Thermal Energy

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Slide 1

Heat and Internal Energy

Internal Energy U is the total 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 the intermolecular potential energy

Does not include macroscopic kinetic energy or external potential energy

Heat refers to the transfer of energy between a system and its environment due to a temperature difference between them

Amount of energy transferred by heat designated by symbol Q

A system does not have heat, just like it does not have work (heat and work speak to transfer of energy)

Slide 2

Units of Heat

The historical unit of heat was the calorie

A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5°C to 15.5°C

A Calorie (food calorie, with a capital C) is 1000 cal

Since heat (like work) is a measure of energy transfer, its SI unit is the joule

1 cal = 4.186 J (“Mechanical Equivalent of Heat”)

New definition of the calorie

The unit of heat in the U.S. customary system is the British thermal unit (BTU)

Defined as the amount of energy necessary to raise the temperature of 1 lb of water from 63°F to 64°F

Slide 3

More About Heat

Heat is a microscopic form of energy transfer involving large numbers of particles

Energy exchange occurs due to individual interactions of the particles

No macroscopic displacements or forces involved

Heat flow is from a system at higher temperature to one at lower temperature

Flow of heat tends to equalize average microscopic kinetic energy of molecules

When 2 systems are in thermal equilibrium, they are at the same temperature and there is no net heat flow

Energy transferred by heat does not always mean there is a temperature change (see phase changes)

Slide 4

Heat Transfer Simulation

Simulation presented in class.

(ActivPhysics Online Exercise #8.6, copyright Addison Wesley publishing)

Slide 5

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, defined as:

Or

DT is always the final temperature minus the initial temperature

When the temperature increases, DT and Q are considered to be positive and energy flows into the system

When the temperature decreases, DT and Q are considered to be negative and energy flows out of the system