Revision: Class 11 >> Thermal Properties of Matter: Heat NEET (UG) Thermal Properties of Matter: Heat

Latent heat capacity of a substance is defined as the amount of heat energy required to change the state of a unit mass of the material.

The temperature of a body determines its hotness, while heat energy is its heat content.

The quantity of heat transferred through a unit length of a material in a direction normal to Unit surface area due to a unit temperature difference under steady-state conditions is known as the thermal conductivity of a material.

Specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of 1 kg of a substance by 1 Kelvin or 1°C.

Molar specific heat capacity is defined as heat energy required to increase the temperature of one mole of a substance by IK or 1°C.

C = `1/μ ((Δ"Q")/(Δ"T"))`

"Heat is energy in transit. When two bodies at different temperatures are brought in contact, they exchange heat."

"Temperature is a physical quantity that defines the thermodynamic state of a system."

One mole of any substance is the amount of that substance which contains the Avogadro number (NA) of particles (such as atoms or molecules).

When two bodies at different temperatures are brought into contact through a diathermic wall, heat flows from the hotter body to the cooler one. This continues until both reach the same temperature, at which point heat flow stops. This state is called thermal equilibrium.

A diathermic wall is a partition that freely allows heat to flow between two systems. It is shown as a thin dark line in diagrams. A thin copper sheet is a good example.

An adiabatic wall is an ideal partition that completely prevents heat transfer between two systems. In diagrams, it is shown as a thick, cross-hatched (slanting lines) region.

Thermometry is the branch of physics dealing with temperature measurement. It relies on the principle that certain physical properties of materials change continuously and predictably with temperature.

The SI absolute temperature scale starting at absolute zero. Written as K (no degree symbol °).

The lowest theoretically possible temperature (0 K = −273.15 °C), where ideal gas molecules have zero kinetic energy.

Extending a graph line beyond the measured data to predict values.

One kelvin = 1/273.16 of the difference between absolute zero and the triple point of water.

A constant in the ideal gas equation; R = 8.31 J mol⁻¹ K⁻¹.

The unique temperature & pressure at which solid, liquid, and gas phases of a substance coexist in equilibrium.

A hypothetical gas whose molecules have no volume and exert no intermolecular forces; obeys PV = μRT exactly.

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.

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.

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 change from vapour to solid is called solidification.

The change from solid to liquid phase is known as melting, while the reverse change from liquid to solid is called freezing. 

The triple point of water is that point where water in a solid, liquid and gas state co-exists in equilibrium and this occurs only at a unique temperature and a pressure.

Sublimation is the process in which a solid changes directly into a gas on heating, without passing through the liquid state.

The process of change from one state to another at a constant temperature is called the change of phase.

The direct change from solid to vapour is called sublimation.

Specific latent heat of a phase is the quantity of heat energy absorbed (or liberated) by the unit mass of the substance for the change in its phase at a constant temperature.

S.I. unit of specific latent heat is J kg⁻¹, and 1 cal g⁻¹ = 4.2 × 10³ J kg⁻¹.

The heat energy absorbed (or liberated) in change of phase is not externally manifested by any rise or fall in temperature, it is called the latent heat.

The constant temperature at which the liquid transforms into gaseous state is called the boiling point of the liquid.

The constant temperature, at which the ice converts into water is called the melting point of ice.

The amount of heat energy absorbed at constant temperature by unit mass of a liquid to convert into gaseous phase is called the specific latent heat of vapourization.

The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion.

The heat energy absorbed at constant temperature during transformation of solid into liquid is called the latent heat of fusion.

The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of unit mass of that substance through 1°C (or 1 K).

OR

Heat capacity of a body when expressed for the unit mass is called the specific heat capacity of the substance of that body.

OR

The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat of that object.

The heat capacity of a body is the quantity of heat required to raise its temperature by 1°C. It depends upon the mass and the nature of the body.

A calorimeter is a cylindrical vessel which is used to measure the amount of heat gained (or lost) by a body when it is mixed with another body or substance.

The coefficient of thermal conductivity of a material is defined as the quantity of heat that flows in one second between the opposite faces of a cube of side 1 m, the faces being kept at a temperature difference of 1°C (or 1 K).

The heating-up of the earth’s atmosphere due to trapped infrared rays reflected from the earth's surface by atmospheric gases is called the greenhouse effect.

\[E_k=\frac{3}{2}k_BT\]

Where:

• E_k = Average kinetic energy of the molecules (in joules)
• k_B = 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 (T_final−T_initial)

Master Conversion Formula:

\[\frac{T_F-32}{180}=\frac{T_C}{100}\] = \[\frac {T_K−273.15}{100}\]

Master Conversion Formula:

\[\frac {T_C}{100}\] = \[\frac {(T_{F}-32)}{180}\] = \[\frac {(T_{K}-273.15)}{100}\]

L = \[\frac{\text{heat absorbed (or liberated) for the change of phase}}{\text{mass}}\]

or

L = \[\frac {Q}{m}\]

Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]

or

Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]

If system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then systems A and B are in thermal equilibrium with each other.

• Gases expand linearly with temperature, making them useful for thermometers. This consistent behaviour suggests the existence of a lowest temperature limit.
• Absolute zero (−273.15 °C or 0 K) is the temperature where an ideal gas would have zero pressure. It is the lowest possible temperature.
• The Kelvin scale begins at absolute zero and uses the triple point of water (273.16 K) as a reference point. It is the SI temperature scale.
• The ideal gas equation (PV = μRT) combines all gas laws into a single relationship among pressure, volume, and temperature. It works best for gases at low pressure and high temperature.

• Heat energy absorbed (Q) depends on: mass (m), rise in temperature (Δt), and specific heat capacity (c), i.e., Q ∝ m × Δt × c.
• Heat capacity (C') and specific heat capacity (c) are related by: C′ = m × c.