Emissivity

Definition

Emissivity is a measure of an object's ability to emit infrared energy or thermal radiation. It ranges from 0 to 1, where 1 represents a perfect blackbody that emits the maximum possible radiation.

5 Must Know Facts For Your Next Test

1. Emissivity is a dimensionless quantity that ranges between 0 and 1.
2. An object with high emissivity emits more thermal radiation compared to one with low emissivity at the same temperature.
3. Perfect blackbodies have an emissivity of exactly 1, while highly reflective surfaces like polished metals have low emissivities close to 0.
4. The Stefan-Boltzmann Law, which states $E = \sigma \epsilon T^4$, relates emissivity ($\epsilon$) to the total radiant heat energy emitted per unit surface area of a blackbody.
5. Emissivity depends on factors such as material properties, surface texture, and temperature.

Review Questions

• What is the range of values for emissivity?
• How does the emissivity of an object affect its thermal radiation emission?
• According to the Stefan-Boltzmann Law, how does increasing an object's emissivity affect its emitted radiant energy?

Related terms

Blackbody: An idealized physical body that absorbs all incident electromagnetic radiation and re-emits it perfectly with no reflection; has an emissivity of 1.

Stefan-Boltzmann Law: A principle stating that the total energy radiated per unit surface area of a blackbody is directly proportional to the fourth power of its absolute temperature: $E = \sigma T^4$.

Thermal Radiation: The emission of electromagnetic waves from all matter that has a temperature greater than absolute zero; this includes visible light and infrared radiation.