1. What Is Temperature?
Temperature tells how hot or cold something is. On a deeper level, it reflects how fast the particles inside a substance are moving. When particles move faster, the temperature rises; when they slow down, the temperature falls.
Two objects may contain different amounts of heat, yet they can have the same temperature. Temperature does not measure total heat—it only indicates the degree of hotness.
2. Temperature and Particle Motion
Every substance is made of tiny particles that constantly move. Heating makes these particles vibrate or move faster, which we observe as an increase in temperature. Cooling has the opposite effect.
Temperature is therefore linked directly to the average kinetic energy of particles.
2.1. Faster Motion Means Higher Temperature
When heat is supplied, particles gain energy and move more vigorously. This increased motion raises the temperature.
Example: When a metal rod is heated at one end, the particles in that region vibrate faster, causing the temperature to rise.
2.2. Slower Motion Means Lower Temperature
Cooling removes energy from particles, making them move more slowly. This reduces the temperature.
Example: When water is placed in a freezer, particle motion slows until the water eventually freezes.
3. Temperature vs Heat
Temperature and heat are related but not the same. Many situations in daily life make this difference clear.
3.1. Heat Is Energy; Temperature Is a Measure
Heat is the energy transferred between objects due to temperature difference. Temperature indicates the average kinetic energy of particles.
Heat flows from higher temperature to lower temperature until both objects reach thermal equilibrium.
3.2. Same Temperature Can Have Different Heat
Two bodies at the same temperature can contain very different amounts of heat.
Example: A bucket of warm water and a cup of warm water have the same temperature, but the bucket holds much more heat because it contains more matter.
4. Scales of Temperature
Temperature is measured using various scales, depending on the system used. The three most common scales are Celsius, Fahrenheit, and Kelvin.
4.1. Celsius Scale
- Freezing point of water: 0°C
- Boiling point of water: 100°C
This scale is widely used in everyday measurements.
4.2. Fahrenheit Scale
- Freezing point of water: 32°F
- Boiling point: 212°F
This scale has a different step size and is used in some countries for weather and body temperature readings.
4.3. Kelvin Scale
Kelvin is the scientific scale for temperature. It starts at absolute zero, the lowest possible temperature where particle motion nearly stops.
The relation is:
\( K = ^\circ C + 273.15 \)
5. Absolute Zero
Absolute zero is the lowest possible temperature at which particles have minimal energy. On the Kelvin scale, this temperature is 0 K. In Celsius, it is −273.15°C.
It is a theoretical limit because no object can be cooled to exactly absolute zero, but matter can be brought very close to it in laboratories.
6. Thermal Equilibrium and Measurement
Temperature measurement is based on the idea of thermal equilibrium. When an object touches a thermometer, heat flows until both reach the same temperature.
At that point, the thermometer reading becomes the temperature of the object.
6.1. How Thermometers Work
Different thermometers use different physical changes to measure temperature:
- Expansion of mercury or alcohol
- Change in electrical resistance
- Voltage produced by thermocouples
7. Real-Life Examples of Temperature
- Metal objects feel cooler than wood at the same temperature because they conduct heat away from the skin faster.
- A cup of tea cools down as heat flows to the surroundings until both reach the same temperature.
- A refrigerator keeps food cool by constantly removing heat from inside.