Physical Science Chapter 17
17-1 Heat: A Form of Energy
Heat is a form of energy caused by the internal motion of molecules of matter.
KMT Kinetic molecular theory
(Hot molecules move faster than cold molecules)
Cold is the absence of heat.
Heat moves from hot to cold and this movement is called heat transfer.
Heat energy is transferred by:
1. conduction: Transfer by direct contact of molecules.
Conduction can take place in solids, liquids, and gases, but is best in solids because the molecules are closest together.
Conductors easily transfer heat energy.
Insulators do not conduct heat energy well.
2. convection: Transfer of heat in fluids (liquids and gases) by the up and down movements called convection currents. (Heated fluid is "less dense" and so rises while cooler fluids are "more dense" and so sink.)
3. radiation: Transfer of heat energy through empty space by electromagnetic waves called "infrared radiation."
17-2 Temperature and Heat
Kinetic molecular theory---Kinetic Energy in molecules
Temperature is a measure of the average kinetic energy of the molecules that make up an object.
Temperature is a measure of how hot or cold something is.
A thermometer measures temperature.
Fahrenheit scale: Who knows why
Celsius scale: metric
based on 100
Kelvin scale: Lord Kelvin
..no negative numbers
Absolute zero: All molecular movement ceases.
17-3 Measuring Heat
Heat energy is needed to set molecules in motion.
Temperature is a measure of that molecular motion.
Heat added - temperature goes up.
Heat being removed - temperature goes down.
Heat is measured in calories (cal).
Heat is also measured in Joules (J).
1 joule is 1 newton-meter: W=Fd
1 calorie equals 4.19 joules.
1 calorie is the heat needed to raise the temperature of 1 gram of water 1 degree Celsius.
Specific Heat: The heat energy needed to raise the temperature of one gram of a substance 1 degree Celsius.
|| Q = m Cp Ät
Ex: How much energy is released when a 25 g sample of waters
|| Heat gained or lost = (mass) (specific heat)(change in temp.)
temperature changes from 50ºC to 25ºC?
Ex: How much energy is absorbed from ammonia when when it is
converted to a vapor by a change in temperature of 45ºC
17-4 Heat and Phase Changes
Heat energy moves from hot to cold.
*(area of high concentration to area of low concentration)
Matter generally exists in three phases: Solid, liquid, and gas. The physical change from one phase state to another is called a phase change.
Heat of Vaporization: The heat energy needed to change from liquid to gas
Hv phase.of water = 2260J/g (chart)
Ex: How much energy is required to change 10ml of water to vapor (gas)
Heat of Fusion: The heat energy needed to change from solid to liquid phase.
Hf of water 334J/g
Ex: How much heat is required to convert a 25g ice cube to water?
Ex: How much more energy is required to convert the water in the above
question to vapor (gas)
A change in phase requires heat energy to be absorbed or released. (Note: Molecular movement increases with temperature)
Freezing Point: The temperature at which a liquid changes to a solid.
Melting Point: The temperature at which a solid changes to a liquid.
A substance's freezing and melting point are the same!
Depends on if heating or cooling
Boiling Point: The temperature at which a liquid changes into a gas.
Condensation Point: The temperature at which a gas changes to a liquid.
The temperature of the boiling point and condensation point are the same!
17-5 Thermal Expansion
Thermal Expansion: The increase in size of a substance caused by heat.
Most substances - solids, liquids, and gases - will expand in volume when their temperature is increased.
As a substance expands, the same "mass" takes up more space (volume) and therefore the substance becomes "less dense."
D = m/v ( remember ice floats for this reason)
Thermostats make use of this principle of expansion with "bimetallic strips."
PH Physical Science Chapter 18
18-4 Heat Engines
Heat engines convert heat energy into mechanical energy in order to do work.
||External combustion engine
||Internal combustion engine 4 stroke engine:
|| Intake, Compression, Power, Exhaust