thermal physics

Chapter 2: States of Matter

Solid

  • Particles are arranged in a regular pattern.

  • Particles are very close together and touching.

  • They can only vibrate at fixed positions.

  • Very strong intermolecular forces hold the particles together.

  • Have a definite shape and volume.

  • Not easily compressible.

Liquid

  • Particles do not follow a regular pattern.

  • They are close together but can move and slide past each other.

  • Strong intermolecular forces keep them close.

  • No definite shape but have a definite volume.

  • Not easily compressible.

Gas

  • Particles are far apart and exhibit no regular pattern.

  • Move randomly at high speeds.

  • Weak intermolecular forces exist between particles.

  • No definite shape and no fixed volume.

  • Easily compressible.

Internal Energy

  • Internal energy includes both kinetic energy (KE) and potential energy (PE) of molecules:

    • Kinetic Energy (KE):

    • Depends on temperature; as temperature increases, KE of molecules increases.

    • Potential Energy (PE):

    • Depends on the separation of molecules; as molecules separate, PE increases.

Changes in States of Matter

Melting

  • Occurs at a constant temperature (melting point).

  • Ice absorbs latent heat.

  • Intermolecular forces are broken, separating molecules, which increases PE while KE remains constant.

Boiling

  • Occurs at boiling point under specific conditions.

  • Energy is absorbed to break intermolecular forces, separating molecules.

  • Average KE remains constant, as does temperature.

Evaporation vs Boiling

  • Boiling:

    • Takes place at the boiling point.

    • Average KE remains constant, and temperature stays constant until all liquid is converted to gas.

  • Evaporation:

    • Takes place at the liquid's surface.

    • Mostly energetic molecules escape into the air, resulting in cooler remaining liquid.

Factors Affecting Evaporation

  1. Temperature: Higher temperature increases evaporation rate.

  2. Surface Area: Larger surface area increases evaporation.

  3. Wind Speed: Removing water vapor allows more to escape.

  4. Humidity: Lower humidity increases the rate of evaporation.

Thermal Energy Transfer

Cooling

  • Lowering temperature results in lower thermal energy and KE of molecules, decreasing internal energy.

Heating

  • Increasing temperature results in increased thermal energy and KE, increasing internal energy.

Kinetic Theory

  • Brownian Motion:

    • Random motion of small particles suspended in liquids or gases due to collisions with air particles.

  • Matter consists of tiny particles in constant motion.

  • Absolute zero is 0 Kelvin, equivalent to -273°C.

Pressure of Gas

  • Defined as force per unit area arising from the collisions of gas molecules with surfaces.

  • The relationship PV = k shows that volume decrease leads to increased pressure due to more frequent collisions.

  • Increasing temperature raises pressure as molecular speed increases.

Thermal Expansion of Solids, Liquids, and Gases

  • Heat transfers kinetic energy, causing particles to move faster and expand:

    • Examples include bimetallic strips, railway lines, thermometers, and car tires.

Specific Heat Capacity

  • Defined as the thermal energy required to change the temperature of a unit mass by one degree Celsius or Kelvin:

    • Formula: E = mcΔT

Modes of Thermal Energy Transfer

Conduction

  • Thermal energy is transferred through vibration and collision of atoms in solids.

  • Metals are good conductors due to free-moving electrons.

Convection

  • Transfer of thermal energy through movement of fluids based on density differences:

    • Warm water rises, cool water sinks, creating convection currents.

Radiation

  • Transfer of thermal energy via electromagnetic waves, primarily infrared radiation.

  • Factors affecting heat emission:

    • Temperature of the surface: hotter surfaces emit more heat.

    • Surface area: larger surface areas emit more heat.

Greenhouse Effect

  • Solar radiation heats the Earth, some is absorbed and re-emitted at longer wavelengths.

  • Greenhouse gases trap some thermal radiation, preventing its escape into space.

Consequences of Thermal Energy Transfer

  • Air Conditioner:

    • Warm air rises, cooler air sinks; positioned strategically for effective cooling.

  • Sea Breezes:

    • Land heats up faster than water, creating temperature differences that generate breezes.

  • Land Breezes:

    • At night, air above land cools faster than air above water, causing breezes in the opposite direction.

Important Equations

  • PV = k

  • P1V1 = P2V2

  • E = mcΔT