ch18-Temperature,Heat, FirstLae_Thermodynamics

Chapter 18: Temperature, Heat, and the First Law of Thermodynamics

18-1 Temperature

  • Thermodynamics: Study and application of thermal energy (internal energy) of systems.

  • Temperature: SI base quantity related to hot and cold, measured with a thermometer.

    • Thermometers: Contain a working substance with measurable properties (length or pressure) that change with temperature.

    • Measurement Scale: Temperature is measured on the Kelvin scale (units: kelvins).

The Zeroth Law of Thermodynamics

  • Two bodies are in thermal equilibrium if they have the same temperature, with no heat flow between them.

Triple Point of Water

  • The triple point of water is where solid ice, liquid water, and water vapor coexist in thermal equilibrium (not at normal atmospheric pressure).

  • Defined temperature of this mixture is 273.16 K at 4.58 mm Hg (611.2 Pa).

Constant-Volume Gas Thermometer

  • Consists of a gas-filled bulb connected to a mercury manometer, maintaining constant gas volume during readings.

  • Temperature measurement formula: T = (273.16 K) (p/p3), where p is observed pressure and p3 is pressure at triple point.

18-2 The Celsius and Fahrenheit Scales

  • Celsius Scale: Defined in relation to kelvins.

  • Fahrenheit Scale: Defined with respect to Celsius and Kelvin scales.

18-3 Thermal Expansion

  • Linear Expansion: All objects change size with temperature changes.

    • Change in linear dimension (ΔL) is proportional to temperature change (ΔT).

    • Many thermostats work on the principle of linear expansion.

  • Volume Expansion: For solids or liquids, increase in volume (ΔV) due to temperature change (ΔT) is given by:

    • ΔV = βVΔT, where β is the coefficient of volume expansion.

18-4 Absorption of Heat

Temperature and Heat

  • Heat (Q): Energy transferred due to a temperature difference, measured in units like joules (J), calories (cal), kilocalories (Cal or kcal), or British thermal units (Btu).

Absorption of Heat by Solids and Liquids

  • Heat Capacity (C): Proportionality constant between heat (Q) absorbed/lost and resulting temperature change (ΔT).

    • Formula: C = Q/ΔT where Ti and Tf are initial and final temperatures.

  • Specific Heat (c): Heat capacity per unit mass.

  • Molar Specific Heat: Specific heats expressed in moles rather than mass.

  • Heat of Transformation (L): Energy required for phase change. Total energy transferred during a phase change is Q = mL.

18-5 The First Law of Thermodynamics

Work and Heat

  • A gas can exchange energy with its surroundings through work.

  • Work done by a gas during volume change (Vi to Vf) is given by integration due to varying pressure (p) during change.

Conservation of Energy

  • The first law of thermodynamics states that energy conservation applies to thermodynamic processes:

    • ΔEint = Q - W

    • For differential changes: dEint = δQ - δW.

  • Special Cases of the First Law:

    • Adiabatic Process: Q = 0 ; ΔEint = -W

    • Constant Volume: W = 0 ; ΔEint = Q

    • Closed Cycle: ΔEint = 0 ; Q = W

    • Free Expansion: Q = W = 0 ; ΔEint = 0

18-6 Heat Transfer Mechanisms

Thermal Conduction

  • Rate of heat conduction (Pcond) through a slab with one side at higher temperature (TH) and the other at lower (TC) depends on:

    • Area (A), Thickness (L), and Thermal Conductivity (k).

Convection

  • Energy transfer within fluids due to temperature differences; observed in natural processes (e.g., atmospheric convection).

  • Example: Sea breezes result from convection.

Thermal Radiation

  • Energy transfer via electromagnetic radiation.

  • Rate of energy emission (Prad) depends on object’s surface area, temperature, and emissivity.

  • Rate of energy absorption (Pabs) depends on the environment's temperature.

Summary

Temperature and Thermometers

  • Temperature as an SI base quantity measured with thermometers.

Zeroth Law of Thermodynamics

  • Equilibrium between bodies related to a third body.

Temperature Scales

  • Kelvin, Celsius, and Fahrenheit comparisons.

Thermal Expansion

  • Linear and volume expansion described.

Heat Capacity and Specific Heat

  • Relationship between heat transfer and temperature change.

First Law of Thermodynamics

  • Conservation of energy outlined through work, heat, and internal energy changes.

Heat Transfer Applications

  • Heat transfer through conduction, convection, and radiation.