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# Heat & Thermodynamics - PowerPoint PPT Presentation

Heat & Thermodynamics. Heat. Heat is the transfer of energy between a system and its environment because of a temperature difference between them The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment. Heat.

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### Heat & Thermodynamics

• Heat is the transfer of energy between a system and its environment because of a temperature difference between them

• The symbol Q is used to represent the amount of energy transferred by heat between a system and its environment

• The process by which energy is exchanged between objects because of temperature differences is called heat

• Objects are in thermal contact if energy can be exchanged between them

• Thermal equilibrium exists when two objects in thermal contact with each other cease to exchange energy

• Calorie

• A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 1° C .

• A Calorie (food calorie) is 1000 cal

• US Customary Unit – BTU

• BTU stands for British Thermal Unit

• A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F

• 1 cal = 4.186 J

• This is called the Mechanical Equivalent of Heat

• Internal Energy, U, is the energy associated with the microscopic components of the system

• Includes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the atoms or molecules

• Also includes any potential energy bonding the particles together

Zeroth Law of Thermodynamics

• If objects A and B are separately in thermal equilibrium with a third object, C, then A and B are in thermal equilibrium with each other.

• Allows a definition of temperature

• We cannot get a greater amount of energy out of a cyclic process than we put in

• We can’t break even

• Means that Qc cannot equal 0

• Some Qc must be expelled to the environment

• Means that e must be less than 100%

• A perpetual motion machine would operate continuously without input of energy and without any net increase in entropy

• Perpetual motion machines of the first type would violate the First Law, giving out more energy than was put into the machine

• Perpetual motion machines of the second type would violate the Second Law, possibly by no exhaust

• Perpetual motion machines will never be invented

• Temperature of an ice-water mixture is defined as 0º C

• This is the freezing point of water

• Temperature of a water-steam mixture is defined as 100º C

• This is the boiling point of water

• Distance between these points is divided into 100 segments or degrees

• When the pressure of a gas goes to zero, its temperature is –273.15º C

• This temperature is called absolute zero

• This is the zero point of the Kelvin scale

• –273.15º C = 0 K

• To convert: TC = TK – 273.15

• The size of the degree in the Kelvin scale is the same as the size of a Celsius degree

• Most common scale used in the US

• Temperature of the freezing point is 32º

• Temperature of the boiling point is 212º

• 180 divisions between the points

• A gas does not have a fixed volume or pressure

• In a container, the gas expands to fill the container

• Most gases at room temperature and pressure behave approximately as an ideal gas

• It’s convenient to express the amount of gas in a given volume in terms of the number of moles, n

• One mole is the amount of the substance that contains as many particles as there are atoms in 12 g of carbon-12

• The number of particles in a mole is called Avogadro’s Number

• NA=6.02 x 1023 particles / mole

• Defined so that 12 g of carbon contains NA atoms

• The mass of an individual atom can be calculated:

Ideal Gas Law, Chemistry Version

• PV = n R T

• R is the Universal Gas Constant

• R = 8.31 J / mole.K

• R = 0.0821 L. atm / mole.K

• Is the equation of state for an ideal gas

Ideal Gas Law, Physics Version

• P V = N kB T

• kB is Boltzmann’s Constant

• kB = R / NA = 1.38 x 10-23 J/ K

• N is the total number of molecules

• n = N / NA

• n is the number of moles

• N is the number of molecules

• Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1° C

• The specific heat, c, of a substance is a measure of this amount

• Q = m c ΔT

• ΔT is always the final temperature minus the initial temperature

• When the temperature increases, ΔT and ΔQ are considered to be positive and energy flows into the system

• When the temperature decreases, ΔT and ΔQ are considered to be negative and energy flows out of the system

• Water has a high specific heat compared to land

• On a hot day, the air above the land warms faster

• The warmer air flows upward and cooler air moves toward the beach

• A phase change occurs when the physical characteristics of the substance change from one form to another

• Common phases changes are

• Solid to liquid – melting

• Liquid to gas – boiling

• Phases changes involve a change in the internal energy, but no change in temperature

• Some substances will go directly from solid to gaseous phase

• Without passing through the liquid phase

• This process is called sublimation

• There will be a latent heat of sublimation associated with this phase change

• Methods include

• Conduction

• Convection

• Energy transferred by the movement of molecules that are in direct contact with each other.

• Energy transferred by the movement of a substance

• When the movement results from differences in density, it is called natural conduction

• When the movement is forced by a fan or a pump, it is called forced convection

• The radiator warms the air in the lower region of the room

• The warm air is less dense, so it rises to the ceiling

• The denser, cooler air sinks

• A continuous air current pattern is set up as shown

• Radiation does not require physical contact

• All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules