Conduction, convection and radiation are the three forms of heat transfer. Heat is the energy that passes from one body or system to another.
Heat transfer only occurs when there is a temperature difference between two things.
A the drivingheat transfer occurs when one body or object is in contact with another.
Convectioninstead, it is produced by the movement of gases or liquids at different temperatures.
On the other hand, the radiation it is a transfer of heat without the bodies being in contact. For this reason, this occurs through the emanation of energy through electromagnetic waves.
An example illustrating this would be a pot of boiling water: the fire heats the pot (radiation), the metal in the pot heats the water (conduction), and the hot water rises by conduction. heat (convection).
|definition||Form of heat transfer by contact.||Form of transfer of heat by movement of matter.||Form of heat transfer by electromagnetic waves.|
|Transfer address||From more to less temperature|
|mechanism||Movement of atoms in a body||Movement of gases and liquids due to temperature difference||Propagation of electromagnetic waves in space|
|examples||The handle of an aluminum pot on a burning stove||Domestic heating systems||The heat of the Sun|
What is heat conduction
Conduction is a form of heat transfer that occurs when two bodies are in contact or when heat passes from side to side of the same body. For example, if we heat one end of an iron rod, the other end will heat up shortly, despite not being in direct contact with the heat source.
The mechanism of heat transfer by conduction is based on the movement of atoms. As the temperature rises, the atoms move faster and also push neighboring atoms, transferring heat to them.
The ability of materials to conduct heat is known as thermal conductivity. For example, air has a low conductivity as well as wood.
On the other hand, metals such as aluminum and iron have a high thermal conductivity. These materials are very effective in conducting heat, as they have free electrons that transfer energy faster from hot areas to cold areas of the body.
Examples of heat conduction
- Cast iron pan on a lit stove: the heat of the oven heats the pan which conducts the heat to the rest of the pan and the contents inside it.
- Melted ice in hand: if we place an ice in the hand, it melts due to the conduction of body heat.
- Hot feet in the sand: on a hot day at the beach, if we walk on the hot sand with bare feet, we will soon feel that we are getting burned by the conduction of the heat from the sand to our feet.
- The cup of hot coffee: when pouring hot coffee (or another hot drink) into a cup, we will eventually feel the heat in our hands. That’s why the cups have a handle so we can hold it without burning ourselves.
- The ironing of clothes: the iron used to remove wrinkles from clothes heats up and when it comes into contact with the clothes, it conducts the heat.
- the thermometer: this instrument is used to measure temperature because it receives or transfers heat by conduction from the object or substance with which it is in contact.
- The metal spoon against the wooden spoon: A wooden spoon conducts heat poorly while a metal spoon heats up quickly. That’s why we can mix the food with the wooden spoon, but when we eat the soup we use a metal spoon.
What is convection heat?
Convection is the form of heat transfer that occurs by the movement of liquids and gases from hot areas to cold areas. When a fluid is heated, it becomes less dense, causing it to rise.
It is thanks to the process of convection that clouds are formed: water vapor and hot air on the surface of the Earth rises and then condenses as clouds in the heights. This type of convection is natural or free, without the intervention of external forces.
On the other hand, forced convection occurs when a force is applied to move the fluid. This is what happens when we use fans to move hot air around a room or when we move the contents of a pot on the stove.
Examples of heat convection
- Domestic heating systems: the air in contact with a heating system is heated, then rises and displaces the cold air causing air currents.
- The internal heat of the Earth: the Earth’s core is hot and fluid, so convection currents are produced that cause earthquakes and volcanic eruptions.
- The fluid in the accumulator thermoses: In the pipes of a solar heater, the fluid heats up and becomes less dense, moving heat to cooler fluids.
- Convection oven: the hot air inside the oven circulates thanks to a fan, which makes the food bake more evenly.
- Boiling water in a pot: when we place water in a pot on a hot stove, the hot water at the bottom rises forcing the movement of the cold water on the surface.
- Hair dryer: cold air enters the dryer, heats up by passing through a resistor and blows hot air through the filter.
- ocean currents: the waters of the oceans are kept in constant motion by convection, the warm water of the tropics moves towards the poles.
- Hot air balloons or hot air balloon: when the air is heated, it becomes less dense, it is trapped inside the fabric of the balloon, causing it to float. That’s why, to go down, a window opens that lets the hot air escape.
What is heat radiation?
Radiation is the transfer of heat without contact between objects. This happens through electromagnetic waves, such as visible light, ultraviolet and infrared, which propagate through space. For heat radiation to occur, the presence of material is not necessary.
Bodies emit heat by radiation, but they also absorb heat, depending on the temperature difference. In fact, bodies that absorb better are also good emitters. For example, a black surface absorbs radiation better, but also emits more than a white surface.
Thus, the emission depends on the temperature of the body, the higher the temperature, the more heat is emitted.
The operation of a thermos is based on concentrating the heat radiation inside. A thermos is constructed with a double wall of glass with no air between them, to prevent heat loss by conduction or convection. The silver inner wall reflects radiation without absorbing it, so the drink stays hot for longer.
Examples of thermal radiation
- The heat of the Sun: the Earth receives heat from the Sun by radiation.
- Hot sand on a beach: on a summer day, solar radiation heats the sand.
- Embers from a bonfire: when we approach a campfire or fireplace, burning coals emit heat by radiation.
- solar oven: the sun’s rays hit a surface heating the air.
- Infrared thermometer: measures the radiation of a body in the infrared range and represents it as temperature.
- Thermal chambers: thermographic cameras record the heat of bodies emitted by radiation.
- Tanning rooms: tanning beds are based on the emission of ultraviolet rays that promote tanning of the skin. Its use is limited by the predisposition to skin damage.
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