Thermodynamics & Statistical Physics

Understanding heat, temperature, energy, and the behavior of systems with many particles

Subcategories

Heat & Temperature

Understanding heat, temperature, and thermal energy transfer

Phase Changes

Understanding states of matter and transitions between them

Statistical Mechanics

Understanding macroscopic properties from microscopic particle behavior

Thermodynamic Laws

Understanding the laws of thermodynamics and energy conservation

Topics

Heat transfers through three mechanisms: conduction (direct contact between objects), convection (movement of fluids), and radiation (electromagnetic waves). Heat always flows from hotter objects to colder objects.

What is heat?

Heat is energy that transfers from one object to another due to a temperature difference. It flows from hotter objects to colder objects until thermal equilibrium is reached.

What is temperature?

Temperature is a measure of the average kinetic energy of particles in a substance. It indicates how hot or cold something is and determines the direction of heat flow—heat flows from higher temperature to lower temperature.

What is the difference between heat and temperature?

Heat is energy being transferred between objects, while temperature measures how hot or cold something is. Heat is the total energy transferred, while temperature measures the average kinetic energy of particles.

Why does metal feel cold?

Metal feels cold because it conducts heat away from your skin very efficiently. Even at room temperature, metal quickly transfers heat from your warmer hand, making it feel cold compared to materials that conduct heat poorly.

How does water boil?

Water boils when its temperature reaches 100°C (212°F) at standard pressure and heat energy is added. Water molecules gain enough energy to overcome liquid bonds and escape as vapor, forming bubbles throughout the liquid.

How does water freeze?

Water freezes when its temperature drops to 0°C (32°F) and heat energy is removed. Water molecules slow down and form a crystalline structure held together by hydrogen bonds, transitioning from liquid to solid ice.

What is evaporation?

Evaporation is the process where liquid molecules at the surface gain enough energy to escape into the gas phase. It occurs at any temperature below the boiling point and is how liquids like water turn into vapor.

Why does ice float?

Ice floats because it is less dense than liquid water. When water freezes, its molecules form a crystalline structure that takes up more space than liquid water, making ice less dense and causing it to float.

Why does water expand when it freezes?

Water expands when it freezes because water molecules form a hexagonal crystalline structure with more space between molecules than in liquid water. This open structure makes ice less dense, causing expansion.

How do heat engines work?

Heat engines convert thermal energy (heat) into mechanical work by using a working fluid that expands when heated and contracts when cooled. They operate in cycles, taking heat from a hot source, converting some to work, and rejecting waste heat to a cold sink.

What is entropy?

Entropy is a measure of disorder or randomness in a system. According to the second law of thermodynamics, entropy always increases in isolated systems, meaning systems naturally become more disordered over time.

What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. The change in a system's internal energy equals the heat added minus the work done by the system.

What is the second law of thermodynamics?

The second law of thermodynamics states that entropy (disorder) always increases in isolated systems, and heat cannot spontaneously flow from cold to hot. This law explains why processes are irreversible and why 100% efficient engines are impossible.

Why can't we have 100% efficient engines?

We can't have 100% efficient engines because the second law of thermodynamics requires that some heat must be rejected to a cold sink. Heat engines need a temperature difference to operate, and some waste heat is inevitable, making perfect efficiency impossible.