EPS 7 Fall 2026 Midterm #1 Notes

What is central to understanding the causes of climate change and what to do about it?

Energy

Why did the Industrial Revolution happen in 1700s Britain?

1. The scientific revolution and Enlightenment encouraged testing hypotheses and discovering physical laws.

2. Britain had abundant coal.

3. Watt made steam engines far more efficient.

What is the Newcomen engine (1712)?

• Coal heats a boiler, and the steam fills a piston chamber.

• Cold water sprayed in condenses the steam, dropping the pressure.

• Atmospheric pressure then pushes the piston down to do work, such as pumping water out of mines.

• It was very inefficient because one chamber was repeatedly heated and cooled.

How did Watt improve the Newcomen engine?

• He was fixing a Newcomen model at the University of Glasgow.

• He separated the hot and cold functions into two chambers, greatly increasing efficiency.

• His engines made steam power economical for factories.

• Example: "Old Bess" (1770s) at London's Science Museum ran industrially for 70 years, though Watt called it one of his worst engines.

What is the watt?

A unit of power, the rate of energy flow. W = 1 J/s, or lifting 1 kg by 10 cm every second.

Who was James Joule?

• Joule was more interested in why engines work than in building them.

• He measured the mechanical equivalent of heat: falling weights turned a paddle, and the temperature rise was measured with very accurate thermometers.

• This showed that a given amount of mechanical work produces a given amount of heat.

What did James Joule find?

• Energy is neither created nor destroyed, only moved between places and forms (gravitational potential, kinetic, thermal, photons).

What is a joule?

A unit of energy. One joule is about the energy needed to lift 1 kg (a MacBook Air) 10 cm.

What is average power?

total joules delivered ÷ time.

What is the Watt Challenge?

Deliver as many joules as possible into a bin in 20 seconds.

• The professor's son Felix delivered 6 joules in 20 seconds.

• 6 J ÷ 20 s = 0.3 W.

What are some power reference points?

• LED bulb: about 10 W

• Human at rest: about 100 W (metabolic rate)

• Toaster or microwave: about 1,000 W (1 kilowatt, kW)

• Hair dryer: about 2,000 W

• Clothes dryer: about 3 kW

• Electric car during moderate to peak acceleration: about 10 kW

Why is it important to understand energy to understand global warming?

• Global warming is caused by our use of energy from coal, oil, and gas.

• Solving it means shifting to energy sources that don't emit carbon dioxide.

• Understanding energy and power is therefore essential to understanding both the problem and the solutions.

What are the three modes of energy transport?

• Convection

• Conduction

• Radiation

Convection

• Matter that holds thermal energy physically moves from one place to another, carrying the energy with it

• Usually a liquid or gas; a solid is possible but unlikely.

• Examples:

  • Hot air balloon

  • Rising clouds, especially rain-producing ones, which are warm and buoyant

  • Fire, where air rises through the combustion zone

  • Cold air pouring in through an open winter window while warm air flows out

• Convection doesn't have to mean "hot stuff rising." It only means matter carrying energy. Hot things usually rise because they are buoyant.

Conduction

• Neighboring molecules and atoms jiggle against each other and pass energy along by bumping

• Thermal energy is microscopic jiggling of molecules and atoms

• Net transport goes from the hotter object to the colder one

• Examples:

  • The handle of a cast iron skillet heating up, through a domino effect of jiggling

  • Touching a hot water pipe (heat flows into your hand)

  • Touching a cold window (heat flows out of your hand)

  • A cold handshake

Radiation

• Matter exchanges energy through photons (light) across space

• No contact, no medium, and no motion of matter is required

• All matter with a line of sight to other matter exchanges radiation

• A hotter object gives net energy to a cooler one

• Examples:

  • The sun heating the Earth

  • A heat lamp warming a person

  • A laser burning paper

What is the National Ignition Facility (NIF)?

• It is at Lawrence Livermore National Laboratory, one of four national labs in the Bay Area. Lawrence Berkeley National Lab is right behind UC Berkeley's campus.

• A 1 W laser can light a match. NIF uses a 500 trillion watt laser system made of 192 lasers focused on a tiny hydrogen target.

• The lasers must fire within a billionth of a second of each other to trigger nuclear fusion.

• Fusion is what powers the sun: crushing hydrogen together turns mass into energy.

• NIF is not for the power grid. Its main purpose is nuclear weapons research.

• Lesson: radiation can deliver a huge amount of energy in a short time.

How does a toaster oven use all three modes of energy transport?

• Conduction: metal coil atoms jiggle and knock air molecules, heating the air. Hot air also conducts heat into the toast.

• Radiation: heating filaments emit photons that the toast absorbs.

• Convection: hot air rises and carries energy from the coils to the bread.

How does a convection oven use all three modes of energy transport?

• A regular oven plus a fan, which enhances convection.

• The fan peels hot air away from the heating element faster, and thins the cool air layer around the food.

• This matters because conduction is efficient over short distances and inefficient over long ones (discovered by Joseph Fourier, who returns later in the course).

  • Example: a 50 ft rod in a fire is safe to hold, but a 2 inch rod is not.

• Result: heat transfers slightly faster and more evenly.

How does an air conditioner (gas cycle model) use all three modes of energy transport?

• Conduction and convection dominate, but all three modes play a part.

• The cycle:

  1. Expand a piston of air inside the room, which cools it.

  2. Room heat conducts into the cold piston.

  3. Take the piston outside and compress it, which makes it hot.

  4. Heat conducts out to the outside air.

  5. Bring it back inside and repeat.

• Net effect: heat is moved from indoors to outdoors.

• It requires work (useful energy), but energy is conserved. The useful energy ends up as heat outdoors.

• It can be efficient: 1 J of electricity can move about 2 J of heat.

• Modern units use different working fluids, but the principle is the same.

• Fans enhance convection, and outdoor radiation speeds up the heat release.

How does energy transport between the sun, the Earth, and space?

Only radiation. Not conduction, because they aren't in contact. Not convection, because the sun and Earth send essentially no mass to each other. Earth losing mass to space would take an apocalyptic event like losing the atmosphere or a meteorite impact. Energy leaves the Earth by radiation at the same rate it arrives from the sun.

How does energy transport within the Earth’s atmosphere?

• Conduction: moves energy from the sun-heated surface into the overlying air.

• Convection: moves energy vertically through the atmosphere, which is key to the atmosphere's temperature structure and to why global warming happens.

• Radiation: exchanged between the ground, clear air, greenhouse gases, and clouds, and between different heights in the atmosphere.

What sets the energy balance of the planet?

Conduction, convection, and radiation, global warming happens when greenhouse gas concentrations throw that balance out of whack.

Why is Fahrenheit “stupid”?

• Zero is the freezing point of a salty, briny slush.

• Water freezes at 32°F and boils at 212°F. The professor says this came from a circle-based idea, with boiling placed 180° from freezing (32 + 180 = 212).

• Per the clip: Fahrenheit wanted 0 to be the coldest mix he could make (snow plus nitric acid) and 100 to be human body temperature, but got it slightly off (normal is about 98.4°F).

Why is Celsius “stupid”?

• Zero is where pure water freezes.

• Nothing is special about water as a molecule, apart from us being mostly water.

• It has 100 steps between freezing and boiling.

At what Kelvin is there no heat? Why is Kelvin better?

Zero Kelvin. Rankine also puts zero at no heat, but its increments match Fahrenheit (while one Kelvin increment equals 1 Celsius increment. Heat is microscopic molecular motion (jiggling). At 0 K, molecules stop moving, whatever the material. Nothing is tied to a specific molecule. Kelvin is the universal convention (no clear reason it beat Rankine). Can never be negative.

How to exactly convert from F to C?

Subtract 32, then multiply by 5/9.

How to approximately convert from F to C?

Subtract 30, then divide by 2.

How to convert from C to K?

Add 273 (since water freezes at about 273 K)

What happens at 0 K?

Nothing moves, no heat

What happens at 273 K?

Water freezes (32 F)

What happens at 288 K?

Earth's average surface air temperature (similar to Berkeley's mean).

What happens at 373 K?

Water boils (212°F).

What happens at ~450 K?

Oven baking a cake.

What happens at ~750 K?

Oven self-clean cycle.

What happens at 2,000 - 3,000 K?

Temperature of a torch

What happens at 6,000 K?

Temperature of sun’s photosphere

What happens at 1,000 K?

Fire/candle flame temperature

How should you treat units?

• Treat units exactly like numbers.

  • 2 × elephant = 2 elephants.

  • 9 elephants ÷ 3 elephants = 3 (the elephants cancel).

  • 12 m ÷ 4 = 3 m.

• Don't drop units, and don't tack them on at the end.

What are SI Units?

• SI is the International System of Units, the modern metric system.

• Base units: kilograms (kg), meters (m), seconds (s), joules (J), watts (W), and Kelvin (K).

• Keeping units helps you avoid errors and can show you how to solve a problem.

What are the two rules of thumb for units?

• Multiply or divide so the units work out.

  • Example: how long to drive 10 km at 20 km/hr?

    • 10 km × 20 km/hr gives km²/hr (wrong).

    • 20 km/hr ÷ 10 km gives 1/hr (wrong).

    • 10 km ÷ 20 km/hr = 0.5 hr (correct).

  • Example: 300 students at 100 W each.

    • 100 W/student × 300 students = 30,000 W = 30 kW.

    • The prefix k means 1,000.

• Multiply by a "clever version of one."

  • Take a true equality, such as 1,000 m = 1 km, and divide both sides to get 1,000 m / 1 km = 1.

  • Example: Cal's campus is 1 km². Multiply by (1,000 m / 1 km)² to get 1 million m². A square meter is about the size of a small coffee table.

  • Example: a factory makes 10 g of fuzz per second, and you want kg per hour.

    • 10 g/s × (1 kg / 1,000 g) = 0.01 kg/s.

    • 0.01 kg/s × (3,600 s / 1 hr) = 36 kg/hr.

What should you wear if it is below 273 K?

Freezing. Ice and snow, earmuffs.

What should you wear if it is between 273 - 285 K?

Cold. Pants, sweater, and jacket.

What should you wear if it is between 285 - 290 K?

Cool. Pants and a sweater, no jacket.

What should you wear if it is between 290 - 295 K?

Ideal. Pants and a shirt.

What should you wear if it is between 295 - 300 K?

Pleasant and warm. Shorts and a shirt.

What should you wear if it is above 300 K?

Hot and uncomfortable.

What does Wien’s law link?

Color (wavelength) of light an object emits to its temperature. Discovered by Wilhelm Wien (around 1911 or earlier). All objects emit radiation, and the peak wavelength depends on temperature. λ_peak = 3,000 micron·K ÷ T

• Use T in Kelvin, so the Kelvin cancels and leaves microns.

• Hotter objects emit at shorter wavelengths.

What is radiation exactly?

• Process, not a physical thing: energy moves from one piece of matter to another via fast-moving elementary particles.

What do elementary particles include?

Electrons, protons, neutrons, and photons.

What is another name for photons?

"Light," "electromagnetic radiation”, an individual particle of light

Properties of Photons

• Light is both a wave and a particle, part of quantum weirdness.

• Photons have no mass but carry energy. Your eyes receive energy from your screen but gain no mass.

• They travel at the speed of light: 300 million m/s (about 600 million mph).

• Different types of photons are like different pasta shapes: same "flour," different sizes.

  • Radio waves, microwaves, infrared, visible light, UV, X-rays, and gamma rays are all photons that differ only in wavelength.

• Photons oscillate as they travel, tracing a sinusoidal pattern.

What is Emission?

• Jiggling or oscillating charged particles (or anything that can create momentary charge separation) emit photons, losing energy.

What is absorption?

• Matter that is "ready to be jiggled" absorbs photons and gains energy.

What is the universe’s speed limit?

The speed of light, which is 300 million m/s

How fast does a signal travel between two points on Earth?

40 million m (circumference) ÷ 300 million m/s ≈ 0.1 second (fast enough for real-time calls).

How fast does a signal travel between the Earth and the Moon?

The distance is about 400,000 km (400 million m). 400 million m ÷ 300 million m/s ≈ 1 second delay (awkward phone calls).

What is a wavelength?

Also known as λ, lambda. Distance between adjacent peaks or adjacent troughs. Only thing that distinguishes types of photons. The wiggles are oscillations in the photon's electric and magnetic fields (imagine a little clock on its belly whose hand traces the wave).

What is the wavelength of radio/TV waves?

1-10 M (invisible)

What is the wavelength of microwaves (including kitchen microwave)?

10 - 40 cm (invisible)

What is the wavelength of visible light (sun, light bulbs)?

0.4 - 0.7 microns

What are the wavelengths of infrared light?

3 - 30 microns (invisible). Example: Warmth from a fire or heat lamp is mostly infrared radiation

What is the Micron (micrometer)?

1 micron = 1 um = one millionth of a meter. A rubber band is about 1 mm thick. A micron is 1/1000 of that.

Visible Light

Spans violet/blue (short, about 0.4 μm) to red (long, about 0.7 μm). Ultraviolet is shorter, and near-infrared and infrared are longer.

Why do we see visible light?

• It's what the sun emits most.

• Water is transparent to the visible range, and the eyeball is a ball of water. Infrared would be absorbed before reaching the retina.

Emitted Light

The object itself produces photons, and Wien's law applies. Examples: light bulbs, fire, electric stovetop coils, glowing molten metal, the sun and other stars, and phone or computer screens (visible in a pitch black room).

Scattered Light

Photons from elsewhere bounce off the object. Examples: books, lawnmowers, chairs, cars, cats, dogs, houses, leaves, canoes, baby chicks, and roses.

Are photons conserved?

Absorbed photons cease to exist, and their energy (joules) goes into jiggling the matter. 

Why is a rose red?

• White light is a mixture of wavelengths (white is a construct of our brains).

• The rose absorbs every visible color except red.

• The red photons scatter to your eye, but they were emitted by the sun, not the rose.

What is shortwave?

• Wavelengths under about 3 microns.

• Emitted by the sun, peaking around 0.5 micron.

What is longwave?

• Wavelengths over about 3 microns.

• Emitted by the Earth and terrestrial objects, peaking around 10 microns.

What is the Stefan-Boltzmann Law?

Gives the total power emitted by light. Power per area 5.67 W/m²/K⁴ × (T / 100 K)⁴. Approximation 6 × (T/100)⁴

How to calculate total power?

Area * power per area

How is radiation emitted and absorbed?

By surfaces of liquids and solid, not the interior.

How much power does one human emit?

1,000 W, this is because we are bathed in a “sea” of infrared radiation even though we consume only 100 W of food energy.

What power is emitted by the sun?

70 million W/m

Does a blue star or red star emit more power?

• Wien's law: blue has a shorter wavelength, so the blue star is hotter.

• Stefan-Boltzmann: power per area goes as T⁴, so the hotter star emits more per area.

• The two stars have equal areas, so the blue star emits more total power.

What is a solar system?

Set of planets orbiting a star. Our solar system has 8 planets.

What are the rocky planets?

Close to the sun. Mercury, Venus, Earth, Mars

What are the gas giants?

Far from the sun. Jupiter, Saturn, Uranus, Neptune

Mercury

• Smallest planet and closest to the sun.

• An 88 Earth-day year.

• No atmosphere, just a spherical rock in orbit.

Energy Balance

• Energy flows must balance. Earth is neither steadily gaining nor losing energy over its 4.5 billion years, though temperatures have varied.

• Energy received as radiation from the sun must be balanced by radiation emitted from the planet back to space.

• The sun emits about 70 million W/m² at its surface (from the Stefan-Boltzmann law, T = 6,000 K).

What is total solar irradiance (TSI)?

The power per area a planet receives from its star, measured at the top of the atmosphere on a surface pointed directly at the star (like a black piece of cardboard). System is S.

What is spreading out?

Sunlight thins as it travels out, not from absorption or scattering, just from spreading. It drops from 70 million W/m² at the sun to 1,360 W/m² at Earth.

What is the inverse square law?

TSI scales as 1/R², where R is the distance from the sun. Power through any imaginary sphere around the sun is the same, but the sphere's area is 4πR². A planet twice as far from the sun gets 4 times less TSI. This explains why planets generally get colder farther from the sun.

What is the average intercepted sunlight per m² of planetary surface?

S / 4

What is Albedo?

The fraction of sunlight reflected or scattered back to space

How to calculate absorbed sunlight

(1 - albedo) * S/4

What is Mercury’s temperature?

440 K

Mars

• The second smallest planet and the last of the rocky planets.

• A 687 Earth-day year.

• Very little atmosphere.

• Albedo: about 25%, so 75% of sunlight is absorbed.

• TSI: 586 W/m², far below Mercury's 9,080 and Earth's 1,360, because Mars is farther from the sun.

• Temperature is 210 K

What are Curiosity and Perseverance?

Two rovers on Mars. Curiosity landed in 2012, still operating today. Perseverance landed in 2021, carried helicopter Ingenuity and catchs rock core samples for planned Mars Sample Return mission.

Earth

Planet we live on. Albedo: 30% (clouds, deserts, glaciers, and ice sheets reflect light), so 70% is absorbed. TSI: 1,360 W/m². Up to about 1,000 W/m² reaches the surface on a sunny day.

How thick is the atmosphere?

10 km thick. That’s the distance from the Berkeley Hills to the bay.

What is the atmosphere made of?

80% nitrogen, 20% oxygen. Both are diatomic molecules (N2 and O2)

Nitrogen

Liquid nitrogen forms at 77K, looks like clear water. Can be used to make ice cream. Origin: Outgases from magma during volcanic activity. It stays because N2 is very inert, with two atoms happily bonded to each other and chemically unreactive.

Relationship Between Oxygen and Cyanobacteria

Highly reactive, but cyanobacteria using photosynthesis release oxygen as waste which is where all the O2 comes from

What are cyanobacteria?

Blue-green algae. Really bacteria.

What is photosynthesis?

Process where sunlight energy combines CO2 and H2O into organic matter. 3 O2 go in, but only one ends up in the organic matter, so the rest are released as waste O2.

What happened during the Great Oxygenation Event (~2.4 billion years ago)?

• Before: Very little O2, methane-rich atmosphere. Methane = potent greenhouse gas, so Earth was warm. Oceans were deep red from dissolved iron, no ozone layer, so the surface was bathed in harmful UV

• After cyanobacteria evolved: O2 accumulated and oxidized the methane, weakening the greenhouse effect. Earth froze over in the Huronian glaciation, the longest snowball Earth period. Iron precipitated out of the oceans, moving them toward their blue color. Oxygen is toxic to life (hence antioxidants), so this was a catastrophe for microbes, but good for later oxygen-breathers like us.

What is pressure?

Molecules bounce off surfaces and impart tiny forces that add up. Like a catcher facing 100 pitchers firing baseballs at once.