What does temperature measure?
All matter is made of tiny particles in constant motion. In a hot cup of cocoa, the particles move fast. In an ice cube, they barely jiggle. Temperature measures the average kinetic energy (energy of motion) of the particles in a substance.
- Higher temperature → faster-moving particles
- Lower temperature → slower-moving particles
Measuring temperature
A thermometer measures temperature.
| Type | How it works | Used for |
|---|---|---|
| Liquid-in-glass | Alcohol (often dyed red) expands up a thin tube when heated | Classrooms, outdoor thermometers |
| Digital | An electronic sensor changes resistance with temperature | Fever thermometers, ovens |
| Infrared | Detects infrared radiation given off by objects | Forehead thermometers, cooking |
| Bimetallic strip | Two metals that expand differently bend as temperature changes | Oven dials, older thermostats |
Old thermometers used mercury, but it’s toxic, so it’s rarely used today.
Temperature scales
| Event | Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20 | 68 | 293 |
| Body temperature | 37 | 98.6 | 310 |
| Water boils (sea level) | 100 | 212 | 373.15 |
Celsius
Created by Swedish astronomer Anders Celsius in 1742. Canada and most of the world use Celsius for weather and daily life. Each degree is 1/100 of the range between water freezing and boiling.
Fahrenheit
Created by Daniel Gabriel Fahrenheit in 1724. The United States still uses it for weather and cooking.
Converting:
- °F = (°C × 9/5) + 32
- °C = (°F − 32) × 5/9
Example: 25 °C = (25 × 9/5) + 32 = 45 + 32 = 77 °F
At −40, Celsius and Fahrenheit are equal!
Kelvin
The kelvin scale, named after Lord Kelvin, is used by scientists. It starts at absolute zero, the coldest possible temperature, where particles have the least possible motion. There are no negative kelvin temperatures.
K = °C + 273.15
A change of 1 K is the same as a change of 1 °C. Kelvin is written without a degree symbol.
Temperature extremes
| Temperature | Example |
|---|---|
| About 0.000 000 000 1 K | Coldest temperatures reached in labs |
| 2.7 K | Average temperature of deep space |
| −63 °C | Coldest temperature recorded in Canada: Snag, Yukon, 1947 |
| −89.2 °C | Coldest natural temperature recorded on Earth’s surface (Vostok, Antarctica, 1983) |
| 56.7 °C | Hottest air temperature recorded on Earth (Death Valley, 1913, though some experts question it) |
| 5500 °C | Surface of the Sun |
| About 15 million °C | Core of the Sun |
Thermal expansion
Most substances expand when heated and contract when cooled. This is thermal expansion, because faster-moving particles push farther apart.
| Example | Explanation |
|---|---|
| Liquid thermometers | The liquid expands up the tube |
| Bridge expansion joints | Gaps let the bridge expand on hot days |
| Sagging power lines | Wires expand and droop in summer |
| Loosening a jar lid with hot water | The metal lid expands more than the glass |
| Sidewalk cracks | Concrete slabs have gaps to allow for expansion |
Water is unusual
Water contracts as it cools, until 4 °C. Below that it expands, and when it freezes it expands by about 9%. That’s why:
- ice floats
- pipes can burst in winter
- lakes freeze from the top down, so fish survive below the ice
Temperature and the human body
Normal body temperature is about 37 °C. The brain’s hypothalamus acts like a thermostat:
- Too hot: you sweat, and blood vessels near the skin widen.
- Too cold: you shiver, and blood vessels near the skin narrow.
A fever (above about 38 °C) helps the body fight infections.
Common mistakes
- Confusing temperature and heat. Temperature is the average particle energy; heat is energy transferred.
- Writing "degrees kelvin." It’s just kelvin (K).
- Thinking you can go below absolute zero.
Try it yourself
- Convert today’s temperature to Fahrenheit and kelvin.
- Make a simple thermometer: fill a bottle with coloured water and rubbing alcohol, seal it with clay around a straw, and put it in warm and cold water.
- Stretch a balloon over an empty bottle’s neck. Put the bottle in hot water, then cold water. Explain what happens.
Quick recap
- Temperature measures the average kinetic energy of particles.
- Thermometers measure temperature in many ways.
- Canada uses Celsius; scientists use kelvin, which starts at absolute zero.
- Most materials expand when heated.
- Water is unusual: it expands when it freezes, so ice floats.