Revision: Thermal Physics Science SSLC (English Medium) Class 10 Tamil Nadu Board of Secondary Education

The total internal energy of all the molecules of a substance is called its thermal energy.

The sum of the potential energy and kinetic energy of a molecule is called its internal energy.

One kilo-calorie of heat is the heat energy required to raise the temperature of 1 kg of water from 14.5°C to 15.5°C.

The kinetic energy due to random motion of the molecules of a substance is known as its heat energy.

The measurement of the quantity of heat is called calorimetry.

Heat is that form of energy which flows from a hot body to a cold body when they are kept in contact.

The temperature at which the pressure and volume of a gas theoretically reach zero is called absolute zero.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C.

Temperature is a parameter which tells the thermal state of a body (i.e., the degree of hotness or coldness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

The increase in length per unit original length of a rod (at 0°C) per unit rise in temperature is called the coefficient of linear expansion.

The increase in volume of a body per unit original volume (at 0°C) per unit rise in temperature is called the coefficient of cubical expansion.

The increase in the dimensions (length, area, or volume) of a body due to an increase in its temperature is called thermal expansion. Conversely, a decrease in temperature causes contraction.

OR

The increase in the dimensions of a body due to an increase in its temperature is called thermal expansion.

OR

When matter changes its shape, area and volume in response to a change in temperature (i.e., an object expands and becomes larger due to a change in its temperature), this is called thermal expansion.

The change in area per unit original surface area of a two-dimensional body (at 0°C) per unit rise in temperature is called the coefficient of superficial expansion.

1. Consider a metallic rod of length l₀ fixed between two rigid supports at T °C.
2. If the temperature of rod is increased by ΔT, length of the rod would become, l = l₀ (1 + αΔT) Where, α is the coefficient of linear expansion of the material of the rod.
3. But the supports prevent the expansion of the rod. As a result, rod exerts stress on the supports. Such stress is termed as thermal stress.

The coefficient of volume expansion is equal to the change in volume of a rod of volume 1m³ when its temperature rises by 1°c.

The coefficient of superficial expansion is equal to the change in the area of a rod of area 1m² when its temperature rises by 1°c.

When a solid is heated and its length increases, the increase in length proportional to the original length and temperature change is called linear expansion.

where α is called the coefficient of linear expansion.

When the area of an object changes with increase in temperature, it is called areal expansion (or superficial expansion).

where β is called the coefficient of areal expansion.

If the Coefficient of Linear expansion is denoted by α
Coefficient of superficial expansion is denoted by β
And Coefficient of volume expansion is denoted by γ
Then the relation between α, β and γ is stated as
β = 2 α and γ = 3 α

α : β : γ : : 1 : 2 : 3

When a solid is heated and its volume increases, the expansion is called volume expansion.

where γ is called the coefficient of volume expansion.

Charles's Law

At constant pressure, the volume of a given mass of a dry gas increases or decreases by 1/273^rd of its original volume at 0°C for each degree centigrade rise or fall in temperature.

V ∝ T (at constant pressure)
At temperature T₁ (K) and volume V₁ (cm³):

`"V"_1 ∝ "T"_1 "or" "V"_1/"T"_1 = "K"` = constant...(i)

At temperature T₂ (K) and volume V₂ (cm³):

`"V"_2 ∝ "T"_2 "or" "V"_2/"T"_2 = "K"` = constant….(ii)

From (i) and (ii),

`"V"_1/"T"_1 = "V"_2/"T"_2` = constant

For Temperature = Conversion from Celsius to Kelvin

1 K = °C + 273

Example:

20°C = 20 + 273 = 293 K

An atom is the smallest particle of an element that can take part in a chemical reaction; however, it may or may not exist independently. 

A molecule is the smallest particle of an element or a compound that can exist by itself; it never breaks up except for taking part in a chemical reaction.

A gas whose molecules are identical, spherical, rigid, and perfectly elastic point masses, which keep colliding among themselves and with the walls of the containing vessel in perfectly elastic collisions (total energy before collision = total energy after collision), and between which no attractive or repulsive force acts, is called an ideal gas.

The equation that combines Boyle's Law, Charles' Law, and Gay-Lussac's Law into a single relation for a fixed mass of gas, relating the quantities pressure (P), volume (V), and temperature (T) which describe the state of the gas, is called the Equation of State.

Combining the above three laws for a fixed mass of gas:

PV = nRT

PV = N_kB​T

where:

• P = pressure, V = volume, T = temperature
• n = number of moles, R = universal gas constant
• N = number of molecules, k_B = Boltzmann's constant

Robert Boyle systematically studied the relationship between pressure and volume of gases. In 1662, he found that, at a constant temperature, the volume of a fixed mass of a dry gas decreased by half when the pressure on it was doubled, and it became four times its original volume when its pressure was decreased to one-fourth. He described this behaviour in the form of a law, known as Boyle’s Law. 

Boyle’s Law: Temperature remaining constant, the volume of a given mass of dry gas is inversely proportional to its pressure.

V α `1/P` = T = Constant

"Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules."

• Proposed by Avogadro in 1811.
• 1 mole of any gas at STP = 22.4 L (at 0°C, 1 atm) or 22.71 L (at 0°C, 1 bar — new IUPAC STP).
• 1 mole of any substance = 6.022 × 10²³ particles.

Avogadro's Law (Volume–Moles Relationship):

At constant temperature (T) and pressure (P), volume is directly proportional to number of moles.

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.

V ∝ \[\frac {1}{P}\]

At constant pressure, the volume of a fixed mass of gas is directly proportional to its temperature.

V ∝ T(at constant P)

At constant volume, the pressure of a fixed mass of gas is directly proportional to its temperature.

P ∝ T(at constant V)

• Heat is the energy that flows from a hot body to a cold body when they are kept in contact.
• The S.I. unit of heat is joule (J), while calorie and kilocalorie are commonly used units.
• One calorie is approximately equal to 4.2 joule, and kilocalorie is used to measure the energy value of foods.

• If no heat flows between two bodies in contact, they are said to be at the same temperature, even though their thermal energies may be different.
• The S.I. unit of temperature is kelvin (K), and absolute zero (0 K) is the temperature at which molecular motion ceases.

• In gases, the intermolecular forces are very weak, causing the molecules to move apart in all directions.
• Gases have no fixed shape and no fixed size — they can be obtained in a vessel of any shape or size.
• Gases expand indefinitely and uniformly to fill any available space.
• Gases exert pressure on their surroundings.