TRANSFER OF THERMAL ENERGY
1.In general, heat travels from a place of higher temperature to a place of lower temperature. The three processes by which heat may be transmitted or heat transfer are:
- conduction
- convection
- radiation
2.Mechanism of transfer of thermal energy
- free electrons in metals or vibrating particles (in conduction)
- differences in density (in convection)
- no medium required (in radiation)
We will consider each of these in turn.
3.Conduction
Conduction is the process by which heat is transmitted through a medium from one particle to another. When one end of a rod is heated, the particles at this end of the rod gain energy and vibrate faster. These particles collide with their less energetic neighbours. Some of their energy is transferred to these neighbouring particles, which in turn gain kinetic energy. In this way, heat is passed along the rod by vibrating particles. There is no net movement of particles during the process.
Atoms in a substance are always vibrating. If the substance gets hotter, the atoms vibrate more. The heat energy is given to the atoms, which makes them move about faster.Note: the atoms don't swap places, or move around they just vibrate more on the spot.
Look under conduction & click on Start heating
Have you ever danced next to someone really energetic? If so, you know that it makes you have to move about more – often just to get out of the way! It is like that for atoms passing heat energy on to each other.
Solids are better at conducting than liquids and gases because the atoms are closer together. If the atoms are too spaced out it makes it harder for the atoms to pass the energy along.
Conduction in solids, Conduction in liquid, Conduction in gas
or to view all
Comparison of conduction in all three states of matter
Metals are the best solids for conducting heat energy. In metals, there are free electrons that can move through the metal. These electrons are free to travel in the spaces between the particles before colliding with other electrons and transferring some of their energy to them. This process is much faster than the conduction by vibration of molecules in the body. This is calledelectron diffusion. Hence, metals conduct heat much faster than non-metals, which have no free electrons.
The poorest conductors are gases as their molecules are too far apart to affect each other much. This means that air is a terrible conductor of heat energy.
3.1.1Different materials transfer heat by conduction at different rates - this is measured by the material's thermal conductivity.
Thus, for a given temperature difference between any two reservoirs, materials with a large thermal conductivity will transfer large amounts of heat over time - such materials, like copper, are good thermal conductors. Conversely, materials with low thermal conductivities will transfer small amounts of heat over time - these materials, like concrete, are poor thermal conductors or insulators.
Materials such as polystyrene foam, wool and fiberglass are effective insulators because they contain pockets of still air. Air is a very poor conductor of heat.
Water is a poor conductor of heat energy. It is demonstrated as follows.
3.1.2Uses of good conductors
Good conductors are used in situations where heat has to be quickly transmitted. Thus pans, kettles and other cooking utensils, which have to be heated directly, are made of metals such as aluminium, copper and steel. The copper bit of a soldering iron also conducts energy readily from the electric heater to the solder because of its high conductivity.
In hot countries, like Singapore, marble and ceramic floor tiles help to keep the feet cool. This because the tiles are made of good conductors and heat is transferred quickly away from the feet into the tiles.
3.1.3Uses of poor conductors (insulators)
Poor conductors such as cloth, plastic and wood are also useful. They help to keep unwanted heat away. For instance, we use oven gloves to take a hot dish out of the oven. Kettles, saucepans and electric irons usually have plastic or wooden handles. You would also prefer to use a plastic ladle to stir boiling soup. A metal ladle would have burnt you very quickly.
Insulators also help to prevent heat loss. Since air is a very poor conductor, good insulation can be achieved by trapping air in between two sheets of glass in double glazed windows. This will reduce conduction of heat through the windows and keep the room warmer during cold seasons and cooler during warm weather.
Birds also use the same principle to keep warm. They fluff up their feathers in cold weather. This action allows air to be trapped in the feathers and cuts down the heat loss from their bodies.
4.Convection
4.1.1Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction.
4.1.2Suppose we consider heating up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. Hot air being less dense than the surrounding cooler air, will subsequently rise due to resultant buoyant forces - this movement of hot air into a cooler region is then said to transfer heat by convection.
4.1.3Heating a pot of water on a stove is a good example of the transfer of heat by convection. When the stove is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually the water starts bubbling - these bubbles are actually local regions of hot water, expanding and rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, denser water at the top will sink to the bottom, where it is subsequently heated. The circulation of a liquid in this manner is called convection current.
4.1.4Convection is the process by which heat is transmitted from one place to another by the movement of heated particles of a gas or a liquid. It takes place only in liquids and gases, not in solids. This is because in solids, the particles cannot move far away from their fixed positions.
These convection currents are illustrated in the following figure.
Look under convection
4.1.5To show convection in air
A simple experiment to show convection currents in air can be done using the apparatus below. The apparatus consists of two glass chimneys fitted onto the top of a wooden box with a plane of glass window so that the interior is visible.
A candle is lit below one chimney. The heat from this initiates convection current. The hot air flows out of this chimney. Cold air flows in through the chimney. You can test this by holding a piece a smouldering paper over the top of the chimney through which the cold air enters the box and observing the path of the smoke. This simple method of air circulation was once used to ventilate mines.
Convection occurs much more readily in gases than in liquids because they expand much more than in liquids when their temperature rises.
4.1.6Land and Sea Breezes
The land and sea breezes over the coastal areas are natural convection currents.
During the day, the sun heats up the land much faster than the sea partly due to efficient conduction in the ground. On the other hand, water is a poor heat conductor. Convection also does not take place efficiently since only the surface water is being heated and not the bottom layer. So the seawater temperature takes a long time to rise. All this while, the air above the land is being heated. It expands and rises. Cool air above the sea then moves inland to take its place. The result is a sea breeze.
At night, the converse happens. The sea temperature drops more slowly, wheras the land cools very rapidly in comparison. The sea is now warmer than the land. The warm air above the sea rises and the cool air above the land moves out to take its place and we have a land breeze.
(See illustrations below)
4.1.7Some everyday applications of convection:
- household hot water systems, electric kettles, air conditioners
- refrigerators (see below)
5.Radiation
5.1The third and last form of heat transfer we shall consider is that of radiation, which in this context means light (visible or not). This is the means by which heat is transferred, for example, from the sun to the earth through mostly empty space - such a transfer cannot occur via convection nor conduction, which require the movement of material from one place to another or the collisions of molecules within the material.
5.2Conduction and convection require a medium to transfer heat. Radiation is a method of heat transfer that does not require any medium – it can take place in a vacuum. In radiation, the source of heat transmits energy in the form of waves. These waves constitute part of the electromagnetic spectrum are known as infra-red radiation. The waves are transformed into heat again when they are absorbed by another body.
5.2Often the energy of heat can go into making light, such as that coming from a hot campfire. This light, being a wave, carries energy, and so can move from one place to another without requiring an intervening medium. When this light reaches you, part of the energy of the wave gets converted back into heat, which is why you feel warm, sitting beside a campfire. Some of the light can be in the form of visible light that we can see, but a great deal of the light emitted is infrared light, whose longer wavelength is detectable only with special infrared detectors. The hotter the object is, the less infrared light is emitted, and the more visible light. For example, human beings, at a temperature of about 37 o Celsius, emit almost exclusively infrared light, which is why we don't see each other glowing in the dark. On other hand, the hot filament of a light bulb emits considerably more visible light.
5.2So “How do we toast bread?”. When a piece of bread is put in a toaster the wires inside the toaster glow red hot on either side of the bread.
How does the heat energy get to the bread?
Is it by conduction?
No, the heat energy cannot conduct through the air to the bread because air is a very bad conductor.
Is it by convection?
No, hardly any of the heat energy could have travelled to the bread by convection, as the hot air particles would rise out of the toaster.
The heat energy must have reached the toast some other way. It travelled as radiated heat. This heat energy movement is sometimes called heat waves, but strictly speaking, it is infrared radiation.
5.2.1Emission of Radiation
The hotter an object is, the more energy it radiates or gives out. For example, the radiation from a red hot iron or grill is more easily felt than the radiation from a hot cup of tea. Not all surfaces emit radiation equally well.
To demonstrate this, take a shiny tin can and paint half of its outer surface dull black. The tin can is then filled with boiling water.
Place your hands at equal distances from the two sides of the can. You will feel that the black side radiates more heat energy than the shiny side. A dull black surface is a better emitter or radiator than a shiny one.
The rate of radiation also depends on the surface area of the body. The ears of an African elephant (over 1m in width) provide a large surface area for the giant mammal to cool off quickly in hot weather.
5.2.2Uses of good and poor emitters of radiation
The properties of surfaces that emit heat have been put to use for years.
For example, a shiny metal teapot is a poor radiator. This helps to keep the tea pot for a longer period.
By contrast, the cooling fins at the back of a refrigerator are painted dull black so that they emit radiation more quickly.
5.2.3Absorption of radiation
We have said that a body’s temperature rises when it absorbs radiation. We will study how the nature of a surface affects its ability to absorb radiation.
Take two tin can lids and paint one of them dull black on the inside surface, leaving the other one shiny.
Stick corks on the outside of each lid with candle wax. Place a bunsen flame midway between the lids. The was on the blackened lid will melt in a short time and the cork on it will fall off.
The shiny lid, however remains fairly cool and the wax on it remains melted.
This experiment shows that a dull black surface is a much better abosorber of radiation than a shiny surface.
In general, a good emitter is also a good absorber. On the other hand, the shiny surface is a good reflector of heat.
5.2.4Uses of good and poor absorbers of radiation
The fact that light colours and shiny surfaces reflect heat can be seen in many examples around us.
Houses in hot countries are often painted white to keep them cooler.
Factory roofs are sometimes coated with aluminium paint. This reduces absorption of heat during the day and cuts down radiation during the night. So a fairly steady temperature is obtained in the factory.
Solar control films are usually made of polyester layers combined with ultraviolet radiation – absorbing dyes and highly reflective metals. They allow light to pass through windows of cars and buildings while keeping the heat out.
Although fashion affects our choice of colours, it is important to remember that dark-coloured clothes absorb heat readily. They are not suitable for someone exposed to the sun for long periods of time. Similarly, choose a light colour to keep your car cooler in hot weather.
5.3Vacuum Flask
A vacuum (thermos) flask is designed to keep hot liquids hot and cold liquids cold.
It consists of a double walled glass container, with a vacuum between the walls. Both walls are silvered on the vacuum side. The container is supported on foam plastic which is a poor conductor of heat.
See illustration below.
No heat can enter or leave the flask by conduction or convection across the vacuum.
The inner silvered surface reflects radiation from hot fluids back into the flask.
The outer silvered surface reflects radiation in the external surroundings away from the flask.
The foam plastic support and the poorly conducting plastic stopper also minimize the heat transmitted by conduction through the thin glass walls of the flask.
The plastic cap to cover the vacuum flask stops convection and evaporation.
5.4Greenhouse effect
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