Radiation Hot Metal When metal is heated enough

Radiation

Hot Metal ] When metal is heated enough it glows. ] The color changes with temperature. • Red – orange – yellow – white ] Light can cross a vacuum. • Carries energy with it.

Radiation from Heat ] Heated objects give off electromagnetic waves. • Higher temperature has more radiation ] intensity low energy high energy A hot object gives off a spectrum of frequencies. • Shifts based on temperature frequency

Stefan-Boltzmann Law ] Radiation increases with temperature. ] Energy rate increases as 4 th power of temperature Stefan-Boltzmann Law. ] • • • P = power (W) A = surface area (m 2) T = temperature (K) e = emissivity (0 to 1) s = constant (5. 67 x 10 -8 W/m 2 K 4)

Blackbody ] ] ] Emissivity measures the ability of an object to radiate heat. It is the same to absorb heat. A perfect emissivity of 1 is a blackbody. Low e near 0 High e near 1

Surface Temperature ] ] Assume the sun is a blackbody radiating 3. 9 x 1026 W, and its radius is 7. 0 x 108 m. Ø First find the surface area. Ø A = 4 p r 2 Ø Now apply the Stefan. Boltzmann Law Ø T = 5. 8 x 103 K What is the surface temperature?

Absorbing Heat ] Objects that radiate also absorb. • Absorption coefficients for different materials ] The radiation heat can be converted into thermal energy. • Angle of radiation reduces energy

Seasons ] Earth absorbs radiation from the Sun. ] The tilt of Earth’s axis changes the radiation absorbed. • Summer vs winter

Greenhouse Effect ] ] The atmosphere only permits visible light and radio waves. Visible light warms the surface. The surface absorbs visible light and radiates infrared. The infrared is trapped and warms the air. visible light atmosphere infrared light earth