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.