(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.
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