Radiation

Radiation

Concepts

• Electromagnetic spectrum

• Emissivity

• Absorptivity

• Spectral response

• Planck Distribution

• Planck Law

• Net Radiative heat transfer

Radiation

• Why do you think radiant heat is important?

• Where do you observe radiant heat?

• What wavelengths are involved?

Thermal radiation

• What wavelengths do we refer to with thermal radiation?

• Why?

Using Thermal Radiation

• If we want something to absorb thermal radiation, what properties are necessary?

• If we want something to shed thermal radiation?

• What if we want to use sunlight to warm something?

• What if we want light but not heat?

Blackbody radiation

• What does the spectrum of radiation look like for the sun?

  • Which part of this is thermal?

• What does the spectrum of radiation look like for the earth?

  • Which part of this is thermal?

Planck's law

• How do the wavelengths emitted changes as a substance increases in temperature?

Planck's law

This law relates the intensity of power at each wavelength with the temperature of an object (ideal black-body).

$B_\lambda(\lambda, T) =\frac{2 hc^2}{\lambda^5}\frac{1}{ e^{\frac{hc}{\lambda k_\mathrm{B}T}} - 1}$

Stefan-Boltzmann law

• What predicts the total radiant power from a hot object?

Stefan-Boltzmann law

This law relates the total power emitted to the temperature, surface area, and emissivity of an object.

$P = A \epsilon \sigma T^4$

$A$	surface area	square meters
$\epsilon$	emissivity	dimensionless
$\sigma$	Stefan-Boltzmann constant	$5.67\cdot 10^{-8}$ W per square meter per K$^4$
$T$	temperature	Kelvin

Emissivity

The emissivity is how much radiant power is emitted by a substance relative to an ideal black-body emitter.

If we have two objects with the same surface area and temperature, the object with the higher emissivity emits more radiation.

Absorptivity

Kirchoff's Law of thermal radiation states that a body in thermodynamic equilibrium, the absorptivity and emissivity are equal.

Emissive Power

Emissivities

Spectrum

Spectrum

Net Radiative Heat Transfer

$P = \epsilon \sigma A (T_1^4 - T_2^4)$

Radiation transfer between surfaces

Radiation transfer between surfaces

• Why is it colder on clear nights?