Thermodynamics Chapters 21-24

Thermodynamics- the study of heat and its transformation into mechanical energy. What is mechanical energy again? Energy due to position or movement of an object; potential and/or kinetic energy. As the MOTION of atoms increases, so does the TEMPERATURE. What will happen to temperature if we decrease the motion of atoms?

Absolute Zero- the limit at which no more energy can be taken from a substance and therefore the temperature cannot be lowered. On the celsius scale absolute zero is 273 degrees below zero. On the Kelvin scale absolute zero is 0 degrees.

The Kelvin scale was named after a british physicist named Lord kelvin who coined the term thermodynamics which come from Greek words meaning “movement of heat”. Which is larger a celsius degree or a Kelvin? Look at figure 24.1 on pg. 355- At what temperature does water boil in Celsius? In Kelvin?

What temperature scale do we use in the U.S.? Convert from Fahrenheit to Celsius- Celsius to Kelvin - ADD 273 Kelvin to Celsius - SUBTRACT 273

Practice Convert 0°C to K. Answer: 273K Convert 546K to °C. Answer: 273°C Convert 100°F to °C. Answer: 38°C Convert 75°F to K. Answer: 297K

We said that as the movement, or kinetic energy of atoms decreases, so does temperature. What happens to the amount of space (also known as volume) a substance will occupy as heat is added? Heat addition will cause expansion; heat reduction (cold) will cause contraction. The tendency of matter to change in volume as a response to temperature is called Thermal Expansion. See pages 315-317.

What is a system????? See page 356. 1st Law of Thermodynamics- when heat is added to a system it is transformed into an equal amount of some other form of energy. Heat added = increase in internal energy + external work done by system What is a system????? See page 356. Heat is energy!! Also known as thermal energy!! Remember* Energy cannot be created or destroyed!!!

Practice If 25J of energy is added to a system that does no external work, by how much will the internal energy of that system be raised? Answer: 25J If 20J of energy is added to a system that does 7J of external work, by how much will the internal energy of that system be raised? Answer: 13J

2nd Law of Thermodynamics- heat only flows in ONE direction- hot to cold. It will never flow from cold to hot by itself. What about air conditioners? They make the air cold. See pg. 360 last paragraph. “Hot” and “cold” are just ways of describing the amount of heat present.

Thermal Equilibrium Two systems are in thermal equilibrium when their temperatures are the same. This is also known as the zeroeth law of thermodynamics.

What are some kinds of heat engines? pg. 361 A heat engine is any device that changes internal energy into mechanical work. What are some kinds of heat engines? pg. 361 In every heat engine only some of the heat can be transformed into work. Every heat engine will: absorb heat from a reservoir of higher temperature (which increases internal energy) convert some of this energy into mechanical work put the remaining energy as heat to a lower temperature reservoir which is usually called a sink.

Ideal efficiency = Thot - Tcold Thot French Engineer Sadi Carnot found that even under ideal conditions, the amount of heat that can be used to do useful work is dependent on the temperature difference between the hot reservoir and the cold sink. The ideal efficiency, also known as Carnot efficiency of a heat engine is expressed below- Ideal efficiency = Thot - Tcold Thot

Practice What is the ideal efficiency of an engine if both the hot reservoir and the exhaust are 500K? Answer: zero efficiency What is the ideal efficiency of a machine having a hot reservoir of 400K and a cold reservoir of 300K? Answer: 25%

Entropy- the measure of the amount of disorder. For natural processes entropy is always increased. This means that as entropy is increased, the available energy of the system for doing work decreases. The exception to this is living organisms- see pg. 366.

Types of Heat Transfer VIDEO- CLICK HERE Conduction- heat transfer due to direct contact between objects Convection- heat transfer through a fluid (liquid or gas) Radiation- transferred by electromagnetic waves. VIDEO- CLICK HERE

Specific Heat- the heat required to raise the temperature of one gram of a substance one degree celsius. Calorie- A unit of heat and energy equal to the amount of energy requires to raise the temperature of 1 gram of water by 1° Celsius. 1 calorie = 4.184 joules

Practice Problem The specific heat capacity of iron is 450 J/(kg°C). How much heat energy does it take to raise the temperature of 500g from 25°C to 100°C? C = 450 J/(kg°C) M = .5 kg Tf = 100°C Q = m c ΔT = .5(450)(75) = 16,875 J Ti = 25°C ΔT = 100-25 = 75°C

Latent heat is the heat released or absorbed by a system or a chemical substance during a change of state that occurs without a change in temperature. Example: a phase transition such as the melting of ice or the boiling of water. VIDEO-CLICK HERE Process Change of State Melting solid to liquid Freezing liquid to solid Vaporization liquid to gas Condensation gas to liquid Sublimation solid to gas Deposition gas to solid