Temperature - Measuring Hot and Cold
Temperature tells us how hot or cold something is. It affects weather forecasts, cooking, medicine, and industrial processes. Three temperature scales are used worldwide: Celsius, Fahrenheit, and Kelvin.
The Three Temperature Scales
| Scale | Symbol | Freezing point of water | Boiling point of water | Used where |
|---|---|---|---|---|
| Celsius | °C | 0 °C | 100 °C | Most of the world |
| Fahrenheit | °F | 32 °F | 212 °F | USA, some Caribbean nations |
| Kelvin | K | 273.15 K | 373.15 K | Science and engineering |
Conversion Formulas
Celsius to Fahrenheit: F = (C × 9/5) + 32
Fahrenheit to Celsius: C = (F − 32) × 5/9
Celsius to Kelvin: K = C + 273.15
Kelvin to Celsius: C = K − 273.15
Common Reference Temperatures
| Situation | Celsius | Fahrenheit |
|---|---|---|
| Absolute zero (coldest possible) | −273.15 °C | −459.67 °F |
| Water freezes | 0 °C | 32 °F |
| Room temperature | ≈ 20 °C | ≈ 68 °F |
| Body temperature | 37 °C | 98.6 °F |
| Water boils | 100 °C | 212 °F |
Worked Examples
Convert 25 °C to Fahrenheit.
F = (25 × 9/5) + 32 = 45 + 32 = 77 °F.
Convert 98.6 °F to Celsius.
C = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 333/9 ≈ 37 °C.
Convert 300 K to Celsius.
C = 300 − 273.15 = 26.85 °C.
The temperature in a city drops from 12 °C to −5 °C overnight. Find the temperature change.
Change = 12 − (−5) = 12 + 5 = 17 °C drop.
Key Takeaways
- Celsius: water freezes at 0 °C, boils at 100 °C. Used in everyday life worldwide.
- Fahrenheit: water freezes at 32 °F, boils at 212 °F. Common in the USA.
- Kelvin: starts at absolute zero (0 K = −273.15 °C). Used in science.
- C to F: multiply by 9/5 then add 32. F to C: subtract 32 then multiply by 5/9.
Practice Questions
- Convert 30 °C to Fahrenheit.
- Convert 59 °F to Celsius.
- Convert 373 K to Celsius.
- A fridge is set to 4 °C. What is this in Fahrenheit?
- The temperature rises from −8 °C to 3 °C. Find the increase.