11.0     INTRODUCTION

CONCEPT OF TEMPERATURE: Temperature is a measure of the average kinetic energy of the particles in a substance. It indicates the hotness or coldness of an object.

TEMPERATURE UNITS: Commonly measured in degrees Celsius (°C), Kelvin (K), and Fahrenheit (°F).

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          11.1     THERMOMETRIC AND EFFECTS

THERMOMETRIC PROPERTIES

• Thermal Expansion: Many substances expand when heated and contract when cooled. This property is utilized in liquid-in-glass thermometers.
• Electrical Resistance: Some materials change their electrical resistance with temperature, which is used in resistance temperature detectors (RTDs).

THERMOELECTRIC EFFECT: The Seebeck effect is used in thermocouples, where a voltage is generated in response to a temperature difference.

          11.2     CALIBRATION OF THERMOMETERS

Calibration of thermometers is the process of adjusting the thermometer to ensure accurate readings.

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METHODS OF CALIBRATION OF THERMOMETERS 

• By using fixed points (like the freezing and boiling points of water) to set the scale.
• Standard Reference Points: For Celsius, 0°C is the freezing point of water, and 100°C is the boiling point at standard atmospheric pressure.

1.     TEMPERATURE SCALE

TYPES OF TEMPERATURE SCALES

• Celsius Scale: Based on the freezing (0°C) and boiling (100°C) points of water. It is widely used in everyday applications.
• Kelvin Scale: Absolute temperature scale starting at absolute zero (0 K), where all molecular motion stops. The relationship is between Celsius scale and Kelvin scale is given by: K = °C + 273.
• Fahrenheit Scale: The Fahrenheit scale measures temperature; water freezes at 32°F and boils at 212°F under standard conditions, mainly used in the US.

CONVERSION BETWEEN TEMPERATURE SCALES

• Celsius to Kelvin: K = °C + 273
• Kelvin to Celsius: °C = K – 273
• Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• Fahrenheit to Celsius: °C = (°F – 32) × 5/9

11.4     TYPES OF THERMOMETERS

• Liquid-in-Glass Thermometers: Use liquids (like mercury or alcohol) that expand and contract with temperature changes.
• Digital Thermometers: Use electronic sensors to measure temperature and display it digitally.
• Infrared Thermometers: Measure the infrared radiation emitted by an object to determine its temperature without contact.
• Thermocouples: Use the voltage generated at the junction of two different metals to measure temperature.

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1.     THERMAL EXPANSION

Thermal expansion refers to the increase in size (length, area, or volume) of a material when it is heated. This occurs because the particles in the material move more vigorously and tend to occupy more space.

LINEAR EXPANSIVITY: Linear expansion is the change in length of a solid when its temperature changes.

Linear expansivity of a substance is defined as the fractional increase in length per unit length per degree rise in temperature.

Linear Expansivity is given by:

ΔL = α * L₀ * ΔT

Where       ΔL is the change in length,

α is the coefficient of linear expansion,

L₀ is the original length, and

ΔT is the change in temperature.

AREA EXPANSIVITY: Area expansion is the change in area of a solid when its temperature changes.

It is defined as the fractional increase in area per unit area per degree rise in temperature.

Area expansivity is given by:

ΔA = 2α * A₀ * ΔT

Where       ΔA is the change in area,

A₀ is the original area.

CUBIC EXPANSIVITY: Cubic expansivity is the change in volume of a solid or liquid when its temperature changes.

It is defined as the fractional increase in volume per unit volume per degree rise in temperature.

Cubic Expansivity is given by:

ΔV = β * V₀ * ΔT

Where                 ΔV is the change in volume,

β is the coefficient of volume expansion,

V₀ is the original volume.

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RELATIONSHIP BETWEEN DIFFERENT EXPANSIVITIES

The relationship between these expansivities is based on the number of dimensions involved:

• Area expansivity is twice linear expansivity i.e.,    β=2a
• Cubic expansivity is three times area expansivity   i.e.,    Y=3a

EFFECTS AND APPLICATIONS OF THERMAL EXPANSION

• Building Strips: In construction, materials expand and contract with temperature changes. Expansion joints are used in buildings and bridges to accommodate this movement.
• Railway Lines: Gaps are left between railway tracks to allow for expansion during hot weather, preventing buckling.

1.     THERMAL EXPANSION IN LIQUIDS

VOLUME EXPANSIVITY: Liquids expand in volume when heated, and this is characterized by the coefficient of volume expansion (β).

REAL AND APPARENT EXPANSIVITIES: Real expansivity refers to the actual change in volume, while apparent expansivity considers the effects of temperature changes in the container as well.

DETERMINATION OF VOLUME EXPANSIVITY: Volume expansivity can be determined experimentally by measuring the change in volume of a liquid with temperature.

1.     ANOMALOUS EXPANSION OF WATER

Water exhibits anomalous expansion, meaning it expands when cooled from 4°C to 0°C. This is why ice floats on water.

          11.8     MODES OF HEAT TRANSFER 

• Conduction: The transfer of heat through a material without the movement of the material itself. Metals are good conductors.
• Convection: The transfer of heat by the movement of fluids (liquids and gases). Hot fluid rises while cooler fluid sinks, creating a convection current.
• Land and Sea Breeze: Land breezes occur at night when land cools faster than water, causing cooler air to flow from land to sea.

Sea breezes occur during the day when land heats up faster than water, causing cooler air to flow from sea to land.

• Radiation: The transfer of heat through electromagnetic waves. All objects emit radiation, and this is how heat can be transferred through a vacuum.

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11.9     CONCEPT OF THERMAL CONDUCTIVITY

Thermal Conductivity (k) is defined as a measure of a material’s ability to conduct heat. It is defined as the amount of heat that passes through a unit area of the material in a unit time for a temperature difference.

It is given by the formula:

CONCEPT OF HEAT FLUX (q) – The rate of heat transfer per unit area, expressed as q = Q/A. It is influenced by the thermal conductivity and the temperature gradient.

          11.10     EFFECTS OF THE NATURE OF THE SURFACE ON ENERGY RADIATED AND ABSORBED

• The nature of a surface affects its emissivity, which is the ability to emit and absorb thermal radiation.
• Darker and rougher surfaces tend to absorb and emit more energy than lighter and smoother surfaces.
• Black bodies are ideal emitters and absorbers of radiation, while shiny surfaces reflect more energy.

11.11     CONDUCTIVITIES OF COMMON MATERIALS

• Metals (e.g., copper, aluminum) have high thermal conductivity.
• Wood and plastics have moderate thermal conductivity.

Insulating materials (e.g., glass wool, Styrofoam) have low thermal conductivity.

CONCEPT OF THERMOS FLASK

A thermos flask is designed to keep liquids hot or cold by minimizing heat transfer.

It consists of two walls with a vacuum in between, which reduces conduction and convection. The inner surface is often reflective to minimize heat loss through radiation.