Heat

Heat
1
Heat
Energy that is transferred from one body to another as the result of a difference in
temperature.
With the increase in the temperature of a body, the vibrations of molecules or atoms
increase. These vibrations are then transferred from one part of the body to another. The
measure of energy with which the molecules vibrate in a system is termed as heat stored in
that object.
The direction of flow of the heat energy takes from the substance of higher temperature to
the substance of lower temperature. This is because the molecules are vibrating faster and
transfer their energy to the molecules vibrating slower.
The vibrational energy is also termed as its heat content. The heat content in the body
makes it hot or cold. Greater the heat content, hotter will be the body.
Stefan’s Law
It states that “The amount of heat energy (E) radiated per second by unit area of perfectly
black body is directly proportional to the fourth power of absolute temperature (T) of the
body.”
E ∝ T4
Good absorbers are good emitters and poor absorbers are poor emitters.
State of Matter
• Solid
• Liquid
• Gas
• Plasma
• Bose-Einstein condensate
A substance may absorb heat without an increase
in temperature by changing from one physical state
to another.
In the process of melting, the substance is changed from solid to liquid.
In the process of sublimation, solid is converted into the vapour state.
In the process of boiling, the liquid is converted to vapour.
Heat is a form of energy that can be converted into work. Amount of energy is expressed in
units of work. It is expressed in joules, foot-pounds, kilowatt-hours, or calories.
Mode of Heat Transfer
• Conduction
• Convection
• Radiation
• Conduction: It is the method in which the transfer
of heat takes place between atoms and molecules
in direct contact.
Conduction
• The heat from a hot liquid makes the cup itself hot.
• An ice cube will soon melt if you hold it in your
hand. The heat is being conducted from your hand
into the ice cube.
• A metal spoon becomes hot from the boiling water
inside the pot.
• Convection: It is the method in which the transfer
of heat happens by the movement of the heated
substance.
Convection
• When a fluid, such as air or a liquid, is heated and then travels away from the source, it
carries the thermal energy along. This type of heat transfer is called convection.
Examples:
• Boiling water - The heat passes from the burner into the pot, heating the water at the
bottom. Then, this hot water rises and cooler water moves down to replace it, causing a
circular motion.
• Hot air balloon - A heater inside the balloon heats the air and so the air moves upward.
This causes the balloon to rise because the hot air gets trapped inside. When the pilot want
to descend, he releases some of the hot air and cool air takes it place, causing the balloon
to lower.
Newton’s Law of Cooling

The rate of loss of heat from a body is directly proportional
to the difference in temperatures of the body and its
surroundings.
If we take hot water and fresh water and put it in a
refrigerator, then rate of cooling of hot water will be faster
than the fresh tap-water.
Sea Breeze During day time, the seashore warms up much
faster than sea water. Hot air over the seashore rises and
cooler air from sea water moves towards seashore to take
its place resulting in a sea breeze.
Land Breeze At night, land cools faster than sea water. Now hot air over sea water rises and
cooler air from land moves towards sea to take its place and resulting in a land breeze.
Cloudy night are warmer than clear night because clouds reflect the radiations emitted by the
earth at night and keep it warm.
Radiation
Radiation: It is the method in which the transfer of heat takes place by

electromagnetic waves.
Scales
Celsius Scale
Fahrenheit Scale
Kelvin Scale
Conversion
Scale of Temperature
Celsius scale of temperature freezing point is 0°C Boiling point of water is 100°C
Fahrenheit scale of temperature ice point or freezing of water = 32° F
Boiling point of water = 212° F
Kelvin or absolute scale of temperature ice point of water = 273° K
Boiling point of water = 373° K
Reaumur scale of temperature ice point of water is 0° R, Boiling point of water = 80°R
Thermometers
The instruments used to measure temperature of a body is called thermometer.
Clinical thermometer It is used to measure human body temperatures and ranges from
96° F to 110°F or 35°C to 43°C.
Electronic thermometer Basic components of an electronic thermometer are thermistors
or thermoresistors. Range of electronic thermometer is –40° to 450°F.
In thermometer, mercury is commonly used through a wide range from –30°C to 300°C.
Thermometer was developed by Galileo who found that the gases expand on heating.
Thermal Expansion of Solids

Expansion in length on heating, is called linear expansion. The increase in length of a rod
of unit length of a substance due to increase in its temperature by 1°C is called the
coefficient of linear expansion of the substance of that rod. It is represented by α.
Expansion in area on heating, is called superficial expansion.
Expansion in volume on heating, is called volume expansion or cubical expansion.
α : β : γ = 1 : 2 : 3
Difference Between Heat and Temperature

Parameter Heat Temperature
Temperature is defined
Heat is defined as the total
as the measure of the
Definition energy of an object that has
thermal energy of an
molecular motion inside it
object
SI Unit Joule Kelvin
Symbol Q T
Specific Heat
The amount of heat required to raise the temperature of unit mass (m) of a substance
through 1°C, is called its specific heat (s).
It is denoted by s and its unit is ‘cal/g°C or Joule/g°/C.
The specific heat of water is 4200 J/kg1/°C or 1000 cal/ g1/° C–, which is high compared
with most other substances. Therefore, water is used as coolant in radiator in vehicle and
hot water is used for the fermentation.
Latent Heat
The heat energy absorbed or released at constant temperature per unit mass for change
of state, is called latent heat.
It is denoted by L and its SI unit is cal/g or kcal/kg.
Heat energy absorbed or released during change of state is given by
Q = mL
where, m = mass of the substance.
Latent heat of fusion of ice is 80 cal/g.
Latent heat of vaporisation of steam is 536 cal/g.
Special char. Of Water
Expansion of Water
• When temperature of water is increased from 0°C, then its volume decreases up to 4°C,
becomes minimum at 4° C and then increases. This behaviour of water expansion around
4°C is called, anomalous expansion of water.
Practice Question
When a bottle is filled with water and it is freezing the bottle breaks down, because-
(a) Water expands upon freezing
(b) Bottles shrinks at freezing point
(c) The temperature outside the bottle is higher than inside
(d) Water expands on heating
Effect of Pressure on
Boiling point
Practice Question
Assertion (A) : The boiling point of water decreases as the altitude increases.
Reason (R) : The atmospheric pressure increases with altitude.

Code:
1. Both (A) and (R) are true, (R) is the correct explanation of (A)
2.Both (A) and (R) are true, but (R) is not the correct explanation of (A)
3.(A) is true, but (R) is false
4.(A) is false, but (R) is true
Thermodynamics
The branch of physics which deals with the study of relation of heat energy with different
types of energy is called thermodynamics.
Zeroth Law
Zeroth law of thermodynamics tells about thermal equilibrium.
First Law
As per first law about energy, heat given to a substance is equal to sum of change in internal
energy and work done.
Second Law
In second law work can be converted into heat and vice-versa but conversion is not
possible with 100% efficiency.
It is impossible for a machine operating in a cyclic process to convert heat completely into
work, it is kelvin’s statement.
Heat engine is a device which converts heat into mechanical work. Internal combustion
and external combustion heat engine are two types of heat engine.
Question
A fan produces feeling of comfort during hot weather, because-
(a) Fan supplies cold air
(b) Our perspiration evaporates rapidly
(c) Our body radiates more heat
(d) Conductivity of air increase
In an earthen pitcher, the water remains cold due to the process of

(a) Condensation
(b) Evaporation
(c) Sublimation
(d) None of the above
Assertion (A) : With the increase of temperature, the viscosity of glycerin increases.
Reason (R) : Rise of temperature increases kinetic energy of molecules
Code:
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Pressure-cooker cooks food in lesser time because

(a) The temperature of boiling water decreases due to high pressure
(b) Air has no effect as it is closed from all sides
(c) Temperature of boiling water increases due to high pressure
(d) Evaporation of the water used is very small
Cloudy nights are warmer compared to clear cloudless nights, because clouds-
(a) Prevent cold waves from the sky from descending on earth
(b) Reflect back the heat given off by earth
(c) Produce heat and radiate it towards earth
(d) Absorb heat from the atmosphere and send it toward earth.
Fruit stored in a cold chamber exhibit longer storage life because-

(a) Exposure to sunlight is prevented
(b) Concentration of carbon dioxide in environment is increases
(c) Rate of respiration is decreased
(d) There is increase in humidity
Assertion (A) : Woollen clothes keeps us warm.

Reason (R) : Woollen fibres are made from a special type of protein, which is a bad conductor of heat
Select the correct answer from the codes given below
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Question Answer
1 b
2 b
3 d
4 c
5 b
6 d
7 a

Heat

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Heat

Newton’s Law of Cooling

Radiation: It is the method in which the transfer of heat takes place by

Thermal Expansion of Solids

Difference Between Heat and Temperature

Special char. Of Water

Reason (R) : The atmospheric pressure increases with altitude.

In an earthen pitcher, the water remains cold due to the process of

Pressure-cooker cooks food in lesser time because

Fruit stored in a cold chamber exhibit longer storage life because-

Assertion (A) : Woollen clothes keeps us warm.

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