(a) State the three processes of thermal energy transfer by which a hot object cools down.
Conduction, convection, radiation.
(b) Of the three processes in (a), which one of them
(i) involves thermal energy travelling from the hot object to its surroundings without heating the air around it? Why?
Radiation. Radiation is the only process that does not need a medium to transfer the energy. The energy can be transferred through vacuum. Hence, it can travel without heating the air around it.
(ii) occurs only in fluids (ie liquids or gases)? Why?
Convection. It occurs by means of convection currents set up in fluids due to differences in density.
(c) Metals are good conductors of heat whereas wood is a poor conductor of heat (i.e. an insulator).Discuss this statement with reference to the mechanism of thermal energy transfer in solids.
For conduction of heat within solids, the two mechanisms are molecular vibrations and free electron diffusion. Since metals contain many free electrons, they transfer heat faster.
Wednesday, June 17, 2009
The Cool Physics of Heat
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The Cool Physics of Heat
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Explain the following observations
(a) Birds usually fluff up their feathers during cold weather.
By fluffing up their feathers, birds trap tiny air pockets within their feathers. Since air is a poor conductor of heat, the bird loses less heat during cold weather.
(b) Cooking utensils, kettles and boilers are usually made of aluminium or stainless steel.
Aluminium and stainless steel (metals) conducts heat well. This helps to conduct heat quickly to the food to enable it to be cooked faster.
(c) Sawdust is used to cover ice blocks.
Sawdust is a bad conductor heat. By using sawdust to cover ice blocks, heat transfer from surroundings to ice is reduced, hence, melting of ice blocks is delayed.
(d) A black car becomes hotter than a white car when both are parked under direct sunlight for several hours.
Black surfaces are better than white surfaces in absorbing radiation. Thus, the black car absorbs more heat from radiation as compared to the white car, resulting in a hotter temperature for the black car.
(a) Birds usually fluff up their feathers during cold weather.
By fluffing up their feathers, birds trap tiny air pockets within their feathers. Since air is a poor conductor of heat, the bird loses less heat during cold weather.
(b) Cooking utensils, kettles and boilers are usually made of aluminium or stainless steel.
Aluminium and stainless steel (metals) conducts heat well. This helps to conduct heat quickly to the food to enable it to be cooked faster.
(c) Sawdust is used to cover ice blocks.
Sawdust is a bad conductor heat. By using sawdust to cover ice blocks, heat transfer from surroundings to ice is reduced, hence, melting of ice blocks is delayed.
(d) A black car becomes hotter than a white car when both are parked under direct sunlight for several hours.
Black surfaces are better than white surfaces in absorbing radiation. Thus, the black car absorbs more heat from radiation as compared to the white car, resulting in a hotter temperature for the black car.
The figure below shows a household hot water system.
(a) State the process by which the hot water rises from the boiler to the hot water tank.
(a) State the process by which the hot water rises from the boiler to the hot water tank.Convection
(b) Through which pipe, P or Q, does the heated water rise from the boiler to the hot water tank? Explain your choice.
Pipe P. Hot water is less dense than cold water. Thus, hot water in the boiler will rise, and exit the boiler through pipe P to reach the hot water tank. In the hot water tank, the colder water, being denser, will sink, and exit through pipe Q to reach the boiler.
(c) State the purpose of pipe R.
Pump cold water to the lower half of the water tank to be transferred to the boiler via pipe Q for boiling/heating.
(d) What is the name and purpose of pipe S.
Pipe S is the overflow pipe attached to the tap of the hot water tank in case the temperature of the water becomes too high and cause a large expansion of the hot water. The excess water will then flow back into the cistern.Tuesday, June 16, 2009
Heat transfer in general
We've looked at the three types of heat transfer. Conduction and convection rely on temperature differences; radiation does, too, but with radiation the absolute temperature is important. In some cases one method of heat transfer may dominate over the other two, but often heat transfer occurs via two, or even all three, processes simultaneously.
A stove and oven are perfect examples of the different kinds of heat transfer. If you boil water in a pot on the stove, heat is conducted from the hot burner through the base of the pot to the water. Heat can also be conducted along the handle of the pot, which is why you need to be careful picking the pot up, and why most pots don't have metal handles. In the water in the pot, convection currents are set up, helping to heat the water uniformly. If you cook something in the oven, on the other hand, heat is transferred from the glowing elements in the oven to the food via radiation.
We've looked at the three types of heat transfer. Conduction and convection rely on temperature differences; radiation does, too, but with radiation the absolute temperature is important. In some cases one method of heat transfer may dominate over the other two, but often heat transfer occurs via two, or even all three, processes simultaneously.
A stove and oven are perfect examples of the different kinds of heat transfer. If you boil water in a pot on the stove, heat is conducted from the hot burner through the base of the pot to the water. Heat can also be conducted along the handle of the pot, which is why you need to be careful picking the pot up, and why most pots don't have metal handles. In the water in the pot, convection currents are set up, helping to heat the water uniformly. If you cook something in the oven, on the other hand, heat is transferred from the glowing elements in the oven to the food via radiation.
Thursday, June 11, 2009
Friday, June 5, 2009
Radiation
The third way to transfer heat, in addition to convection and conduction, is by radiation, where energy is transferred in the form of electromagnetic waves. An electromagnetic wave is basically an oscillating electric and magnetic field traveling through space at the speed of light. All of us are already familiar with many kinds of electromagnetic waves, such as radio waves, microwaves, the light we see, X-rays, and ultraviolet rays. The only difference between the different kinds is the frequency and wavelength of the wave.
The radiation here, in regard to heat transfer, is not the same thing as the dangerous radiation associated with nuclear bombs. That radiation comes in the form of very high energy electromagnetic waves, as well as nuclear particles. The radiation associated with heat transfer is entirely electromagnetic waves, with a relatively low (and hence relatively safe) energy.
Everything around us takes in energy from radiation, and gives it off in the form of radiation. When everything is at the same temperature, the amount of energy received is equal to the amount given off. Because there is no net change in energy, no temperature changes occur. When things are at different temperatures, however, the hotter objects give off more energy in the form of radiation than they take in; the reverse is true for the colder objects.
The third way to transfer heat, in addition to convection and conduction, is by radiation, where energy is transferred in the form of electromagnetic waves. An electromagnetic wave is basically an oscillating electric and magnetic field traveling through space at the speed of light. All of us are already familiar with many kinds of electromagnetic waves, such as radio waves, microwaves, the light we see, X-rays, and ultraviolet rays. The only difference between the different kinds is the frequency and wavelength of the wave.
The radiation here, in regard to heat transfer, is not the same thing as the dangerous radiation associated with nuclear bombs. That radiation comes in the form of very high energy electromagnetic waves, as well as nuclear particles. The radiation associated with heat transfer is entirely electromagnetic waves, with a relatively low (and hence relatively safe) energy.
Everything around us takes in energy from radiation, and gives it off in the form of radiation. When everything is at the same temperature, the amount of energy received is equal to the amount given off. Because there is no net change in energy, no temperature changes occur. When things are at different temperatures, however, the hotter objects give off more energy in the form of radiation than they take in; the reverse is true for the colder objects.
Thursday, June 4, 2009
Conduction
In metals, the dominant method of conduction is through the movement of electrons. This method of conduction does not work in non-metals as there are no free electrons. When a metal is heated, the electrons closest to the heat source will vibrate more rapidly. Electrons then collide with these atoms and gain more kinetic energy. The electrons therefore move around faster and collide with other free electrons which then gain more kinetic energy. Kinetic energy is therefore transferred between the electrons and through the metal from the point closest to the heat source towards points futher away. The electrons all travel very short distances but are very fast moving therefore conduction of heat happens very quickly.
In metals and in insulators, there is conduction of heat due to the vibration of the atoms. As atoms closest to the heat source absorb heat/thermal energy, they make their neighbouring atoms vibrate more rapidly which then in turn make their neighbouring atoms vibrate more.
This is how conduction works... :)
Examples of conduction:
- wire gauzes used on tripods are metal therefore they are good heat conductors.
Gauzes on cookers are also metal so that heat is conducted quickly and food is cooked fast.
- Poor thermal conductors (insulators) are used for saucepan handles
so that they don't heat up and can still be handled.
- Metals are used for the containers which heat liquids e.g. pans, kettles on hobs
- Air is a poor conductor
so materials that trap air are used for insulation in lofts and hot water cylinders.
In metals, the dominant method of conduction is through the movement of electrons. This method of conduction does not work in non-metals as there are no free electrons. When a metal is heated, the electrons closest to the heat source will vibrate more rapidly. Electrons then collide with these atoms and gain more kinetic energy. The electrons therefore move around faster and collide with other free electrons which then gain more kinetic energy. Kinetic energy is therefore transferred between the electrons and through the metal from the point closest to the heat source towards points futher away. The electrons all travel very short distances but are very fast moving therefore conduction of heat happens very quickly.
In metals and in insulators, there is conduction of heat due to the vibration of the atoms. As atoms closest to the heat source absorb heat/thermal energy, they make their neighbouring atoms vibrate more rapidly which then in turn make their neighbouring atoms vibrate more.
This is how conduction works... :)
Examples of conduction:
- wire gauzes used on tripods are metal therefore they are good heat conductors.
Gauzes on cookers are also metal so that heat is conducted quickly and food is cooked fast.
- Poor thermal conductors (insulators) are used for saucepan handles
so that they don't heat up and can still be handled.
- Metals are used for the containers which heat liquids e.g. pans, kettles on hobs
- Air is a poor conductor
so materials that trap air are used for insulation in lofts and hot water cylinders.
Wednesday, June 3, 2009
Convection
The cool particles gain kinetic energy when they are heated from the source and expands as it heats up. The particles become less dense than the surrounding cold air therefore it rises and displace the cool air. Cool particles are more dense therefore they fall and move towards the heat source to take the place of the warm particles. They then heat up and rise while other particles cool down and fall.
Example of convection:
- in fridges to cool it down.
Heat is carried away, therefore the back of fridges are always warm.
- Land & sea breezes are due to convection.
- Atmospheric winds.
- Hot water systems.
The cool particles gain kinetic energy when they are heated from the source and expands as it heats up. The particles become less dense than the surrounding cold air therefore it rises and displace the cool air. Cool particles are more dense therefore they fall and move towards the heat source to take the place of the warm particles. They then heat up and rise while other particles cool down and fall.
Example of convection:
- in fridges to cool it down.
Heat is carried away, therefore the back of fridges are always warm.
- Land & sea breezes are due to convection.
- Atmospheric winds.
- Hot water systems.
Monday, June 1, 2009
There are three basic ways in which heat is transferred. They are conduction, covection and radiation. In fluids, heat is often transferred by convection, in which the motion of the fluid itself carries heat from one place to another. Another way to transfer heat is by conduction, which does not involve any motion of a substance, but rather is a transfer of energy within a substance (or between substances in contact). The third way to transfer energy is by radiation, which involves absorbing or giving off electromagnetic waves.
Conduction: The transfer of energy through matter from particle to particle. It is the transfer and distribution of heat energy from atom to atom within a substance. For example, a spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon. Conduction is most effective in solids-but it can happen in fluids.
When heat is transferred via conduction, the substance itself does not flow; rather, heat is transferred internally, by vibrations of atoms and molecules. Electrons can also carry heat, which is the reason metals are generally very good conductors of heat. Metals have many free electrons, which move around randomly; these can transfer heat from one part of the metal to another.
Convection: Convection currents can transfer heat in a fluid, which is a liquid or a gas, but can not transfer heat in a solid. The fluid circulation carries thermal energy from the heat source to the other portions of the fluid. Convection currents are set up in the fluid because the hotter part of the fluid is not as dense as the cooler part, so there is an upward buoyant force on the hotter fluid, making it rise while the cooler, denser, fluid sinks. Birds and gliders make use of upward convection currents to rise.
Forced convection, where the fluid does not flow of its own accord but is pushed, is often used for heating (e.g., forced-air furnaces) or cooling (e.g., fans, automobile cooling systems).
Radiation: Electromagnetic waves that directly transport ENERGY through space. Sunlight is a form of radiation that is radiated through space to our planet without the aid of fluids or solids. The energy travels through nothing! Just think of it! The sun transfers heat through 93 million miles of space. Because there are no solids touching the sun and our planet, conduction is not responsible for bringing heat to Earth. Since there are no fluids (like air and water) in space, convection is not responsible for transferring the heat. Thus, radiation brings heat to our planet.
Conduction: The transfer of energy through matter from particle to particle. It is the transfer and distribution of heat energy from atom to atom within a substance. For example, a spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon. Conduction is most effective in solids-but it can happen in fluids.
When heat is transferred via conduction, the substance itself does not flow; rather, heat is transferred internally, by vibrations of atoms and molecules. Electrons can also carry heat, which is the reason metals are generally very good conductors of heat. Metals have many free electrons, which move around randomly; these can transfer heat from one part of the metal to another.
Convection: Convection currents can transfer heat in a fluid, which is a liquid or a gas, but can not transfer heat in a solid. The fluid circulation carries thermal energy from the heat source to the other portions of the fluid. Convection currents are set up in the fluid because the hotter part of the fluid is not as dense as the cooler part, so there is an upward buoyant force on the hotter fluid, making it rise while the cooler, denser, fluid sinks. Birds and gliders make use of upward convection currents to rise.
Forced convection, where the fluid does not flow of its own accord but is pushed, is often used for heating (e.g., forced-air furnaces) or cooling (e.g., fans, automobile cooling systems).
Radiation: Electromagnetic waves that directly transport ENERGY through space. Sunlight is a form of radiation that is radiated through space to our planet without the aid of fluids or solids. The energy travels through nothing! Just think of it! The sun transfers heat through 93 million miles of space. Because there are no solids touching the sun and our planet, conduction is not responsible for bringing heat to Earth. Since there are no fluids (like air and water) in space, convection is not responsible for transferring the heat. Thus, radiation brings heat to our planet.
What is HEAT?
Heat is a form of energy. When something is heated, it's molecules gain kinetic energy and so they move around more rapidly. The molecules knock off each other, moving further apart which generally makes the substance expand.
HEAT IS MEASURED IN JOULES(J) OR KILOJOULES(kJ).
What is HEAT TRANSFER?
Heat moves from an area of high temperature to an area of low temperature, until both areas are of the same temperature.
For example, if you pour warm water over ice cubes, heat moves from the the water to the ice. The ice will melt and the water will be cooler than before. What has happened is that the water lost heat and the ice gained heat until both mixed and were of the same temperature.
HEAT CAN BE TRANSFERED IN THREE WAYS: CONDUCTION, CONVECTION AND RADIATION. :)
Heat is a form of energy. When something is heated, it's molecules gain kinetic energy and so they move around more rapidly. The molecules knock off each other, moving further apart which generally makes the substance expand.
HEAT IS MEASURED IN JOULES(J) OR KILOJOULES(kJ).
What is HEAT TRANSFER?
Heat moves from an area of high temperature to an area of low temperature, until both areas are of the same temperature.
For example, if you pour warm water over ice cubes, heat moves from the the water to the ice. The ice will melt and the water will be cooler than before. What has happened is that the water lost heat and the ice gained heat until both mixed and were of the same temperature.
HEAT CAN BE TRANSFERED IN THREE WAYS: CONDUCTION, CONVECTION AND RADIATION. :)
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