Energy in Thermal Processes Essay

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PHYS117 – Introductory Physics
Energy in Thermal Processes

§11.1 – Heat and Internal Energy
§11.5 – Energy Transfer


PHYS117 – Introductory Physics
Recall Temperature (Fall term, lecture 11-1) :
 a measure of the hotness or coldness of a material
 a measure of the average internal kinetic energy of the molecules of a material (average heat energy per molecule)
 temperature is a macroscopic property describing a system of interacting particles (not derived from Newton’s laws)

 temperature describes the system as a whole
(not meaningful for just a few molecules)
 the system is in thermal equilibrium when all parts of the system are at the same temperature



PHYS117 – Introductory Physics
Chapter 11 - Energy in Thermal Processes :
A system consists of a large number of particles (atoms or molecules) that can exchange energy with each other.
The system is described by macroscopic properties (eg. temperature or pressure) on a length scale much larger than the size of the individual components. Heat is a form of energy associated with the motion of atoms or molecules.
For a solid, heat energy is found primarily in the vibrations of its atoms about their (fixed) equilibrium positions.
Refer to heat when discussing the flow or transfer of energy between systems at different temperatures.
The transfer of heat energy from a hot system to a colder system can take place in different ways but the direction of the transfer will always depend only on the temperature difference.



PHYS117 – Introductory Physics
Units of heat energy (referring to Q)
SI unit of the Joule (J  1 J = 1 Nm) historical unit of the Calorie (from 1824)

 1 calorie (cal) = energy necessary to raise the temperature of 1 g of water from 14.5 ºC to 15.5 ºC
= 4.186 J
 1 food calorie (Cal) = 1000 cal
US unit of the BTU (British Thermal Unit)
 1 BTU = (heat) energy necessary to raise the temperature of 1 pound of water from 3.8 ºC to 4.4 ºC
 1054 J



PHYS117 – Introductory Physics
Which of the following phrases best describes heat?

The total energy possessed by a body.


The flow of temperature to or from a body.


The amount of energy dissipated by friction.


The total energy flowing between two bodies at different temperatures.


The useful work that could be extracted from a body.



PHYS117 – Introductory Physics
Consider 3 mechanisms of heat transfer:
Conduction is heat transfer by the interaction between neighbouring vibrating molecules
(moves from molecule to molecule, from hot to cold). Convection is heat transfer by large-scale (bulk) fluid motion (liquid or gas).

Radiation is heat transfer by the emission and absorption of photons
(electromagnetic radiation).

Generally, heat flow occurs with all three mechanisms acting simultaneously.


PHYS117 – Introductory Physics
Heat flow and the human body: food input

food energy energy for work (20%)

heat energy

excess heat removed by 3 mechanisms of heat transfer: convection (blood flow to body surface)

conduction (from capillaries at skin surface through the skin layer) radiation (emission of thermal radiation from skin)



PHYS117 – Introductory Physics
Free or natural convection is based on thermal expansion (in fluids) and the effect of the buoyant force of
Archimedes’ principle.
The warmer liquid on the bottom (nearer the heat source) becomes less dense (thermal expansion).

The warm, lower-density air moves upward due to the resulting increase in buoyant force. The cooler, denser air is displaced and sinks lower where it is heated by the heat source.

The resulting circular flow movement of volumes of fluid is the (natural) convection mechanism of heat energy transfer.
Forced convection relies on the use of external sources to generate fluid