Energy-saving bits of knowledge "How heat is transmitted"
How heat is transferred
We introduced the principle of heat transfer from a high temperature to a low temperature in the Energy Saving Bits of Knowledge "The Principle of Heat Transfer".
There are three methods of transferring heat: conduction, convection, and radiation (radiation).
transmission
Heat transfer caused by contact.
When a hot object and a cold object are brought into contact, thermal energy is transferred from the hot object to the cold object, activating the cold object and increasing its temperature.
Familiar examples include the transfer of heat when a person touches a warm object such as a hot carpet or a hot water bottle, or the loss of heat when a person touches a cold object such as an ice pillow.
The reason why you feel hot when you touch a hot object such as a boiling kettle is that the atoms and molecules in the human body are activated by the thermal energy of the hot object and you feel hot. Conversely, when you touch a cold object such as a glass filled with ice, it feels cold because the heat energy of the human body activates the atoms and molecules of the cold object, slowing the movement of the atoms and molecules in the human body and making it feel cold. is.
convection
Heat transfer that occurs when air or liquid circulates.
High temperature air and liquids rise and low temperatures fall, circulating in the space and equalizing the temperature.
As a familiar example, air conditioners and fans that circulate air use heat transfer by convection.
The air conditioner is installed near the ceiling because the cooling air descends.
By the way, regarding the reason why hot air and liquids rise, when the temperature rises, the air and liquids expand and their densities decrease. A decrease in density means a decrease in weight, so air and liquids rise.
radiation (radiation)
Heat transfer occurs when an object emits far infrared rays (electromagnetic waves).
Radiation is heat transfer using far infrared rays, and is also called radiation, thermal radiation, or thermal radiation. It transfers heat without a medium such as air or water.
As a familiar example of radiation, microwave ovens and oil heaters that heat objects by emitting far infrared rays use heat transfer by radiation.
In fact, various objects with heat (including the human body) emit far-infrared rays to a greater or lesser extent. Therefore, far infrared rays transfer heat from a hot object to a cold object even if the objects are not in contact with each other or there is no wind. A hot object absorbs heat from a cold object, and the movement of atoms and molecules slows down, causing the temperature to drop.
The reason why the sun feels warm and the inside of the tunnel feels cold
The reason why the sunlight feels warm and the inside of the tunnel feels cold is because heat transfer occurs due to radiation.
The sun is in space, and there is no air or water in space. However, people feel warm when exposed to sunlight.
This is because the movement of the atoms and molecules in the human body is activated by the energy of invisible far-infrared light emitted from the sun. (If you compare the temperature of the sun and humans, naturally the sun is much higher, so heat transfer from the sun to humans occurs.)
Also, remember when you entered the tunnel. Contrary to the sunlight, you may have felt a chill.
This is because far infrared rays are exchanged between the tunnel and the human body. When comparing the temperature of the human body and the tunnel, the human body has a higher temperature, so far-infrared heat transfer occurs from the human body to the tunnel. Therefore, the tunnel is gradually warmed by the far infrared rays of the human body, but conversely, the tunnel absorbs the far infrared rays, slowing down the movement of the atoms and molecules of the human body and making the body feel cooler. . It's the same principle that makes you feel cold inside a cave.
What are emissivity and spectral emissivity?
Emissivity ε is the ratio of the radiant energy per unit time of a comparison object and a reference object.
The emissivity value ranges from 0 to 1. The higher the emissivity, the easier it is to radiate. The reference object, called a black body, is a theoretical object that emits ideal radiation at all wavelengths and temperatures, and does not exist.
Spectral emissivity is the ratio of the radiant energy of a comparable object and a black body at a specific wavelength, not at all wavelengths. In the case of thermal radiation, spectral emissivity refers to the ratio of radiant energy in the far-infrared region.
Emissivity and absorptivity have a relationship of = (equal), and a high far-infrared emissivity (easy to radiate) also means a high far-infrared absorptivity (easy to absorb). In other words, the higher the emissivity, the more far-infrared rays are emitted, making it easier to warm the object.
- Emissivity ε (total emissivity): The ratio of radiant energy per unit time at all wavelengths of the object and black body to be compared
- Spectral emissivity: the ratio of the radiant energy of a comparable object and a blackbody at a specific wavelength
- Blackbody (perfect radiator): A theoretical thermal radiator that emits ideal radiation at all wavelengths and temperatures
metal emissivity
The emissivity varies depending on the material, and even with the same material, it varies depending on the surface condition.
For metals, generally polishing the surface reduces the emissivity, while oxidizing or roughening the surface increases the emissivity. The general emissivity guideline for each metal is shown in the table. (*1)
In the case of aluminum, it is possible to increase the emissivity to 0.9 or higher by anodizing.
*1: Values may vary depending on measurement conditions, so consider these values as reference values.