Temperature is a physical quantity that indicates the thermal state of an object or system. It is a measure of the average kinetic energy of the particles in a substance.

1. Definition of Temperature

Temperature is a scalar quantity that expresses how hot or cold an object is. It is not the same as heat, which is the energy transfer between systems due to temperature difference.

2. Scales of Temperature

1. Celsius Scale (°C)

Water freezes at 0°C and boils at 100°C (at standard atmospheric pressure).

This scale is commonly used in daily life and most scientific measurements.

2. Fahrenheit Scale (°F)

  - Water freezes at 32°F and boils at 212°F.

  - The Fahrenheit scale is primarily used in the United States.

3. Kelvin Scale (K)

  - The Kelvin scale is the SI unit of temperature.

  - It is an absolute temperature scale with its zero point at absolute zero, the point where molecular motion theoretically stops.

  - 0 K = -273.15°C

  - Water freezes at 273.15 K and boils at 373.15 K.

4. Relation Between Temperature Scales:

T(K) = T(°C) + 273.15

T(°F) = 9/5 T(°C) + 32

Read Also: Applications of elastic behaviour of materials

3. Types of Thermometers

1. Mercury-in-glass Thermometer

  - Uses mercury, which expands uniformly with temperature.

  - Commonly used for measuring room temperature or body temperature.

2. Alcohol Thermometer

  - Uses alcohol, which expands more than mercury and can measure lower temperatures.

3. Thermocouple

  - Based on the Seebeck effect, which occurs when two different metals are joined at two junctions and experience a temperature difference.

  - Commonly used in industries for measuring a wide range of temperatures.

4. Resistance Thermometer (RTD)

  - Uses the principle that the electrical resistance of a material changes with temperature.

  - More accurate and precise, often used in laboratories and industry.

5. Infrared Thermometer

  - Measures the infrared radiation emitted by an object to determine its temperature without direct contact.

  - Commonly used for remote temperature measurements.

4. Concept of Thermal Equilibrium

- Two systems are said to be in thermal equilibrium if they are at the same temperature and no heat flows between them.

- The concept is essential for understanding the Zeroth Law of Thermodynamics: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

5. Absolute Zero

- Absolute zero (0 K) is the lowest temperature possible, where all molecular motion theoretically stops. It corresponds to:

  - 0 K

  - -273.15°C

  - -459.67°F

At absolute zero, the entropy of a perfect crystal is zero, and the system contains no thermal energy.

6. Measurement of Temperature in Various Fields

1. Meteorology:

  - Temperature is measured using thermometers placed in meteorological stations, often shielded from direct sunlight to avoid measurement errors.

2. Body Temperature:

  - Measured using clinical thermometers, typically in Celsius or Fahrenheit. Normal body temperature is around 37°C (98.6°F).

3. Astronomy:

  - In space, temperature is measured using infrared telescopes and radiometers to observe celestial bodies' thermal radiation.

7. Important Concepts Related to Temperature Measurement

1. Heat vs. Temperature:

  - Heat is the energy transferred due to a temperature difference.

  - Temperature is a measure of the average kinetic energy of particles in a substance.

2. Thermal Expansion:

  - Most materials expand when heated and contract when cooled. This principle is used in devices like bimetallic strips and liquid-in-glass thermometers.

3. Specific Heat Capacity:

  - The amount of heat required to raise the temperature of a unit mass of a substance by one degree.

4. Thermal Conductivity:

  - A measure of a material's ability to conduct heat. Materials with high thermal conductivity, like metals, transfer heat efficiently.

8. Units of Temperature

1. Kelvin (K) – SI unit of temperature.

2. Celsius (°C) – Used in most scientific and everyday contexts.

3. Fahrenheit (°F) – Primarily used in the U.S. for non-scientific applications.

4. Rankine (°R) – Absolute temperature scale in Fahrenheit, mainly used in thermodynamic calculations in engineering.

9. Instruments and Methods for Advanced Temperature Measurement

1. Pyrometers:

  - Measures high temperatures by detecting thermal radiation, often used in furnaces or for molten metals.

2. Platinum Resistance Thermometer:

  - High-precision temperature measurement over a wide range, used in calibrating other thermometers.

These concepts cover the basics of temperature measurement in physics, explaining the different scales, types of thermometers, and practical applications.