Thermal Physics A Common Exam Traps

Overview

This page is a compact revision-support guide for common mistakes in Thermal Physics A.

It focuses on:

• definition errors
• wrong formula choice
• unit mistakes
• graph misreading
• practical misconceptions
• careless assumptions

Use this page alongside:

• Thermal Physics A

Definition

An exam trap is a predictable thermal-physics mistake caused by weak definitions, wrong process identification, careless unit handling, or ignoring experimental assumptions.

Why It Matters

Thermal Physics A uses short formulas, but many marks are lost by choosing the wrong model for the process. Clear distinction between temperature change, phase change, and practical heat loss is usually more important than algebra.

Key Representations

Trap 1: Heat vs Temperature

Wrong Idea

“A body contains heat.”

Correct Idea

Heat is energy transferred because of temperature difference.

Temperature is a measure of hotness.

Internal energy is what is stored.

Trap 2: Celsius vs Kelvin Misuse

Wrong Idea

Use Celsius everywhere.

Correct Idea

For ordinary temperature differences:

• (1^\circ\mathrm{C}=1\mathrm{K})

For absolute temperature formulas, use Kelvin.

Conversion:

$$T(\mathrm{K})=T{(}^{\circ }\mathrm{C})+273.15$$

Trap 3: Writing Kelvin With Degree Symbol

Wrong

(300^\circ\mathrm{K})

Correct

$$300\mathrm{K}$$

Kelvin has no degree sign.

Trap 4: Wrong Formula During Phase Change

Wrong

Using:

$$Q=mc\Delta T$$

during melting or boiling.

Correct

During phase change, temperature stays constant.

Use:

$$Q=ml$$

Trap 5: Thinking Temperature Always Rises When Heated

Wrong Idea

If energy enters, temperature must increase.

Correct Idea

During:

• melting
• freezing
• boiling
• condensing

temperature remains constant while state changes.

Trap 6: Confusing Heat Capacity and Specific Heat Capacity

Heat Capacity

Whole object:

$$Q=C\Delta T$$

Unit:

$$\mathrm{J}{\mathrm{K}}^{-1}$$

Specific Heat Capacity

Per kg of substance:

$$Q=mc\Delta T$$

Unit:

$$\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$$

Trap 7: Forgetting Final Common Temperature in Mixing

Wrong Idea

Use separate final temperatures.

Correct Idea

At equilibrium:

• both substances reach the same final temperature

Use one common (T_f).

Trap 8: Assuming Heat Lost = Heat Gained Automatically

Wrong Idea

Always true in any question.

Correct Idea

Only if system is effectively insulated or losses are negligible.

If calorimeter or container absorbs heat, include it.

Trap 9: Wrong Sign or Negative Heat Values

Wrong Practice

Substituting negative temperature drops carelessly.

Better Method

Use magnitudes clearly:

$$\text{Heat lost}=\text{Heat gained}$$

Then assign sensible temperature differences.

Trap 10: Unit Conversion Errors

Common mistakes:

• g not converted to kg
• min not converted to s
• kW not converted to W

Example

$$2.0\mathrm{kW}=2000\mathrm{W}$$ $$5\min =300\mathrm{s}$$

Trap 11: Misreading Heating Curves

Sloping Sections

Temperature changes:

$$Q=mc\Delta T$$

Flat Sections

State changes:

$$Q=ml$$

Flat line does not mean no energy supplied.

Trap 12: Assuming Boiling Means Temperature Must Exceed 100°C

For pure water at standard pressure:

• boiling occurs at (100^\circ\mathrm{C})

But boiling point depends on pressure.

Do not memorise (100^\circ\mathrm{C}) blindly for every context.

Trap 13: Confusing Fusion and Vaporisation

Fusion

Solid (\rightarrow) liquid

Use:

$$l_f$$

Vaporisation

Liquid (\rightarrow) gas

Use:

$$l_v$$

Usually:

$$l_v>l_f$$

Trap 14: Ignoring Electrical Energy Formula

In practical questions, energy often comes from heater:

$$E=IVt$$

Not from (Q=Pt) unless power is already given.

Trap 15: Forgetting Control Correction in Ice Practical

If ice melts naturally from surroundings:

Measured melt mass is too large.

Correct mass melted by heater:

$$m=m_1-m_2$$

where:

• (m_1) = with heater
• (m_2) = control

Trap 16: Assuming Thermometer Reading Equals True Temperature Immediately

Thermometers need time to respond.

Wait for:

• stable reading
• proper mixing
• equilibrium

Trap 17: Thinking Absolute Zero Means Zero Energy

Better Statement

At absolute zero, substances have minimum internal energy.

Not simply “no energy”.

Trap 18: Using Celsius Difference Incorrectly

Temperature rise:

• (20^\circ\mathrm{C}\to35^\circ\mathrm{C}) is (15^\circ\mathrm{C})
• same as (15\mathrm{K})

Temperature intervals are numerically identical in Celsius and Kelvin.

Quick Checklist Before Final Answer

Definitions

• Heat vs temperature?
• (C) vs (c)?
• Fusion vs vaporisation?

Formula Choice

• Temperature change → (mc\Delta T)
• State change → (ml)
• Electrical heating → (IVt)

Units

• kg?
• s?
• W?

Logic

• Common final temperature?
• Any heat losses?
• Correct graph interpretation?

Links

• Thermal Physics A
• Thermal Measurement and Scales
• Heat Capacity and Latent Heat
• Thermal Practicals

Summary

Most Thermal Physics A mistakes come from choosing the wrong thermal model for the process. If you separate temperature change, phase change, and practical corrections clearly, the rest is usually routine.