Definitions [14]
The sum of the potential energy and kinetic energy of a molecule is called its internal energy.
The total internal energy of all the molecules of a substance is called its thermal energy.
Heat is that form of energy which flows from a hot body to a cold body when they are kept in contact.
Define one kilo-calorie of heat.
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.
Define the term heat.
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.
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.
Answer the following question.
What is thermal stress?
- Consider a metallic rod of length l0 fixed between two rigid supports at T °C.
- 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.
- 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 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.
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.
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
Define the following term:
Coefficient of cubical expansion
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.
Define the following term:
Coefficient of superficial expansion
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.
Key Points
- 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.
- Solids have three types of expansion - Linear (1D), Superficial (2D), and Cubical (3D) - with β = 2α and γ = 3α.
- Change in dimensions: ΔL = L0αΔT, ΔA = A0βΔT, ΔV = V0γΔT.
- Liquids have only volume expansion; real expansion = apparent expansion + vessel expansion, i.e., γr = γa + γv.
- Gases have only real expansion as the container expansion is negligible.
- Final quantity after heating: L = L0(1 + αΔT), A = A0(1 + βΔT), V = V0(1 + γΔT).
