Thermodynamics and Magnetism Notes

Thermodynamics

• Heat flows from hotter to colder objects normally.
• Thermodynamics: physics field governing heat movement.
• Fog created by cooling humid air quickly.
• Experiment to make fog in a bottle: requires sudden temperature drop of humid air.

Experiment: Making Fog in a Bottle

• Materials: clean plastic bottle, water.
• Shake bottle with water to increase humidity.
• Compress bottle by stepping on it (wait 20-30 seconds for heat to flow out).
• Release compression quickly and illuminate with bright light to see fog form.
• If fog doesn't form, add a smoking match to introduce smoke particles.

Chapter Itinerary

• Examine heat movement in air conditioners and automobiles.
• Air Conditioners: Laws of thermodynamics permit heat transfer from cold to hot using electric energy.
• Automobiles: Thermal energy converts to work as heat flows from hot fuel to cold air.
• Principles also apply to refrigerators, heat pumps, steam engines, hot-air balloons.

Air Conditioners

• Air conditioners cool room air by removing thermal energy.
• Air conditioners transfer heat from cooler room air to warmer outdoor air.
• Air conditioners are heat pumps, using ordered electric energy.
• Questions:
  • Why doesn't heat naturally flow from cold to hot?
  • Why do air conditioners need electric energy?
  • Why do they have indoor and outdoor components?
  • What happens if an air conditioner is in the middle of a room?
• Experiment: Compare temperatures of air leaving indoor and outdoor vents.

Moving Heat Around: Thermodynamics

• Air conditioners transfer heat against natural flow, from colder indoor air to hotter outdoor air.
• This requires ordered energy and is done by a heat pump.

Harebrained Cooling Alternatives (Discredited)

1. Letting heat flow to a neighbor's home.
2. Destroying thermal energy.
3. Converting thermal energy into electric energy.

Law of Thermal Equilibrium (Zeroth Law of Thermodynamics)

• Two objects in thermal equilibrium with a third object are also in thermal equilibrium with each other.
• Meaningful temperature system based on this law.
• Example: No heat flow between homes in thermal equilibrium with outdoor air.

Law of Conservation of Energy (First Law of Thermodynamics)

• Energy cannot be destroyed; it must be converted or transferred.

• Two ways to transfer energy: work and heat.

• Increase object's internal energy by doing work or transferring heat.

• Word Equation:

  • change in object’s internal energy = heat added to object - work done by object

• Symbols:

  • \Delta U = Q - W

Disorder and Entropy

• Ordered energy easily converts to thermal energy, but the reverse is difficult.
• Creating disorder out of order is easy, but recovering order from disorder is nearly impossible.
• Systems become more disordered over time.
• Entropy is a measure of total disorder.
• Energy is conserved; entropy generally increases.
• Law of Entropy (Second Law of Thermodynamics): Entropy of a thermally isolated system never decreases.
• Cooling a home requires exporting thermal energy and entropy.

Pumping Heat Against Its Natural Flow

• Entropy redistribution allows one object to become colder if another becomes hotter.

• Heat is more disordering to cold objects than to hot objects.

• Analogy: Trading a lively 4-year-old for a quiet octogenarian increases overall disorder.

• Air conditioners transfer heat from cold to hot, which seems impossible.

• Air conditioners convert ordered energy (electricity) into thermal energy, increasing overall entropy.

• The amount of ordered energy consumed depends on the temperature difference.

• Ideally efficient air conditioner equations:

  • heat removed from cold object = work consumed * (temperaturecold / (temperaturehot - temperaturecold))
  • \text{heat removed from cold object} = \text{work consumed} \times \frac{\text{temperature}{\text{cold}}}{\text{temperature}{\text{hot}} - \text{temperature}_{\text{cold}}}
  • heat added to hot object = heat removed from cold object + work consumed
  • Qh = Qc + W

How an Air Conditioner Cools the Indoor Air

• Working fluid transfers heat from colder indoor air to hotter outdoor air.
• Components: evaporator, condenser, compressor.
• Evaporator: Heat moves from warm indoor air to cool working fluid; working fluid evaporates from liquid to gas, absorbing heat (latent heat of evaporation).

How an Air Conditioner Warms the Outdoor Air

• Compressor: Squeezes low-pressure gaseous working fluid into smaller volume, increasing its density and pressure, and raising its temperature, then delivers it as a hot high-pressure gas to the condenser.
• Condenser: Hot working fluid condenses from gas to liquid, releasing latent heat of evaporation, transferring the heat into the cooler outdoor air.
• Cycle repeats, extracting heat from indoor air and releasing it to outdoor air.
• Refrigerators and drinking fountains also use heat pumps.
• Heat pumps can run backward to heat homes in mild climates.
• Working fluids were chlorofluorocarbons (Freons), now hydrofluorocarbons (less ozone damage but are greenhouse gases).

Automobiles

• Internal combustion engine converts thermal energy from burning fuel into work.
• Questions:
  • Obstacles to using fuel's thermal energy?
  • Why need hot and cold objects?
  • Hot and cold objects in a car?
  • Why a cooling system instead of converting all heat to work?
  • Premium vs. regular gasoline?
  • Why aren't gasoline and diesel interchangeable?

Using Thermal Energy: Heat Engines

• Heat engine: Converts thermal energy into ordered energy as heat flows from hot to cold objects.
• Hot object: Burning fuel; cold object: outdoor air.
• Limited thermal energy converts because the engine must add entropy to the cold outdoor air.
• Ideally efficient heat engine equations:
  • work provided = heat removed from hot object * (temperature hot - temperaturecold) / temperature hot
  • W = 2Qh \frac{Th - Tc}{Th}
  • heat added to cold object = heat removed from hot object - work provided
  • Qc = 2Qh - W

The Internal Combustion Engine

• Four tasks in sequence: Fuel and air mixture introduction, ignition, work done by burned gas, get rid of exhaust gas.
• Four-stroke engine: induction, compression, power, exhaust.
• Induction: Piston moves away, inlet valves open, fuel injector adds fuel.
• Compression: Piston moves toward, mixture becomes denser, temperature rises.
• Power: Spark plug ignites, hot gas pushes the piston, doing work.
• Exhaust: Piston moves toward, outlet valves open, exhaust gas is released.

Engine Efficiency

• Goal: extract as much work as possible from fuel.
• Burning fuel produces thermal energy and unnecessary entropy.
• Hotter burned gas helps the engine extract ordered energy.
• Atkinson cycle engines: popular recently to build more fuel-efficient vehicles. Longer power stroke than compression stroke.

Improving Engine Efficiency

• Compression stroke: should squeeze the fuel-air mixture into the smallest volume possible.
• Compression ratio: volume at the start of compression stroke / volume at the end of compression stroke.
• Preignition (knocking): fuel-air mixture ignites all by itself due to overcompression.
• Reduce knocking via uniformly mixing fuel/air and using proper fuel.
• Octane numbers: Higher octane number, more resistant to knocking.

Diesel Engines and Turbochargers

• Diesel engines: no spark plug, compress pure air with very high compression ratio.
• Turbocharger: pumps outdoor air into the cylinder during induction to increase power output.
• Intercooler: removes heat from air passing through the turbocharger.

Multicylinder Engines

• Four or more cylinders timed to always have one cylinder in the power stroke (provides work for other cylinders).
• Crankshaft: converts each piston's reciprocating motion into rotary motion.

Resonance and Mechanical Waves: Additional Notes

• Natural Resonance: The state when the energy in an isolated object causes it to perform a certain motion over and over again.
• Harmonic Oscillator: Simplest and best understood mechanical system in nature. Pendulum, a weight hanging from a pivot, can be said to follow this structure.
• Simple harmonic motion: Regular and predictable oscillation that makes it a superb timekeeper.
• a word equation:
  • Period of pendulum = 𝐵 length of pendulum / acceleration due to gravity.
  • 2 √length of pendulum / acceleration due to gravity. Period of pendulum 2B length acceleration due to gravity
• Electronic Clocks: These modern clocks use quartz oscillators as their timekeepers. Have a Piezoelectric material, which is ideal for electronic clocks.
• Piezoelectric Material: Crystalline quartz. Have mechanical and electrical behavior.
• Sound and Music: In air, sound consists of density waves, patterns of compressions, and rarefactions that travel outward rapidly from their source.
• Transverse wave: A wave in which the underlying oscillation is perpendicular to the wave itself is called a transverse wave.
• Longitudinal Wave: A wave in which the underlying oscillation is parallel to the wave itself is called
  a longitudinal wave.