What is the most interesting or surprising thing you learned about heat transfer and conduction?

What is Heat?

Índice Show

Methods of Heat Transfer

All matter is made up of molecules and atoms. These atoms are always in different types of motion (translation, rotational, vibrational). The motion of atoms and molecules creates heat or thermal energy. All matter has this thermal energy. The more motion the atoms or molecules have the more heat or thermal energy they will have.

This is an animation made from a short molecular dynamics simulation of water. The green lines represent hydrogen bonds between oxygen and hydrogen. Notice the tight structure of water

Hydrogen bonds are much weaker than covalent bonds. However, when a large number of hydrogen bonds act in unison they will make a strong contributory effect. This is the case in water shown here.

Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and breaking up. Because of the short time scale (on the order of a few picoseconds) few bonds

What is temperature?

From the video above that shows movement of atoms and molecules it can be seen that some move faster than others. Temperature is an average value of energy for all the atoms and molecules in a given system. Temperature is independent of how much matter there is in the system. It is simply an average of the energy in the system.

How is heat transferred?

Heat can travel from one place to another in three ways: Conduction, Convection and Radiation. Both conduction and convection require matter to transfer heat.

If there is a temperature difference between two systems heat will always find a way to transfer from the higher to lower system.

CONDUCTION--

Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred. Metal is a good conduction of heat. Conduction occurs when a substance is heated, particles will gain more energy, and vibrate more. These molecules then bump into nearby particles and transfer some of their energy to them. This then continues and passes the energy from the hot end down to the colder end of the substance.

CONVECTION--

Thermal energy is transferred from hot places to cold places by convection. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas which have risen higher. This results in a continous circulation pattern. Water boiling in a pan is a good example of these convection currents. Another good example of convection is in the atmosphere. The earth's surface is warmed by the sun, the warm air rises and cool air moves in.

RADIATION--

Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted through empty space by thermal radiation often called infrared radiation. This is a type electromagnetic radiation . No mass is exchanged and no medium is required in the process of radiation. Examples of radiation is the heat from the sun, or heat released from the filament of a light bulb.

SOURCES AND READERS CHOICES --

Heat and Temperature from Cool Cosmo -- NASA

Here is a good applet to show motion in molecules -- you can control the temperature and see in this applet how to movements of the molecules change.

Important Temperatures in Cooking and Culinary Skills

In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation.

EVAPORATIONCONVECTIONCONDUCTIONRADIATIONFIND OUT MORE

Heat energy can be transferred from one place to another by three main processes. In CONVECTION, heat energy is carried by the movement of particles of matter. In CONDUCTION, heat is transferred by particles vibrating. In RADIATION, heat is carried directly by electromagnetic waves. When a hot object touches a cool object, heat moves from the hot one to the cool one. When objects transfer heat, they cool down to a lower temperature unless the heat energy they lose is constantly replaced.

Another process by which heat is transferred is called evaporation. When a dog sticks out its tongue and breathes hard (pants), the moisture on the tongue turns into water vapour – it evaporates. Heat energy is needed to turn a liquid into a gas, so heat is removed from the dog’s tongue in the process. This helps to cool the dog down. People cool themselves down by sweating through pores (tiny holes) in their skin, which removes heat from their bodies in the same way.

Convection is like an invisible conveyor belt that can transfer heat through fluids (liquids and gases). When part of a fluid is heated up, it expands. This makes it lighter and less dense than the fluid around it, so it rises upwards. As it rises, it moves away from the source of heat. Then it starts to cool and move downwards, before starting the cycle over again.

A burner at the base of the balloon warms the air inside. As the air warms up, it moves upwards, cools, and moves round in a circular pattern known as a convection current. When the balloon is full of hot air, it lifts off the ground because the hot air inside it is less dense and lighter than the cold air outside it.

During the day, sunlight warms the land more quickly than the sea. Warm air rises from the land by convection, moves out to sea, and cools, creating a circular current. This is why, at ground level, sea breezes blow from sea to land during the day. At night, the land cools more quickly than the sea. Warm air rises from the sea, the convection current reverses, and the breezes blow from land to sea.

Explorer Alexander von Humboldt explained how the oceans circulate using convection. Water warms and rises at the Equator, where Earth is hottest, then flows along the surface before cooling and sinking at the poles. He gave his name to the Humboldt Current, which travels up the South American coast.

Heat travels through solids by conduction. If one end of a metal bar is heated, heat energy moves rapidly along the bar. The hot particles do not move along the bar, but vibrate and pass energy to their neighbours. Materials that conduct electricity are also good conductors of heat. Metals conduct heat well, but wood, plastics, and glass conduct heat only poorly.

Cooking pots are made from metal, often aluminium. This metal is a good conductor of heat, so it rapidly transfers heat energy from the stove to the food. The handles of cooking pots are often made of wood or plastic. These materials do not conduct heat very well and are called insulators.

All the light and heat energy we receive on Earth comes from the Sun, and travels through space in invisible electromagnetic waves known as radiation. Space is vast and empty, so heat energy cannot travel from the Sun by conduction or convection. Hot objects on Earth, such as fires and radiators, also radiate heat.

Conductors

Energy Waves

Heat

Oceans

Sun

I'm a lecturer teaching Heat Transfer at freshmen (first year undergraduate) level. I'm always trying to make sure that what I teach my students has relevance and use, especially to teach them material beyond their tests and assignments etc.

Sometimes I have found that academia is woefully disconnected from real world. I'm not a practicing chemical engineer, so I want to know from your experience(s) what did you learn about Heat Transfer that was pertinent to your job/career/success.

I would like to use your responses to update my course if needed. If there was something you wish was taught in Heat Transfer, let me know

Thank you everyone.

Heat transfer is the transfer of thermal energy from one object to another. It occurs between two objects when they are at different temperatures – the heat will transfer from hot object to cold object. In our environment heat transfer occurs all the time. The sun is transferring some of its heat energy towards earth, and we get days hotter than nights. We cook our food on stoves – it is possible due to heat transfer.

Methods of Heat Transfer

There are 3 methods of heat transfer, which are conduction, convection, and radiation. Let’s discuss each of them individually.

Conduction

When two objects with different temperatures come in contact with each other, the heat is transferred from the hot object towards the cold object. This happens due to the collision of the molecules at the contact area of two objects.

In solids, molecules of hot objects vibrate more frequently than cold objects. When hot and cold objects come in contact with each other, their molecules collide. As a result, the collisions of molecules transfer the energy from the hot object to cold object. This transfer of thermal energy continues until hot and cold objects reach the same temperature. For example, when you boil water on a gas stove, the heat is conducted to the pot from fire.

Convection

Convection is the heat transfer method that only occurs in fluids (liquid and gases). It is caused by the flow and diffusion of fluids. This flow happens in the closed path – from hot regions of fluids towards cold regions and back again.

For example, when water is boiled on the gas stove, the heat is conducted by metal pot. The temperature of metal pot increase, which increases the temperature of the water that is in contact with it. Hot water rises to the surface and is replaced by cold water. This process continues and causes circular motion. As a result, the heat is distributed towards all the regions of water.

Radiation

In this process, heat is transferred by electromagnetic waves – also known as light. This process doesn’t depend on any medium or material to transfer the heat. Instead, the radiations travel through space or air to transfer the heat.

The heat is transferred to objects that absorb the radiation. As a result, their temperature increases. For example, the Sun transfer the heat energy as radiations that heats-up our Earth. Furthermore, the reflecting objects like mirrors can’t completely absorb the radiations. So, their temperature doesn’t increase.

Facts

Insulators don’t conduct the heat very well. So, they are used to cover the objects whose heat we want to preserve. For example, when ice-creams are taken on a ride, they are stored in a styrofoam box to prevent the heat by conduction.
Materials that are good conductors of electricity are also good conductors of heat. For example, copper is the best conductor of electricity, so it is also the best conductors of heat. Copper heat sinks are used on computer processors to remove the excess heat quickly by conduction.
Sometimes it is necessary to transfer the heat from cold object to hot object – opposite to the natural heat transfer. This kind of heat transfer is called forced heat transfer. Today, we use forced heat transfer in air-conditioners and refrigerators to cool-down things.