Thermal Physics Common Exam Traps

Overview

Thermal Physics Common Exam Traps is now a legacy umbrella note for the split thermal topics.

Use:

• Thermal Physics A Common Exam Traps
• Thermal Physics B Common Exam Traps

for the current canonical revision sheets.

Many errors come from weak definitions, careless units, wrong sign conventions, mixing Celsius and kelvin, or using the wrong formula for the process.

Definition

An exam trap is a predictable mistake caused by misunderstanding a concept, applying a formula outside its conditions, poor graph interpretation, or careless algebra and units.

Why It Matters

Thermal physics formulas are short, but many marks are lost through incorrect assumptions rather than difficult calculations:

$$Q=mc\Delta T$$ $$pV=nRT$$ $$\Delta U=Q+W$$

Key Representations

Heat is not stored in an object. Internal energy is stored; heat is energy transferred due to temperature difference.

Temperature does not flow. Heat flows from higher temperature to lower temperature.

Same temperature does not imply same internal energy.

Negative Celsius temperature does not mean no molecular motion. Only $0\text{K}$ is absolute zero.

Use $Q=mL$ during phase change, not $mc\Delta T$.

For pure substances at constant pressure, melting and boiling occur at constant temperature.

Multi-stage thermal problems must be split into stages, such as warming ice, melting ice, and warming water.

Use kelvin in gas laws:

$$\frac{V_1}{T_1}=\frac{V_2}{T_2}$$

not Celsius temperature.

Simple gas laws assume constant amount of gas. If gas leaks or is added, use:

$$pV=nRT$$

Temperature relates to average kinetic energy per molecule:

$$⟨E_k⟩=\frac{3}{2}kT$$

This wiki uses:

$$\Delta U=Q+W$$

where $W$ is work done on the system. If using work done by gas:

$$W_{\text{by}}=-W$$

Signed area under a $p$-$V$ graph gives:

$$W_{\text{by}}=\int pdV$$

not heat.

Common Exam Traps

Do not say “the object contains heat”; say internal energy is stored.

Do not use $mc\Delta T$ during melting or boiling.

Do not use Celsius in proportional gas-law calculations.

Do not assume isothermal means no heat transfer.

Do not assume adiabatic means constant temperature.

Do not assume a closed cycle means zero work; it means $\Delta U=0$.

Do not confuse vertical and horizontal lines on $p$-$V$ graphs: vertical means constant volume and horizontal means constant pressure.

Do not ignore heat losses in practicals:

$$IVt\ne mc\Delta T$$

exactly unless losses are negligible.

Before submitting, check kelvin use, whether the process is temperature change or phase change, whether work sign is correct, whether graph area means work, and whether units have been converted.

High-scoring explanations use phrases such as average kinetic energy increases, intermolecular potential energy increases, more energetic molecules escape, collisions with walls become more frequent, and no net heat transfer at equilibrium.

Links

• Thermal Physics (Transition Hub)
• Thermal Physics A
• Thermal Physics B
• Thermal Physics A Common Exam Traps
• Thermal Physics B Common Exam Traps
• Temperature and Heat
• Thermal Properties of Matter
• Ideal Gases
• First Law and Thermodynamic Processes
• p-V Diagrams and Cycles
• Thermal Practicals