Fundamental Energy Systems, Radiation, and Climate Physics

Posted on Jan 14, 2026 in Physics

1. Energy and Systems (Weeks 3–4)

  • Energy is a real, quantifiable physical quantity.
  • Exists in many forms: kinetic, potential, thermal, electromagnetic, nuclear, chemical, sound.
  • Energy can change form, but total energy is conserved.
  • Open system: exchanges both energy and mass.
  • Closed system: exchanges energy only.
  • Isolated system: exchanges neither energy nor mass.
  • Scale and duration matter: a system may look isolated on short timescales but open on long ones.

2. Sound Energy and Intensity

  • Sound energy is described by intensity (power per area).
  • The decibel (dB) scale is logarithmic.
  • Doubling intensity does not double dB.
  • Sound spreads spherically, so intensity drops with distance.
  • MC cue: “Two identical sound sources together” → intensity doubles, dB increases slightly.

3. Gravitational Potential Energy (Week 4)

  • Near Earth: gravitational potential energy depends on height.
  • Far from Earth: depends on distance from the center.
  • Negative potential energy means an object is bound.
  • If an object moves closer → potential energy becomes more negative.
  • MC trap: moving closer = losing potential energy, not gaining it.

4. Electromagnetic Radiation and Light (Weeks 5–6)

  • Includes: radio → microwave → infrared (IR) → visible → ultraviolet (UV) → X-ray → gamma.
  • Higher frequency = higher energy.
  • Light behaves as both a wave and a particle (photon).
  • Earth’s atmosphere is transparent mainly to visible light and radio waves.
  • If asked “Which EM regions reach Earth easily?” → visible + radio.

5. Radiative Intensity and Distance

  • Light from stars spreads spherically.
  • Intensity decreases with distance.
  • Power of the source stays constant.
  • MC cue: farther ≠ dimmer source, just spread over a larger area.

6. Radiative Equilibrium of Planets (Week 11)

  • Planet temperature is set by the balance of incoming and outgoing radiation.
  • Size of the planet does not matter for equilibrium temperature.
  • Albedo matters: high albedo → cooler; low albedo → warmer.
  • If two planets or asteroids differ only in size → they have the same temperature.

7. Greenhouse Effect and Infrared Opacity

  • Atmospheres trap outgoing infrared (IR) radiation.
  • Surface temperature becomes greater than the radiative equilibrium temperature.
  • The greenhouse effect is not caused by sunlight being trapped.
  • It is caused by outgoing IR being absorbed and re-emitted by atmospheric gases.
  • MC cue: greenhouse ≠ glass simply trapping heat directly; it’s about IR opacity.

8. Seasons and Earth’s Axial Tilt

  • Seasons are caused by axial tilt, not distance to the Sun.
  • Tilt changes the solar angle and daylight length.
  • If tilt were 90° → extreme seasons, repeating yearly.
  • If asked “Why do seasons exist?” → tilt + changing sunlight angle.

9. Electricity Basics (Week 12)

  • Current is the rate of charge flow.
  • Direction is defined by positive charge motion.
  • Electric field points from high voltage → low voltage.
  • Positive charges move with the electric field.
  • Quick cue: “Field direction?” → high → low voltage.

10. Batteries and Chemical Energy

  • Batteries do not store charge.
  • They store chemical energy.
  • Inside the battery: charges are pushed uphill.
  • Outside the battery: charges flow downhill.
  • MC cue: battery magic = internal energy pump.

11. Photovoltaic Cells and Charge Separation (Week 13)

  • PV cells act as current pumps, similar to batteries.
  • Light ejects electrons in the depletion (PN junction) region.
  • The electric field sweeps charges out.
  • Maximum output occurs when sunlight is normal to the panel.
  • Effective area decreases with angle.
  • MC cue: electron ejection location → depletion zone.

12. Heat Engines and Efficiency

  • Thermal efficiency = useful work / input heat.
  • Carnot efficiency = maximum possible efficiency.
  • No heat engine can be 100% efficient.
  • Typical components:
    • Boiler → heat source
    • Turbine → does work
    • Generator → electrical output
    • Condenser → heat sink
  • MC cue: heat sink = condenser; useful work → turbine → generator.

13. Power Generation and Energy Sources

  • Combined-cycle natural gas plants achieve the highest efficiency.
  • Hydro > wind (water is a much denser fluid than air).
  • Natural gas emits approximately 50% less CO₂ than coal.
  • Main pollutant: CO₂.
  • MC cue: switching from coal to gas helps but does not eliminate emissions.

14. Nuclear Energy and Mass–Energy Conversion

  • The proton–proton chain converts ~0.7% of mass into energy.
  • This process requires quantum tunneling.
  • Fission example: U-235 + neutron → Ba + Kr + neutrons + energy.
  • Positron + electron → annihilation → photons.

15. Ecology and Light Pollution

  • Artificial night light:
  • Attracts nocturnal insects.
  • Disrupts feeding and mating.
  • Increases predation risk.
  • MC cue: negative impact = behavioral disruption, not physical damage.