Temperature and Heat Thermal Basics

Does a glass of water sitting on a table have any energy?

Thermal Physics-1 II Fluids and thermodynamics B. Temperature and heat AP Physics 2 Thermal Physics-1 AP Physics 2 Objectives II Fluids and thermodynamics B. Temperature and heat Mechanical equivalent of heat Heat transfer and thermal expansion C. Kinetic theory and thermodynamics Ideal gases Kinetic model Ideal gas law Laws of thermodynamics First law (including processes on PV diagrams) Second law (including heat engines)

Heat vs Temperature Heat: Transmission of energy from one body to another due to a temperature difference (hot to cold) – unit is Calorie or Joule Is there another way to increase the energy of an object? Temperature: physical property of matter that quantitatively expresses the common notions of hot and cold. you can do work on the object - The temperature varies with the microscopic speed of the fundamental particles that it contains (or their kinetic energy).

Heat, Thermal Energy and Internal Energy-words of caution! “To describe the energy that a high temperature object has, it is not a correct use of the word heat to say that the object "possesses heat" - it is better to say that it possesses internal energy as a result of its molecular motion. The word heat is better reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one. You can take an object at low internal energy and raise it to higher internal energy by heating it. But you can also increase its internal energy by doing work on it, and since the internal energy of a high temperature object resides in random motion of the molecules, you can't tell which mechanism was used to give it that energy.”

Chemistry Review: The mole A mole is a word describing a specific number of particles. One mol = 6.02 × 1023 (atoms or molecules) Know these variables and how to convert between them. N = number of actual particles (atoms or molecules) n = number of mol

Chemistry Review: Atomic and Mass Numbers Atomic Number ( Z ), which tells us two things 1. Number of protons in the atom 2. Defines the atom. Each type of atom has a unique atomic number. Mass Number ( A ), which tells us… Number of protons plus neutrons It is also the atomic/molecular mass (mass of one atom/molecule), in amu 3. It is also the molar mass (mass of a mol of atoms), in grams per mole (g/mol)

Chemistry Review: Using the Mass Number Example The mass number of an element is 10. Determine both its molecular mass in kg and its molar mass in kg/mol. The molecular mass μ is 10 amu The conversion from amu to kg is The molar mass M is 10 g/mol

So try # 3 0.016 kg/mol 2.66 x 10-26 kg 8 = p+ = e- = no

Temperature Scales Example 1 Convert 25oC to Kelvin 25oC + 273 = 298 K

The Speed of the Molecules & Maxwell Distribution A system of gas at a given temperature will exhibit a variety of speeds Three speeds are of interest: Most probable Average rms

Maxwell Distribution, cont For every gas, vmp < vav < vrms As the temperature rises, these three speeds shift to the right The total area under the curve on the graph equals the total number of molecules want to learn more or review… Warning! sign convention for first law is different https://www.youtube.com/watch?v=NZZQLYcgZZs

Speed of the Molecules Expressed as the root-mean-square (rms) speed At a given temperature, lighter molecules move faster, on average, than heavier ones Lighter molecules can more easily reach escape speed from the earth want to learn more or review

Root Mean Squared Velocity vrms Root Mean Squared Velocity (m/s) R Universal Gas Constant (R = 8.31 J/mol K) kb Boltzman Constant (kb = 1.38 × 10-23J/K) T Temperature in Kelvin (K) M Molar mass (kg/mol) Mass of one mole of gas. Convert g/mol to kg/mol. μ Molecular mass (kg/atom) Mass of one molecule of gas. Convert (amu’s) to kg (1.66×10−27 kg/amu)

Example (# 5) A gas at 25oC has a mass number of 10. c. Determine the molecular mass in kilograms d. Determine the root mean squared velocity of the gas.

What’s hot ? What’s not? https://www.youtube.com/watch?v=vqDbMEdLiCs

Thermal Physics: Kinetic Energy of a Gas If gas particles have velocity, then they have kinetic energy. * kb Boltzman Constant (kb = 1.38 × 10-23J/K) T Temperature in Kelvin (K) Example 6B Determine the average kinetic energy of a gas at 25oC.

Thermal Physics: Some Trends to note If the temperature of a gas doubles a. the speed of the molecules increase/decreases by a factor of… Increases by a factor of root 2 b. kinetic energy of molecules increase/decreases by a factor of… Increase by a factor of 2 As temperature increases the molecules move faster and as a result gain kinetic energy.

Internal Energy Internal Energy, U, is the energy associated with the microscopic components of the system Includes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the atoms or molecules Also includes any potential energy bonding the particles together We all know that U is potential energy in mechanics. However… U is Eint (thermal energy) in Thermodynamics! Yuk

Internal Energy N - # of molecules   N - # of molecules Kb – Boltzmann’s constant 1.38 x 10-23 J/K T – Temperature Kelvin (K) n - # of moles R – gas constant 8.315 J/mol∙K ˚

ΔU = 3/2 (1 mol)(8.31 J/mol K)(303 - 293 K) Example 7 Determine the change in thermal energy (or internal energy) when one mole of a gas is heated from 20 to 30o C. ΔU = 3/2 (1 mol)(8.31 J/mol K)(303 - 293 K)

Relationship between ΔU and ΔT The trend revealed in the above equation may be more important than the values that can be derived from the equation. While temperature and internal energy are not the same thing, they are directly proportional. If temperature increases, then ΔT is positive & ΔU is positive If temperature decreases, then ΔT is negative & ΔU is negative If temperature stays the same, then ΔT = 0 & ΔU = 0

Take Home Message(s) Heat is a process Matter contains internal energy NOT HEAT Heat is the transfer or conversion of energy Compare to Work and Mechanical Energy… How?(tomorrow) Conduction: molecular collisions Convection: motion of fluid Radiation: no medium necessary (EM waves)