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Class 9–10 · Physics
Effects of heat on matter
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Effects of heat on matter
Lessons in this chapter
Lessons
1Heat and temperature
Heat is energy transferred because of a temperature difference. Temperature indicates thermal state, not total stored energy. Net heat flows from warmer to cooler objects until thermal equilibrium. Internal energy also depends on amount and material.
Key relationship
heat unit J; temperature unit K
Worked example
A large bucket at 40°C can contain more internal energy than one drop at 80°C.
Next lesson →2Thermometers and scales
Thermometers use a property that varies with temperature, such as expansion or electrical resistance. Kelvin and Celsius have equal-size intervals but different zeros. Fahrenheit uses a different interval size.
Key relationship
K = °C+273.15; °F = (9/5)°C+32
Worked example
30°C = 303.15 K = 86°F; temperature rise 10°C = rise 10 K
Next lesson →3Expansion of solids
Most solids expand as average particle separation increases. For small changes with approximately constant coefficients, linear, area and volume expansions are proportional to temperature rise. For isotropic solids, area and volume coefficients are approximately 2α and 3α.
Key relationship
ΔL = αL₀ΔT; ΔA ≈ 2αA₀ΔT; ΔV ≈ 3αV₀ΔT
Worked example
α = 12×10⁻⁶ K⁻¹, L₀ = 10 m, ΔT = 40 K ⇒ ΔL = 4.8 mm
Next lesson →4Real and apparent liquid expansion
A heated liquid and its container both expand. The level change relative to the container gives apparent expansion. Add the container's expansion to obtain the liquid's real expansion, using compatible coefficients and temperature ranges.
Key relationship
γreal ≈ γapparent+γcontainer
Worked example
γreal = 9×10⁻⁴, γcontainer = 3×10⁻⁵ ⇒ γapparent = 8.7×10⁻⁴ K⁻¹
Next lesson →5Anomalous expansion of water
Water contracts when warmed from 0°C to about 4°C and has maximum density near 4°C. Above that it expands normally. Ice's more open structure gives it lower density, allowing it to float.
Key relationship
fixed mass: ρ = m/V
Worked example
Cooling water from 4°C to 1°C increases its volume.
Next lesson →6Heat capacity and specific heat
Heat capacity concerns a whole object; specific heat capacity is per unit mass. With no phase change and approximately constant c, multiply mass, specific heat and temperature rise. Water's high specific heat makes it useful for thermal regulation.
Key relationship
C = Q/ΔT; c = Q/(mΔT); Q = mcΔT
Worked example
0.5 kg water, c = 4200 J/(kg K), rise 10 K ⇒ Q = 21000 J
Next lesson →7Calorimetry
In an insulated mixture, heat lost equals heat gained. Include each material, the container and any phase change. Temperatures can be averaged arithmetically only when the relevant heat capacities are equal.
Key relationship
ΣQ = 0
Worked example
Equal masses of water at 80°C and 20°C, ideal container ⇒ 80−T = T−20 ⇒ T = 50°C
Next lesson →8Phase changes and latent heat
During melting or boiling at fixed pressure, energy can change phase without raising temperature. Evaporation occurs at a surface at many temperatures; boiling occurs throughout a liquid at its pressure-dependent boiling point.
Key relationship
Q = mL
Worked example
0.1 kg ice at 0°C, L = 3.34×10⁵ J/kg ⇒ melting requires 33400 J
Next lesson →9Conduction, convection and radiation
Conduction transfers energy without bulk flow; convection uses moving fluid; radiation uses electromagnetic waves and needs no medium. A vacuum flask limits conduction and convection, while reflective surfaces reduce radiative exchange.
Key relationship
radiation travels through vacuum
Worked example
Spoon handle warms: conduction; rising warm water: convection; sunlight: radiation.
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. A temperature rise of 20°C in kelvins?
ΔT = 20 K
2. What can stay constant while melting absorbs heat?
Temperature at fixed pressure; Q = mL changes phase
3. Main transfer mechanism from the Sun?
Electromagnetic radiation through vacuum
4. At what temperature is water densest?
Approximately 4°C
5. Heat 1 kg water by 2 K, c = 4200: Q?
Q = 1 × 4200 × 2 = 8400 J
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.