Transfer of Thermal Energy

Conductors and insulators have different mechanisms

Good conductors

In metals, another much faster mechanism of thermal energy takes place at the same time (free electron diffusion)

Free electrons gain kinetic energy and move faster

Fast moving electrons then diffuse into cooler parts of metals

Insulators

In insulators, the transfer of thermal energy is solely the results of vibrating atoms and molecules
No free electrons

No free electrons

Metals contain free electrons which move randomly between the atoms and molecules

Non metals do not have free electrons

When thermal energy is supplied to one end of a rod, the particles at the hot end vibrate vigorously

These particles collide with neighbouring particles, making them vibrate as well

Conduction in liquids and gases

process of conduction is inefficient

liquid particles are further apart and collisions of particles are less frequent and even lesser in gases

thus, transfer of kinetic energy from fast moving molecules to neighbouring molecules is slower

Hence, air is poor conductorof heat compared to water, which is in turn poor compared to solids

Transfer of heat energy in fluids due to the difference in density

Particles do not expand/contract, it is the substance/object that expands/contracts.

Kettle water heater

Water at the base gets heated up

volume increases

density decreases, hence it rises

colder water at the top is denser, hence it sinks

Cold water gets heated up by the water

Air-con {convection current}

Air near the air-con is cooled

The air contracts and its volume is decreased

D=m/v , hence the density of the air increases and it sinks to the bottom

Warm air at the bottom which is less dense rises

Warm air reaches the air-con gets cooled

Continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium

All object radiate heat in the form of infrared radiation as they are above 0 kelvin

thermal energy from infrared waves is called radiant heat

The hotter the object, the greater the radiant heat emitted

Factors affecting rate of infrared radiation

Colour and texture of the surface

Surface temperature

Surface area

infrared radiation is absorbed by all objects and surfaces

causes temperature rise

An object emits heat when its temperature is higher than its surrounding

Good emitter is also a good absorber

Poor emitter is also a poor absorber

An object absorbs heat when its temperature is lower than the surrounding

Conduction

takes place in solid, liquid, gas

Due to vibration of particles

Can happen in solid, liquid, gas

Requires a medium

Cannot happen in vacuum

Convection

takes place in only liquid and gas

Due to changes in density

Can happen in liquid and gas

Requires a medium

Cannot happen in vacuum

Subtopic

Subtopic

Radiation

does not require a medium

Due to infra-red radiation

Requires no medium

Can happen in vacuum

An object with more heat than another does not necessary means it has a higher temperature than the other object

difference in temperature

flows from a region of higher temperature to a region of lower temperature

no transfer of heat at thermal equilibrium

Common applications of conduction

Uses of good conductors of heat

Cooking utenseils

Soldering iron rods

Heat exchanges

Uses of bad conductors of heat (insulators)

Table mats- made of cork

Sawdust

Wooden ladles

Woolen clothes

fiberglass, felt and expanded polystyrene foam

Common applications of convection

Heating water in electric kettles

Household hot water system

Air-conditioners

Refrigerators

Common applications of radiation

Teapots

shiny teapots can keep tea warm longer than black teapots

Greenhouses

Vacuum flasks

prevents heat from exiting or entering