Thermodynamics
Class 11 · Physics
4 ideas in Class 11 Physics, explained in plain words, with games and quizzes to practise.
Heat, Work and Internal Energy
Part of: Energy Rules and Gas Processes
Bodies sharing temperature sit in equilibrium, which is the zeroth law. Internal energy belongs to the state; heat and work only travel during change. Pushing a piston or warming the walls both feed the same store, and heat supplied splits into store plus work done.
Why it matters. Calling heat a stored fluid wrecks every later argument. The split between state and transit runs all engines.
Formulas
- Change in store = heat in minus work done
- Zeroth law: equilibrium with a third body is mutual.
Key words
- Zeroth law
- : Bodies each in equilibrium with a third are mutually in equilibrium.
- Thermal equilibrium
- : The state of shared temperature with no net heat flow.
- Internal energy
- : State energy of a system, not heat or work.
- State variable
- : A quantity fixed by the state, like energy or temperature.
Try it in two minutes
- Warm a gas and squeeze it, comparing effects.
- Push a syringe sealed and feel it warm.
- Track where supplied heat goes.
Try at home. Pump a cycle pump fast and feel the barrel warm.
First Law and Thermodynamic Processes
Part of: Energy Rules and Gas Processes
The first law banks heat as store plus work. Fixed temperature trades all heat into work; insulation trades work against store and temperature; fixed volume banks all heat as store; fixed pressure splits heat between store and work. Cycles return to start, turning net heat into net work.
Why it matters. Each process is a different deal between heat, work and store. Naming the deal unlocks every gas calculation.
Formulas
- Isothermal work W = n R T ln(V2 / V1)
- Adiabatic work W = n R (T1 - T2) / (gamma - 1)
- Fixed pressure work W = P (volume change)
Key words
- Isothermal
- : Fixed temperature, trading all heat into work.
- Adiabatic
- : No heat exchange, trading work against store and temperature.
- Isochoric
- : Fixed volume, banking all heat as store.
- Isobaric
- : Fixed pressure, splitting heat between store and work.
- Cyclic process
- : A loop returning to start, turning net heat into net work.
Try it in two minutes
- Squeeze a syringe fast and slow, feeling the difference.
- Heat a sealed can and watch pressure climb.
- Run a full squeeze-release cycle and net the work.
Try at home. Compare fast versus slow squeezes of a pump.
Second Law and Irreversibility
Part of: Direction of Change and Best Engines
Energy conservation is not enough: heat never fully becomes work unaided, nor creeps cold to hot by itself. Real processes rush through unsettled states and waste work as friction, so they never run backward. Only infinitely gentle, waste-free steps qualify as reversible.
Why it matters. The second law draws the arrow of time and caps every engine. Spotting one-way streets saves chasing impossible machines.
Key words
- Kelvin-Planck statement
- : No process fully turns heat into work with nothing else changing.
- Clausius statement
- : Heat cannot flow cold to hot unaided.
- Irreversible process
- : A process that cannot run backward exactly.
- Reversible process
- : An infinitely gentle waste-free process that can reverse.
- Dissipative effect
- : Waste like friction that blocks reversal.
Try it in two minutes
- Stir water and try to unstir it.
- Release scent and watch it never return.
- Rub hands and feel work become heat.
Try at home. Mix two colours and try to unmix them.
Carnot Engine and Refrigerators
Part of: Direction of Change and Best Engines
The best engine runs two gentle isotherms joined by two sealed adiabats, reaching efficiency one minus cold over hot. No engine between the same pair beats it. A refrigerator reverses the flow, and its performance is heat removed over work paid.
Why it matters. Every power plant and fridge lives under this ceiling. The temperature ratio alone sets the limit.
Formulas
- Carnot efficiency = 1 - Tc / Th
- Efficiency = W / Qh
- COP = Qc / W; dumped heat = Qc + W
Key words
- Carnot cycle
- : Two gentle isotherms joined by two sealed adiabats.
- Efficiency
- : Work out over heat in.
- Coefficient of performance
- : Heat removed over work paid for a refrigerator.
- Reservoir
- : A body holding fixed temperature while giving or taking heat.
Try it in two minutes
- Compute efficiency from two Kelvin readings.
- Raise the hot end and watch the ceiling lift.
- Compare a fridge's inside gain with its bill.
Try at home. Read a fridge label and estimate its performance.
What your child will practise
- Carnot Engine and Refrigerators: Desert Quick Fire (Quick Fire)
- First Law and Thermodynamic Processes: Space Puzzle (Clue Puzzle)
- First Law and Thermodynamic Processes: Space Quick Fire (Quick Fire)
- Heat, Work and Internal Energy: Ocean Quick Fire (Quick Fire)
- Second Law and Irreversibility: Jungle Puzzle (Clue Puzzle)
- Quiz: Carnot Engine and Refrigerators (8 questions)
- Quiz: First Law and Thermodynamic Processes (8 questions)
- Quiz: Heat, Work and Internal Energy (8 questions)
- Quiz: Second Law and Irreversibility (8 questions)