(455) HL Thermodynamics 1st law and basics [IB Physics HL]

Thermodynamics Overview

• Focus on the first law of thermodynamics.

• Basic example: coffee cooling down from hot to cold.

Systems in Thermodynamics

Closed Systems

• No mass transfer in or out.

• Energy transfer (heat) is possible.

Isolated Systems

• No mass or energy transfer.

First Law of Thermodynamics

• Applies to closed systems.

• Equation: Q = ΔU + W

  • Q: Heat transferred (joules)

  • ΔU: Change in internal energy (joules)

  • W: Work done (joules)

• Significance: Describes how internal energy relates to energy transfer.

Work in Thermodynamics

• Work Equation: W = P * ΔV

  • Work done (joules).

  • Pressure (pascals).

  • ΔV: Change in volume (cubic meters).

• Area under the PV curve represents work.

Clausius Conventions

• Work direction impacts sign:

  • Positive if volume expands (right arrow).

  • Negative if volume contracts (left arrow).

Heat and Energy Terms

• Heat transferred to surroundings: Q is positive.

• Internal energy change:

  • Positive ΔU: Internal energy increases.

• Work done by a system: Positive work.

• Work done on a system: Negative work.

Change in Internal Energy

• Internal energy equation: ΔU = 3 * n * R * ΔT or ΔU = 3/2 * N * kB * ΔT.

• Need to know units:

  • ΔU: joules

  • ΔT: kelvin

  • n: number of moles

  • R: gas constant

  • N: number of molecules

  • kB: Boltzmann's constant

Example Problem

Context

• Gas undergoes isovolumetric change (volume stays constant).

• Initial (A) and final (B) states known: pressure, volume, and temperature.

Query

• Find thermal energy removed when transitioning from A to B.

Approach

1. Understand that thermal energy means finding Q.

2. From the first law: Q = ΔU + W

   • Work done is zero in isovolumetric process.

   • Therefore, Q = ΔU.

3. Calculate ΔU using: ΔU = 3 * nR * ΔT.

4. Utilize the ideal gas law to determine n:

   • PV = nRT implies n = PV/T.

Calculation

• Substitute known values from state A into ΔU, taking care of temperature change (ΔT).

• Example calculation yields: Q = -332,353 joules (or -332 joules for significant figures).

Summary

• Key takeaways include understanding the first law of thermodynamics, conditions of closed vs. isolated systems, work definition, Clausius conventions, and applying these to problems involving internal energy and heat transfer.