Conduction
1. Is the heat transferred from a campfire to a person standing in front of it a good example of heat transfer by conduction?
No, because air is generally a poor conductor of heat.
2. Why is it more effective to wear a few layers of thin clothing compared to a single thick layer of clothing on a cold day?
Wearing a few layers of the thin clothing actually traps more air. Because air is a poor conductor of heat and so wearing a few layers of thin clothing is more effective than a single thick layer of clothing.
Convection
1. Is the heat transferred from a hot iron to the person holding it in a downward position for ironing a good example of heat transfer by convection?
Yes. The iron heats up the air around it. And hot air would rise and a convection current can be set up, because it is less dense. However, much of the heat transfer from a hot iron to the person doing it is by radiation.
Radiation
1. Is the heat transferred from a hot zinc roof to a person below it a good example of heat transfer by radiation? Explain.
Yes. The heat transfer could not be due to conduction or convection. Because the cold air below the zinc roof is a poor conductor of heat. Thus the convection currents of air cannot be set up from the roof to the person standing below.
2. What is the purpose of the vacuum flask that is used to store cold liquids?
It is to prevent heat gain by conduction and convection.
Sunday, July 5, 2009
Radiation
Radiation is the continual emission of infared waves from the surface of all bodies, transmitted without the aid of a medium.
This means that radiation does not require a material medium for heat transfer for it can take place in a vacuum. The Sun emits electromagnetic waves of different frequencies and wavelengths but with the same speed in vacuum. The part of the spectrum that makes us feel warm is the infra-red waves. Heat due to the infra-red waves is called radiant heat. The hotter the object, the greater the amount of radiant heat emitted. In general, a good emitter of radiant heat is also a good absorber of it. Conversely, a poor emitter of radiant heat is also a poor absorber. Dull, black surfaces are better emitters and absorbers of infra-red radiation as compared to shiny, white surfaces.
Background Information
Astronomers have found that infrared radiation is especially useful when trying to probe areas of our universe that are surrounded by clouds of gas and dust. Because of infrared's longer wavelength, it can pass right through these clouds and reveal details invisible by observing other types of radiation. Especially interesting are areas were stars and planets are forming and the cores of galaxies where it is believed huge black holes might reside.
The image on the left shows an optical view of a star forming region. The same area is shown on the right in infrared radiation. Notice how the infrared observations penetrate the obscuringcloud to reveal many new details.
Factors affecting the rate of transfer of energy by radiation
1. Colour and texture of the surface
As mentioned earlier, dull, black objects are better emitters and absorbers of infra-red radiation compared to shiny, white objects.
2. Surface temperature
The higher the temperature of the surface of the object relative to the room temperature, the higher the rate of energy transfer.
3. Surface area
When comparing 2 objects of similar material and mass, the object with the larger surface area actually emits more infra-red radiation and at a higher rate.
Applications of Radiation
1. Greenhouses
As seen from the picture, the greenhouse is being used to help plants grow better by trapping heat from the Sun and then to the ground. Thus, it actually warms up the contents of the greenhouse and it starts to emit infared radiation. The infared radiation emitted by the contents of the greenhouse cannot pass through the glass. Therefore, the infared radiation emitted is trapped. It increases over time and also causes the temperature in the greenhouse to increase.
2. Teapots
Chinese teapots that are painted in white and silver allows the heat of the contents (e.g. water) to be retained for a longer time. This is because silver and/or white is a poor emitter of heat, which is why its used for teapots.
This means that radiation does not require a material medium for heat transfer for it can take place in a vacuum. The Sun emits electromagnetic waves of different frequencies and wavelengths but with the same speed in vacuum. The part of the spectrum that makes us feel warm is the infra-red waves. Heat due to the infra-red waves is called radiant heat. The hotter the object, the greater the amount of radiant heat emitted. In general, a good emitter of radiant heat is also a good absorber of it. Conversely, a poor emitter of radiant heat is also a poor absorber. Dull, black surfaces are better emitters and absorbers of infra-red radiation as compared to shiny, white surfaces.
Background Information
Astronomers have found that infrared radiation is especially useful when trying to probe areas of our universe that are surrounded by clouds of gas and dust. Because of infrared's longer wavelength, it can pass right through these clouds and reveal details invisible by observing other types of radiation. Especially interesting are areas were stars and planets are forming and the cores of galaxies where it is believed huge black holes might reside.
The image on the left shows an optical view of a star forming region. The same area is shown on the right in infrared radiation. Notice how the infrared observations penetrate the obscuringcloud to reveal many new details.
Factors affecting the rate of transfer of energy by radiation
1. Colour and texture of the surface
As mentioned earlier, dull, black objects are better emitters and absorbers of infra-red radiation compared to shiny, white objects.
2. Surface temperature
The higher the temperature of the surface of the object relative to the room temperature, the higher the rate of energy transfer.
3. Surface area
When comparing 2 objects of similar material and mass, the object with the larger surface area actually emits more infra-red radiation and at a higher rate.
Applications of Radiation
1. Greenhouses
As seen from the picture, the greenhouse is being used to help plants grow better by trapping heat from the Sun and then to the ground. Thus, it actually warms up the contents of the greenhouse and it starts to emit infared radiation. The infared radiation emitted by the contents of the greenhouse cannot pass through the glass. Therefore, the infared radiation emitted is trapped. It increases over time and also causes the temperature in the greenhouse to increase.
2. Teapots
Chinese teapots that are painted in white and silver allows the heat of the contents (e.g. water) to be retained for a longer time. This is because silver and/or white is a poor emitter of heat, which is why its used for teapots.
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