Heat thermodynamics
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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

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Heat thermodynamics

Heat & Thermodynamics


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 thermodynamics

Heat

  • 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


Units of heat

Units of Heat

  • 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

Internal Energy

  • 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

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


First law of thermodynamics

First Law of Thermodynamics

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


Second law of thermodynamics

Second Law of Thermodynamics

  • 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%


Perpetual motion machines

Perpetual Motion Machines

  • 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


Celsius scale

Celsius Scale

  • 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


Kelvin scale

Kelvin Scale

  • 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


Fahrenheit scales

Fahrenheit Scales

  • 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


Comparing temperature scales

Comparing Temperature Scales


Converting among temperature scales

Converting Among Temperature Scales


Ideal gas

Ideal Gas

  • 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


Moles

Moles

  • 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


Avogadro s number

Avogadro’s Number

  • 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

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

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


Specific heat

Specific Heat

  • 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


Heat and specific heat

Heat and Specific Heat

  • 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


A consequence of different specific heats

A Consequence of Different Specific Heats

  • 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


Phase changes

Phase Changes

  • 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


Sublimation

Sublimation

  • 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 of heat transfer

Methods of Heat Transfer

  • Methods include

    • Conduction

    • Convection

    • Radiation


Conduction example

Conduction example

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


Convection

Convection

  • 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


Convection current example

Convection Current Example

  • 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

Radiation

  • Radiation does not require physical contact

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


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