Section 2: Concepts of Energy and Entropy

Heat Transfer

The movement of thermal energy from a warmer object or region to a cooler one.

Thermal Energy

The internal energy of a system due to the motion of its particles.

Thermal Equilibrium

The state in which two objects in contact reach the same temperature and no net heat transfer occurs.

Conduction

The transfer of heat through direct contact between particles, typically in solids.

Mechanism of Conduction

Faster-moving (hotter) particles collide with slower-moving (cooler) particles, transferring energy.

Thermal Conductivity (k)

A material property that measures how well heat flows through a material.

Good Thermal Conductor

A material, such as metal, that allows heat to flow quickly.

Thermal Insulator

A material, such as wood, rubber, or Styrofoam, that slows down heat transfer.

Factors Affecting Conduction Rate

Temperature difference, material thickness, surface area, and thermal conductivity.

Temperature Gradient

The difference in temperature between two regions that drives heat flow.

Conduction Example

Heat moving from a stove burner through a metal pan to food.

Convection

The transfer of heat through the movement of fluids (liquids or gases).

Mechanism of Convection

Warm, less dense fluid rises while cool, denser fluid sinks, creating a circulating current.

Natural Convection

Heat transfer caused by temperature and density differences without external assistance.

Forced Convection

Heat transfer caused by external forces such as fans, pumps, or wind.

Convection Current

The continuous cycle of rising warm fluid and sinking cool fluid.

Convection Example

Boiling water circulating in a pot or warm air rising in a heated room.

Atmospheric Convection

The movement of warm and cool air that drives weather patterns.

Radiation

The transfer of heat through electromagnetic waves, without the need for matter.

Thermal Radiation

Heat energy emitted mainly as infrared radiation.

Electromagnetic Waves

Energy waves that can travel through empty space, including infrared, visible light, and ultraviolet waves.

Radiation from the Sun

Heat reaching Earth through empty space via electromagnetic waves.

Emissivity

A measure of how efficiently a surface emits and absorbs thermal radiation.

High Emissivity Surface

A dark, matte surface that absorbs and emits heat well.

Low Emissivity Surface

A shiny, reflective surface that reflects heat and emits less radiation.

Radiation Example

Feeling warmth from a campfire without touching it.

Conduction vs. Convection

Conduction transfers heat through direct contact; convection transfers heat through fluid motion.

Convection vs. Radiation

Convection requires a fluid; radiation does not require matter.

Conduction vs. Radiation

Conduction requires contact; radiation travels through electromagnetic waves.

Thermodynamics

The branch of physics that studies heat, temperature, work, energy, and how energy moves within systems and their surroundings.

System

The part of the universe being studied in thermodynamics.

Surroundings

Everything outside the system that can exchange energy with it.

Boundary

The real or imaginary line that separates a system from its surroundings.

Heat (Q)

Energy transferred between objects or systems due to a temperature difference, flowing from hot to cold.

Work (W)

Energy transferred when a force acts through a distance, such as gas expanding or compressing.

Internal Energy (U)

The total microscopic kinetic and potential energy of particles within a system.

First Law of Thermodynamics

Energy cannot be created or destroyed; the change in internal energy equals heat added to the system minus work done by the system.

First Law Equation

Δ𝑈=𝑄−𝑊

Heat Added to a System

Increases the system’s internal energy if no work is done.

Work Done by a System

Decreases the system’s internal energy.

Closed System

A system that can exchange energy but not matter with its surroundings.

Pressure-Volume (P-V) Diagram

A graph that shows how pressure and volume change in a thermodynamic process; area under the curve represents work.

Isobaric Process

A process in which pressure remains constant while volume changes.

Isochoric Process

A process in which volume remains constant and no work is done.

Isothermal Process

A process that occurs at constant temperature.

Adiabatic Process

A process in which no heat is exchanged with the surroundings.

Work in Thermodynamics

Represented by the area under the curve on a P-V diagram.

Heat Engine

A device that converts thermal energy into mechanical work by transferring heat from a hot reservoir to a cold reservoir.

Hot Reservoir

The heat source that supplies energy to a heat engine.

Cold Reservoir

The heat sink that absorbs unused heat from a heat engine.

Heat Engine Energy Relationship

𝑊=𝑄ℎ−𝑄𝑐

Second Law of Thermodynamics

Heat transfers spontaneously from hot objects to cold objects, and total entropy in an isolated system never decreases.

Spontaneous Process

A process that occurs naturally without external energy input.

Irreversible Process

A process that cannot return to its original state without external energy and increases entropy.

Reversible Process

An ideal process that can be reversed without increasing entropy.

Entropy (S)

A measure of disorder or randomness in a system.

Entropy and Time

Entropy increase gives direction to time, known as the “arrow of time.”

Entropy in an Isolated System

Never decreases over time.

Useful Energy

Energy that can be converted into work.

Waste Energy

Energy that spreads into the surroundings and cannot do useful work.

Efficiency

The ratio of useful work output to heat input from the hot reservoir.

Efficiency Formula

Efficiency=𝑊𝑄ℎ

Efficiency and the Second Law

No heat engine can be 100% efficient; some energy is always lost as waste heat.

Refrigerator

A device that removes heat from its interior and transfers it to the surroundings.

Heat Pump

A device that moves heat and can provide both heating and cooling.

Living Organisms and Entropy

Living systems maintain local order by increasing entropy in their surroundings.

Work (Physics)

Energy transfer that occurs when a force causes an object to move in the direction of that force.

Work Formula

Work equals force multiplied by displacement in the direction of the force: 𝑊=𝐹𝑑

Joule (J)

The SI unit of work and energy; one joule equals one newton-meter.

When Work Is Done

Work is done only when a force causes displacement in the direction of the force.

Zero Work

Occurs when there is no movement, no net force, or the force is perpendicular to motion.

Positive Work

Work done when force and motion are in the same direction, adding energy to the system.

Negative Work

Work done when force and motion are in opposite directions, removing energy from the system.

Energy

The ability to do work or cause change.

Kinetic Energy (KE)

The energy an object has due to its motion.

Kinetic Energy Formula

𝐾𝐸=1/2𝑚𝑣^2

Mass and KE Relationship

Increasing mass increases kinetic energy.

Velocity and KE Relationship

Doubling velocity quadruples kinetic energy.

Potential Energy (PE)

Stored energy due to position or configuration.

Gravitational Potential Energy

Energy stored due to an object’s height in a gravitational field.

Gravitational PE Formula

𝑃𝐸=𝑚𝑔ℎ

Height and PE Relationship

Increasing height increases gravitational potential energy.

Elastic Potential Energy

Energy stored when an elastic object is stretched or compressed.

Elastic PE Formula

𝑃𝐸𝑠=1/2𝑘𝑥^2

Hooke’s Law

The force exerted by a spring is proportional to its displacement:

𝐹=𝑘𝑥

Spring Constant (k)

A measure of a spring’s stiffness.

Power

The rate at which work is done or energy is transferred.

Power Formula

𝑃=𝑊𝑡

Watt (W)

The SI unit of power; one watt equals one joule per second.

Conservative System

A system where energy stays within the system and is not lost.

Nonconservative Forces

Forces that remove mechanical energy from a system, such as friction or air resistance.

Law of Conservation of Energy

Energy cannot be created or destroyed; it only changes form.

Isolated System

A system where no energy enters or leaves.

Energy Transformation

The process of energy changing from one form to another, such as potential to kinetic.

KE and PE Relationship

As kinetic energy increases, potential energy decreases, and vice versa, in a closed system.

Pendulum Energy

Maximum PE at highest point, maximum KE at lowest point.

Roller Coaster Energy

PE at the top of the hill converts to KE as it descends.