L04 - Temp and Heat

Lecture Overview

  • Physics 1003 Lecture 4: Temperature and Heat

  • Instructor: Mr. Chan Chung Yuen (TA-in-charge) - Contact: cychandg@connect.ust.hk

Temperature and Temperature Scales

  • Definition: Temperature gauges how hot or cold an object is relative to a reference point.

  • Temperature Scales:

    • Celsius (°C): 0°C (freezing point of water) and 100°C (boiling point of water) at 1 standard atmosphere.

    • Fahrenheit (°F):

    • Kelvin (K): Absolute temperature; conversion: 𝑇𝐾 = 𝑇𝐶 + 273.15

Key Temperature Points

  • Absolute zero:

    • 0 K = -273.15 °C = -459.67 °F

  • Ice melts:

    • 273.15 K, 0 °C, 32 °F

  • Water boils:

    • 373 K, 100 °C, 212 °F

  • Average human body temperature: 309.95 K, 36.8 °C, 98.24 °F

  • Highest recorded temperature on Earth: 331 K, 58 °C, 136.4 °F

  • Lowest recorded temperature: -89.2 °C (184 K) at Vostok Station, Antarctica.

Cold Fact Summary

  • Atmospheric pressure: N2 gas condenses at 77 K and freezes at 63 K.

  • Helium condenses at 4.2 K.

  • Oxygen's boiling point: 90.188 K.

  • CO2 sublimates directly to gas above -78.5 °C.

  • Absolute Zero:

    • The ultimate cold limit; 0 K where atoms cease motion.

Ideal Gas Description

  • Ideal Gas Characteristics:

    • Inert gas; molecules interact minimally except during elastic collisions.

    • Temperature reflects the average kinetic energy of gas molecules.

Avogadro's Number

  • Avogadro’s Number: NA = 6.02 × 10²³ - number of molecules in a mole.

Kinetic Theory and Gas Behavior

  • Kinetic theory links temperature to molecular motion, defining states of matter in terms of kinetic and potential energy.

Thermal Equilibrium and Energy Transfer

  • Objects of differing temperatures reach thermal equilibrium through heat transfer.

  • Heat flows from hot to cold objects until thermal equilibrium is attained.

  • Zeroth Law of Thermodynamics:

    • If A is in equilibrium with B, and B with C, then A and C are in equilibrium.

Heat and Internal Energy

  • Internal Energy (U):

    • Comprises Kinetic Energy (KE) + Potential Energy (PE) of the system's molecules.

    • KE reflects molecular movement; PE is the result of intermolecular interaction.

Degrees of Freedom in Gases

  • Monatomic Gas: 3 translational degrees of freedom (DOF)

  • Diatomic Gas: 5 DOF (3 translational + 2 rotational)

  • Internal energy for gases:

    • Monatomic: U = (3/2)NkT

    • Diatomic: U = (5/2)NkT

Effects of Temperature Increase

  • As temperature increases:

    • Solids absorb heat (kinetic energy) and transition to liquids.

    • Liquids boil into gases.

    • Sublimation (solid to gas) occurs in certain materials.

Heat Capacity

  • Definition: The energy required to increase the temperature of a substance by 1°C per mole.

  • Varies based on material composition and internal energy distribution.

Heat Transfer Mechanisms

  • Methods of Heat Transfer:

    1. Conduction: Direct contact transfer (via electrons and lattice vibrations).

    2. Convection: Fluid particle movement transfers heat.

    3. Radiation: Emitted energy via electromagnetic radiation, requiring no medium.

Thermal Radiation

  • Objects emit thermal radiation based on their temperature.

  • Mechanism involves temperature-induced vibrations of charged particles resulting in electromagnetic waves.

Electromagnetic Radiation Basics

  • Wave Properties:

    • Frequency (f): Measured in Hertz (Hz).

    • Wavelength (λ): Measured in micrometers (µm) or nanometers (nm).

  • Relationship: c = λf (c = speed of light).

Spectra

  • Electromagnetic Spectrum:

    • Ranges from gamma rays to radio waves, with varying frequency and wavelength characteristics.

  • Black-Body Radiation:

    • Planck's theory explains energy emission in quantized form, highlighting particle-nature of light.

Particle-Wave Duality

  • Louis de Broglie: 1924, proposed matter behaves as both particle and wave, earning the Nobel Prize in Physics in 1929.

  • Energy-wavelength frequency relation: E = hf = hc/λ (h = Planck constant).

Summary Report on Heat Transfer and Internal Energy

  • Heat transfer is critical in thermodynamics, impacting physical states and energy dynamics within systems. Understanding these principles is essential for grasping larger physics concepts.