Temperature, Heat, and the First Law of Thermodynamics Chapters 19, 20 Temperature, Heat, and the First Law of Thermodynamics

Temperature Thermodynamics – branch of physics studying thermal energy of systems Temperature (T), a scalar – measure of the thermal (internal) energy of a system SI unit: K (Kelvin) Kelvin scale has a lower limit (absolute zero) and has no upper limit William Thomson (Lord Kelvin) (1824 - 1907)

Kelvin scale Kelvin scale is defined by the temperature of the triple point of pure water Triple point – set of pressure and temperature values at which solid, liquid, and gas phases can coexist International convention: T of the triple point of water is

The zeroth law of thermodynamics If two (or more) bodies in contact don’t change their internal energy with time, they are in thermal equilibrium 0th law of thermodynamics: if bodies are in thermal equilibrium, their temperatures are equal

Measuring temperature Temperature measurement principle: if bodies A and B are each in thermal equilibrium with a third body C, then A and B are in thermal equilibrium with each other (and their temperatures are equal) The standard temperature for the Kelvin scale is measured by the constant-volume gas thermometer

Constant-volume gas thermometer

Celsius and Fahrenheit scales Celsius scale: Fahrenheit scale: Anders Cornelius Celsius (1701 - 1744) Gabriel Daniel Fahrenheit (1686 - 1736)

Thermal expansion Thermal expansion: increase in size with an increase of a temperature Linear expansion: Volume expansion:

Thermal expansion

Chapter 19 Problem 5 A copper telephone wire has essentially no sag between poles 35.0 m apart on a winter day when the temperature is – 20.0°C. How much longer is the wire on a summer day when T = 35.0°C?

Temperature and heat Heat (Q): energy transferred between a system and its environment because of a temperature difference that exists between them SI Unit: Joule Alternative unit: calorie (cal):

Q Q Absorption of heat Specific heat (c): heat capacity per unit mass Common states (phases) of matter: solid, liquid, gas Latenet heat (L): the amount of energy per unit mass transferred during a phase change (boiling, condensation, melting, freezing, etc.) Q Q

Absorption of heat Q Q

Absorption of heat

Absorption of heat

Chapter 20 Problem 17 A 1.00-kg block of copper at 20.0°C is dropped into a large vessel of liquid nitrogen at 77.3 K. How many kilograms of nitrogen boil away by the time the copper reaches 77.3 K? (The specific heat of copper is 0.0920 cal/g °C. The latent heat of vaporization of nitrogen is 48.0 cal/g.)

Avogadro’s number Mole – amount of substance containing a number of atoms (molecules) equal to the number of atoms in a 12 g sample of 12C This number is known as Avogadro’s number (NA): NA = 6.02 x 1023 mol -1 The number of moles in a sample N – total number of atoms (molecules) m – total mass of a sample, m0 – mass of a single atom (molecule); M – molar mass Amedeo Avogadro (1776 -1856)

Ideal gases Ideal gas – a gas obeying the ideal gas law: R – gas constant R = 8.31 J/mol ∙ K kB – Boltzmann constant kB = 1.38 x 1023 J/K Ludwig Eduard Boltzmann (1844-1906)

Heat and work Thermodynamic cycle

Heat and work Work is done by the system: Work is done on the system :

The first law of thermodynamics Work and heat are path-dependent quantities Quantity Q + W = ΔEint (change of internal energy) is path-independent 1st law of thermodynamics: the internal energy of a system increases if heat is added to the system or work is done on the system

The first law of thermodynamics Adiabatic process: no heat transfer between the system and the environment Isochoric (constant volume) process Free expansion: Cyclical process:

Chapter 20 Problem 29 Consider the cyclic process depicted in the figure. If Q is negative for the process BC and ΔEint is negative for the process CA, what are the signs of Q, W, and that are associated with each process?

Heat transfer mechanisms Thermal conduction Conduction rate: Thermal resistance: Conduction through a composite rod: Thermal conductivity

Heat transfer mechanisms Thermal radiation Radiation rate: Stefan-Boltzmann constant: Absorption rate: Emissivity Josef Stefan (1835-1893)

Chapter 20 Problem 46 At high noon, the Sun delivers 1000 W to each square meter of a blacktop road. If the hot asphalt loses energy only by radiation, what is its steady-state temperature?

Heat transfer mechanisms Convection

Heat transfer mechanisms

Questions?

Answers to the even-numbered problems Chapter 19 Problem 2 810°F (b) 450 K

Answers to the even-numbered problems Chapter 19 Problem 6 1.20 cm

Answers to the even-numbered problems Chapter 19 Problem 18 (a) 2.99 mol (b) 1.80 × 1024 molecules

Answers to the even-numbered problems Chapter 20 Problem 26 (a) 12.0 kJ (b) –12.0 kJ