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 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.

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.

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

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.

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.

Avogadro s law states that "equal volumes of all gases under similar conditions of temperature and pressure contain the same number of molecules."

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

• 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.