Grade 10 physics Mechanics and Thermal Physics – Temperature and Thermal Expansion Notes

Specific learning outcomes

• (a) Explain the meaning of temperature as used in thermal physics.
• (b) Measure temperature using different technologies.
• (c) Investigate thermal expansion and contraction in solids and fluids.
• (d) Describe applications of thermal expansion in solids and fluids.
• (e) Appreciate everyday applications of thermal expansion.
• (f) Know temperature measurement techniques: liquid expansion devices, bimetallic devices, thermocouples, RTDs, thermistors, infrared radiators, molecular change-of-state, silicon diodes, motion sensors.
• (g) Know linear expansivity (coefficients) of common metals: iron, steel, copper, etc.

1. Meaning of temperature

Temperature is a measure of the average kinetic energy of particles in a substance. In thermal physics it tells us how hot or cold something is and determines the direction of heat flow (heat flows from higher temperature to lower temperature). Common scales: Celsius (°C) used in Kenya schools and Kelvin (K) used in physics (K = °C + 273.15).

2. Measuring temperature — devices & principles

• Liquid expansion thermometers (mercury, alcohol): liquid expands and rises in a capillary. Alcohol safer than mercury; calibrate with ice point (0°C) and boiling water (≈100°C at sea level).
• Bimetallic devices: two metals with different expansion are bonded; bending with change of temperature is used in thermostats, electric irons and kettles.
• Thermocouples: junction of two different metals produces a small voltage dependent on temperature (Seebeck effect). Good for wide temperature ranges and fast response.
• RTDs (Resistance Temperature Detectors): resistance of a metal (usually platinum) changes nearly linearly with temperature. Used in laboratories and industry for accurate readings.
• Thermistors: semiconductor resistors whose resistance changes strongly with temperature (NTC decreases with temp, PTC increases). Good for precise low-range sensing.
• Infrared (IR) detectors / radiometers: measure emitted IR radiation from an object—useful for non-contact temperature measurement (hot surfaces, cooking pans, electrical equipment).
• Molecular change-of-state: fixed points like melting ice (0°C) and boiling water (≈100°C) used for calibration. Triple point of water (0.01°C) is a precise reference.
• Silicon diodes and IC sensors: electronic sensors whose voltage drop or output varies with temperature—used in digital thermometers and electronics.
• Motion sensors (indirect): some devices convert thermal expansion into motion and then into a readable signal (e.g., bimetal thermostats driving switches).

Class practical: compare readings of an alcohol thermometer, a thermistor (multimeter + simple circuit) and an infrared thermometer on a hot water surface and on a metal spoon heated by a flame.

3. Thermal expansion — basic ideas & formulas

When temperature of a material changes, its particles move more (or less) and the material expands (or contracts). Types:

• Linear expansion (one dimension, typical for rods): ΔL = α L0 ΔT
• Area expansion (two dimensions): ΔA = 2α A0 ΔT (approx.)
• Volumetric expansion (three dimensions, liquids and solids): ΔV = β V0 ΔT, where β ≈ 3α for isotropic solids

Where α is coefficient of linear expansion (per °C), ΔT = (Tfinal − Tinitial).

4. Typical linear expansivity (α) of metals

Values are approximate in units of 10⁻⁶ °C⁻¹ (per °C):

Use: If a 2.0 m steel rail (α = 12×10−6/°C) warms by 30°C, ΔL = αL0ΔT = 12×10−6 × 2.0 × 30 = 0.00072 m = 0.72 mm. Rails need expansion gaps to avoid buckling.

5. Applications (everyday & Kenyan context)

• Railway expansion gaps and sleeper allowances — prevent buckling in hot weather.
• Overhead power lines — lines sag more on hot days due to expansion; design allows sag.
• Bridge expansion joints — allow safe expansion of concrete/steel in daytime heat.
• Thermostats and electric irons — use bimetallic strips to switch heaters on/off.
• Thermometers in clinics and weather stations — liquid-in-glass or electronic sensors measure temperature.
• Cookware lids and jars — heating can loosen stuck lids; cooling can tighten jars.
• Plumbing — thermal expansion of hot water in pipes; some installations use expansion loops or valves.
• Temperature sensors in hospitals and labs — RTDs, thermistors and thermocouples used for accurate control.

6. Suggested learning experiences & practical activities

1. Ball-and-ring experiment — heat a metal ball and try to pass it through a ring; demonstrate expansion. Discuss safety: use tongs, heat source (spirit lamp/Bunsen) and protective gloves.
2. Measure linear expansion of a rod — secure a metal rod with one end fixed, mark position of free end, heat with flame or hot water and measure ΔL with a vernier or metre rule. Calculate α.
3. Volume expansion of liquids — place water (or coloured alcohol) in a narrow-neck flask with capillary tube. Heat and record rise in column; find β experimentally.
4. Compare temperature sensors — use an alcohol thermometer, a simple thermistor + multimeter and an IR thermometer to measure same object. Record differences and think about response time and contact vs non-contact reading.
5. Bimetal strip demonstration — build a simple bimetal strip (two thin metals glued) and observe bending when heated with a hairdryer; connect to a small switch to buzz a buzzer (safety and teacher supervision required).
6. Field observation — visit a nearby bridge or railway (if safe/allowed) and observe expansion joints; explain their purpose to classmates.

Assessment ideas: short report of experimental procedure, graph of ΔL vs ΔT, calculation of α with error discussion. Group presentations on safety and applications in Kenyan contexts (railways, power lines, household).

7. Safety and practical tips

• Never use mercury thermometers for broken devices — mercury is toxic. Prefer alcohol thermometers where possible.
• Use tongs, heat-proof gloves and goggles when heating metals.
• Ensure good ventilation when using spirit lamps or Bunsen burners.
• Calibrate thermometers using ice water and (if safe) boiling water adjusted for altitude.

Quick revision checklist

• Define temperature and state the common scales (°C, K).
• Know how to use and read liquid-in-glass thermometers.
• State and use ΔL = αL0ΔT and ΔV = βV0ΔT in simple problems.
• Give examples of where thermal expansion matters in daily life and Kenyan infrastructure.
• Be able to describe how thermocouples, RTDs and thermistors work in simple terms.