Definitions [23]
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 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.
The measurement of the quantity of heat is called calorimetry.
Heat energy lost by the hot object = Heat energy gained by the cold object. This is called as ‘Principle of heat exchange’
Define coefficient of linear expansion.
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.
Define absolute zero.
The temperature at which the pressure and volume of a gas theoretically reach zero is called absolute zero.
Define one mole.
One mole of any substance is the amount of that substance which contains the Avogadro number (NA) of particles (such as atoms or molecules).
"Heat is energy in transit. When two bodies at different temperatures are brought in contact, they exchange heat."
OR
The form of energy which is exchanged among various bodies or a system on account of temperature difference is called heat.
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Units: joule (J), calorie (cal), BTU (British Thermal Unit)
"Temperature is a physical quantity that defines the thermodynamic state of a system."
OR
The degree of hotness or coldness of a body, whose natural flow is from higher temperature to lower temperature, is called temperature.
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SI unit: kelvin (K) | Scalar quantity
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 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.
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.
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 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.
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.
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.
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.
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
Formulae [2]
\[E_k=\frac{3}{2}k_BT\]
Where:
- Ek = Average kinetic energy of the molecules (in joules)
- kB = Boltzmann constant = 1.380649 × 10−23 J/K
- T = Absolute temperature (in kelvin)
Q = mcΔT
Where:
- Q = Heat absorbed or released (in joules)
- m = Mass of the substance (in kg)
- c = Specific heat capacity (J/kg·K)
- ΔT = Change in temperature (Tfinal−Tinitial)
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.
- 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.
- 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).
