Temperature and Heat: Foundations of Thermodynamics

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Temperature and Heat

Introduction to Temperature and Heat

Temperature and heat are fundamental concepts in thermodynamics, describing the energy content and energy transfer between objects. Understanding these concepts is essential for analyzing thermal phenomena in physics.

Temperature is a measure of the average kinetic energy of the particles in a substance.
Heat is the energy transferred between objects due to a temperature difference.
An object does not "contain" heat; it contains internal energy, and heat refers specifically to energy in transit.

Temperature Scales

There are three primary temperature scales used in physics: Fahrenheit, Celsius, and Kelvin. Each scale has different reference points for freezing and boiling of water.

Fahrenheit (°F): Freezing point of water is 32 °F, boiling point is 212 °F.
Celsius (°C): Freezing point of water is 0 °C, boiling point is 100 °C.
Kelvin (K): Freezing point of water is 273.15 K, boiling point is 373.15 K. Kelvin is the SI unit for temperature and starts at absolute zero.

Comparison of Fahrenheit, Celsius, and Kelvin temperature scales

Converting Between Temperature Scales

Temperature values can be converted between scales using the following equations:

From Celsius to Fahrenheit:
From Fahrenheit to Celsius:
From Celsius to Kelvin:
From Fahrenheit to Kelvin:

Graph of Celsius to Fahrenheit conversion

Absolute Zero

Absolute zero is the lowest possible temperature, at which all molecular motion ceases. It is 0 K, or -273.15 °C, or -459.67 °F. This concept is fundamental in thermodynamics and sets the baseline for the Kelvin scale.

Graph showing absolute zero as the temperature where gas pressure becomes zero

Key Thermal Terms

Heat (Q): Energy transferred due to temperature difference.
Thermal energy (internal energy): Total energy contained within a substance.
Thermal contact: Condition where heat can flow between objects.
Thermal equilibrium: State where objects in thermal contact have equal temperatures and no net heat flows between them.
Thermodynamics: The study of heat, work, and energy transfer.

The Zeroth Law of Thermodynamics

The Zeroth Law establishes the concept of temperature and thermal equilibrium. It states: If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with object B, then objects A and C are in thermal equilibrium with each other.

Illustration of the Zeroth Law of Thermodynamics with two blocks in equilibrium

Thermal Expansion

Introduction to Thermal Expansion

Most substances expand when heated due to increased molecular motion. This phenomenon is important in engineering and everyday life, affecting structures like bridges, power lines, and hot air balloons.

Linear expansion: Change in length.
Area expansion: Change in surface area.
Volume expansion: Change in volume.

Thermal Expansion Equations

Linear expansion:
Area expansion:
Volume expansion:
Where is the coefficient of linear expansion, is the coefficient of volume expansion, is original length, is original area, is original volume, and is the temperature change.

Examples of Thermal Expansion

Linear Expansion Example: The Eiffel Tower, made of iron, increases in height on hot days due to thermal expansion.
Area Expansion Example: When a washer is heated, the hole expands along with the rest of the washer, not shrinks or stays the same.

Washer with expanding hole due to thermal expansion Diagram showing area expansion of a washer and inner disk

Bimetallic Strips

Bimetallic strips are made of two metals with different coefficients of linear expansion. When heated or cooled, the strip bends due to the different rates of expansion or contraction. This principle is used in thermostats and other temperature-sensing devices.

Bimetallic strip bending due to differential expansion

Special Properties of Water

Density Anomaly of Water

Unlike most substances, water is densest at 4 °C. As water cools from 4 °C to 0 °C, it expands, which is why ice floats on liquid water. This property is crucial for aquatic life and environmental processes.

Graph showing density of water as a function of temperature Graph showing density of freshwater as a function of temperature Graph showing density anomaly of water near 4°C

Example: Ice floats because its density is lower than that of liquid water, a result of the molecular structure of ice.

Additional info: The density anomaly of water is due to hydrogen bonding, which causes water molecules to arrange in an open hexagonal structure in ice, making it less dense than liquid water.