(455) HL Thermodynamic processes [IB Physics HL]

Thermodynamic Processes Overview

• Focus on work and energy relations.

• Reference: "When you really want to play hard without working hard" (The Office).

Key Concepts

First Law of Thermodynamics

• Energy transfer relates to change in internal energy (ΔU) and work done.

• Work done (W) = P ΔV (area under PV diagram).

  • Rightward movement = Positive work.

  • Leftward movement = Negative work.

Ideal Gas Equation

• P = nRT

• As temperature increases, internal energy (ΔU) increases.

Types of Thermodynamic Processes

1. Isovolumetric Process (Constant Volume)

• PV = nRT; volume is constant.

• Pressure (P) is proportional to temperature (T).

• Graph: P vs. T is a straight line.

  • Increasing T leads to increasing P and vice versa.

• Change in Internal Energy (ΔU): If P decreases, T decreases → ΔU is negative.

• Work done (W): Area under curve is zero → W = 0.

• Heat transfer (Q): Q = ΔU + W → Q is negative.

2. Isobaric Process (Constant Pressure)

• Ignore pressure in PV = nRT; focus on volume.

• Volume is proportional to temperature.

• As temperature increases, volume increases.

• ΔU: If volume increases, T increases → ΔU is positive.

• Work done (W): Positive area under the graph → W is positive.

• Heat transfer (Q): Q = ΔU + W → Q is positive.

3. Isothermal Process (Constant Temperature)

• Ignore temperature in PV = nRT; focus on pressure and volume.

• Pressure is proportional to 1/volume.

• As pressure decreases, volume increases.

• ΔU: If T is constant, ΔU = 0.

• Work done (W): Area under the curve → W is negative.

• Heat transfer (Q): Q = ΔU + W → Q is negative.

4. Adiabatic Process

• Q = 0 (no heat transfer).

• Looks similar to isothermal but steeper.

• Work done (W): Area is positive, moving to the right.

• ΔU: Based on the relation Q = ΔU + W, if Q is zero, ΔU must be negative, because it compensates for the positive work done.

• Adiabatic pressure relation: PV^(5/3) = constant for ideal monatomic gas.

  • Link between states: P_A * V_A^(5/3) = P_B * V_B^(5/3).

  • Useful for solving thermodynamic problems efficiently.