Heat

· All substances around us are made up of matter.

· This matter is made up of atoms and molecules

· These atoms vibrate since they are in vibratory motion.

· These vibrations produce heat energy.

· It is unidirectional i.e. heat flows from hotter region to colder region.

· When heat is provided it increases the energy of atoms leading to increase in vibrations.

· These vibrations of one atom in a substances sets the adjacent atom in a vibratory motion.

· Thus heat energy is transferred from one place to substance via vibrations.

Effects of heat: - There are three main changes in the substance when heat is applied.

Expansion:

ü When the ball is heated the atoms in the ball gain heat energy.

ü They start vibrating and force each other apart.

ü As a result an expansion takes place.

ü Thus the ball expands which does not allow the ball to pass through the hole.

ü During the course of time the ball cools down and the ball is back to its original shape.

ü This demonstrates that the expansion takes place in solids.

ü Expansion also occurs in liquids and gases.

ü The maximum expansion takes place in gases.

Rise in temperature:

ü When water is heated, water molecules receive heat energy.

ü We know that as heat is applied it increases the kinetic energy of the molecules.

ü Increase in energy leads to the increase in temperature.

ü Thus heat energy causes increase in temperature.

Change of state:

ü Ice cubes are solid in nature which means the force of attraction in the molecules is much greater than in water molecules.

ü When we heat them the force of attraction decreases and the ice cubes convert into water leading to the transformation in state.

ü Similarly when we heat water the fore of attraction decreases and it turns into vapor.

ü As water vapor is gaseous in nature it escapes into the surrounding.

ü Thus the water level decreases.

ü Heat energy applied or taken changes the state of matter.

1. Solid to liquid (melting)

2. Liquid to gas (vaporization)

3. Solid to gas (sublimation)

4. Gas to liquid (condensation)

5. Liquid to solid (freezing)

6. Gas to solid (deposition)

Transfer of heat: - When heat energy is applied to a substance it transfers from one part to another. This transfer takes place in any of the three ways

a) Conduction

b) Convection

c) Radiation

Conduction:

ü When we touch a spoon which was held over a burner it feels hot. Whereas if we touch wood over a burner it turns into ashes.

ü Thus metals are good conductors of heat i.e. they allow the transfer of heat from one place to another.

ü But nonmetals like carbon, phosphorous are bad conductors of heat as they do not allow the transfer of heat from one to another.

ü We use conduction in daily life like for ironing our clothes, handles of utensils are made up of wood or plastic etc.

ü This transfer of heat only occurs in solids.

Convection:

ü When water in a vessel is heated the water molecules at the bottom of the vessel heat up first.

ü As they receive energy they become more active and lighter compared to the other molecules of water on top.

ü Thus these lighter energized water molecules move to the top of the water level i.e. towards the surface.

ü Leading to the top, molecule to sink down as it is less active.

ü The above steps are followed leading to a convection transfer of heat energy.

ü This is how the hot air is lighter and stays at the top

ü Convection in daily life is the raising of hot air balloon, the flowing of cool air from colder to a hotter region, etc.

ü This type of transfer of heat only occurs in liquids and gases.

Radiation:

ü It is defined as the way of heat transfer from one to another in the form of electromagnetic waves.

ü Heat energy from the sun reaches the earth in this form.

ü This type of transfer of heat only occurs in vacuum i.e. in space

ü Radiation is used in everyday life as we are advised to wear white clothes in summer as white reflects radiation.

Calorimetry:-

ü Heat energy along with physical changes also brings about chemical changes.

ü In order to know about the chemical and physical changes produced we need to know the amount of heat energy involved.

ü The technique used to measure the amount of heat involved in a physical or chemical process is known as calorimetry.

Temperature: It is a physical quantity which expresses whether an object is hot or cold.

It is measured with the help of thermometer and has three scales to measure it

a. Celsius scale

b. Kelvin scale

c. Fahrenheit scale

ü Among them Kelvin scale is most commonly used.

ü Unit of heat

ü Heat is a form of energy

ü The unit of energy in SI unit is joule

ü So heat is also measured in joule

ü Represented by J.

ü Although most commonly used unit is calorie

ü 1 calorie=4.0186J

ü One calorie- It is the amount of heat energy required to raise the temperature of 1 gram of water through 1 degree Celsius.

Heat capacity:

The amount of heat energy gained or lost by a substance is determined by three factors

a. Mass of the substance

b. Change in temperature of the substance

c. Nature of the material of the substance

ü Different substances require different amount of heat energy to reach a particular temperature this is called as heat capacity.

ü It is defined as the amount of heat energy required by a substance to raise its temperature by 1K

ü And is denoted by C

Heat capacity= amount of heat energy required (Q)

Raise in temperature ( T)

C’= Q/ T

ü The unit of heat capacity is J/K.

Specific heat capacity:

It is defined as the amount of heat energy required to raise the temperature of 1 kilogram of a substance by 1K.

Specific heat capacity = amount of heat energy required (Q)

Mass X raise in temperature

C=Q /m X T

The SI unit of specific heat capacity is J/Kg K.

Calorimeter:-

A colorimeter is a device used to measure the amount of heat gained or lost by a substance .

Construction:

ü It consists of a vessel made up of metals like copper aluminum which are good conductor of heat and electricity

ü The metallic vessel is kept in an insulating jacket to prevent heat loss through environment

ü There are two holes in it.

ü Through one hole a thermometer is inserted to measure the temperature of the contents

ü A stirrer is inserted through another hole for stirring the contents

ü The vessel is filled with liquid which is heated by passing currents through heating element.

Thermostat:-

ü A thermostat is a device which maintains the temperature of a place or an object constant.

ü It turns an appliance or circuit on or off when a particular temperature is reached

ü The thermostat may function as a sensor and the controller of a thermal system.

ü The devices which uses thermostat are water heater, air conditioner, etc.

Thermos flask (vacuum flask):-

ü It is an insulating storage vessel made to keep its storage hotter or cooler than the surrounding for a longer time

ü It is primarily meant to enhance the storage period of a liquid by maintaining a uniform temperature

ü This avoids possibilities of bad taste and also the liquid becomes less perishable

Working:

ü A thermos flask has double walls in which vacuum is created.

ü It is painted with silver on the inside

ü The vacuum between the two walls prevent heat being transferred from the inside to the outside by conduction and convection

ü With very little air between the walls there is almost no transfer of heat from then inner wall to the outer wall or vice versa.

ü Conduction occurs at only two points where the two walls meet at the top of the bottle and through an insulated support at the bottom.

ü The silvered walls reflect and radiate heat back to the liquid in the bottle