Mechanical And Thermal Properties Of Matter Notes And PYQs by Physics Wallah

Preparing for Mechanical and Thermal Properties of Matter for NEET requires a clear understanding of formulas, physical principles and their applications in numerical problems. Topics such as thermal expansion, thermal stress, calorimetry, heating curves, heat conduction, radiation and Newton's law of cooling can involve direct formula-based questions as well as multi-step calculations. Revising these concepts together can help you identify the right approach when solving NEET Physics questions.

To support your preparation, PW brings you the Mechanical & Thermal Properties of Matter in One Shot: All Concepts, Tricks and PYQs for NEET. The session combines concept revision, useful problem-solving tricks and previous year questions (PYQs). You can use it alongside Mechanical and Thermal Properties of Matter Notes to revise key formulas, practise numerical problems and strengthen your preparation.

Thermal Expansion

Linear, Area, And Volume Expansion

For an initial length L0, the change in length after a temperature change Delta T is:

Delta L = L0 alpha Delta T

The final length is:

Lf = L0(1 + alpha Delta T)

Here, alpha is the coefficient of linear expansion. These are the two important linear-expansion formulae.

For area expansion:

Delta A = A0 beta Delta T

Af = A0(1 + beta Delta T)

For an isotropic material:

beta = 2 alpha

For volume expansion:

Delta V = V0 gamma Delta T

Vf = V0(1 + gamma Delta T)

For an isotropic material:

gamma = 3 alpha

Therefore:

alpha : beta : gamma = 1 : 2 : 3

This ratio may be asked directly in examinations.

Density Variation With Temperature

When a liquid is heated, its mass remains constant but its volume changes. Hence:

rho f = rho 0 / (1 + gamma Delta T)

For small changes:

rho f ≈ rho 0(1 - gamma Delta T)

The density formula is important because examination questions can be solved by direct substitution into it. Density decreases when volume increases at constant mass.

Expansion Of A Sheet With A Hole

When a sheet containing a hole is heated, every dimension increases. The hole expands as if it were made of the same material as the sheet.

(*Memory Tip: A hole in a heated sheet expands as though it were made of the same material as the surrounding sheet.)*

Thermal Stress

A rod fixed between rigid walls cannot expand freely when heated. It develops compressive stress:

Stress = Y alpha Delta T

F = A Y alpha Delta T

where Y is Young’s modulus, and A is the cross-sectional area.

During cooling, a fixed wire tends to contract. The supports prevent contraction, so tensile force develops.

Pendulum Clock And Temperature

The time period of a simple pendulum is:

T = 2 pi square root of (L/g)

Heating increases the length, so the time period increases and the clock becomes slow. For small changes:

Delta T / T = half alpha Delta temperature

(*Memory Tip: Heating increases length, increases time period, and makes the clock slow. Cooling produces the opposite effect.)*

Liquid In A Beaker

For an isotropic beaker, the volume expansion coefficient is 3 alpha. Compare the liquid coefficient gamma liquid with 3 alpha:

The overflow volume is:

V overflow = V0(gamma liquid - 3 alpha) Delta T

The apparent coefficient of expansion is:

gamma apparent = gamma liquid - 3 alpha

Always compare the liquid’s volume expansion with the container’s volume expansion.

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Calorimetry

Sensible Heat And Latent Heat

When there is no phase change:

Q = m s Delta T

Use this when ice remains ice, water remains water, or steam remains steam.

During a phase change:

Q = m L

Temperature remains constant during melting, boiling, condensation, or freezing.

Important values are:

• Specific heat of water = 1 calorie per gram per degree C

• Specific heat of ice = 1/2 calorie per gram per degree C

• Latent heat of fusion = 80 calories per gram

• Latent heat of vaporisation = 540 calories per gram

The mass in Q = mL is only the mass that changes phase.

Water Equivalent

The water equivalent of a body is:

W = m body s body

It represents the mass of water that absorbs the same heat for the same temperature change. It may be added to the water mass only when no phase change occurs.

If phase change is present, treat the body and the phase-changing substance separately.

Ice-To-Water And Ice-To-Steam Problems

For ice below zero degrees C becoming water above zero degrees C, use three stages:

1. Heat ice to zero degrees C: Q1 = m s ice Delta T

2. Melt ice: Q2 = m L fusion

3. Heat water above zero degrees C: Q3 = m s water Delta T

Total heat:

Q total = Q1 + Q2 + Q3

For ice becoming steam, add a fourth stage: vaporisation of only the mass converted into steam.

Principle Of Calorimetry

In an isolated system:

Heat given = Heat absorbed

For steam entering water, steam releases heat through:

1. Condensation: m L vaporisation

2. Cooling of condensed water from 100 degrees C to the final temperature.

The final temperature is common to both the original water and the condensed steam.

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Heating Curves

A heating curve shows temperature against time. Its sequence is:

1. Ice temperature rises to zero degrees C.

2. Melting occurs at constant temperature.

3. Water temperature rises to 100 degrees C.

4. Vaporisation occurs at constant temperature.

5. Steam temperature may rise further.

For a constant heating power:

Slope = P / (m s)

Therefore, the slope is inversely proportional to specific heat. The constant-temperature sections depend on latent heat. Since latent heat of vaporisation is much greater than latent heat of fusion, the boiling section is longer than the melting section.

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Heat Conduction

The rate of heat flow through a rod is:

Heat current = K A Delta T / L

K is thermal conductivity, A is cross-sectional area, and L is length.

The differential form is:

Heat current = -K A dT/dx

The negative sign shows that heat flows toward decreasing temperature.

Thermal resistance is:

R thermal = L / (K A)

Thus:

Heat current = Temperature difference / Thermal resistance

Series And Parallel Systems

For rods in series:

R equivalent = R1 + R2 + R3

The heat current is the same through every rod.

For rods in parallel:

1 / R equivalent = 1/R1 + 1/R2 + 1/R3

The temperature difference is the same across each branch, while total heat current is the sum of branch currents.

(*Memory Tip: Replace the rod with a resistance. Then use the same methods as current electricity.)*

For radial conduction:

• Hollow sphere: R = (R2 - R1) / (4 pi K R1 R2)

• Hollow cylinder: R = ln(R2/R1) / (2 pi K L)

At a junction in steady state, there is no heat accumulation. Therefore, heat entering equals heat leaving.

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Thermal Radiation

Radiation transfers heat without a material medium. Solar heat reaching Earth is an example. All bodies above zero kelvin emit thermal radiation.

For a black body:

P = sigma A T^4

For a real body:

P = emissivity sigma A T^4

The net radiative heat loss is:

P net = emissivity sigma A(T^4 - Ts^4)

All temperatures must be in kelvin.

Wien’s Displacement Law

The wavelength of maximum emission satisfies:

lambda max T = constant

As temperature increases, peak wavelength decreases and total emitted power increases. Hence, a blue-emitting object is hotter than a red-emitting object.

(*Memory Tip: Higher temperature means shorter peak wavelength and greater total emission.)*

Newton’s Law Of Cooling

For a small temperature difference between a body and its surroundings:

Rate of cooling is proportional to (T - Ts)

The integrated form is:

ln[(Tf - Ts)/(Ti - Ts)] = -k t

This law is valid when the temperature difference is small compared with the absolute temperature.

Revise Mechanical and Thermal Properties of Matter with PW! This NEET 2026 one-shot covers important concepts, formula-based problems, useful tricks and PYQs to help you strengthen your Physics preparation.

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Regular revision of Mechanical and Thermal Properties of Matter can help you recall important formulas and apply them to NEET-style numerical problems. Focus on understanding thermal expansion, calorimetry, conduction, radiation and cooling laws, then practise PYQs to test your understanding. Use the PW one-shot video alongside your notes and question practice to reinforce key concepts and continue preparing for NEET 2027 with PW's learning resources.

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