Physics 2 Chapter 12 Sections 2-4

First Law When a system absorbs an amount of heat Q and performs an amount of work W, the internal energy of the system changes by an amount ΔU where ΔU = Q – W Expression of conservation of energy If heat flows out of a system or a system does work it loses energy

First Law ΔU = Q – W If heat flows into a system or work is done on a system it gains energy By convention, work done by a system is + and heat absorbed by a system is +

Thermal Processes 4 common thermal processes that any system can undergo: - isobaric, isochoric, isothermal, adiabatic Assume that all are quasi-static (occurs slowly enough that uniform pressure and temp exist at all times)

Thermal Processes Isobaric process – occurs at constant pressure

Thermal Processes Isochoric process – occurs at constant volume - sometimes called isovolumetric or isometric

Thermal Processes Isothermal process – occurs at constant temperature

Thermal Processes Isothermal process – occurs at constant temperature If ideal gas W = nRT ln (Vf / Vi) where n = number of moles R = 8.31 J/K T – temp in Kelvin

Thermal Processes Adiabatic process – occurs without the transfer of heat

Thermal Processes Adiabatic process – occurs without the transfer of heat If monatomic ideal gas W = 3/2 nR (Ti – Tf)

Thermal Processes

Sample Problem Heat is added to 1000 cm3 of water at atmospheric pressure. The water’s temperature increases by 50 ⁰C. What type of process is it? Find the work done by the water as it expands. Find the increase in the water’s internal energy.

Sample Problem Find the work done by 2 moles of an ideal gas if its volume doubles at a constant temperature of 23 ⁰C. What type of process is this? Find the heat absorbed by the gas.

Sample Problem Consider the PV diagram on your study guide for an ideal gas. Processes I, II, and III take place at constant volume, temperature, and pressure respectively. In process III the heat added to the gas is -61 cal and the work done by the gas is -24 cal. Find the change in internal energy for process III. Find Q, ΔU, and W for process I.

Second Law Heat energy flows spontaneously from a substance at a high temperature to a substance at a low temperature and does not flow spontaneously in the reverse direction Ex – cold drink in a warm room warms up cold drink in a warm room won’t get cooler

Entropy Measure of the disorder of a system Defined by Clausius as heat divided by the temperature at which the heat flows S = Q / T where S – entropy Q – heat flow T – temp in Kelvin [S] = J / K If you were to see a bunch of trees arranged in neat rows you would assume they were planted. Why? If left to its own devices, nature tends to produce a disorderly arrangement. Order is possible but not probable. Toss 3 pennies into a cup. Possible outcomes are HHH, TTT, HHT, TTH, HTT, THH, HTH, and THT. Which outcomes are the most ordered? HHH and TTT Likelihood of ordered outcome? 2/8 Likelihood of a disordered outcome? 6/8

Entropy cont’d. Increasing entropy decreases the energy available to do work - if all gas particles are headed toward piston the energy could be used to do work on piston - highly efficient -if gas particles were moving randomly only some particles would collide with piston to do work

Second Law of Thermo Heat flows spontaneously from a substance at high temperature to a substance at low temperature OR Total entropy of the universe increases when an irreversible process occurs Consider an ice cube in the experimental fluid at Chalky’s Ice melts because heat flows into the system This heat breaks down the ordered crystalline arrangement of molecules and makes them more disordered ---heat and entropy are related

Sample Problem 1200 J of heat energy flows spontaneously from a hot reservoir at 650 K to a cold reservoir at 350 K. Determine the amount by which this irreversible process changes the entropy of the universe. 1.58 J/K Note: individual parts can have a decrease in S but a greater increase in S will take place somewhere else

Sample Problem Suppose that by stirring 100 g of water you are able to increase its temperature from 19 C to 21 C. Find the increase in the water’s entropy. 2.9 J/K

Heat Death of the Universe Since entropy of the universe is always increasing, less energy is available to do work At some point there will be no energy available to do any work

Third Law It is not possible to lower the temperature of any system to absolute zero in a finite number of steps Can’t reach absolute zero