Heat transfer by conduction

1. Definition of the heat transfer by conduction

If you sit in a wooden chair, the surface of the wooden chair becomes warm. Conversely, if you sit in a chair made of plastic or metal, the surface of a metal or plastic chair does not feel warm after sitting. Why is the surface of the wooden chair heated, while the metal chair surface is not? Why can sleep on a cold floor without a mattress cause pain? Have you ever worn an anti-cold jacket? Why are most anti-cold jackets made of wool? There are still many things that can be considered and questioned related to the subject of heat transfer by conduction.

When one part of a high-temperature object in contact with a low-temperature object, energy moves from a high-temperature object to a low-temperature part of the object. The addition of energy causes atoms and molecules to make things move faster. When moving, the molecule has kinetic energy (KE = 1/2 m v2). The molecules that move faster (greater kinetic energy) pound the molecules next to them. The molecule hits another molecule next to it. So on. So molecules collide with each other while moving energy. The heat transfer that occurs through collisions between the molecules of the object is called heat transfer by conduction.

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2. The equation of the heat transfer by conduction

Heat transfer conduction 1The object located on the left has a higher temperature (T1) while the object located on the right has a lower temperature (T2). Because of the temperature difference (T1 T2), heat moves from a high-temperature object to a low-temperature object (the direction of heat flow to the right). The object passed by heat has a cross-sectional area (A) and length (l).

Based on the results of the experiment, heat that moves during a certain time interval (Q/t) is directly proportional to the temperature difference (T1 – T2), cross-sectional area (A), the nature of an object (k = thermal conductivity) and inversely proportional to the length of the object. The formula for heat transfer by conduction:

Heat transfer conduction 2

Description: Q/t = heat transfer rate, k = thermal conductivity, A = cross-sectional area, T1 = high temperature, T2 = low temperature, l = length of object.

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3. The thermal conductivity (k)

Why are most cookware made of aluminum? If your left-hand hold the iron, your right-hand holds the glass, then iron and glass are touched to the fire. Left hand or right hand feeling faster? These questions and perhaps other questions that you will ask relate to the thermal conductivity of objects. The thermal conductivity of an object is the ability of an object to move heat through the object. Objects that have a large thermal conductivity (k) are good heat conductors (good thermal conductors). Conversely, objects that have small thermal conductivity are bad heat conductors (bad thermal conductors).

4. Application of the heat transfer by conduction

Why does the tile feel cooler than the carpet? Tiles have a greater thermal conductivity than carpets. Therefore the tile is a good heat conductor, while the carpet is a bad heat conductor. When we walk on the carpet, heat flows from the foot towards the carpet. This happens because our body temperature is higher than the carpet temperature. The carpet is a bad heat conductor, so the heat flowing from our feet accumulates on the surface of the carpet, so the carpet becomes warmer. When we walk on a tile or ceramic, heat flows from the foot to the tile or ceramic. Because tiles are a good heat conductor, the heat is not stuck on the tile surface. Heat flows smoothly so that our feet feel cold.

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Sleeping on the floor of the house without a pedestal can cause pain. Why? This is due to the amount of heat that moves from the body to the floor. Heat is the energy that moves. When your body loses a lot of heat, the energy in your body decreases. If you lack energy, you can get sick!

What are the functions of the windows and doors? Why should doors and windows be made of wood? At night, the temperature outside the home is lower than the air temperature inside the house. This difference in air temperature causes heat to move from inside the house to the outside of the house. Therefore, usually at night, we close doors or windows. The air trapped on the inside of the window or door serves as a good insulator. The smaller the thermal conductivity of an object, the more difficult heat transfer by conduction through the object.

In addition to maintaining our status as civilized and normal human beings, clothing also functions to keep our body temperature stable. The clothes we use are usually adjusted to air temperature. When the air temperature is low enough, the clothes we use are thicker. Thick blankets or clothes (jackets etc.) make the air unable to move. The air is trapped between the skin and the jacket or blanket. Because there is a temperature difference between our body and trapped air, heat moves from the body to the air. The addition of heat from the body causes the temperature of the trapped air to increase (the air becomes warmer). The value of air thermal conductivity is very small. Therefore, heat cannot get out of the body. Our body temperature is also maintained. If we don’t use a jacket when the air is cold enough, heat out of our body. The more heat coming out of the body, the body loses a lot of energy!

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