FLOW OF HEAT

• Heat is a form of energy that always flows from a hot body to a cold body. This flow of heat is known as transmission or transfer of heat.
• Heat travels from a region of higher temperature to a region of lower temperature in three ways.
  • Conduction
  • Convection
  • Radiation

CONDUCTION

• When the transfer of heat takes place in solid, that flow of heat is called conduction.
• The process of transmission of heat from one particle to another particle in solids, without actual movement of molecules from their position is called conduction.
• Thus, in conduction their is no movement of particles, its the energy that will flow from hotter to colder area, not the particles.

CONDITIONS FOR CONDUCTION:

• Conduction will only occur when the two objects are in contact with each other.
• Their should be a temperature difference between the two objects through which heat has to travel. Eg: One object or one part of the object has to be hot and the other object or the other part of the object has to be cold.
• Conduction will only occur till both the body attains uniform temperature, afterwards the flow of heat will stop because both the content have attained the same temperature.

CONVECTION

• The flow of heat requires a medium and the medium is either liquid or gases.
• Their is movement of particles in case of liquids and gases and thus the particles carry heat from hotter part to colder part.
• This process in which the particles of matter collect heat energy from the hotter part and carry it themselves to the cooler parts by moving is called convection. 
• Eg: in liquids and gases.
• When the earth gets heated by the sun rays the hot air starts rising, that hot air is an example of convection.
• The hot air is lighter in weight and thus rises, whereas the cool air is heavier in weight thus falls down and this rise and fall of air is called convection currents.

APPLICATION OF CONVECTION

SEA BREEZE

• Land has the property that it cools faster than the sea and warms faster than the sea, thus Solar radiation warms the land faster than the water.
• During day time, the sun rays reaches the earth and heats up the land by conduction.
• Air over the land is heated by convection and the hot air rises.
• The hot air keep on increasing and the area above of cold air , which is denser.
• Cooler, denser air has higher pressure and falls towards the sea.
• Thus, convection currents starts.
• The cold air is pushed towards the land.
• Sea breezes often occurs during summer days.
• Sea breezes occur when the temperature of the land is higher than the temperature of the water, thus giving cooler air to the land surface.

LAND BREEZE

• This process is just the reversal of Sea breeze.
• Land breeze occurs at night.
• As land cools faster , so during night air over land cools faster than the sea.
• Air over the warmer water (sea) begins to rise due to convection.
• Cooler, denser air over the land has higher pressure and falls toward the land.
• The land pushes the cooler air of the land towards the sea.

WORKING OF AIR CONDITIONER:

• The air conditioner works on the same principle that the cold air will fall and hot air will rise.
• During summer, the room is hot due to hot air.
• The air conditioners when switched on, start blowing the cold current, and being heavy they descend downwards thus at the bottom of room their is cold air and the hot air rises.
• The hot air interacts over the cooler surface and starts descending downwards and thus makes the room cooler.

WORKING OF ROOM HEATER:

• The room heaters are placed at lower surface.
• During winters, the room is filled with cold air
• As the heater is turned on, the hot air blows from it .
• As hot air is lighter, it starts rising up and the cool air starts settling and thus the whole room becomes hot as compared to the cold climate.

RADIATIONS

• Radiation is a transfer of heat which requires no medium.
• Radiation is transferred from a hot body to a cold body directly without any contact with the medium in between the two.
• The energy transferred from one point to another is called radiant energy.
• Every body emit energy at all temperature.
• Eg: Sun, the heat energy from the sun reaches the earth through radiation.

CONDITION FOR RADIATIONS:

• Radiation depends upon the nature of the material
  • Eg: metals absorbs more heat than non-metals.
• Distance from the heat source
• Colour of the object.
  • Eg:  Dark coloured objects absorb more heat than the light coloured objects.
  • Good absorbers are good radiators of heat.
  • Good reflectors are bad radiators of heat.
  • That is why the colour of house are painted light in colour, so that maximum amount of heat gets reflected.
  • We are advised to wear light colour clothes during summer so that maximum amount of heat gets reflected from it.