Part 3! Radiation

What is RADIATION?????

-Radiation is the process of heat transfer by electromagnetic waves

-Radiation is the only form of heat that can travel through a vacuum.

-The sun shines and warms our body. The energy is carried through the vast space between the sun and the earth by radiation. Unlike conduction and convection, radiation does not need a medium to transfer energy; the energy is carried by electromagnetic (EM) waves. Light is a kind of EM waves that is visible to naked eyes.

-A hot object gives out energy by radiation in the form of EM waves. Objects on fire give out visible light because of their high temperatures. Objects at normal temperature range give out mainly infrared radiation that is detected by our skin as warmth. Place your hand near the side, but without touching, a glass of hot water and you will feel what I mean. If the emitted radiation is absorbed by a colder object, the colder object will gain internal energy and rise in temperature.

Heat from the sun reaches us by radiation.

Absorption and emission of radiation

-The higher the temperature of the object, the higher the amount and more energetic the radiation it emits. A cool object still radiates energy but less than a hot object.Consider your hands as being warmed by the infrared radiation from a lamp for example. Though your hands also radiate energy, the lamp radiates energy to your hands at a much higher rate and thus your skin has a net gain in energy and an increase in temperature.

-The appearance of a surface determines its rates of emission and absorption of radiation. In general, shiny coloured (e.g. white or silvery) and metallic surfaces emit or absorb radiation energy slowly since they reflect the radiation. Dark coloured (e.g. black) surfaces emit or absorb radiation energy more effectively. A good emitter of radiation is also a good absorber of radiation and vice versa.

-Now you can see how people use different colours to enhance or reduce heat gain. People usually wear light-coloured clothing in hot summer to reduce radiation heat gain from the sun, and dark coloured clothing to increase heat gain in cold winter. Solar panels are usually black in colour to increase radiation gain.

The surface of this balloon is silvery white in colour. This can help to reduce radiation energy absorption.

The Hubble Space Telescope is covered by a layer of silvery material.

REMEMBER!:D

Shiny or white surfaces do not radiate heat well which is why water in a shiny jug will stay warm longer than one in a black jug.
Dark or dull surfaces absorb heat radiation much more easily than white shiny surfaces.This is why in hot countries the houses and clothing are often white because white reflects the heat better.

The vacuum flask has a double glass shell and a vacuum in between. The glass shell is coated with silvery reflective material. The shell is protected by an outer casing, usually made of plastic or metal.
The vacuum prevents any energy transfer through conduction and convection. The silver coating reflects much of the radiation and thus radiation energy transfer is also minimized. So the contents inside a vacuum flask can be kept at a more or less constant temperature for a long time

-A vacuum flask has a vacuum in a sealed double-walled container. This is to reduce heat loss or heat gain by conduction and convection.

-The walls of the double-walled container are coated with aluminium. This is designed to reduce heat loss or heat gained by radiation.

-The stopper is usually made from plastic or cork. If hot liquid is placed in the container a stopper will reduce heat losses by conduction and convection.

-Modern vacuum flasks are made from double-walled steel containers instead of double-walled glass containers. The main reason for this change is to reduce damage by impact.

Refrigerator door

A refrigerator has many features which lower conduction, radiation and convection energy transfer to reduce the consumption of electrical energy.
Refrigerators usually have a light coloured outer surface to reflect radiation, and so reduce heat entering the refrigeration compartment. The edges of a refrigerator's doors of the refrigerator have flexible seals to prevent cold air inside from mixing with hot air outside, thus reducing convection. The flexible seals are made of good insulating material to further reduce energy transfer through conduction. The thick walls and doors of the refrigerator are also well-insulated to reduce heat transfer through conduction.