A doctor's thermometer, a burn from steam, dal that takes long to cook in the hills, an iron ship that floats, a tyre with deep treads — all of this is everyday physics. Exams ask you to convert Celsius, Fahrenheit and Kelvin, to explain boiling and pressure, to say why things float, to rank kinds of friction, and to match an instrument with what it measures.
On this page you will learn:
- Heat vs temperature, the three scales, thermometers and the thermostat
- Latent heat, steam burns and the odd behaviour of water
- Conduction, convection, radiation and sea breeze
- Air pressure, boiling in the hills and Bernoulli's principle
- Floating, density, the lactometer, viscosity and surface tension
- Friction, SI units and measuring instruments
Heat and temperature
Rule in one line: heat is energy that flows from hot to cold; temperature tells how hot a body is.
On the Celsius scale, water freezes at 0 °C and boils at 100 °C. On the Fahrenheit scale, these are 32 °F and 212 °F. The Kelvin scale is the SI scale. It starts at absolute zero, 0 K, about −273 °C. It is written K, with no degree sign. Use F = 9C/5 + 32 and K = C + 273.
| Point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Ice melts | 0 °C | 32 °F | 273 K |
| Normal human body | 37 °C | 98.6 °F | 310 K |
| Water boils at sea level | 100 °C | 212 °F | 373 K |
| °C equals °F | −40 °C | −40 °F | 233 K |
A thermometer only measures temperature. A thermostat regulates it. It senses the temperature and switches a heater or cooler on and off to hold it steady. Electric irons, geysers, fridges and ovens have one. Many use a bimetallic strip, two metals that bend when heated.
While ice melts or water boils, the temperature does not rise. The heat goes into changing the state. This hidden heat is latent heat. Steam at 100 °C holds about 22.5 × 10⁵ J per kg more than water at 100 °C. When steam touches skin it condenses and gives out this extra heat. So a steam burn is worse than a burn from boiling water.
Most things shrink when cooled. Water shrinks down to 4 °C, then expands from 4 °C to 0 °C. Water also expands when it freezes, so ice is lighter and floats. Water is densest at 4 °C. In winter a lake freezes at the top; the water below stays near 4 °C, and fish live. Frozen water pipes can burst for the same reason.
How heat travels
Rule in one line: solids conduct, liquids and gases convect, and radiation needs no medium.
| Mode | How heat moves | Example |
|---|---|---|
| Conduction | From particle to particle; mainly in solids | A steel spoon in hot tea gets hot |
| Convection | Warm liquid or gas rises; cool part takes its place | Water heating in a pan; sea breeze |
| Radiation | Travels with no medium, even through space | Sun's heat reaching Earth |
Dark surfaces absorb more heat; light colours reflect it. So we wear light clothes in summer. Wool keeps us warm because it traps air, a poor conductor. Two thin blankets are warmer than one thick one, because of the air layer between them.
A thermos blocks all three modes of heat transfer. Its double glass wall has a vacuum in between, which stops conduction and convection. The walls are silvered, which reflects radiation. An insulating stopper closes the top. When a system can exchange no heat with its surroundings, the process is called adiabatic.
| Process | What stays fixed |
|---|---|
| Adiabatic | No heat goes in or out |
| Isothermal | Temperature |
| Isochoric | Volume |
| Isobaric | Pressure |
Pressure, air pressure and boiling
Rule in one line: pressure = force ÷ area; less air pressure means a lower boiling point.
Pressure = Force ÷ Area. Its SI unit is the pascal (Pa), 1 newton per square metre. The same force on a smaller area gives more pressure. So a sharp knife cuts better, and school bags have wide straps.
Liquid pressure grows with depth. Water spurts farthest from the lowest hole of a bottle. A dam wall is thicker at the bottom to hold the greater push there. Gases press on their container too; that is why a balloon swells.
The air around us presses on everything. At sea level this pressure equals a mercury column about 76 cm high, about 1 × 10⁵ Pa. A barometer measures it. Air pressure falls with height.
- 1Go up a mountain
- 2Air pressure is lower
- 3Water boils below 100 °C
- 4Food gets less heat; cooks slowly
Where a fluid moves faster, its pressure is lower. Air rushes faster over the curved top of an aeroplane wing, so the pressure there falls and the wing is lifted. A strong wind over a roof can lift a tin sheet for the same reason.
Floating, density and liquids
Rule in one line: the upward push equals the weight of the liquid pushed aside.
A liquid pushes up on anything put in it. This is buoyant force or upthrust. Archimedes' principle: the upthrust equals the weight of the liquid displaced. It does not depend on the body's shape or depth. Ships, hydrometers and lactometers work on this.
| Idea | Meaning |
|---|---|
| Density | Mass per volume; water is 1000 kg/m³ |
| Floats | Less dense than the liquid, like cork and ice |
| Sinks | Denser than the liquid, like an iron nail |
| Relative density | Density ÷ density of water; no unit |
A lactometer floats in milk. How deep it sinks shows the milk's density. Water added to milk lowers its density. But fat is lighter than milk, so skimmed milk is denser than whole milk. Add water to skimmed milk and its density can look normal again. So a lactometer cannot catch watered skimmed milk.
Friction
Rule in one line: friction opposes motion; static is the largest, then sliding, and rolling is the smallest.
Friction is the force that opposes motion between two touching surfaces. Tiny bumps on the surfaces lock into each other. Rough surfaces and harder pressing give more friction. Friction always acts against the motion, and it makes heat — rub your palms and feel it.
- LargestStatic frictionBefore motion starts
- Sliding frictionOne surface slides on another
- SmallestRolling frictionA wheel or ball rolls
- Treads on tyres
- Grooves on shoe soles
- Brake pads pressing the wheel
- Kabaddi players rub soil on hands
- Oil and grease between moving parts
- Ball bearings in fans and cycles
- Wheels on suitcases
- Powder on a carrom board
Air and water also slow down things moving through them. This is drag. It depends on speed and shape. A streamlined shape — pointed in front, like a fish or a bird — cuts drag. Aeroplanes, boats and racing cars are built this way.
Without friction we could not walk, write or stop a cycle. But friction also wears out tyres and machine parts, and wastes energy as heat. So we add it where we need grip and cut it where we need smooth motion.
Units and measuring instruments
Rule in one line: the SI system has seven base units; every other unit is built from them.
| Quantity | SI base unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
| Quantity | Unit | Made of |
|---|---|---|
| Force | newton (N) | kg m/s² |
| Work, energy, heat | joule (J) | N m |
| Power | watt (W) | J/s |
| Pressure | pascal (Pa) | N/m² |
| Frequency | hertz (Hz) | 1 per second |
| Instrument | Measures |
|---|---|
| Thermometer | Temperature |
| Barometer | Air pressure |
| Hygrometer | Humidity of air |
| Hydrometer | Density of liquids |
| Lactometer | Purity of milk |
| Anemometer | Wind speed |
| Seismograph | Earthquake waves |
| Spring balance | Force, such as weight |
Earthquakes: more factsTap to open
- The Earth's outer layer is made of moving plates. When plates rub, collide or slide under each other, an earthquake can occur.
- Size is given on the Richter scale. A rise of 2 on the scale means about 1000 times more destructive energy.
- The point on the surface right above the start of an earthquake is the epicentre.
- Indoors, take cover under a strong table, away from glass. Outdoors, move to an open place away from buildings and wires.
- The scales are named after Anders Celsius, Daniel Gabriel Fahrenheit and Lord Kelvin. The pascal honours Blaise Pascal.
Traps and practice
Last revision — where the options try to fool you.
- A thermostat regulates temperature; a thermometer measures it.
- Steam burns more because of latent heat, not because it is hotter than 100 °C.
- In the hills water boils at a LOWER temperature, not a higher one.
- Water expands on freezing and is densest at 4 °C, not 0 °C.
- Skimmed milk is DENSER than whole milk.
- Hygrometer = humidity; hydrometer = liquid density; anemometer = wind.
- Rolling friction is the smallest, static the largest.
Convert 100 °C into Fahrenheit and Kelvin.
Show answer
F = 9 × 100/5 + 32 = 180 + 32 = 212 °F. K = 100 + 273 = 373 K.
- Body temperature
- 37 °C = 98.6 °F
- Same on °C and °F
- −40
- Regulates temperature
- Thermostat
- No heat exchange
- Adiabatic
- Faster flow, lower pressure
- Bernoulli's principle
- Stokes is a unit of
- Kinematic viscosity
- Measures humidity
- Hygrometer
- Records earthquake waves
- Seismograph
- F = 9C/5 + 32; K = C + 273; −40 °C = −40 °F
- Thermostat regulates; steam burns more (latent heat)
- Water expands on freezing; densest at 4 °C
- Low pressure in hills → lower boiling point
- Upthrust = weight of liquid displaced
- Static > sliding > rolling friction
- Hygrometer humidity; anemometer wind; seismograph earthquakes