Heat Transfer, Phase Changes, and Thermodynamics

Heat is energy in transit because of a temperature difference. Once the transfer stops, the energy is part of the receiving system’s internal energy.

This lesson supports Chapter 12 of High School Physics.

Energy moves by three pathways

Conduction transfers energy through particle interactions, convection carries energy with moving fluid, and radiation transfers energy by electromagnetic waves. A vacuum blocks conduction and convection, but radiation crosses it. That is how energy from the Sun reaches Earth.

Heating and changing phase use different equations

Without a phase change, \(Q=mc\Delta T\). For a pure substance melting, boiling, freezing, or condensing at constant pressure, energy changes molecular arrangement while temperature can remain constant. The phase-change relation is \(Q=mL\).

The first law tracks internal energy

Using the convention that \(W\) is work done by the system, \[\Delta U=Q-W.\] Energy added as heat raises internal energy unless the system sends some energy out by doing work. The second law gives natural processes a direction and rules out a cyclic heat engine whose sole effect is to absorb heat from one reservoir and convert it entirely into work.

Worked example

How much energy warms \(0.50\text{ kg}\) of water by \(10^\circ\text{C}\)? With \(c=4186\text{ J/(kg}\cdot^\circ\text{C)}\), \(Q=mc\Delta T=0.50(4186)(10)=20{,}930\text{ J}\).

Watch the idea in action

Khan Academy gives a focused explanation of the chapter’s central idea. Pause before each calculation and predict the next step on paper.

Practice questions

1. Name the three heat-transfer pathways.
2. Which pathway can cross a vacuum?
3. Write the sensible-heating equation.
4. Write the phase-change equation.
5. Can temperature stay constant while heat enters a substance?
6. State the first law using work done by the system.

Answers

1. Conduction, convection, and radiation.
2. Radiation.
3. \(Q=mc\Delta T\).
4. \(Q=mL\).
5. Yes, during a phase change.
6. \(\Delta U=Q-W\).

Keep studying

High School Physics includes 12 full-length practice tests: two in the book and ten online.

Return to the High School Physics learning hub, or move between the previous chapter lesson and the next chapter lesson. Each lesson opens in a new tab so the hub stays available.