Atlas MCAT Course
Physical Foundations → Thermodynamics
Thermodynamics
Energy accounting for heat, work, and gases.
01The idea
Thermodynamics does not explain how molecules move. It explains where energy goes.
If you track heat (Q), work (W), and internal energy (ΔU), everything else follows.
THREAD · Energy vs. rate
02Build the model
AThe first law of thermodynamics
Here ΔU is the change in internal energy, Q is heat added to the system, and W is work done by the system. This equation governs all thermodynamic processes.
BWork done by a gas
Expansion: the gas does work, and energy leaves the system. Compression: work is done on the gas, and energy enters the system. On a P–V diagram, work = area under the curve.
CPressure–volume diagrams
On a P–V diagram, volume is on the x-axis and pressure on the y-axis. Different thermodynamic processes trace different curves. Understanding the shape is more important than memorizing formulas.
DThe four processes
D.1Isothermal: temperature constant
For an ideal gas, PV = constant. Internal energy is unchanged (ΔU = 0), so heat added equals work done (Q = W). The curve is a smooth, downward-sloping hyperbola.
D.2Adiabatic: no heat exchange
Q = 0, so the first law becomes ΔU = −W. For an ideal gas, PV^γ = constant (γ > 1). Expansion cools the gas; compression heats it. The curve is steeper than isothermal on a P–V diagram.
D.3Isobaric: pressure constant
Work is W = PΔV. Heat added changes both volume and temperature. The curve is a horizontal line on the P–V diagram.
D.4Isochoric (isovolumetric): volume constant
Work is W = 0, so the first law reduces to ΔU = Q: all added heat increases internal energy. The curve is a vertical line on the P–V diagram.
EComparing the four processes
| Process | Constant | Q | W | ΔU |
|---|---|---|---|---|
| Isothermal | T | ≠ 0 | ≠ 0 | 0 |
| Adiabatic | — | 0 | ≠ 0 | ≠ 0 |
| Isobaric | P | ≠ 0 | ≠ 0 | ≠ 0 |
| Isochoric | V | ≠ 0 | 0 | ≠ 0 |
This table is worth deep understanding, not memorization.
FIdeal gas law (context anchor)
This is not a process — it’s a state equation. It tells you which variables are linked, and how changing one affects the others.
GThermodynamic cycles (brief)
If a system returns to its initial state, ΔU_cycle = 0, so Q_net = W_net. The area enclosed on a P–V diagram equals the net work done.
HEfficiency (high-level only)
Efficiency relates useful work output to heat input. You do not need engine derivations — just the logic: not all heat can be converted to work.
IConnecting to biology and physiology (context lock-in)
These principles explain breathing mechanics, gas exchange, pressure–volume relationships in lungs, and energy efficiency limits. Again: physics constrains physiology.
03See the connections
The lesson names these links itself. Read the map left to right.
Accounting
- ΔU = Q − W
- W = area under P–V curve
Processes
- Isothermal: ΔU = 0
- Adiabatic: Q = 0
- Isobaric: W = PΔV
- Isochoric: W = 0
Big picture
- PV = nRT (state)
- Cycles: Q_net = W_net
- Not all heat → work
This completes the physics block.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A gas is heated in a rigid, sealed container. Where does the heat go?
Think first.
A gas expands quickly with no time to exchange heat. What happens to its temperature?
Think first.
During an isothermal expansion of an ideal gas, how do Q and W compare?
Think first.
On a P–V diagram, how do you find the work done by a gas?
05MCAT lens
Equation-level checks
ΔU = Q − W (W by the system)
Work = area under the P–V curve
Isothermal ΔU = 0; adiabatic Q = 0; isochoric W = 0
PV = nRT is a state equation, not a process
06Common traps
Trap 1: Isothermal means no heat flows
Misconception
Heat flows; temperature doesn’t change.
Correct model
ΔU = 0, so Q = W.
Trap 2: Adiabatic means temperature is constant
Misconception
No heat is exchanged; temperature changes.
Correct model
Expansion cools; compression heats.
Trap 3: PV = nRT describes a process
Misconception
It is a state equation.
Correct model
It links variables; processes are the paths between states.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
What the first law means
Check against 02 · A: energy bookkeeping.
How work relates to P–V curves
Check against 02 · B and C: area under the curve.
How isothermal and adiabatic processes differ
Check against 02 · D: T constant vs Q = 0.
Why different processes trace different paths
Check against 02 · D and E: different constraints.
If something feels unclear, the missing idea is usually thisThermodynamics tracks energy flow, not molecular detail.
08Remember
REMEMBER · if you keep only a few things
- ΔU = Q − W (W done by the gas).
- Work = area under the P–V curve.
- Isothermal: ΔU = 0, Q = W.
- Adiabatic: Q = 0; expansion cools.
- Isobaric: W = PΔV. Isochoric: W = 0, ΔU = Q.
- Cycle: ΔU = 0, Q_net = W_net.
09Forward hook
This completes the physics block. The next part of the course moves from physical systems to minds and societies.
How do perception, learning, and social structure shape behavior and health?
That leads to psychology and sociology.
Next lessonEntropy & the Second Law of Thermodynamics→