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

A heat pump works by using a refrigerant cycle to transfer heat from a cooler area to a warmer area, opposite to natural heat flow. It compresses refrigerant to heat it, then expands it to cool it, moving heat against the temperature gradient using mechanical work.

How does heat transfer work?

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 absolute zero?

Absolute zero is the theoretical lowest possible temperature, at which particles have minimal motion and no thermal energy can be removed. It's defined as 0 Kelvin (-273.15°C or -459.67°F) and represents the point where entropy reaches its minimum value.

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.

What is thermal conductivity?

Thermal conductivity is a material property that measures how well a material conducts heat. Materials with high thermal conductivity (like metals) transfer heat quickly, while materials with low thermal conductivity (like wood or insulation) transfer heat slowly, making them good insulators.

Why do different materials have different specific heat?

Different materials have different specific heat because they have different molecular structures and bonding. Materials with stronger bonds and more complex molecular structures require more energy to increase temperature, giving them higher specific heat. Water has unusually high specific heat due to hydrogen bonding.

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.

Why does pressure affect boiling point?

Pressure affects boiling point because boiling occurs when vapor pressure equals atmospheric pressure. Higher pressure requires higher temperature to reach the vapor pressure needed for boiling, so water boils at higher temperatures under high pressure and lower temperatures under low pressure.

How does condensation work?

Condensation occurs when water vapor in the air loses energy and transitions from gas to liquid. This happens when air cools below its dew point or when vapor comes into contact with a cold surface, causing water molecules to slow down and form liquid droplets.

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.

What is sublimation?

Sublimation is the phase change where a substance transitions directly from solid to gas without passing through the liquid phase. This occurs when the substance's vapor pressure exceeds atmospheric pressure at temperatures below its melting point, allowing solid particles to escape directly into the gas phase.

What is the triple point?

The triple point is the unique temperature and pressure condition where a substance can exist simultaneously in solid, liquid, and gas phases in equilibrium. At the triple point, all three phases coexist, and changing conditions slightly causes one phase to disappear.

Why does dry ice sublimate?

Dry ice sublimes because carbon dioxide's triple point is above atmospheric pressure. At normal atmospheric pressure, solid CO₂ cannot melt to liquid—it can only transition directly from solid to gas (sublimation) or from gas to solid (deposition), skipping the liquid phase entirely.

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 boil at different temperatures?

Water boils at different temperatures because boiling point depends on atmospheric pressure. Higher pressure requires higher temperature to reach the vapor pressure needed for boiling, so water boils at higher temperatures under high pressure and lower temperatures under low pressure (like at high altitude).

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.

How does a heat engine work?

A heat engine works by converting thermal energy into mechanical work through a cyclic process. It absorbs heat from a hot reservoir, converts some of it to work, and rejects waste heat to a cold reservoir. The efficiency is limited by the temperature difference between hot and cold reservoirs.

How does entropy increase?

Entropy increases through irreversible processes that create disorder. When energy spreads out, systems become more disordered, or heat flows from hot to cold, entropy increases. The second law of thermodynamics states that entropy of an isolated system always increases or stays constant, never decreases.

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 Carnot cycle?

The Carnot cycle is an idealized thermodynamic cycle that represents the maximum possible efficiency for a heat engine operating between two temperature reservoirs. It consists of four reversible processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression.

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.

What is the third law of thermodynamics?

The third law of thermodynamics states that as temperature approaches absolute zero, the entropy of a perfect crystal approaches zero. This law explains why absolute zero cannot be reached and establishes absolute zero as the reference point for entropy calculations.

Why can't we create a perpetual motion machine?

We can't create a perpetual motion machine because it would violate the laws of thermodynamics. The first law (energy conservation) prevents machines that create energy, and the second law (entropy increase) prevents machines that convert heat completely to work without energy input. Friction and other losses also ensure energy is always lost.

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.