Physics

Heat and temperature are different

Heat is energy that flows from a hotter body to a colder one (unit: joule; older unit calorie). Temperature tells how hot a body is — the average energy of its particles (unit: kelvin). A bucket of warm water holds more heat than a cup of boiling water, though the cup is at a higher temperature.

Temperature scales

Scale	Ice melts	Water boils
Celsius	0 °C	100 °C
Fahrenheit	32 °F	212 °F
Kelvin	273 K (273.15)	373 K
Conversions: $F = \frac{9}{5}C + 32$ and $K = C + 273$. Celsius and Fahrenheit agree at −40°. Normal body temperature is 37 °C = 98.6 °F. The lowest possible temperature is 0 K (absolute zero) = −273.15 °C. A clinical thermometer reads about 35–42 °C; a pyrometer measures very high temperatures (furnaces).

Specific heat and latent heat

• Specific heat is the heat needed to raise 1 kg of a substance by 1 °C. Water has a very high specific heat, so it heats and cools slowly. That is why coastal places have mild climates, why water is used in car radiators and hot-water bottles.
• Latent heat is heat taken or given during a change of state without any change in temperature. Steam at 100 °C burns more than water at 100 °C because it also releases its large latent heat of vaporisation when it condenses on skin. Ice at 0 °C cools a drink better than water at 0 °C because melting absorbs latent heat.
• Evaporation causes cooling: water in an earthen pot (matka) stays cool, sweating cools the body, a wet cloth feels cold. Evaporation is faster with more surface area, heat, wind and dry air.

Boiling, melting and pressure

Higher pressure raises the boiling point — a pressure cooker cooks faster because water boils above 100 °C inside it. On mountains air pressure is low, water boils below 100 °C, and food cooks slowly. Pressure lowers the melting point of ice (regelation — two ice blocks pressed together join).

Anomalous expansion of water

Most things expand on heating. Water contracts from 0 °C to 4 °C and then expands, so it is densest at 4 °C. In winter the top of a lake freezes first and floating ice insulates the water below, which stays near 4 °C — fish survive. Pipes burst in frost because water expands on freezing. Gaps in railway tracks and sagging electric wires in summer are examples of ordinary thermal expansion.

Three ways heat travels

• Conduction — through solids, particle to particle (hot spoon handle). Metals conduct well; wood, plastic and air are insulators.
• Convection — by movement of the liquid or gas itself. Sea breeze (day) and land breeze (night), boiling water, room heaters.
• Radiation — no medium needed; the Sun's heat reaches us this way. Dark, rough surfaces absorb and emit well; white clothes keep us cool in summer. A thermos flask blocks all three: vacuum (conduction and convection) and silvered walls (radiation).

Sound — a mechanical wave

Sound is produced by vibration and travels as a longitudinal wave that needs a medium; it cannot travel in vacuum (astronauts use radio). Speed of sound: about 343 m/s in air at 20 °C (≈ 332 m/s at 0 °C), ≈ 1500 m/s in water, ≈ 5000 m/s or more in steel — solids > liquids > gases. Speed rises with temperature and humidity; pressure alone does not change it.

Frequency ranges and characteristics

• Human audible range: 20 Hz to 20,000 Hz. Below 20 Hz = infrasonic (earthquakes, elephants, whales). Above 20 kHz = ultrasonic (bats, dolphins, SONAR, ultrasound scans, cleaning).
• Pitch depends on frequency (women and children have higher-pitched voices). Loudness depends on amplitude, measured in decibel (dB). Quality (timbre) depends on the waveform — it lets us tell a sitar from a flute at the same note.

Echo, Doppler effect and resonance

• Echo: reflected sound heard separately if it returns after at least 0.1 s — needs a reflector about 17 m away. Distance = speed × time ÷ 2.
• Doppler effect: an approaching horn sounds higher-pitched, a receding one lower.
• Resonance: a body vibrates strongly when pushed at its natural frequency — soldiers break step on bridges; tuning a radio.
• Sonic boom: produced by objects moving faster than sound (supersonic).

Quick revision

• F = 9C/5 + 32; K = C + 273; −40 °C = −40 °F; body 37 °C = 98.6 °F.
• Water: highest specific heat, densest at 4 °C.
• Steam burns worse — latent heat. Pressure cooker — higher boiling point.
• Conduction (solids), convection (fluids, breezes), radiation (Sun, no medium).
• Sound: solids fastest, vacuum none; 20 Hz–20 kHz; pitch ↔ frequency, loudness ↔ amplitude.