Heat & Temperature

Chapter 14, Section 1 - Temperature                                                                                                                                        

Temperature - A measure of the average kinetic energy of the particles in an object.  (A measure of how hot or cold something is.)

Thermometer - A device that measures temperature.
Use the expansion of liquids (mercury or colored alcohol) to indicate temp.  (Physical property - most substances expand when heated.)
As temp. rises, particles have more KE, move faster, and take up more space.
Use the expansion of metals in a bimetallic strip.
Two different metal strips pressed together - one side expands faster when heated, making the strip curl.

Temperature Scales
  • Fahrenheit Scale
    • Uses degrees Fahrenheit (oF).
    • Water freezes at 32 oF, boils at 212 oF.
    • Used in the US.
  • Celsius Scale
    • Uses degrees Celsius (oC).
    • Water freezes at 0 oC, boils at 100 oC.
    • Used in most of the rest of the world (and in the US for science).
  • Kelvin Scale
    • Uses kelvins (K).
    • 0 K = absolute zero (the temperature at which particles stop moving - in theory!)
    • Used in science.
Converting Between Temperature Scales
  • 1 Celsius degree = 1.8 Fahrenheit degrees
  • Celsius degree = 1 kelvin (but the scales are offset by 273!)

When two objects at different temperatures touch, energy will be transferred from the warmer object to the cooler one.

Heat - The energy transferred between two objects at different temperatures.
The greater the difference in temp, the faster energy will be transferred.
(The transfer slows down as temps become closer to equal.)


Homework:
    Pg. 479, #1-7

Chapter 14, Section 2 - Energy Transfer                                                                                                                                   

Heat energy can be transferred in three ways...
  1. Thermal Conduction
    • The transfer of energy as heat through a material.
    • Occurs between objects in direct contact.
    • Atoms that are heated vibrate more rapidly, bump into neighboring atoms, and cause them to vibrate more rapidly.
  2. Convection
    • The movement of matter due to differences in density, which are cause by differences in temperature.
    • Occurs in fluids (unevenly heated).
    • Atoms that are heated vibrate faster and take up more space, so the fluid expands, becomes less dense, and rises.
    • Convection Current - The movement of a fluid that is heated and rises, then cools and sinks.
  3. Radiation
    • Energy that is transferred as electromagnetic waves (ex. - light, infrared, etc.).
    • Does not require physical contact.  (Can travel through space.)
    • All hot objects give off infrared (IR) radiation - the hotter the object, the more IR it gives off.
    • When atoms absorb IR, they move faster, so the temp rises.

Conductor - A material that transfers energy (heat) easily.
Insulator - A material that transfers energy (heat) poorly.
  • Heat energy is transferred by particles colliding.
    • Gases - particles spread out, few collisions => poor conductors!
    • Metals are dense -> many collisions => good conductors!
    • Plastics are not dense -> fewer collisions => poor conductors (good insulators)!
Specific Heat (c) - The amount of energy (J) needed to raise the temperature of 1 kg of a substance by 1K (or 1oC).
Measured in J/kg*K.
Specific heat is a physical property that is different for different materials.

You can use specific heat to calculate the energy needed to change an object's temperature.

Adding heat can raise an object's temperature OR change its state - not both at the same time!


Homework:
    Do the Quick Lab on pg. 484.  Write down your results and answers to questions.
    Pg. 487, #1-7


Chapter 14, Section 3 - Using Heat                                                                                                                                          

First Law of Thermodynamics
The total energy used in any process is conserved, whether it is transferred as a result of work, heat, or both.

Second
 Law of Thermodynamics
Energy transferred as heat always moves from an object at a higher temperature to an object at a lower temperature.

Work can increase average KE (temperature).
As work is done on an object, energy is transferred to the object's molecules, which increases their KE, thus raising their temperature.
Friction turns the work into KE.
Mechanical Process - A process in which energy is transferred by work.

Entropy - A measure of the randomness/disorder in a system.
  • Left alone, any system will move towards disorder.
    • (ex. - It takes energy to clean up your room, so a clean room is a high-energy state.  If you stop maintaining the state of your room, it will become messier and messier - moving towards a lower-energy state.  This is the trend towards entropy.)
  • Work done on a system to decrease its entropy causes more entropy in the larger system.
    • (ex. - The work you do in cleaning your room uses energy in your body, from the food you eat.  Breaking down complex molecules to release the energy is a change from a high-energy state - the complex molecules - to a low-energy state - the smaller molecules.  Entropy has increased.)
Useable energy decreases in all energy transfers.
Some energy is lost to the environment, some is converted into unusable forms.
(When using a pulley, some of the energy you put into using it is converted into heat by friction.  This is not used to help lift the load, so it is not considered usable energy.)

Heat Engine - A machine that converts heat energy into mechanical energy (work), usually through combustion (burning of fuels).
Two main types...
  • Internal Combustion Engine - Burns fuel inside the engine.  (ex - car engines)
  • External Combustion Engine - Burns fuel outside the engine.  (ex. - steam engine)
Internal Combustion Engine
Fuel burns in cylinders, which moves the pistons.
Pistons move the crankshaft, which causes the wheels to turn.
Four strokes...
  • intake
  • compression
  • power
  • exhaust
(Diesel is similar, but without spark plugs.  It relies on compression to ignite the gas.)

Internal combustion engines always generate heat - not all of the fuel's chemical energy is converted to mechanical energy.  (Enough heat is generated to require a cooling system!)

Carburetor - A device that mixes air and gasoline before injecting it into the cylinder.
Fuel Injector - A device that injects fuel vapor directly into compressed air during the compression stroke.  (Cars have either a carburetor or a fuel injector - not both.)


Homework:
    Pg. 493, #1-8



Resources
Internal Combustion Engine - Putting it all togther.