Thermodynamics, Heat Engines, and Cooling Systems Study Notes

Convection cells drive land and sea breezes through rising low pressure (warm air) and falling high pressure (cool air).
Adiabatic Process: A process where energy is transferred only by work ( $W$ ), and no heat ( $Q$ ) enters or leaves the system.
Compression and Expansion: Work done on a gas (compression) reduces volume and increases temperature/pressure (e.g., a bicycle pump). Conversely, expansion (work done by the gas) decreases temperature.
Atmospheric Adiabatic Effects: Rising air expands and cools, leading to condensation and rainfall. Descending air compresses and warms, resulting in a rain shadow.

Work ( $W$ ): Energy transferred by a force $F$ over a distance $s$ .
- Formula: $W = F \times s$
- Units: Newton-metres ( $Nm$ ) or Joules ( $J$ ), where $1\,J = 1\,kg\,m^2\,s^{-2}$ .
Heat ( $Q$ ): Energy transferred due to a temperature difference.
Power ( $P$ ): The rate at which energy ( $E$ ) is transferred.
- Formula: $P = \frac{E}{t}$
- Unit: Watt ( $W$ ), where $1\,W = 1\,J\,s^{-1}$ .
Efficiency (n): The ratio of useful work output to total heat input.
- Formula: $\eta = \frac{W}{Q_{in}} \times 100\%$
Energy Signs: Positive for energy entering a system ( $+Q$ , $+W$ ) and negative for energy leaving ( $-Q$ , $-W$ ).

Heat Engine: A system that converts heat into work (e.g., steam, petrol, or diesel engines).
Energy Balance: $\Delta E = Q - W$ . In stable operation (constant temperature), $\Delta E = 0$ .
External Combustion Engines: Fuel is burned outside the engine cylinder (e.g., James Watt's 1736-1819 steam engine; George Stephenson's 1781-1848 Rocket, built in 1829).
Internal Combustion Engines: Invented by Nikolaus Otto (1832-91) in 1867. Uses a four-stroke cycle:
1. Intake stroke: Petrol-air mixture drawn in.
2. Compression stroke: Mixture compressed.
3. Combustion/Power stroke: Ignition pushes the piston down.
4. Exhaust stroke: Burnt gases expelled.

Heat-Exchange System: Transfers heat between locations (e.g., capillaries in the human nasal passage).
Heat-Conversion System: Transforms internal energy (e.g., adiabatic cooling of air blown through a small mouth opening).
Heat Pump: Moves energy from a cooler area to a warmer area using external work (e.g., refrigerators, reverse-cycle air conditioners).
Coefficient of Performance (COP): Ratio of heat removed to work done.
- Formula: $COP = \frac{Q}{W}$
- Example: A $100\,W$ refrigerator removing $200\,W$ of heat has a $COP = 2.0$ .
Refrigeration Cycle: Involves an Evaporator (absorbs heat), a Compressor (adds work, increases internal energy), a Condenser (radiates heat), and an Expansion valve (rapid adiabatic expansion for cooling).

Countercurrent Heat Exchanger: Overlapping arterial and venous blood vessels reduce heat loss.
In Biology: Whales use this in their tongues; wading birds use it in their legs. Emperor penguins use it in their feet to survive Antarctic winters while preventing tissue freezing.

Professor Hui Tong Chua Interview: Discusses the development of cost-effective freshwater desalination plants and the use of low-grade waste heat.
Applications: Converting natural gas into hydrogen for non-polluting fuel cells or ammonia fertilizers to reduce carbon emissions and slow climate change.
Project Team: Hui Tong Chua, Alexander Christ (PhD student), Cameron Bruce McKenzie, and James Maddock (Final Year Project students).
Desalination Context: Particularly suitable for the arid inland areas of Australia.

Efficiency Loss: Modern internal combustion engines are roughly 35% efficient; the remaining 65% is lost as waste heat.
Calculations:
- A heat engine performing $100\,kJ$ of work with $250\,kJ$ input loses $150\,kJ$ as heat (Efficiency = 40%).
- A $120\,W$ refrigerator removing heat at $560\,W$ has a $COP = 4.67$ .