Chapter 22: Heat Transfer

43704_3Chapt22Notes.pdf

Chapter 22: Heat Transfer

When objects are in thermal contact, heat is transferred between them until all are at a common temperature. This heat transfer occurs in three ways, conduction, convection and radiation.

Conduction

Conduction occurs within a material or between different materials that are in direct contact.

Conduction refers to collision between atoms or

molecules and the actions of loosely bound electrons.

If one part of a piece of iron is heated, its

electrons in that part will have their kinetic energy increased. Because electrons in metals are loosely bound to the nucleus, the heated electrons will bump into many other electrons as they vibrate. This causes a chain reaction of collisions, so eventually all the electrons will have their kinetic energy increased and the iron will heat up evenly.

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rd Edition - Paul Hewitt Chapter 22 - Heat Transfer Page 1 of 7 Conductors are materials that heat up in this manner efficiently. They conduct heat. Good heat conductors are also good electricity conductors. Poor conductors are called insulators. Insulators slow down the transfer of heat.

N.B. No insulator can stop heat transfer, only

slow it down.

Convection

We all know that less dense materials rise. We also know that when a material is heated it becomes less dense due to its vibrating molecules requiring more space. This is how convection occurs. When a fluid (a liquid or a gas) is heated it will not heat evenly as they are usually poor conductors. This means that some molecules will have more translational kinetic energy than others will. The warm molecules will rise whilst the cold molecules fall, causing a current to form. This current keeps the fluid moving so all

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rd Edition - Paul Hewitt Chapter 22 - Heat Transfer Page 2 of 7 the molecules will eventually come into contact with the heat source and all will heat up.

Winds

Convection currents in air cause winds. As the ground heats and cools faster than water, the air above the ground will be hotter than the air above the water during the day. As the hot air above the ground rises, the cool air from above the water takes its place. As the hot air rises it expands. This will cause the air molecules to convert their translational kinetic energy into linear kinetic energy, in the action of spreading out. As translational kinetic energy is lost, so the temperature of the air decreases, thus the warm air that has just risen will cool and begin the fall again. This continuous process of heating and cooling produces convection currents.

Radiation

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rd Edition - Paul Hewitt Chapter 22 - Heat Transfer Page 3 of 7 How is it that we can feel the heat of the sun on our bodies? There is no matter between the Earth and the Sun for heat to travel through by conduction or convection, so how is it that the Earth warms up? It heats up through radiation. Any energy that is transmitted by radiation is called radiant energy. These energies are transmitted as electro-magnetic waves (which are transverse waves). Some types of radiant energy are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays and gamma rays. All these radiants differ only in their wavelength (listed here in longest to shortest).

All objects continually emit radiant energy with

mixtures of wavelengths. The hotter the objects, the shorter the wavelength of energy it emits. Thus a hot object will emit infrared radiation, which we can feel, and as it gets hotter the object will emit visible light which we can see. (Initially the longest wavelength of visible light, red, and then all the wavelengths, white light, will be emitted).

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rd Edition - Paul Hewitt Chapter 22 - Heat Transfer Page 4 of 7 When this radiant energy falls on an object, some is reflected and some is absorbed. The absorption increases the internal energy of the object.

Absorption of Radiant Energy

Absorption and reflection are opposite processes.

As different objects have different levels of

absorption, a good absorber will therefore be a poor reflector. Thus when we talk about radiant energy, a good absorber will appear dark, as it will reflect little visible light.

Everyone knows that if a dark coloured object is

left is the sun it will be hotter than a light coloured object. This is because the darker object absorbs more radiant energy, which will increase its internal energy.

Emission of Radiant Energy

Good absorbers of radiant energy are also good

emitters. We know this because otherwise all good absorbers would always have more internal energy

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rd Edition - Paul Hewitt Chapter 22 - Heat Transfer Page 5 of 7 than poor absorbers. Good absorbers also need to be good emitters in order to lose the internal energy as readily as they absorb it. While a black object left in the sunlight will be hotter than a white one, it will cool down faster in the dark. This means that the black object is a net absorber in the sun and a net emitter in the dark.

Newton's Law of Cooling

We know that if there is a temperature difference between an object and its surrounds, both will change temperature until equilibrium is reached. Common sense tells us that the greater the temperature difference the higher the rate of temperature change. ie ice will melt faster in hot water than in water at room temperature - there is a greater difference in temperature.

Newton's Law of Cooling states that the rate of

cooling is proportional to the difference in temperature;

Rate of cooling T

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Global Warming and the Green House Effect

The Earth is enveloped in an atmosphere of gases, mainly nitrogen. This atmosphere lets the short wave radiant energy from the Sun (solar radiation) through and onto the Earth, where much of it is absorbed. The Earth in turn emits radiant energy (terrestrial radiation), but because it is nowhere near the temperature of the Sun it has a much longer wavelength. Most of this longer wavelength energy will not pass through the atmosphere. Atmospheric gases (mainly carbon dioxide and water vapour) will absorb and re-emit much of it back to Earth. This is a natural process, which makes it possible to sustain life on Earth. The concerning aspect of

Global Warming is that we a continuing to pump

huge amounts of carbon dioxide (CO 2 ) into the atmosphere. As this is one of the gases which re- emits terrestrial radiation back to Earth, the temperature of the Earth will increase. We a not yet sure what effect this will have on our planet.