--- title: "Stefan-Boltzmann Law of Radiation | College Physics I" description: "Stefan-Boltzmann law of radiation says a black body’s emitted power per area equals σT^4, showing why hotter objects radiate much more in College Physics I." canonical: "https://fiveable.me/intro-college-physics/key-terms/stefan-boltzmann-law-of-radiation" type: "key-term" subject: "College Physics I – Introduction" unit: "Unit 14" --- # Stefan-Boltzmann Law of Radiation | College Physics I ## Definition The Stefan-Boltzmann law of radiation says a black body’s radiant power per unit area is proportional to the fourth power of its absolute temperature: E = σT^4. In College Physics I, it is used to model thermal radiation and heat loss. ## What It Is The Stefan-Boltzmann law of radiation is the physics rule that tells you how much thermal energy an object emits from its surface because of its temperature. For an ideal black body, the emitted power per unit area is E = σT^4, where T must be in kelvins. That fourth-power dependence means small temperature changes can create big changes in emitted radiation. In College Physics I, this law shows up when you study thermal radiation, the one heat-transfer process that works even through empty space. Unlike conduction or convection, radiation does not need air, water, or any other medium. That is why the Sun can warm Earth across a vacuum. The law starts with a simplified model called a black body. A black body absorbs all incoming radiation and also emits the maximum possible radiation at a given temperature. Real objects are not perfect black bodies, so you usually adjust the equation with emissivity: E = εσT^4. Emissivity tells you how closely a real surface behaves like the ideal model. The Stefan-Boltzmann constant, σ, has a value of about 5.67 × 10^-8 W/m^2K^4. The units matter because the law gives you radiant power per area, not total energy for a whole object. If you want total emitted power, you multiply by surface area after finding the emission rate per square meter. A good way to read the law is as a before-and-after relationship: before, an object has a certain absolute temperature; after, that temperature determines how strongly it radiates. For example, if temperature doubles, radiation does not double, it increases by 2^4, or 16 times. That is why very hot objects glow so strongly compared with cooler ones. ## Why It Matters This law is one of the cleanest examples of how temperature connects to energy flow in College Physics I. It lets you predict why a warm object in a room loses heat slowly, but a much hotter object, like a kiln or the filament in a lamp, gives off far more radiation. It also bridges the idea of thermal energy with electromagnetic waves. In this course, you are not just memorizing that hot things glow. You are using a formula to relate surface temperature, emissivity, and emitted power, then interpreting what that means for heat transfer. You will also see it in problems that compare surfaces, estimate heat loss, or explain why black surfaces radiate differently from shiny ones. When a problem mentions vacuum, space, stars, or infrared emission, Stefan-Boltzmann is often the tool that makes the situation measurable instead of just descriptive. ## Connections ### Black Body The Stefan-Boltzmann law is built around the black body model. A black body is the ideal surface that absorbs all incoming radiation and emits the maximum possible thermal radiation at a given temperature. Real objects are compared to it so you can estimate how much radiation they actually give off. ### Emissivity Emissivity adjusts the ideal black-body version of the law for real materials. A surface with emissivity near 1 behaves more like a black body, while a low-emissivity surface, like a shiny metal, emits less radiation at the same temperature. That correction is what makes the law useful for real objects. ### Thermal Radiation Thermal radiation is the process the law describes. The Stefan-Boltzmann law tells you how much energy leaves an object as electromagnetic waves because of its temperature. In this course, that is the heat transfer mode that can happen through empty space. ### [Wien's Displacement Law](/intro-college-physics/key-terms/wiens-displacement-law) Wien's Displacement Law and the Stefan-Boltzmann law answer different questions about the same radiation. Wien’s law tells you the wavelength where emission peaks, while Stefan-Boltzmann tells you the total emitted power. Together, they describe both the color and the strength of thermal radiation. ## On the AP Exam A problem set may give you a temperature, emissivity, and surface area, then ask for radiated power or compare two objects. Your job is to plug the temperature into E = εσT^4, keep T in kelvins, and pay attention to whether the question wants power per area or total power. Lab questions may also ask you to explain why a black-painted surface cools faster by radiation than a shiny one, or why the Sun’s energy reaches Earth without a medium. If you see a graph or table of temperature versus emitted energy, look for the fourth-power pattern, not a linear one. ## Key Takeaways - The Stefan-Boltzmann law says emitted radiant power per unit area scales with the fourth power of absolute temperature. - Use kelvins, not Celsius, because the law depends on absolute temperature. - Real surfaces are usually handled with emissivity, so the practical form is E = εσT^4. - This law explains why hot objects radiate so much more energy than cooler ones. - In College Physics I, it shows up in thermal radiation, heat transfer, and comparisons of surface materials. ## FAQs ### What is the Stefan-Boltzmann law of radiation in College Physics I? It is the equation that gives the radiant power emitted per unit area by a black body: E = σT^4. In College Physics I, you use it to model thermal radiation and to compare how strongly objects emit energy at different temperatures. ### Why does the Stefan-Boltzmann law use T to the fourth power? The fourth-power relationship means radiation rises very fast as temperature increases. That is why heating something a little can make it radiate a lot more energy, especially once you get into high-temperature situations like furnaces or stars. ### How do I use emissivity with the Stefan-Boltzmann law? For real objects, multiply the black-body form by emissivity: E = εσT^4. Emissivity ranges from 0 to 1 and tells you how efficiently a surface emits radiation compared with an ideal black body. ### What is the difference between Stefan-Boltzmann law and Wien's displacement law? Stefan-Boltzmann law tells you how much total energy an object radiates. Wien's displacement law tells you the wavelength where its emission is strongest. One gives the amount, the other gives the peak color. ## Related Study Guides - [14.7 Radiation](/intro-college-physics/unit-14/7-radiation/study-guide/1UwAANEpkVywHpfX) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/intro-college-physics/key-terms/stefan-boltzmann-law-of-radiation#resource","name":"Stefan-Boltzmann Law of Radiation | College Physics I","url":"https://fiveable.me/intro-college-physics/key-terms/stefan-boltzmann-law-of-radiation","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/intro-college-physics/key-terms/stefan-boltzmann-law-of-radiation#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:17.061Z","isPartOf":{"@type":"Collection","name":"College Physics I – Introduction Key Terms","url":"https://fiveable.me/intro-college-physics/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/intro-college-physics/key-terms/stefan-boltzmann-law-of-radiation#term","name":"Stefan-Boltzmann law of radiation","description":"The Stefan-Boltzmann law of radiation says a black body’s radiant power per unit area is proportional to the fourth power of its absolute temperature: E = σT^4. 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