Bell Ringer Name: 2/23/2009 What do you believe color is? Response

Radiation Radiation, is one more way in which heat transfer occurs Radiation from heat transfer must not be confused with radioactivity!

Radiation All objects emit radiant energy in a mixture of different wavelengths. We will study waves and their wavelength in later chapters, but a brief introduction will have to do here.

Radiation Picture a smooth lake or pond. A stone is tossed into the water. After the initial splash waves move away from the center in evenly spaced increments called waves.

Radiation A wavelength is the distance between the tops (crests) of consecutive waves. If the distance between the crests is reduced the wavelength shortens.

Radiation A few wavelengths that you may be familiar with are: –Infrared –Visible light –Ultraviolet

Radiation The sun is our largest source of radiant energy. There is nothing but vacuum between the sun and Earth’s atmosphere, so how can you feel the energy from the sun on your skin on a sunny day?

Radiation The further away an object is from a radiant energy source, the less energy it will receive.

Radiation Back to weather –We know the sun releases approximately the same amount of radiant energy from every spot on its surface –How does the amount of radiant energy received on the Earth’s equator, compare to that received at the poles?

Radiation The equator is closer to the sun, than the poles, and will receive more energy per square meter!

Radiation Mr. Everson like the farm and cars, I like astronomy. Who can provide a good reason why the planets Mercury and Venus are hot, while Mars and Pluto are cold?

Radiation There are two important radiation laws that are used in physics and especially in astronomy. –One law relates the amount of radiant energy released with an object’s temperature –The other law relates an object’s temperature to it’s color

Radiation We won’t be studying the details but it’s good to know that there is mathematics that describe how this is possible. We will look at some of the simple math in a couple more chapters.

Radiation Remember the demonstration where I held my fingers on the side of the flame without being burnt. This has more to do with radiation than convection.

Radiation The Earth receives a lot of radiant energy from the sun in the infrared wavelengths. If you sit close to a fireplace, you feel the heat on your eyelid if they are closed. –Put on a pair of sunglasses and the heat is reduced, why?

Radiation The Earth receives a lot of radiant energy from the sun in the infrared wavelengths. If you sit close to a fireplace, you feel the heat on your eyelid if they are closed. –Put on a pair of sunglasses and the heat is reduced, why? –The sunglasses block some of the infrared wavelengths and allow the visible light wavelengths through! The same with sunlight.

Radiation Why is sun block used for?

Radiation Why is sun block used for? To block ultraviolet wavelengths from the sun and prevent burning?

Absorption Absorbing and reflecting are opposite processes

Absorption An object or material that is a good absorber of radiant energy, reflects very little. Can you think of any objects or materials that absorb a lot of radiant energy?

Absorption How about a color of clothing that absorbs energy?

Absorption How about a color of clothing that reflects energy?

Absorption Perfect absorbers are black because they absorb almost all visible light radiation and reflect almost none. –They don’t absorb because they are black. There is a difference. The pupil of your eye is a good example.

Absorption Why do you suppose photographs taken with a flash sometimes show red pupils?

Absorption Why do you suppose photographs taken with a flash sometimes show red pupils? Light is reflected off the retina in the back of the eye!

Absorption Why don’t you see white in the closed box?

Absorption Why don’t you see white in the closed box? The light enters, is reflected many times and loses a little energy with each reflection. Very little comes back out.

Emission Good absorbers are good emitters Poor absorbers are poor emitters Objects with mirror-like surfaces reflect most radiant energy they encounter

Emission Tall radio antennas are good absorbers and emitters in the radio wavelengths. Extending the example, dark objects that absorb a lot of radiant energy must also emit a lot of radiant energy.

Emission As an example: –Imagine two water containers, one white or with a mirror-like surface and one black –Add water of the same temperature into both of them –Place a thermometer in each Which cools faster?

Emission When absorption rate equals emission rate the temperature remains constant or unchanged.

Emission Back to weather again. On a sunny day, the Earth’s surface is a good absorber At night, the Earth’s surface is a good emitter

Emission Is it a good idea to paint a radiator in your home black or silver?

Emission Is it a good idea to paint a radiator in your home black or silver? Black. Although most of the heat is transmitted via convection to the air, a dull black color would increase the contribution made through radiation.

Newton’s Law of Cooling An object with a temperature different from its surroundings will eventually reach thermal equilibrium. The rate of object cooling depends upon how much hotter the object is than the surroundings.

Newton’s Law of Cooling Newton studied many things besides gravity. He also studied how objects cool.

Newton’s Law of Cooling The rate of cooling of an object, whether by conduction, convection or radiation is approximately proportional to the temperature difference between the object and its surroundings. Rate of cooling ~

Newton’s Law of Cooling An object with a temperature different from its surroundings will eventually reach thermal equilibrium. The rate of object cooling depends upon how much hotter the object is than the surroundings.

Newton’s Law of Cooling An object with a temperature different from its surroundings will eventually reach thermal equilibrium. The rate of object cooling depends upon how much hotter the object is than the surroundings.

Global Warming The Greenhouse Effect Almost everyone has experienced the temperature difference in an automobile or greenhouse due to window glass.

Global Warming The Greenhouse Effect This is a true because of a combination of things, some of which we’ve looked at. –All things radiate energy –The radiation wavelength depends upon temperature

Global Warming The Greenhouse Effect This is a true because of a combination of things, and some that are new. –High temperature objects radiate short waves –Low temperature objects radiate long waves –Substances are transparent to some wavelengths

Global Warming The Greenhouse Effect Let’s consider the short and long waves The sun is a high temperature object compared to a fire –High temperature objects radiate short waves –We block Ultraviolet (UV) rays from the sun to prevent burning. Therefore UV rays are short waves.

Global Warming The Greenhouse Effect Let’s consider the short and long waves Fire is a low temperature object compared to the sun –Low temperature objects radiate long waves –We block Infrared rays from a fire and feel cooler Therefore Infrared rays are long waves.

Global Warming The Greenhouse Effect Now let’s consider transparent and opaque, which are opposites Glass is transparent and lets visible light through A piece of metal is opaque and does not let visible light through, it is reflected

Global Warming The Greenhouse Effect A substance that is transparent allows waves through. –These waves lose some energy when traveling through the substance –These waves reflect off surfaces that don’t absorb them and lose a little more energy

Global Warming The Greenhouse Effect For radiation from the sun reaching Earth this means –The atmosphere is opaque to some waves and these are reflected –Some waves are absorbed by the atmosphere –The atmosphere is transparent to some waves and these are transmitted through the atmosphere

Global Warming The Greenhouse Effect The atmosphere lets through short waves, just like glass –These waves lose energy and get a little longer –Some of these waves are reflected and eventually reach the glass or atmosphere again –When they cross the threshold from being transparent to the glass or atmosphere, they are trapped inside because they reflect

Global Warming The Greenhouse Effect Before man came along, radiation from the sun balanced the natural terrestrial radiation from Earth’s surface (on average) –We did have extremes called ice ages But man is releasing energy from fossil fuels and nuclear energy In addition, man is changing the composition of the atmosphere and changing its transparent and opaque abilities –This is global warming