What is Temperature?
Temperature is the measure of how hot or cold a body is. It tells us the thermal condition of a body and helps us compare whether one object is hotter or colder than another.
For example, hot tea has a higher temperature than cold water. Ice has a lower temperature than boiling water.
Temperature is closely related to the motion of particles in matter. When the particles of a substance move faster, its temperature increases. When the particles move slower, its temperature decreases.
Temperature and Heat
Temperature and heat are related, but they are not the same.
Temperature tells how hot or cold a body is.
Heat is energy that flows from a hotter body to a colder body because of temperature difference.
For example, when a hot spoon is placed in cold water, heat flows from the spoon to the water. The spoon cools down and the water becomes warmer.
So, temperature decides the direction of heat flow.
Heat always flows from a body at higher temperature to a body at lower temperature.
Measurement of Temperature
Temperature is measured using an instrument called a thermometer.
A thermometer works because some physical property of a substance changes when temperature changes.
For example:
- A liquid expands when heated.
- A gas changes pressure when heated.
- The electrical resistance of a material may change with temperature.
These changes can be used to measure temperature.
Temperature Scales
Temperature can be measured using different scales.
The common temperature scales are:
- Celsius scale
- Fahrenheit scale
- Kelvin scale
Celsius Scale
The Celsius scale is based on two fixed points.
- The freezing point of water is taken as 0°C.
- The boiling point of water is taken as 100°C.
The interval between these two points is divided into 100 equal parts.
Each part is called one degree Celsius.
Fahrenheit Scale
In the Fahrenheit scale, the freezing point of water is 32°F.
The boiling point of water is 212°F.
This scale is commonly used in some countries for measuring body temperature and weather temperature.
Kelvin Scale
The Kelvin scale is the scientific scale of temperature.
The SI unit of temperature is kelvin, written as K.
On the Kelvin scale, the lowest possible temperature is called absolute zero.
Absolute zero is 0 K, which is equal to −273.15°C.
At absolute zero, the motion of particles is minimum.
The Kelvin scale is important in physics, especially in gas laws and thermodynamics.
Conversion Between Temperature Scales
Temperature can be measured using different scales such as Celsius, Fahrenheit, and Kelvin.
Sometimes, the same temperature has to be expressed in another scale. For example, a temperature given in Celsius may have to be converted into Kelvin for scientific calculations.
Temperature conversion formulae help us change a temperature value from one scale to another.
Celsius to Kelvin
To convert Celsius into Kelvin, add 273.
K = °C + 273
For example:
25°C = 25 + 273 = 298 K
So, 25°C = 298 K
Kelvin to Celsius
To convert Kelvin into Celsius, subtract 273.
°C = K − 273
For example:
300 K = 300 − 273 = 27°C
So, 300 K = 27°C
Celsius to Fahrenheit
To convert Celsius into Fahrenheit, use the formula:
°F = (9/5 × °C) + 32
For example:
20°C = (9/5 × 20) + 32
20°C = 36 + 32 = 68°F
So, 20°C = 68°F
Fahrenheit to Celsius
To convert Fahrenheit into Celsius, use the formula:
°C = 5/9 × (°F − 32)
For example:
68°F = 5/9 × (68 − 32)
68°F = 5/9 × 36 = 20°C
So, 68°F = 20°C
Kelvin to Fahrenheit
To convert Kelvin to Fahrenheit, first convert Kelvin to Celsius, and then convert Celsius to Fahrenheit.
Fahrenheit to Kelvin
To convert Fahrenheit to Kelvin, first convert Fahrenheit to Celsius, and then convert Celsius to Kelvin.
Absolute Zero
Absolute zero is the lowest possible temperature.
It is 0 K or −273.15°C.
At this temperature, particles have minimum possible motion.
Absolute zero cannot be reached exactly in practice, but scientists can reach temperatures very close to it.
Importance of Temperature
Temperature is important in many areas of life and science. It helps us understand:
- Weather conditions
- Body temperature
- Cooking and boiling
- Melting and freezing
- Expansion of substances
- Heat transfer
- Gas laws
- Thermodynamics
Temperature is one of the most basic and important quantities in Thermal Physics.
Glossary of Key Terms
Recap of the Key Terms in Temperature
- Absolute Zero: The lowest possible temperature, equal to 0 K or −273.15°C.
- Boiling Point: The temperature at which a liquid changes into a gas.
- Celsius Scale: A temperature scale in which water freezes at 0°C and boils at 100°C.
- Fahrenheit Scale: A temperature scale in which water freezes at 32°F and boils at 212°F.
- Freezing Point: The temperature at which a liquid changes into a solid.
- Heat: Energy transferred from a hotter body to a colder body due to temperature difference.
- Kelvin Scale: The scientific temperature scale used in physics.
- Kelvin: The SI unit of temperature.
- Temperature: A measure of how hot or cold a body is.
- Thermometer: An instrument used to measure temperature.
Quiz
Recap the concepts you have learnt. Try to answer the questions. You can find the answer to any question by clicking on the icon.
What is temperature?
Temperature is the measure of how hot or cold a body is.
What instrument is used to measure temperature?
A thermometer is used to measure temperature.
What is the SI unit of temperature?
The SI unit of temperature is kelvin.
Name three temperature scales.
The three common temperature scales are Celsius, Fahrenheit, and Kelvin.
What is the freezing point of water on the Celsius scale?
The freezing point of water is 0°C.
What is the boiling point of water on the Celsius scale?
The boiling point of water is 100°C.
What is absolute zero?
Absolute zero is the lowest possible temperature, equal to 0 K or −273.15°C.
How are Celsius and Kelvin related?
Kelvin = Celsius + 273.
What is the difference between heat and temperature?
Temperature tells how hot or cold a body is. Heat is energy transferred from a hotter body to a colder body.
Why is temperature important in heat transfer?
Temperature difference decides the direction of heat flow.