Atlas MCAT Course
Physical Foundations → Work, Heat, and the First Law of Thermodynamics
Work, Heat, and the First Law of Thermodynamics
How energy is tracked.
01The idea
Chemical and biological systems do not violate physics — they rearrange energy.
Every reaction, phase change, and physiological process can be understood by tracking:
- Where energy comes from
- Where it goes
- How it is transferred
THREAD · Gradients and flow
The First Law sets the bookkeeping rules.
02Build the model
AInternal energy: the state of the system
Internal energy (U) is the total microscopic energy contained within a system:
- Kinetic energy of particles
- Potential energy from interactions
Internal energy is a state function — it depends only on the current state, not on how the system got there.
BHeat: energy transferred by temperature difference
Heat (q) is energy transferred due to a temperature gradient.
Key properties:
- Flows spontaneously from hot → cold
- Not a property of the system itself
- Exists only during transfer
CWork: energy transferred by force
Work (w) is energy transferred when a force acts over a distance.
In chemistry and biology, work commonly involves:
- Expansion or compression of gases
- Movement against pressure gradients
Work, like heat, is path-dependent.
DThe First Law: energy conservation
The First Law states:
In equation form (conceptually):
Meaning:
- Internal energy increases when energy enters the system
- Internal energy decreases when energy leaves the system
This equation is bookkeeping, not prediction.
EInterpreting signs: direction matters
| Positive | Negative | |
|---|---|---|
| Heat, q | q > 0: heat enters the system | q < 0: heat leaves the system |
| Work, w | w > 0: work done on the system | w < 0: work done by the system |
FExpansion work
When a gas expands:
- It does work on the surroundings
- Internal energy decreases unless heat enters
GBiological context
In biology:
- Cells convert chemical energy into work
- ATP hydrolysis redistributes energy, not creates it
- Gradients store usable energy
03See the connections
The lesson names these links itself. Read the map left to right.
Two ways energy moves
- Heat (temperature difference)
- Work (force over distance)
Both change
- Internal energy, U (a state function)
- ΔU = q + w
Applied
- Expansion work
- Cells converting chemical energy into work
- Gradients that store usable energy
Next: entropy and the Second Law of Thermodynamics.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
A gas expands against its surroundings and no heat enters. What happens to its internal energy?
Think first.
What is the difference between heat and temperature?
Think first.
Internal energy is a state function. Heat and work are not. What does that mean?
Think first.
A process is called “inefficient.” What happened to the energy that didn’t become useful work?
05MCAT lens
The MCAT emphasizes
Energy flow direction
System vs surroundings
Conceptual interpretation over calculation
You are rarely asked to compute work. You are often asked to reason about energy transfer.
06Common traps
Trap 1: Heat and temperature are the same
Misconception
Temperature measures average kinetic energy.
Correct model
Heat is energy transfer.
Trap 2: Energy is lost in inefficient processes
Misconception
Energy is conserved.
Correct model
Energy is redistributed.
Trap 3: Biological systems violate thermodynamics
Misconception
They obey it precisely.
Correct model
They couple processes.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
How heat and work change internal energy
Check against 02 · D and E: ΔU = q + w; energy in raises U, energy out lowers it.
Why expansion can cool a system
Check against 02 · F: an expanding gas does work, so its internal energy falls unless heat enters.
Why ATP hydrolysis releases usable energy
Check against 02 · G: ATP hydrolysis redistributes energy that cells convert into work. Why it is favorable comes in the Gibbs lesson.
Why energy conservation always holds
Check against 02 · D: energy cannot be created or destroyed, only transferred or transformed.
If something feels unclear, the missing idea is usually thisThermodynamics tracks energy, not intent.
08Remember
REMEMBER · if you keep only a few things
- The First Law is bookkeeping: energy is transferred or transformed, never created or destroyed.
- ΔU = q + w. Energy into the system raises U; energy out lowers it.
- Internal energy is a state function. Heat and work depend on the path.
- Heat is energy in transit from hot to cold. It isn’t “contained.”
- An expanding gas does work and loses internal energy unless heat enters.
- Cells don’t break the rules. They redistribute and couple energy.
09Forward hook
The First Law tells us energy is conserved. The next question is:
Why do some processes occur spontaneously while others do not?
That leads directly to entropy and the Second Law of Thermodynamics.
Next lessonThermodynamics→