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Chapter 3: Heat, Work and Energy

Chapter 3: Heat, Work and Energy. Definitions. Force Work: motion against an opposing force dw = - f dx Examples: spring, gravity Conservative Force: absence of friction Energy: capacity to do work Kinetic En: by virtue of motion; K = ½ mv 2 Potential En: by virtue of position; V

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Chapter 3: Heat, Work and Energy

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  1. Chapter 3: Heat, Work and Energy

  2. Definitions • Force • Work: motion against an opposing force dw = - f dx Examples: spring, gravity • Conservative Force: absence of friction • Energy: capacity to do work • Kinetic En: by virtue of motion; K = ½ mv2 Potential En: by virtue of position; V • Total (Internal) Energy: E = K + V

  3. Law of Conservation of Energy (First Law) • The internal energy of an isolated system is constant; is an invariant of motion. • ΔE = 0 = q + w = heat plus work • In an isolated system, no energy or matter is exchanged with the surroundings. • Other conserved properties are mass and momentum’ they cannot be created or destroyed but can flow.

  4. Heat • Interesting historical theories about heat. • Heat, q, can flow, is not conserved, is related to the temperature change of matter.

  5. Kinetic Theory of Gases • Matter = particles (atoms, molecules) that move through space. • Heat is the exchange of energy due to motions and collisions of particles. Collisions are assumed to be elastic (limit of ideal case) • During collisions, energy and momentum are exchanged. As a result, a piston may move (work is done) or heat can be transferred to the surroundings. In either case, q decreases.

  6. KTG (2) • Electromagnetic radiation can influence the motions of molecules (radiant heat) • Mechanical model on a microscopic scale • Explains many macroscopic observations: IGL, P, diffusion rates, η, κ,, velocity of sound in a gas… In 3 dimensions, <K> = 3/2 kT = ½ m<v>2

  7. KTG + QM = Improved Model • QM  energy levels are quantized; the population or occupancy number, Ni, of each level depends on T and the set of εi. • The internal total energy is U = Σ Ni εi

  8. Heat Flows to Maximize W • Ex 3.3 and Fig 3.7 conclude that W increases with U. • Ex 3.4 and Fig 3.9 explains that heat flow maximizes W. (And the flow continues until the two temps are identical.) • Ex 3.5 shows that max W is not associated with equal final energies.

  9. Second Law of Thermodynamics • Systems tend toward their states of maximum multiplicity. • The entropy of the universe is increasing. • Carnot Cycle: no perpetual motion machines; you cannot convert all heat to work because there is always residual heat loss.

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