HEAT TRANSFER OPERATION

CONDUCTION

Modes of Heat Transfer

•Conduction

•Convection

•Radiation

Transfer of Heat

(Thermal Energy)

Thermal energy always flows from a region of higher temperature to a region of lower temperature.

Net flow of thermal energy occurs only when there is a difference in temperature (Driving Force).

Conduction is the transfer of thermal energy through a medium without any flow of the medium.

•Particles at the heated end vibrate vigorously.

•They collide with neighboring particles and transfer their energy.

•Eventually the particles at the cooler end are also set into vigorous vibration.

Heat from the source as causing the atoms of the solid to vibrate and gain kinetic energy.

These atoms cause neighbouring atoms to vibrate.

Kinetic energy is transferred from one atom to the next.

Heat energy is conducted through the solid in this way. As the atoms of the solid gain kinetic energy the temperature of the solid increases.

•In all solids, thermal energy is transferred through the vibration and collision of particles.

•However, in metals, due to the presence of free electrons, thermal energy is also spread through electron diffusion.

•Electrons gain kinetic energy and move more rapidly. They collide with atoms in the cooler parts of the metal and pass on their energy in the process.

Conduction in Liquids and Gases

•Particles in liquids and gases are spaced farther apart those in solids

•Collisions between particles occur less frequently; slow transfer of kinetic energy

•Poor conductors of heat

Convection

Convection is the transfer of thermal energy by means of convection currents in a fluid (liquid or gas), due to a difference in density.

Conduction vs Convection

•In conduction, thermal energy is transferred from one particle to another.

•In convection, thermal energy is transferred through the actual movement of the heated particles from the warmer to cooler parts of the fluid.

Convection in Liquids

•When the water at the bottom of the flask is heated, it expands.

•The expanded water is less dense than

the surrounding water and rises.

•Since the upper region is cooler, it is

denser and therefore sinks.

•The difference in the densities of water in the different regions sets up a convection current.

•This is shown by purple streams rising from the bottom and sinking at the sides.

Convection in Gases

•When the air above the candle is heated, it expands.

•The expanded air is less dense than the surrounding air and rises out of chimney B.

•Since the surrounding air is cooler, it is denser and therefore sinks into chimney A.

•The difference in the densities of the air at the different chimneys sets up a convection current.

Land & Sea Breeze

•In the day, the land heats up faster than the sea. The air above the land is heated, expands and rises.

•Cool air above the sea is denser and moves in to replace the warmer air. This sets up a sea breeze.

•At night, the land cools faster than the sea.

•The air above the land is now cooler than the

air above the sea.

•A convection current is set up in the opposite direction. This is called a land breeze.

Radiation

Radiation is the transfer of thermal energy in the form of electromagnetic waves without the aid of a medium.

•All bodies emit infrared radiation

•Infrared radiation does not require a medium to be transmitted. This is how the Earth is warmed by the Sun

Effect of Colour and Texture on Emission Rate of Infrared Absorption

The pieces of foil need to be positioned at equal distances from the source of heat in order for the experiment to be fair

Other Factors affecting Rate of Infrared Radiation Absorption/Emission

•Surface area

For two objects of identical mass and material, the object with the larger surface area will emit or absorb infrared radiation at a higher rate.

•Surface temperature

The higher the temperature of an object relative to its surrounding temperature, the higher the rate of emission of infrared radiation.

Applications of Transfer of Thermal Energy

Conductors

A material through which heat can move easily is called a conductor.

Iron, silver, aluminum, stainless steel, and copper are all examples of metals that are good conductors.

Insulators

A material that heat does not move through easily is an insulator.

Plastic, wood, rubber, and glass are all good insulators.

Many cooking utensils and handles are made out of plastic, wood, or rubber.

Air is a good insulator.

Saucepan

A saucepan with a thick copper base contains water and is placed on a flat electric hot plate.

(a)State the process by which thermal energy is

(i)transferred from the hot plate to the water;

(ii)transferred through the water.

(b)The sides of a saucepan are often polished, but the base is usually dark-coloured. Why is this so?