Heat

1st Law of Thermodynamics

• Julius Robert von Mayer, and James Prescott Joule experiments helped establish the first law.
• The relationship between mechanical energy and heat is called the mechanical chemical equivalent of heat.
• First law of thermodynamics states: the energy gained (or lost) by a system is equal to the energy lost (or gained) by its surroundings.
• A practical restatement of the law of conservation of energy.

2nd Law of Thermodynamics

Factors Affecting Thermal Energy

Heat Transferring Energy

• Sadi Carnot is considered the founder of the science of thermodynamics for his studies of steam engines.
• Carnot's research led to the conclusion that mechanical energy can be produced from thermal energy as heat flows from a high-temperature source to a low-temperature receiver.
• Law states: natural processes tend to go only one way, toward less usable energy and greater disorder.
• Entropy- a measure of the amount of disorder in the system.
• All natural processes tend to increase entropy
• 2nd law makes a perpetual motion impossible.

• Temperature- an object's thermal energy is closely related to its temp.
• Explained by KE Theory- molecules in rapid motion have more KE than slower molecules
• State- if all other factors are the same a gas has more thermal energy than a liquid, and a liquid has more than a solid G > L > S
• Explained by KE Theory- similar to temp. Gms > Lms > Sms
• Mass- even though each molecule of a small hot object has a lot of KE, a cooler larger object will have more KE
• Explained by KE Theory- larger object has more molecules so more KE

• Thermal Equilibrium- the state in which no thermal enrgy is transferred between objects because they are at the same temperature
• Heat- the transfer of thermal energy from an object of higher temp to an object of lower temp.
• Heat is measured in J
• Heat and thermal energy are NOT the same thing
• Thermal energy vs Heat
• Thermal- internal KE
• Heat- the transfer of thermal

5.3 Heat as Energy

Holding Thermal Energy

5.1 Nature of Heat

• Heat capacity- the amount of heat needed to change its temperature by a certain amount; indicates its ability to absorb and store thermal energy
• Heat capacity=heat/temp change
• Delta- change in value
• so change in T= Tf - Ti
• Q= + number if gaining energy
• Q= - number if losing energy
• Specific heat- the heat capacity of an object divided by the objects mass
• Substance's c is the amount of heat needed to cause a uit rise in the temp of a unit mass of the substance

• Thermodynamics- the branch of physics that deals with thermal energy, heat, and their relationships to other forms of energy and energy transfer.
• System- the matter or portion of the universe being studied
• Surroundings- the entire universe except the system, although for practical purposes only the immediate surroundings need to be considered.

• Kinetic theory of matter explains that the molecules of an object move faster as the object is heated
• Increased motion of molecules causes the molecules to gain KE
• Hotter=more internal KE
• Thermal energy- an objects internal energy due to the random motions of its individual molecules

Measuring Thermal Energy

• Calorimeter- device designed to measure the heat involved in physical and chemical changes; an insulated container that isolates a sample from the surrounding environment
• Can measure specific heat
• To calculate the amount of thermal energy gained use the following equation:
• Q=cm(delta)T
• a rearrangement of the specific heat formula

Heat Causes Changes

• Thermal Energy has the ability to cause changes as it is transferred
• Examples:
• Heat of sun produces Earth's winds and helps power windmills etc
• Gas expansion cause by heat of burning fuel propels modern cars, airplane, etc
• Geothermal energy- the thermal energy of the Earth itself

Heat

Physical Science

• Heat flows naturally from higher temps to lower temps
• Conduction- the process by which heat flows through materials and between objects in direct contact
• No matter is transferred only energy
• Sir Humphry Davy invented a miner's safety lamp in 1815. It operates through heat conduction
• Convection
• Transfer of heat by moving substances.
• Ex: as water heats in a pot near the bottom expands cools and falls
• Archimedes' principle-cooler denser water sinks to take the place of warmer water
• Convention currents.
• Radiation- the transfer of energy as radiant energy, transfers heat without matter.
• Energy transmitted by radiation at the speed of light
• Can pass through an object without heating it.
• All objects release some energy through radiation.

• Sublimation- substances skip the liquid state and change direction into a gas
• Ex: dry ice
• Deposition- a gas may change directly into a solid
• Plasma- special state of matter that exists when temps are too high for matter to exist in ordinary state
• glows brightly due to heat.

Types of Heat Transfer

Sublimation and Deposition

• Matter occurs in 3 states normally- gas, liquid, and solid.
• Kinetic theory of matter leads us to expect that an object's state depends upon its temperature.
• Thermal energy affects motion of molecules of matter, a change from one state to another involves the addition or subtraction of thermal energy

• According to Charles's law gases expand dramatically when heated; as the molecules gain kinetic energy, they push each other farther apart.
• Expansion of water- water contracts as temperature drops
• Unlike most substances, water stops contracting at 4 degrees C.
• The density of ice at 0 C is much less than the density of liquid water at 0 C
• Expansion and thermostats
• a device that automatically regulates the temperature.
• Bimetallic strip that responds to temp changes.
• Copper expands when heated
• Steal expands when coolded

5.4 Heat and Changes of State

• Devices that use mechanical energy and latent heat to transfer thermal energy from a colder location to a warmer location
• 4 main components
• Refrigerant- most important part; the fluid used to transfer thermal energy throughout the system.
• Compressor- compresses the gaseous refrigerant to very high pressure which causes an increase in temp.
• Condenser- radiator where the refrigerant cools to room temp.
• Evaporator- small opening into a winding tube that causes a large drop in pressure and some of the liquid changes instantly to vapor absorbing the thermal energy and cools dramatically

5.2 Expansion and Heat Transfer

Heat Pumps

• Vacuum bottles- Sir James Dewar make the Dewar flask to store liquid oxygen, liquid hydrogen, and other liquid gases
• Ex: Thermos
• Heating homes- many homes are heated by central heating systems
• Hot-water heating systems- warm a room by convection currents
• Forced-warm-air systems- commonly used for central heating in America.

Applications of Heat Transfer

• Melting- fusion; the changing of a solid into a liquid.
• Melting point- temp at which melting occurs.
• Freezing- the changing of a liquid into a solid
• Freezing point- temp at which freezing occurs.
• Freezing point and melting point are often the same temp just in opposite directions.
• Freezing point depression- the freezing point of liquid is lowered by the addition of solutes(dissolve in liquid)
• Latent heat- the heat used to melt a solid produces no rise in temp.
• Heat of fusion- the latent heat required to change a given mass of a solid into a liquid without changing its temp

• Condensation- the process of a gas changing into a liquid
• Heat of vaporization is involved in both evaporation and condensation
• Vapor pressure- affects the rates of evaporation and condensation
• At certain combos of pressure and temp both evaporation and condensation occur at the same time.
• As temp increases vapor pressure also increases.
• The temp at which the vapor pressure equals the pressure of the air above the liquid is the boiling point.
• The vapor pressure equals the pressure needed to keep a liquid from boiling at a given temp
• Critical temp-water cannot exist in liquid state but behaves as a gas.

• Evaporation- vaporization; the change of a liquid to a gas.
• Evaporation occurs more frequently at higher temps.
• Other factors include surface area and pressure.
• Volatile- liquids that evaporate quickly
• Nonvolatile- liquids that don't evaporate quickly
• Boil- evaporation occurs throughout the liquid
• Boiling point- the temp at which a substance boils
• Rises with increased pressure.
• Can be controlled if the pressure can be controlled
• Boiling point elevation- addition of solutes changes the boiling point.
• Nonvolatile solutes raise it.
• Heat of vaporization-quantity of heat needed to change a liquid already at its boiling point into a gas without raising the temp.
• Heat vaporization is much greater than heat of fusion.

Condensation

Melting and

Freezing

Evaporation