Chapter 6: Heat, Waves, Light, and Sound for Technology

Heat and temperature

Temperature is a measure of how hot something is; heat is energy transferred because of a temperature difference. The kelvin scale is . To change temperature, (where is the specific heat capacity; water has ). To change state, (where is the specific latent heat). Example: heating 5 kg of water by 30 K needs kJ. A 2 kW element at 100% efficiency does this in s.

Thermal expansion

Solids expand when heated: , where is the coefficient of linear expansion (steel about ). A 10 m steel rail heated by 40 K lengthens by mm, so expansion gaps are left in rails, bridges, and pipelines.

Heat transfer

Conduction: through solids, as in a metal rod; metals are good conductors; air and foam are good insulators.

Convection: by the movement of fluids, which drives sea breezes and room heating.

Radiation: by electromagnetic waves, which needs no medium; dark, rough surfaces absorb and emit more.

Applications: insulation in roofs, cool-roof paint, cooking vessels, heat sinks for electronics, solar water heaters, and refrigeration. A refrigerator circulates a refrigerant through a compressor, condenser (heat is rejected), expansion valve, and evaporator (heat is absorbed from the cold space).

The gas laws

For a fixed mass of gas: Boyle’s law (constant ); Charles’s law (constant ); pressure law (constant ); combined: . Temperatures must be in kelvin. Example: a tyre at 27 °C and 200 kPa heats to 57 °C at constant volume: kPa.

Waves and sound

A wave carries energy without carrying matter. Wave equation: . Sound is a longitudinal wave; its speed in air at 20 °C is about . Pitch relates to frequency, loudness to amplitude. Echo: a sound that returns after time has travelled to the obstacle and back, so the distance is (for s, m). Sound intensity level is measured in decibels; prolonged exposure to loud noise damages hearing, so ear protection is required in workshops. Ultrasound is used for cleaning, flaw detection, and medical imaging.

Light and optics

Light travels in straight lines, reflects () and refracts (bends on entering another medium, ). Total internal reflection makes optical fibres work. Lenses form images: a converging lens can form a real image (cameras, projectors) or a magnified virtual image (magnifying glass). The electromagnetic spectrum runs from radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays; all travel at in a vacuum. Solar energy reaches Sri Lanka in abundance and can be used directly as heat (solar water heaters) or converted to electricity by photovoltaic cells.

Common mistakes

Using Celsius in gas-law calculations.

Forgetting that an echo’s time includes the round trip.

Confusing heat (energy) with temperature.

Practice questions

How much heat is needed to raise 2.5 kg of water from 25 °C to 85 °C? [630 kJ]

A 30 m steel bridge section undergoes a temperature rise of 25 K. Find the expansion. () [9 mm]

A sound takes 1.2 s to return from a cliff. How far is the cliff? () [204 m]