Definitions [22]
"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)
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).
"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 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 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.
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 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 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.
OR
The amount of heat per unit mass absorbed or given out by a substance to change its temperature by one unit (one degree), i.e., 1°C or 1 K, is called specific heat capacity.
OR
The quantity of heat required to raise the temperature of a unit mass of a gas by one degree, whose exact value depends upon the mode of heating the gas and can range from zero to infinity or even be negative, is called the specific heat capacity of a gas.
The quantity of heat needed to raise the temperature of the whole body by 1°C (or 1 K) is called heat capacity.
OR
The amount of heat ΔQΔQ supplied to a substance to change its temperature from T to T + ΔT, per unit mass per unit degree change in temperature, is called specific heat:
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Unit: J kg⁻¹ K⁻¹
The amount of heat required to raise the temperature of one mole of a substance through a unit degree Celsius or Kelvin is called molar heat capacity.
Define heat capacity.
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.
Calorimetry is the science of measuring heat exchange during physical or chemical processes. The word comes from the Latin calor (heat) + Greek metron (measure).
OR
An experimental technique for the quantitative measurement of heat exchange is called calorimetry.
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 quantity of heat required to convert unit mass of a substance from its liquid state to vapour state, at its boiling point without any change in its temperature is called its latent heat of vapourization (Lv).
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.
OR
Latent heat is the quantity of heat energy required to change the state of unit mass of a substance from one phase to another, at constant temperature and constant pressure.
OR
The quantity of heat absorbed or given out by unit mass of a substance during change of state of the substance at a constant temperature is called the latent heat of the substance.
The quantity of heat required to convert unit mass of a substance from its solid state to the liquid state, at its melting point, without any change in its temperature, is called its latent heat of fusion (Lf).
OR
The heat energy absorbed at constant temperature during the transformation of solid into liquid is called the latent heat of fusion. 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.
Substances that do not conduct heat easily are called bad conductors of heat.
Conduction is the process by which heat flows from the hot end to the cold end of a solid body without any net bodily movement of the particles of the body.
OR
The process by which heat flows from the hot end to the cold end of a solid body without any net bodily movement of the particles of the body is called conduction.
Solid substances that conduct heat easily are called good conductors of heat.
Convection is the process by which heat is transmitted through a substance from one point to another due to the actual bodily movement of the heated particles of the substance.
OR
The process by which heat is transmitted through a substance from one point to another due to actual bodily movement of the heated particles of the substance is called convection.
OR
The mode of heat transfer by actual motion of matter (bulk transport of fluid) from the source of heat, which occurs only in fluids, is called convection.
Define 'Greenhouse effect'.
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.
Formulae [6]
\[E_k=\frac{3}{2}k_BT\]
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)
\[Q=mc\Delta T\]
Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]
or
Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]
C = M × c = Q/(nΔT)
Unit: J/mol · K
Q = m × L
where,
Q = Heat energy absorbed or released during phase change
m = Mass of the substance undergoing phase change
L = Specific Latent Heat (characteristic of the substance & process)
SI Units = J kg⁻¹
Theorems and Laws [3]
Statement: When different parts of an isolated system are at different temperatures, heat transfers from the part at higher temperature to the part at lower temperature. The heat lost by the hot object is equal to the heat gained by the cold object, provided no heat is allowed to escape to the surroundings.
(For liquid in calorimeter: m1c1Δθ + mcccΔθ)
Key Points:
- A system is said to be isolated if no exchange of heat occurs between the system and its surroundings.
- Calorimetry literally means measurement of heat.
- Energy supplied by heater = VIt (voltage × current × time).
- This principle is based on the Law of Conservation of Energy.
Statement: In steady-state heat flow by conduction in a bar with ends maintained at different temperatures TC and TD, the heat flow is proportional to the temperature difference and the area of cross-section A, and inversely proportional to the length L.
Also written as:
Where K is the thermal conductivity of the material.
Key Points:
- Gases are poor conductors; liquids have intermediate conductivities; solids are generally good conductors.
- The greater the value of K, the more rapidly the material conducts heat.
The wavelength (λm) for which the emissive power of a blackbody is maximum is inversely proportional to the absolute temperature of the blackbody:
With increase in temperature, λm decreases (shifts towards shorter wavelengths). Also, the energy Emax emitted at λm increases with the fifth power of temperature, i.e., Emax ∝ T5.
Key Points
- Heat is energy in transit — it transfers between bodies due to a temperature difference.
- Temperature is a physical quantity that defines the thermodynamic state and measures the average kinetic energy of particles.
- When two bodies at different temperatures come in contact, heat flows from the hotter to the cooler body until thermal equilibrium is reached.
- In solids, heating increases particle vibration (kinetic energy); near the melting point, heat weakens bonds without raising the temperature.
- Liquids have weaker bonds than solids but stronger than gases; they have definite volume but no definite shape.
- In gases (ideal), there are no intermolecular forces; particles move freely with no definite shape or volume.
- Interatomic spacing: solids ~10−10 m, gases at NTP ~10−9 m.
- SI unit of heat: Joule (J); CGS unit: Erg; 1 J = 107 erg.
- 1 calorie = 4.184 J.
- Dimensional formula: Heat = [M1L2T−2]; Temperature = [M0L0T0K1].
- Average kinetic energy formula: Ek = \[\frac {3}{2}\]kBT, linking microscopic particle energy to macroscopic temperature.
- 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).
- 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.
- A calorimeter is an insulated device used to measure heat transfer; measurement of specific heat of a substance is carried out using it.
- Principle of Calorimetry: Heat lost by hot body = Heat gained by cold body, which represents the law of conservation of heat energy.
- In the method of mixtures, a heated sample is placed in the calorimeter and the temperature change is measured to calculate specific heat using the formula Q = msΔt.
- Specific heat of a substance depends on the nature of the substance; water is preferred in calorimetry due to its high specific heat, allowing it to absorb large amounts of heat with minimal temperature change.
- For accurate results, the sample must be transferred quickly into the calorimeter and stirred well to ensure uniform heat distribution.
- Formula: Q = mL. Specific latent heat L has SI unit J kg⁻¹.
- Temperature stays constant during any phase change. Heat energy goes into breaking or forming intermolecular bonds, not into raising kinetic energy.
- Latent Heat of Fusion (water): Lf = 3.33 × 10⁵ J kg⁻¹ = 80 cal/g. Heat needed to melt 1 kg of ice at 0°C.
- Latent Heat of Vaporisation (water): Lv = 22.6 × 10⁵ J kg⁻¹ = 540 cal/g. Heat is needed to convert 1 kg of water to steam at 100°C.
- Lv ≫ Lf because vaporisation requires complete molecular separation and work against atmospheric pressure during expansion.
- All latent heat values depend on atmospheric pressure. Standard values quoted at 1 atm. Increasing pressure raises the boiling point (pressure cooker effect).
- The transfer of heat from the hot part to the cold part of an object is called conduction of heat.
- Conduction takes place through solid substances only — it requires a medium.
- Heat travels by molecular collisions: fast-vibrating molecules pass energy to slower neighbours.
- Copper conducts heat faster than aluminium, which conducts faster than steel.
- Conduction of heat through a substance depends on the property of that substance.
- Good conductors: silver, copper, aluminium, brass — all metals.
- Bad conductors: wood, cloth, air, paper — most non-metals.
- Good conductors of heat are also good conductors of electricity, and bad conductors of heat are also bad conductors of electricity.
- Convection occurs only in fluids (liquids and gases) — not in solids.
- In conduction, molecules vibrate but stay in place.
- In convection, molecules physically move from one place to another.
- Heating reduces density → hot fluid rises; cool fluid sinks → a convection current is set up.
- Convection currents transfer heat to the entire mass of the fluid.
- Potassium permanganate makes convection currents visible as magenta-coloured streams.
- The greenhouse effect is a naturally occurring phenomenon that heats Earth's surface. Without it, Earth's temperature would be -18°C instead of 15°C.
- Greenhouse gases are transparent to solar radiation but retain and reflect back long-wave heat radiation. Main gases — CO₂ (60%), CH₄ (20%), CFCs (14%), N₂O (6%).
- Earth's surface re-emits heat as infrared radiation. Greenhouse gases like CO₂ and CH₄ absorb this and return heat to Earth's surface — causing the greenhouse effect.
- Rising CO₂ due to the burning of fossil fuels and deforestation intensifies the greenhouse effect, causing global warming.
- Global warming leads to melting of polar ice, rising sea levels, changes in rainfall patterns and loss of biodiversity.
