ceiling of a house. Warm air is less dense than cool air, so it rises.

How convection happens

Convection currents form because a heated fluid expands, becoming less dense. The less-dense heated fluid rises away from the heat source. As it rises, it pulls cooler fluid down to replace it. This fluid in turn is heated, rises and pulls down more cool fluid. This cycle establishes a circular current that stops only when heat is evenly distributed throughout the fluid. For instance, a hot radiator heats the air immediately around it. The air rises towards the ceiling, pulling cooler air down from the ceiling into the radiator to be heated. This process repeats until the air in the room is evenly heated.

Effects of convectional currents in the atmosphere

The atmosphere is in constant motion of magma that are less dense, therefore they rise.
When air in the atmosphere is heated, it rises. As air rises, air pressure at the surface is lowered. Rising air expands and cools. The result is condensation.

When temperature lowers, the air becomes cold and, as a result, it sinks. Sinking air compresses and heats. As air sinks, air pressure at the surface is raised. Cold air holds less moisture than warm air.
Because of these variations, air then travels from high to low pressure, creating wind.

If the Earth did not rotate, what would wind patterns look like?

Assignment 6.1

Researching and reporting on the causes and effects of convectional currents on climate/ weather change

In groups, carry out comprehensive research and make a report on the causes and effects of convection currents in the atmosphere and how they give rise to climate or weather change.
Represent your work on posters and present it to the whole class.

Land and sea breeses

Generally, land heats up faster than water. However, water seems to retain more heat than land. So, during the day, the land is heated by the sun to a higher temperature than the sea.

Air over the land is therefore heated, expands and rises, while cooler air blows in from the sea to replace it – hence sea breeze.
Wind in the upper atmosphere blows in the opposite direction to complete the circulation.

At night, the land cools rapidly while the temperature of the sea remains unchanged. So, the land becomes cool while the sea remains warm Therefore, warm air over the sea rises while cool air blows from the land to replace it – hence land breese.

6.5: Applications of Convection

1. The domestic hot-water supply system.

Hot water rises to the top of the hot water storage tank. Therefore, hot water to the taps is drawn from the top of this tank and cold water to replenish it enters at the bottom.
Whatever water is heated keeps rising to the top. The expansion pipe provides room for the heated water to expand into and also permits dissolved air to escape.

2. Ventilation by convection
This may be demonstrated using the following set-up consisting of two wide glass tubes projecting from the top of a rectangular box having a transparent front.

Figure 6.5: Ventilation
A lit candle is placed at the base of one of the tubes.

A smouldering piece of paper is placed above the edge of the other tube

It is observed that smoke is drawn down the tube and expelled via the one above the candle.

Ventilation is an application of convection. It is a process of providing cool fresh air to an enclosure, for example, a room or building. When air in the room is heated up by the occupants, the warm air rises, escaping through the ventilators, and is replaced by fresh air flowing through the windows.

Figure 6.6: Ventilation

6.6: Radiation

Radiation is the way we receive heat energy from the sun. It does not require a medium for its transmission (i.e. it can travel through empty space). When the radiated energy waves falls on a body, the energy may be:
1. absorbed.

2. transmitted.

3. reflected.
When radiant energy is absorbed, the body will rise in temperature. A rack of clothes left in front of a radiant heater will continue to absorb heat until it catches fire.

NOTE:
1. Black and dull surfaces absorb (and radiate) heat much more efficiently than white shiny surfaces.
2. Shiny, silver surfaces will reflect radiant energy and not heat up. This is the reason for the silver coating on a fire fighter’s jacket.

In general, good radiators are good absorbers of radiation and vice versa.

Activity 6.7 Comparing how heat is absorbed or reflected bydifferent surfaces
What you need
Small piece of white cloth, black cloth, water and basin.
What to do

1. Soak the two pieces of black and white clothes in basin filled with water.
2. Hang them outside on sun’s radiation.
3. At intervals of 5 minutes, keep checking on the drying of the two pieces.
4. Continue checking until they are all dry.
5. Make a report containing your observation.
6. Present your findings to the whole class.

Questions

1. Basing on your report, how did heat reach the pieces of clothes from the sun?
2. Which clothes became dry first? And why do you think they dried first?
3. Basing on your observation, explain why you think it is scientifically advisable to put on white clothes on a sunny day.

Radiant energy is transmitted through clear materials such as glass. The glass does not heat up. Radiant heat from the sun may be concentrated by means of a magnifying glass, which is sufficient to ignite flammable material.

Activity 6.8 Comparing absorption of radiation by different surfaces
What you need
Two sheets of tin plate (one polished and the other dull black, vertically a short distance apart), source of flame, two identical corks and wax.
What to do
a) Arrange the flame and sheets of plate as shown in the following figure.

b) Using wax, attach a cork on the back side of each sheet.

c) Place a burner midway between the plates.

d) Discuss what happens to both plates after some time when the Bunsen burner is lit.
e) Explain how heat reaches the different plates.
f) Explain all your observations and finally present your results to the whole class.

The dull black plate must have absorbed heat faster than the polished one. So, dull black surfaces are better absorbers than polished ones.

6.7: The Vacuum Flask

A vacuum flask, or thermos, minimises heat transfer by the three ways that heat can travel. The silver coating on the inner bottle reduces heat transfer by radiation, and the vacuum between its double wall reduces heat moving by convection. The thinness of the glass walls stops heat entering or leaving the flask by conduction. The case surrounding the flask provides additional insulation.

6.8: Greenhouse Effect

The “greenhouse effect” is the warming that happens when certain gases in the Earth’s atmosphere trap heat.
Sunlight shines onto the Earth’s surface, where the energy is absorbed and then radiates back into the atmosphere as heat. In the atmosphere, greenhouse gases trap some of the heat, and the rest escapes into space. The more greenhouse gases concentrate in the atmosphere, the more heat gets locked up in the molecules.

6.9: Greenhouse Effect and Global Warming

The Earth’s atmosphere behaves like a greenhouse. Radiation from the sun easily passes through the atmosphere and is absorbed by the Earth. The Earth’s surface warms up and re-radiates heat energy. So the atmosphere, consisting of water vapour, carbon dioxide and other gases, which do not allow this radiation to pass through it, absorbs it. Thus, the Earth gets warmer than it would be.

The greenhouse effect is the process by which radiation from a planet’s atmosphere warms the planet’s surface to a temperature above what it would be without this atmosphere.

Gases such as carbon dioxide, water vapour, methane and others present in our atmosphere are called greenhouse gases. Greenhouse gases are those that absorb and radiate heat energy in all directions. The earth’s atmosphere behaves as a greenhouse and its natural effect is critical in supporting life. Human activities, however, mainly the burning of fossil fuels and cutting down of forests, have led to an accumulation of greenhouse gases in the earth’s atmosphere.

Activity 6.9 Researching and discussing applications of greenhouse effect
In groups, using the internet and familiar examples in Uganda, discuss how a greenhouse is used in agriculture/horticulture.

Assignment 6.2 Researching and reporting the causes and effects of global warming and how greenhouse works
In groups, research from the internet (if available) or library on the causes and the effects of global warming.
Also, research on how a greenhouse works.
Make a report of your findings and finally present your work to the
whole class.

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