Heat:
Heat is defined as the sum of kinetic energy
contained by the molecules of that object. The
SI unit of heat is Joules. In c.g.s system it is
measured calorie.
1calorie= 4.2joules
Heat depends on mass of the object and
average kinetic energy of the molecules.
Heat is directly proportional to the mass of
object and average kinetic energy.
Effects of heat are:
a. It changes the state of matter
b. It changes the temperature of an object.
c. It changes the solubility of a substance.
d. It changes the size of an object.
e. It changes the color of the body.
f. It changes the volume of the body.
Temperature: Temperature is the degree of
hotness or coldness of an object. Temperature
is the thermal condition of the body that
determines rate of flow of heat from one body
to another. It is the average kinetic energy of
the molecule.
Temperature scale
Centigrade scale:
Lower fixed point: 0°C
Upper fixed point: 100° C
Fahrenheit scale:
Lower fixed point: 32° F
Upper fixed point: 212° F
Kelvin scale:
Lower fixed point: 273K
Upper fixed point: 373K
Anomalous expansion of water:
Generally all substance expands on heating and
contracts on cooling. But in case of water the
behavior is different, when water at 0c is heated
its volume decrease up to 4 and density
increases. At 4°C the density becomes maximum and beyond this temperature volume start to increase. This unusual expansion of water is called anomalous of water.
Graph between the volume and temperature:
Heat equation:
The amount of heat gained or lost by a body is
equal to the product of the mass (m), the
specific heat capacity (s) and the change in
temperature (dt) of that body is called heat
equation.
Q= msdt
Proof:
Let us consider the temperature of the body
changes to t₁ to t₂ when it is heated and m be
mass of the body then, change in temperature
dt = t₂ - t₁
If Q is amount of heat supplied then,
Q α m ……..1
Q α dt ……..2
Then combining 1 and 2 we get Q=msdt ………3
Where s is proportionality constant called the
specific heat capacity of the material of the
body which is constant for the given material
and independent of the shape size and mass of
the body.
Principle of calorimetry:
When two body of different temperature are
kept in thermal contact, heat transfers takes
place from the hot body to cold body and the
process continues until the equilibrium (same
temperature) state is maintained. This principle
works on the principle of the conservation of
energy.
Heat lost = Heat gained
Specific heat Capacity:
The amount of heat required to change the
temperature of a body of unit mass by 1°C is
known as specific heat capacity of that body.
Or s = Q/mdt
S.I unit of specific heat capacity is J/kg°C
Body having he more heat capacity changes the
temperature slowly then the body having less
heat capacity.
Specific heat capacity of water (s) = 4200 J/kg°C means 4200J of energy is needed to raise
the temperature through 1⁰C
Q] When we get out of the bed on a very cold
morning, we feel air of the room cold, but when
we come back staying outside for sometimes, we
feel air of the room warm , why?
Ans] When we get out of the bed on a very cold
morning, we feel air of ht room cold, but when
we come back staying outside for sometimes, we
feel air of the room warm because heat always
flows form high temperature to lower
temperature. In fact, when we get out of the bed,
our body surface temperature is higher than air
temperature of the room. Therefore, heat flows
from the body to the air and we feel cold. When
we come back to the same room, the situation is
just opposite. Our body surface temperature is
lower than that of air temperature, therefore,
heat flows form the air to our body and we feel
warm.
Q] During high fever a wet cloth is kept on the
forehead of the patient, why?
Ans] During high fever a wet cloth is kept on the
forehead of the patient, because heat always
flows from high temperature to low temperature.
Therefore, the wet cloth is kept in order to
reduce the high temperature due to fever as the
heat flows form the body to the wet cloth.
Q] Water in an earthen pot remains cold in
summer. Why?
Ans] Water in an earthen pot remains cold in
summer, because water gets evaporated coming
out from a large number of small pores of the
pot. The heat lost by water molecules for the
evaporation makes inside eater cool.
# Reasonable Facts:
When a red hot iron nail is plunged into boiling
water, heat flows from the iron nail to
the water. Here, the amount of heat is more in
heater than the red hot iron nail, but the iron nail
is at higher temperature than that of water.
Hence, heat always flows from high temperature
to low temperature.
When water at 0°C is heated, its volume
decreases up to 4°C and begins to increase after
4°C only. This unusual behavior of water is
known as anomalous expansion of water.
When a beaker completely filled with water of
4°C is heated or cooled, water overflows because
of the anomalous expansion of water. The
volume of water increases even when
temperature falls (becomes cool) and as usual
the volumes rise with rise in temperature.
Water taps may burst at very cold night in
Himali region, because of the anomalous
expansion of water. The volume of water
trapped at 4°C is minimum due to which on
further cooling to 0°C, ice made from water gets
expanded. It gives outward pressure and the
pipe may burst.
Aquatic animals can stay alive even in a frozen
pond because of the anomalous expansion of
water. In fact, the density of water of 4°C
becomes maximum and sinks to the bottom. On
further cooling to 3°C, 2°C,1°C, the water forms
a successive layer. Finally, water at 0°C changes
into ice, while aquatic animals remain safe at
4°C below the ice layer.
The amount of heat needed to raise the
temperature of 1kg of a substance through 1°C
(or 1K) is known as specific heat capacity. Its SI
unit is Jkg⁻¹°C⁻¹.The quantity of heat supplied
(Q) to a Q = ms dt where, s is specific heat
capacity
Specific heat capacity of some substance is more.
The substances having more specific heat
capacity need more heat to raise their
temperature.
Specific heat capacity of water is 4200Jkg⁻¹°C⁻¹.
It means, in order to raise the temperature of 1
kg water through 1⁰C, 4200J of heat should be
supplied.
As the specific heat capacity of water is very
high, it is used for cooling hot engines as well as
for heating system. Higher the value of specific
heat capacity, higher will be the amount of heat
absorbed by the body. Therefore, it is used for
cooling hot engines, because it can absorb large
amount of heat of the engines. Similarly, large
quantity of heat may be available by cooling hot
water and it is suitable for heating purpose.
A key made up of aluminum is colder than that
of iron, because specific heat capacity of
aluminum is higher than that of iron. Due to this
reason, equal amount of heat available to both
leads to fewer rises in temperature of the
aluminum key. Therefore, it becomes colder than
the iron key.
Sea breeze occurs at day and land breeze occurs
at night, because specific heat capacity of land
(of soil) is less than that of water. During
daytime the temperature of the land becomes
higher than that of the sea due to the heat of the
sun. It makes the pressure of air reduced.
Therefore, air blows from land to sea. Due to this
reason sea breeze occurs at days and land
breeze occurs at night.
# Specific heat capacity of some materials:
S.N. Materials In Jkg⁻¹°C⁻¹.
1. Mercury 138
2. Copper 380
3. Iron 470
4. Sand 800
5. Aluminum 910
6. Air 993
7. Ice 2100
8. Paraffin oil 2200
9. Water 4200
# Exercise for Practice:
⊕Define a calorie? What effects would have been
seen in the life of aquatic organisms in the very
cold regions if water hadn't shown anomalous
expansion of water? Explain your answer.
⊕The specific heat capacity and mass of the two
substances A and B are shown in the given table.
Which substance will get more heated if they are
heated equally? Give reason.
Matter Mass Sp. Heat Capacity
A 1 Kg 390 Jkg⁻¹°C⁻¹.
B 1 Kg 380 Jkg⁻¹°C⁻¹.
⊕What is mean by specific heat capacity of
mercury is 138 Joule per kg°C? Write down the
cause that the night of desert is very cold and
day of that place is very hot.
⊕What is the temperature of water at the bottom
of a pond when its surface water freezes?
Explain with reasons.
Define the term 'heat'. If heat is supplied to a
solid continuously, it melts. Give reason to
this.
Mention any two effects of heat. Calculate the
Sp. Heat capacity of water if 2 kg water at 25°C
required 2.1 × 105J heat energy to increase its
temperature to 50°C.
Define specific heat capacity. If 10 kg water is
heated from 50°C to 100°C, how much heat is
required for it?
A glass tumbler cracks when hot water is poured
in it. Why?
Give a difference between heat and
temperature.
Explain why a beaker full of water at 4°C
overflows if the temperature is decrease or
increased.
Aquatic animals can survive in a pond in a cold
region even though the outer surface of the pond
is frozen. Mercury is heated faster than water.
Why?
Give the concept of heat and temperature on the
basis of molecules.
What is meant by specific heat capacity? How
much heat is required to raise the temperature
of 150 gm of lump of iron from 25°C to 15°C.?
(Specific heat capacity of iron = 480J/kg°C)
What do you mean by the specific hat capacity of
water is 4200J/kg°C. Write down the cause that
the night of desert is very cold and day of that is
very hot.
Write down the cause of using water to cool the
engine of motor car.
Due to winter season the temperature of water is
5°C. If 20 liters of water has to be heated to 35°C
for taking bath. Calculate the amount of heat
required for it. (The specific heat capacity of
water is 4200J/kg°C and the mass of 1 liter of
water is 1 kg)
Heat capacity of 1000 Joules is released while
a lump of iron of mass 2 kg at 90°C is cooled to
15°C. Calculate the specific heat capacity of
iron.
Write two effects of heat with an example of
each.
Why do ice made of water float in it? Why water
is called anomalous liquid?
Calculate the amount of heat required to raise
the temperature of a frying pan by 80°C if the
mass of the pan is 0.5 kg and its specific heat
capacity is 480J/kg°C.
Thankyou for Reading....
