10.4 Heat Transfer
Heat Transfer by Radiation
Both conduction and convection involves the movement of particles. Radiation does not. Radiation is the transfer of energy by invisible waves given off by the energy source.
Conduction
Conduction is the transfer of heat through a solid or between a solid and another solid, a liquid, or a gas that is in contact with it. Conduction also occurs where energy is transferred between a liquid and a solid or a gas and a solid. Conduction occurs in one direction only — from a region that is warmer to a region that is cooler. Ex: shows a pot of soup heating up on the element of a stove. The particles in the stove element are moving rapidly. They are vibrating rapidly, bumping into their neighbours — the particles on the bottom of the pot. Some of the energy of the particles in the red-hot element transfers to the metal pot. This makes the particles of the pot vibrate faster. Some of this energy transfers to the particles of the soup at the bottom of the pot, making the soup hotter.
Convection
Heat first transfers from the hot element to the bottom of the pot by conduction. In turn, heat transfers from the hot bottom of the pot to the soup at the bottom of the pot. This is also convection.
Infrared Waves
Heat is transferred by invisible waves called Infrared Waves . Scientists use inferred waves to detect many things in nature that otherwise could not be observed.
Examples of Infrared Waves
Satellites that orbit Earth can detect infrared waves that reflect off Earth and into space. These infrared images help people to discover how pollution spreads, where insects are damaging forests, and what the weather might be like in your region for the next several days
Movement of particles
The hot soup begins to rise to the surface of the soup, pushing the cooler particles at the surface to the sides of the pot. There, the cooler particles sink to the bottom to take the place of the rising hot particles. When the cooler particles reach the bottom, they bump into the hot bottom of the pot and the circular pattern continues. When the circulating hot liquid reaches the top of the pot, energy from the particles of the liquid transfers to the air particles. These particles of liquid, therefore, become somewhat cooler. They are pushed to the side of the pot as more hot liquid rises to the surface. This continuous motion sets up a pattern that continues as the soup is heated.
Three Types of Heat Transfer
The word Transfer means to carry across. There are three types of heat transfer, Conduction, Convection and radiation.
Types of Radiant Energy
Thermal Energy is one of the many forms of energy radiated by the sun and other stars. Thermal Energy form the Sun reaches earth by radiation.
Transfer of thermal energy
This transfer of thermal energy by moving particles occurs in fluids — liquids and gases. It is called convection. The circular pattern of moving particles within fluids is called a convection current. Convection currents transfer heat from the hotter region to the cooler region, just as in conduction.
How Radiant Energy Warms Up Objects
When invisible waves of radiant energy come into contact with a solid, the particles in the solid vibrate faster. The solid becomes hotter. The solid can in turn, reradiate some of this energy back into the area where it is standing. Coloured solids can absorb and reradiate infrared waves, but transparent solids, liquids and gases allow inferred waves to pass easily.
