Thermodynamics

Grade 11 · Science · Study Guide

Explore the laws of thermodynamics, heat engines, entropy, and energy transfer processes at an advanced level.

Thermodynamics governs how energy transforms and flows as heat and work. The Zeroth Law establishes thermal equilibrium as a transitive property: if body A is in equilibrium with B, and B with C, then A and C are also in equilibrium — this underpins our ability to define temperature.

The First Law states that energy is conserved: ΔU = Q - W, where ΔU is the change in internal energy, Q is heat added to the system, and W is work done by the system. This means you cannot create energy from nothing.

The Second Law introduces entropy (S) — a measure of disorder. Spontaneous processes in isolated systems always increase total entropy. No heat engine can be 100% efficient; some energy is always lost to the cold reservoir. The Carnot engine defines the theoretical maximum efficiency: η = 1 - T_cold/T_hot, where temperatures are in Kelvin.

Thermodynamic processes include: isothermal (constant temperature, ΔU = 0), adiabatic (no heat exchange, Q = 0), isochoric (constant volume, W = 0), and isobaric (constant pressure). The ideal gas law PV = nRT connects pressure, volume, and temperature for ideal gases.

The Third Law states that the entropy of a perfect crystal approaches zero as temperature approaches absolute zero (0 K), establishing an absolute scale for entropy.

Key terms

Entropy (S)
A thermodynamic quantity representing the degree of disorder or randomness in a system; increases in spontaneous irreversible processes.
Carnot Efficiency
The maximum theoretical efficiency of a heat engine: η = 1 - T_cold/T_hot (in Kelvin).
Adiabatic Process
A thermodynamic process in which no heat is exchanged between the system and surroundings (Q = 0).
Internal Energy (U)
The total kinetic and potential energy of all molecules within a system; related to temperature.
Heat Engine
A device that converts heat energy into mechanical work by operating between a hot and cold reservoir.
Isothermal Process
A process occurring at constant temperature; for an ideal gas, internal energy does not change (ΔU = 0).

Flashcards

Zeroth Law of Thermodynamics
If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other.
First Law of Thermodynamics
Energy conservation: ΔU = Q - W. Internal energy changes equal heat added minus work done by the system.
Second Law of Thermodynamics
Entropy of an isolated system never decreases; heat naturally flows from hot to cold.
Third Law of Thermodynamics
The entropy of a perfect crystal at absolute zero (0 K) is zero.
Specific Heat Capacity
The heat required to raise 1 kg of a substance by 1°C (or 1 K): Q = mcΔT.
Latent Heat
Heat absorbed or released during a phase change at constant temperature: Q = mL.
PV Diagram
A graph of pressure vs. volume for a gas; the area under the curve represents work done by or on the gas.
Isochoric Process
Constant volume process; no work is done (W = 0) and all heat input changes internal energy.

Questions

  1. A Carnot engine operates between 600 K and 300 K. What is its maximum efficiency?

    1. 25%
    2. 33%
    3. 50%
    4. 67%
    Show answer

    Answer: 50%

    Carnot efficiency = 1 - T_cold/T_hot = 1 - 300/600 = 1 - 0.5 = 0.5 = 50%.

  2. In an adiabatic process, the internal energy of the system can still change even though no heat is exchanged.

    Show answer

    Answer: True

    In an adiabatic process Q = 0, but the First Law gives ΔU = -W. Work done on or by the system still changes internal energy.

  3. The First Law of Thermodynamics can be expressed as ΔU = Q - _____, where the blank represents work done by the system.

    Show answer

    Answer: w

    The First Law: ΔU = Q - W, where W is work done by the system on its surroundings.

  4. Which process involves no change in pressure?

    1. Isothermal
    2. Adiabatic
    3. Isochoric
    4. Isobaric
    Show answer

    Answer: Isobaric

    Isobaric means constant pressure (iso = same, baric = pressure). Isothermal = constant T, adiabatic = no heat, isochoric = constant volume.

  5. The entropy of an isolated system can decrease spontaneously according to the Second Law of Thermodynamics.

    Show answer

    Answer: False

    The Second Law states total entropy of an isolated system can only increase or remain constant in reversible processes, never spontaneously decrease.

Get a personalized course on Thermodynamics built around your child.