Three ways energy crosses a temperature difference
You will be able to: Distinguish conduction, convection and radiation in familiar situations.
How does energy reach you from a hot surface?
A metal spoon warms in soup, moving warm water carries energy around the pot, and you feel warmth from the stove without touching it. These situations show three different mechanisms that can operate together.
A useful starting point: Heat flows until temperatures agree →
Words and symbols before equations
- Conduction
- Transfer through microscopic interactions without bulk transport of the material.
- Convection
- Energy transported by bulk movement of fluid.
- Radiation
- Energy carried by electromagnetic waves; no material medium is required.
- Net transfer
- The difference between energy going in opposite directions.
What this picture assumes
Conceptual energy-path diagram, not a quantitative rate model. Highlighted arrows indicate a possible hot-to-cold net transfer. Multiple mechanisms can coexist in a real setup; electromagnetic radiation can cross vacuum.
Connect the picture to the physics
Conduction transfers energy through interactions among nearby particles and, in metals, mobile electrons. The entire spoon need not travel from the soup to your hand. We will quantify conduction through a slab later.
Convection transports warmer or cooler fluid from one location to another. A fan or pump can force the motion; buoyancy can also drive it. Saying “heat rises” is too broad: buoyant warm fluid may rise in gravity, but conduction and radiation can transfer energy in any direction.
Objects emit and absorb radiation. A colder object radiates too; between a hotter and colder object, the net radiative transfer is typically toward the colder object under the stated surroundings. The diagram below is a mechanism map, not a calculation of actual transfer rates.
A worked example, step by step
A hot plate loses 30 J by conduction, 50 J by convection and 20 J by radiation in a chosen interval. No work is done. Find its energy change.
- Choose the plate as the system and count energy entering as positive.
- All three listed transfers leave the plate.
- Q=−30−50−20=−100 J.
- Its internal energy decreases by 100 J if there are no other transfers or changes of macroscopic energy.
Radiation does not require air. Convection requires moving fluid; a temperature difference alone does not identify the mechanism.
Which mechanism can transfer energy across empty space?
Compare with an explanation
Radiation; neither conduction nor convection can cross an ideal vacuum gap without matter.
Predict. Change one thing. Explain.
Select a mechanism to highlight its energy path. Explain what moves: microscopic energy between neighbors, bulk fluid, or electromagnetic radiation. The arrows show direction only, not measured rates.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Selected: Conduction. Energy passes through microscopic interactions without bulk transport of the material. Arrows indicate net direction only.
Conceptual energy-path diagram, not a quantitative rate model. Highlighted arrows indicate a possible hot-to-cold net transfer. Multiple mechanisms can coexist in a real setup; electromagnetic radiation can cross vacuum.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use the relevant particle, temperature or energy relationship to justify your prediction.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA room has a warm radiator, circulating air and a metal support attached to the radiator. (a) Identify transfer through the support. (b) Identify transport by the moving air. (c) Identify direct transfer through electromagnetic waves. (d) Explain why “heat always rises” is inadequate.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: Conduction through microscopic interactions in the support.
- 1 point: Convection carried by bulk air motion.
- 1 point: Radiation.
- 1 point: Buoyant warm fluid may rise, but the direction of conduction/radiation is not universally upward.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Does conduction require bulk flow?
No.
RECALL 2Does convection occur in a solid block?
Not by bulk fluid motion within an ordinary solid block.
RECALL 3Which process works through vacuum?
Radiation.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Three ways energy crosses a temperature difference
- Conduction: microscopic interaction, without bulk material flow.
- Convection: moving fluid carries energy.
- Radiation: electromagnetic waves, including through vacuum.
Remember: Radiation does not require air. Convection requires moving fluid; a temperature difference alone does not identify the mechanism.
Conditions: Conceptual energy-path diagram, not a quantitative rate model. Highlighted arrows indicate a possible hot-to-cold net transfer. Multiple mechanisms can coexist in a real setup; electromagnetic radiation can cross vacuum.
Refresh Kid · AP Physics 2 Unit 1 (official Unit 9) · Objectives 9.3.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.3, objectives 9.3.A. CED effective Fall 2024, current PDF ©2026; checked September 16, 2026. Refresh Kid calls this the first AP Physics 2 unit; College Board numbers it Unit 9, continuing after AP Physics 1. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
Want to work through this with a tutor?
Bring your question about Three ways energy crosses a temperature difference. Your explanation and answers remain free to access.