Laws of Thermodynamics Chapter 11 Physics Ms. Hanlin

Thermal energy is conserved  Sliding a book across a table leads to a decrease in kinetic energy over time from friction= produces heat.  Thermal energy=kinetic + potential energy of the system  If energy is added to the system, or taken out, the thermal energy(E) changes.  Demonstration: paper clip

Adding Heat and Work to a system…  Example: You have a container with a specific initial thermal energy (E i ). If heat (Q) is added to the system, Ef is equal to the initial thermal energy + heat added.  Because of this, the change in thermal energy is Q= E f - E i  Q positive when heat is added to the system  Q is negative when heat is taken away from the system  The same thing can happen when work is added to a system (form of energy)  When a system does work, the system loses final thermal energy (W=negative)  Ef =initial thermal energy – work OR Ei-W = Ef  When work is done on a system, thermal energy increases (W=positive)

Combining Work and Heat…  Change in thermal energy = heat – work  ΔE = Q -W  Adding heat to the system increases the thermal energy  When work is done by a system, uses the heat and decreases the thermal energy  Transfer energy from work to heat to create end thermal energy

Example: Jogging on the beach  Jogging on the beach one day, you do 4.3 X 10 5 J of Work and give off 3.8 X 10 5 J of heat. What is the change in your thermal energy?  Give off heat (exothermic) heat is negative

Heat Engines  Heat engine converts thermal energy into work.  Gas engines in cars turn thermal energy from burning fuel into mechanical energy (motion).  YOU consume food and “burn” it during digestion and use this energy to survive.

Thermal Reservoir and Efficiency  Heat supplied to the heat engine come from a Hot thermal reservoir. This heat will be denoted as Q h.  A thermal reservoir is an area that receives heat, with little to no change in thermal temperature.  Source of heat at a constant temperature.  Of the heat going into the engine, some is used as work, while the rest is given off as waste heat at a relatively low temperature  This is denoted as Qc  The energy into the system is equal to the energy out  Q h = W + Q c  *Rearrange equation to find Work

Thermal Reservoir Efficiency  Efficiency of a heat engine is found by calculating how efficiently thermal energy is converted into work (not given off as waste).  Efficiency=work done by engine/heat supplied by reservoir  SI unit: percent  Since Work is equal to the Initial heat minus the waste heat…  Q h – Q c / Q h = efficiency

The Second Law has Limitations  Second Law of Thermodynamics: When objects at different temperatures are brought in contact, energy flows from the high temp object to the low temp object, until equilibrium  Because of this, efficiency can never reach 100 %  We can use an equation to find the maximum efficiency  Max efficiency= 1 – (temp. of cold reservoir/temp of hot reservoir)  Max Work= (1 – T c /T h ) Q h  All that matters in engine efficiency is the hot and cold reservoir, not the material or construction.  Heat engines require a temperature difference to operate

Example Problem  Suppose you have an ideal heat engine that can operate in two different modes. In mode 1, the temperature of the two reservoirs are T c =200 K and T h =400 K. In mode 2, the temperature of the two reservoirs are T c =400 K and T h =600 K. Which of these models in more efficient?

Entropy  Entropy is the measure of disorder in a system.  S is the value we use and ΔS is used for change of entropy  ΔS = heat/ Temperature  Heat= Q  Temperature= T  The entropy of the universe never decreases, either increases or stays the same.

Third Law of Thermodynamics  Absolute zero can never be reached. It is impossible to reach temperatures lower than absolute zero.

11.2 Thermal Processes  Thermal energy in a system can be changed in a number of ways.  1. Constant volume: The temperature and pressure can increase, while the volume stays constant (holding a cold soda can)  Can does no work (work=force X displacement)  Δ E = Q – W so… ΔE =Q change in thermal energy = heat  2. Constant pressure: Heating and expansion occur with no change in pressure.  Hot air balloon: the gas expands as it is heated  As gas expands with heat, it can do work (F=pressure X area)  Force X distance = Work  Work= Pressure X volume