Chapter 14.3 – Using Heat

thermodynamics –

study of heat and temperature

there are three laws of thermodynamics

First law of thermodynamics –

the total energy used in any process is conserved, whether that energy is transferred as a result of work, heat, or both

- restatement of Law of Conservation of Energy

Chapter 14.3 – Using Heat

Second law of thermodynamics –

heat always moves from an object at higher temperature to an object at lower temperature

Third law of thermodynamics –

nothing can reach the temperature of absolute zero

Why not?

mechanical process –

process in which energy is transferred by work

ex.

Chapter 14.3 – Using Heat

entropy –

measure of the randomness or disorder of a system

- the more ordered the system, the lower the entropy

- most highly ordered systems have high amounts of energy

- this is because to order them you have to do a large amount of work on the system

- over time entropy increases and systems become disordered

- the universe wants to become more disordered

- more disorder the lower the energy

Chapter 14.3 – Using Heat

- we can decrease entropy and increase order in small areas, but overall the entropy will increase

ex. clean your room it becomes more ordered, but the ordered food you ate becomes more disordered providing the energy to clean the room

we can restate the 2nd and 3rd laws of thermodynamics using entropy

Second Law –

the entropy of the universe tends to increase

Third Law –

the entropy of an object goes to zero at 0 K

Chapter 14.3 – Using Heat

two analogies for the 3 laws of thermodynamics

1. you can’t win

2. you can’t tie

3. you can’t get out of the game

1. you can’t get anything without working for it

2. the most you can accomplish by work is to break even

3. you can’t break even

Chapter 14.3 – Using Heat

- the amount of useable energy always decreases whenever energy is transferred or transformed

- always lose energy to friction, heat loss

heat engine –

chemical energy is converted to mechanical energy through the process of combustion

external combustion engine –

combustion takes place outside the engine

ex. steam engine

Chapter 14.3 – Using Heat

internal combustion engine –

combustion takes place inside the engine

ex. cars, trucks, planes

- most internal combustion engines are four stroke engines

stroke –

up or down movement of a piston

Chapter 14.3 – Using Heat

1. intake stroke –

gas is mixed with air and drawn into the cylinder as the piston moves downs

2. compression stroke –

the piston moves up and compresses the fuel – air mixture

3. power stroke –

the spark plug ignites the fuel – air mixture, gases expand and push the piston down

Chapter 14.3 – Using Heat

4. exhaust stroke –

piston moves up and pushes the exhaust gases out

- gasoline engines are 12% efficient

- diesel engines have no spark plug and are 25% efficient

carburetor –

part of the engine that vaporizes the fuel

fuel injector –

adds the fuel vapor during the compression stroke instead of intake stroke, used in place of a carburetor