Temperature

Heat

Heat is the total internal energy of a body. It is a form of energy that flows due to a temperature difference. Heat is measured in Joules (J).

Temperature

Temperature refers to the degree of hotness or coldness of a body. The unit of temperature is either degrees Celsius (°C) or Kelvin (K).

Measurement of Temperature

Temperature is measured using thermometers. Thermometers have two reference temperatures, known as fixed points:

• Upper Fixed Point: The temperature of steam from pure boiling water at a standard atmospheric pressure of 760 mm of mercury, which is 100°C.
• Lower Fixed Point: The temperature of pure melting ice at a standard atmospheric pressure of 760 mm of mercury, which is 0°C.

Temperature Scales

The difference between the upper and lower fixed points is called the fundamental interval of a thermometer. The calibration of this interval is based on different temperature scales, including:

• Celsius Scale (°C)
• Fahrenheit Scale (°F)
• Kelvin Scale (K) or Absolute Scale

The SI unit of temperature is the Kelvin (K), but it is also commonly measured in degrees Celsius (°C) and Fahrenheit (°F).

Thermometers

Thermometers are instruments used to measure temperature. They are classified based on the thermometric properties they utilize. Each thermometer functions by detecting changes in the physical properties of its materials, which correspond to temperature changes.

Types of Thermometers

Liquid-in-Glass Thermometer

The liquid-in-glass thermometer operates based on the uniform expansion of a liquid with temperature changes. A suitable thermometric liquid should:

1. Be a good conductor of heat
2. Be clearly visible in glass
3. Have a high boiling point
4. Have a low freezing point
5. Have a low specific heat capacity
6. Not adhere to glass
7. Expand uniformly with temperature

Clinical Thermometer

This thermometer is designed for measuring body temperature and has a narrow range of 35°C to 43°C. It includes a constriction that prevents mercury from flowing back into the bulb immediately after being removed from the patient’s body.

Platinum Resistance Thermometer

This thermometer works by measuring the variation in the electrical resistance of a conductor with temperature. The relationship is expressed as:

\( \frac{R_t - R_0}{R_{100} - R_0} = \frac{t}{100} \)

• \( R_t \) : Resistance at temperature \( t°C \)
• \( R_0 \) : Resistance at \( 0°C \)
• \( R_{100} \) : Resistance at \( 100°C \)

Thermocouple

A thermocouple consists of two different metals connected in a circuit with a galvanometer. It operates based on the variation of the electromotive force (e.m.f) between junctions. The equation governing the relationship is:

\( E = a + bt + ct^2 \)

where a, b, and c are constants.

Constant-Volume Gas Thermometer

This thermometer measures temperature by observing the variation in the pressure of a gas at a constant volume. The governing equation is:

\( \frac{P_1}{T_1} = \frac{P_2}{T_2} \)

Absolute Scale of Temperature

Temperature has magnitude but no direction. The absolute scale, also called the thermodynamic scale, defines the concept of absolute zero, the lowest possible temperature, at -273 K.

At absolute zero, the molecules of a substance have no kinetic energy and remain stationary.

Celsius Scale

The Celsius scale is the most widely used temperature scale. It is based on two fixed points:

• Lower Fixed Point: 0°C (melting point of ice)
• Upper Fixed Point: 100°C (boiling point of water)

The difference between these points is known as the fundamental interval.

Heat Transfer

Conduction

Conduction is the process of heat transfer within a substance without the movement of its particles from one place to another.

When one end of a metal rod is placed in a hot liquid, heat travels through the rod, making the other end warmer.

At the heated end, molecules gain energy and vibrate more rapidly. These energized molecules transfer their energy to neighboring molecules through collisions, gradually spreading heat along the rod.

While conduction occurs in solids, liquids, and gases, metals are the most efficient conductors, whereas gases are poor conductors and act as insulators.

Important Note: Heat cannot be conducted through a vacuum.

Convection

Convection is the transfer of heat within a fluid (liquid or gas) through the actual movement of the fluid itself, carrying heat from a warmer region to a cooler one.

Natural Convection: This occurs when a fluid moves due to differences in temperature and density. For example, when water is heated on a stove, the warmer, less dense water near the base rises, while the cooler, denser water sinks, creating a convection current.

Forced Convection: In this process, a fluid is actively circulated by external forces, such as a fan or pump, to transfer heat. This can be seen in heating and cooling systems where hot or cold air is pushed into different areas.

Important Note: Convection cannot occur in a vacuum.

Radiation

Radiation is the transfer of heat energy between objects through electromagnetic waves, without requiring a material medium. This explains how the Sun's heat reaches Earth through the vacuum of space.

All surfaces emit heat energy in the form of electromagnetic waves, specifically infrared radiation, which has wavelengths slightly longer than visible light.

The amount of heat radiated by a surface depends on:

• The surface area: Larger surfaces emit more radiation.
• The absolute temperature: Hotter surfaces radiate more heat.
• The surface properties: Dark, dull surfaces are better at absorbing and emitting heat, while shiny, reflective surfaces are poorer radiators and absorbers.