QUESTIONS!!!!

LARGE CLOUDS ARE MORE LIKELY TO FORM DURING A HOT DAY WITH HIGH HUMIDITY.WHY?

• sun heats land up,through radiation
• land absorbs heat and becomes hot thus heating the air above it
• hot air rises while the colder air ontop sinks due to difference in density
• convection currents are thus set up
• as the hot air rises,it cools condenses and forms clouds
• on cold days,there is a decrease in rate of covection currents being set up=less clouds form

EXPLAIN WHY WATER WOULD BOIL FASTER IN A KETTLE IF A LID IS PLACED ON IT?

• when heat is applied to kettle(metal),kettle boils water
• kettle lid is made of metal which is shiny and smooth
• little heat will thus be emitted through radiation
• also,when lid is covered over kettle,less heat will be lost through convection
• because the hot air that rises cannot escape

APPLICATIONS OF THERMAL ENERGY

APPLICATION-CONDUCTION

• COOKING UTENSILS

cooking utensils such as kettles,saucepans,rice cooker,woks are usually made of aluminium or steel(conductor).metal conduct heat well,thus when direct heat is applied to the utensils,it will heat up very fast

• ELECTRICAL CABLES

electrical cables are made up of electirical wires which are usually made of copper and a rubber covering. the wires are made of copper(conductor) so as to conduct electricity well while the exterior is made of rubber(insulator),so as to prevent users from getting an electric shock

• WOOLLEN CLOTHES
  

wool is an insulator and it traps heat,thus it keeps us warm on cold days

APPLICATION-CONVECTION

• AIR CONDITIONERS
  

air conditioners are installed near the ceiling of a room to facilitate convection currents.the air becomes cooler when its near the air con and thus sink downwards while the warm air below rises due to a difference in density

• HEATER

heaters are installed/placed at the bottom of a room so that convection currents can be set up.when the air at the bottom is heated up,it rises,causing the cooler part of the air to sink downards due to a difference in density

• REFRIGERATOR

the freezing unit is usually placed at the top of the fridge to create convection currents.the cooled air at the top sinks while the warm air at the bottom rises


APPLICATION-RADIATION

• SOLAR PANELS
  

the surface of the solar panels are usually painted black so as to absorb as much heat from the sun as possible

• FIREMEN CLOTHES

firemen clothes are usually white,shiny and smooth.this ensures that the firemen do not absorb much of the heat from the fire

• GREENHOUSES

greenhouses are used in cold climates.greenhouses are made of glass,thus the infrared radiation from the sun passes into the greenhouses and these greenhouses traps most of the heat which will warm up the plants

RADIATION

THERMAL ENERGY- RADIATION

radiation is the emission of infrared waves from the surface of all bodies-without the aid of a medium

• radiation does not require a medium


• therfore,can take place in a vacuum
  
• e.g heat from the sun reaches earth by radiation
  

RADIATION

• absorber : substance that takes in heat in the form of radiation (temp rises)


• radiator/emitter: substance that gives off heat in the of radiation (temps falls)


• reflector: substance that cannot absorb radiation (gd reflector=poor absorber,poor reflector=gd absorber)
  

the factors that affect the rate of heat transfer by radiation

1.colour and texture of the surface

• black,dull and rough surfaces are better absorbers of heat than shiny,white and smooth surfaces


• black,dull and rough surfaces are better radiators of heat than shiny,white and smooth surfaces


• black,dull and rough surfaces are poor reflectors or heat than shiny,white smooth surfaces

2. surface temperature

• the higher the temperature of the surface of an object when compared to the surrounding temperature,the higher the rate of infrared radiation from the object to the surrounding
  

3.surface area

-comparing between objects of the same mass and material,but with different surface area

• the object with the larger surface area will absorb heat better than the other


• the object with the larger surface area will also emit heat at a faster rate than the other

CONVECTION

THERMAL ENERGY-CONVECTION

convection is the transfer of thermal energy by means of currents in a fluid(liquid or gas)

• convection requires the movement of particles
  
• thus this process can only take place in liquid/gas


• and not solids,because the particles in solids are not able to move about freely
  
• convection current = hot fluid rise, cold fluid sinks (change in density)


• when a fluid is heated, its density decreases due to the increase of volume
  
• heated fluid therefore rises


• cooler part of the liquid has a greater density now,thus sinks downwards
  
  

convection in liquids

* liquid is heated

*the density of the heated liquid decreases because the liquid expands

*the density of the cooler part of the liquid is now greater then the heated part of the liquid

*therfore,the heated liquid rises and the cooler liquid sinks

convection in gas

*air is heated

*the density of the heated air decreases

*the density of the cooled part of the air is now
greater then the density of the heated air

*the heated air rises and the cold air sinks

*thus,creating a current which cost the spiral to spin

(sea breezes and land breezes are examples of convection currents)

CONDUCTION

THERMAL ENERGY- CONDUCTION

conduction is the process of thermal energy transfer without any flow of the material medium

• different materials conduct heat at different rates
  

metals(iron,copper,silver) are generally good conductors of heat while non metals(wood,plastic,glass) are poor conductors of heat

why? because conductors & insulator have different mechanisms to transfer thermal energy

conduction

• all solids(metals & non metals) are made up of atoms and molecules
  
• (DIFFERENCE) metals contain free electrons which move randomly between atoms and molecules while non metals do not contain such free electrons
  
• When a metal rod(conductor)and a non-metal rod are heated from one end, the particles at the hot end vibrates and collides with neighbouring particles, causing them to vibrate and thus the kinetic energy at the hot end is transferred on
  
• The transfer of energy through the atoms and molecules vibration is quite slow.
  
  conduction in metals

• However, metals have free electrons in them which allows thermal energy to be transferred by free electron diffusion

• When a metal rod is heated(conductor), the free electrons gains kinetic energy, and thus move faster and collide randomly with the particles in the cooler region of the metal and transfer kinetic energy to them
  
  
• Therefore, metals are better conductors than non metals :)
  

*thermal energy can also be conducted in liquid and gas.however,because the particles are spaced further apart in liquid and gases than in solids, the collisions between the particles are less frequent and thus the transfer of kinetic energy between particles is slower.

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


*when both region are of the same temperature,there is no loss or gain or thermal energy = thermal equilibrium


Thermal energy is transferred by -

• conduction
• convection
• radiation