Chapter 10: Thermal Properties of Matter Short Questions | physics Class 11 CBSE

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Chapter 10: Thermal Properties of Matter Short Questions | physics Class 11 CBSE | ByteNotes.in

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Question 1

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Define heat and state its SI unit. How does it differ from temperature?

Heat is the form of energy transferred between two systems or between a system and its surroundings by virtue of a temperature difference between them.
Temperature is a measure of the degree of hotness or coldness of a body, while heat is energy in transit due to that temperature difference.
The SI unit of heat is joule (J); the SI unit of temperature is kelvin (K).

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Question 2

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Write the relation between Celsius and Fahrenheit temperature scales and convert 45°C into Fahrenheit.

The relation is (tF – 32)/180 = tC/100, which can be rewritten as tF = (9/5)tC + 32.
Substituting tC = 45°C: tF = (9/5)(45) + 32 = 81 + 32 = 113°F.
On the Fahrenheit scale there are 180 equal intervals between the fixed points, while on Celsius there are 100.

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Question 3

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State the ideal gas equation and identify each term with its SI unit.

The ideal gas equation is PV = μRT, combining Boyle's law and Charles' law for low-density gases.
P is pressure (Pa), V is volume (m³), μ is number of moles (mol), T is absolute temperature (K).
R is the universal gas constant with value 8.31 J mol⁻¹ K⁻¹; this equation holds for any quantity of any low-density gas.

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Question 4

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What is absolute zero? At what temperature does it correspond on the Celsius scale?

Absolute zero is the lowest possible temperature, defined as the temperature at which the pressure of an ideal gas would become zero if it remained gaseous.
It is found by extrapolating the pressure-temperature graph of an ideal gas to zero pressure, giving –273.15°C.
On the Kelvin scale, absolute zero is taken as 0 K and forms the origin of the absolute temperature scale named after Lord Kelvin.

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Question 5

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Define the coefficient of linear expansion and write its mathematical expression. State its SI unit.

The coefficient of linear expansion αl is defined as the fractional change in length of a solid per unit change in temperature: αl = (Δl/l)/ΔT.
It is a characteristic of the material and has SI unit K⁻¹; its value varies slightly with temperature but is treated as constant over normal ranges.
For metals, αl is relatively large (e.g., copper ~1.7×10⁻⁵ K⁻¹), while for glass it is much smaller (~0.32×10⁻⁵ K⁻¹).

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Question 6

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State the relation between the coefficient of volume expansion and the coefficient of linear expansion for an isotropic solid, and derive it briefly.

For an isotropic solid, αV = 3αl, where αV is the coefficient of volume expansion and αl is the coefficient of linear expansion.
Derivation: for a cube of side l, ΔV = (l+Δl)³ – l³ ≈ 3l²Δl = 3l³αlΔT, so ΔV/V = 3αlΔT, giving αV = 3αl.
Terms (Δl)² and (Δl)³ are neglected since Δl is very small compared to l for typical temperature changes.

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Question 7

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Explain the anomalous expansion of water and state its temperature range.

Water contracts on heating between 0°C and 4°C, which is opposite to the behaviour of most substances — this is called anomalous expansion.
Below 4°C, water expands as temperature decreases, so water has its maximum density at 4°C.
This property causes lakes and ponds to freeze at the top first, preserving aquatic life beneath the ice during winter.

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Question 8

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What is thermal stress? Give a numerical example from the chapter.

Thermal stress is the compressive or tensile stress developed in a body when its thermal expansion or contraction is prevented by rigid external supports.
The thermal stress equals Y × αl × ΔT, where Y is Young's modulus of the material.
Example: a steel rail (αl = 1.2×10⁻⁵ K⁻¹, Y = 2×10¹¹ N m⁻²) prevented from expanding for a 10°C rise develops a thermal stress of 2.4×10⁷ N m⁻².

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Question 9

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Define specific heat capacity and heat capacity of a substance. Write the relation between them.

Heat capacity S of a substance is the heat required to raise its temperature by one degree: S = ΔQ/ΔT; its SI unit is J K⁻¹.
Specific heat capacity s is the heat required per unit mass per unit temperature change: s = ΔQ/(mΔT) = S/m; SI unit is J kg⁻¹ K⁻¹.
Specific heat capacity depends on the nature of the substance and its temperature; water has the highest value of 4186 J kg⁻¹ K⁻¹ among common substances.

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Question 10

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Distinguish between molar specific heat capacity at constant pressure (Cp) and at constant volume (Cv) for gases.

Cp is the molar specific heat capacity when the gas is heated at constant pressure, allowing it to expand and do work on the surroundings.
Cv is the molar specific heat capacity when the gas is heated at constant volume, so all supplied heat goes into raising the internal energy.
Since a gas at constant pressure does extra work during expansion, Cp > Cv for all gases; the difference Cp – Cv = R (universal gas constant) for ideal gases.

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Question 11

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State the principle of calorimetry. Describe the basic construction of a calorimeter.

Principle: in an isolated system, the heat lost by a hotter body equals the heat gained by a cooler body, provided no heat escapes to the surroundings.
A calorimeter consists of a metallic vessel (copper or aluminium) with a stirrer of the same material, placed inside a wooden jacket containing heat-insulating material like glass wool.
A mercury thermometer is inserted through an opening in the outer jacket to measure the temperature of the contents inside.

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Question 12

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What is the melting point of a substance? Explain how pressure affects the melting point with an example.

The melting point is the temperature at which the solid and liquid states of a substance coexist in thermal equilibrium; it is characteristic of the substance.
Melting point depends on pressure: increase in pressure lowers the melting point of ice (an exception), enabling phenomena like regelation.
Example: a metallic wire weighted at both ends passes through an ice slab because increased pressure below the wire causes ice to melt at a lower temperature; water refreezes above the wire once the wire has passed.

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Question 13

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Define latent heat. Distinguish between latent heat of fusion and latent heat of vaporisation with values for water.

Latent heat L is the heat per unit mass absorbed or released by a substance during a change of state at constant temperature: L = Q/m; SI unit is J kg⁻¹.
Latent heat of fusion Lf is the heat per unit mass required to convert a solid to liquid at its melting point; for water, Lf = 3.33×10⁵ J kg⁻¹.
Latent heat of vaporisation Lv is the heat per unit mass required to convert a liquid to vapour at its boiling point; for water, Lv = 22.6×10⁵ J kg⁻¹, about 6.8 times Lf.

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Chapter 10: Thermal Properties of Matter | Short Questions

Define heat and state its SI unit. How does it differ from temperature?

Write the relation between Celsius and Fahrenheit temperature scales and convert 45°C into Fahrenheit.

State the ideal gas equation and identify each term with its SI unit.

What is absolute zero? At what temperature does it correspond on the Celsius scale?

Define the coefficient of linear expansion and write its mathematical expression. State its SI unit.

State the relation between the coefficient of volume expansion and the coefficient of linear expansion for an isotropic solid, and derive it briefly.

Explain the anomalous expansion of water and state its temperature range.

What is thermal stress? Give a numerical example from the chapter.

Define specific heat capacity and heat capacity of a substance. Write the relation between them.

Distinguish between molar specific heat capacity at constant pressure (Cp) and at constant volume (Cv) for gases.

State the principle of calorimetry. Describe the basic construction of a calorimeter.

What is the melting point of a substance? Explain how pressure affects the melting point with an example.

Define latent heat. Distinguish between latent heat of fusion and latent heat of vaporisation with values for water.

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