ASTA02 Ch-5

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University of Toronto St. George
Astronomy & Astrophysics
Dr.Parandis Khaveri

Astronomy – 5.2 • Yellow hot is hotter than red hot but not as hot as white-hot Definitions: Atom – the smallest unit of chemical element consisting of a nucleus containing positively charged protons, no charged and mass same as proton, neutrons and negatively charge electrons Nucleus – central core of an atom containing protons and neutrons that carries a net positive charge. Molecule – two or more atoms bonded together Heat – energy stores in a material as agitation/movement among its particles Temperature – a measure of agitation/movement among the atoms and molecules of a material - a number related to average speed of particles, intensity of particle motion Kelvin temperature – temp. scale starting at absolute zero (-273 degrees Celsius). Change of 1 Kelvin is 1 degrees in Celsius Absolute zero – theoretical lowest possible temperature at which a material contains no heat energy that can be extracted. 0 on Kelvin scale temperature. Blackbody radiation – radiation emitted by a hypothetical perfect radiator – spectrum is continuous and wavelength of maximum emission depends on blackbody’s temperature Wavelength of maximum intensity – the wavelength at which a perfect radiator emits the maximum amount of energy – depends on object’s temperature Atoms and Subatomic particles – - Charged particles (protons and electrons) are surrounded by electric fields that they produce. When particles change, they emit energy outward at speed of light almost in the form of electromagnetic radiation - Sun is really hot and it has lots of electrons surrounding that change its direction, speed, collide and hence, create electromagnetic radiation. - Although protons can also generate electromagnetic radiation, but because protons has more mass than electrons, electrons gets to move around more than protons Temperature, heat and blackbody radiation - Electrons inside atoms are wandering lose, in constant motion. In hot object, they are more excited/agitated, hence objects gains more thermal energy (in other words, becomes heated) - Temperature of sun and other objects in astronomy is measured in Kelvin scale – used in astronomy because it is based on absolute zero and is directly related to motion of particles in object - Temperature at which objects contains no thermal energy that can be extracted is known as absolute zero – (0 K, -273 degree Celsius) - Water freezes at 273 K and boils at 373 K When electrons are accelerated, they collide with each other and produce heat (thermal energy) but part of that energy is carried away as electromagnetic radiation. The radiation released by an opaque object is called blackbody radiation - At room temperature, these objects (absorber) look black. At high temperature, they visibly glow (like sun) - Blackbody – is a type of light emitted by ordinary incandescent light bulb – electricity flowing through filaments rises the bulb heat to really high temperature and it glows Two laws of blackbody: 1. Wien’s law – states that the hotter a glowing object is, the shorter will be its wavelength of maximum intensity will be, inversely proportional to temperature – because in hot object, particles travel faster, collide more violently and emit more energy photons with shorter wavelength a. Means hot objects tend to emit radiation at shorter wavelengths and look more bluer – hot stars looks blue 2. Stefan-Boltzmann law – states that hotter objects emit more energy than cooler objects of same size, directly proportional to 4 power of temperature. - Hotter objects have stronger intensity at shorter wavelengths, so emits more blue light - Cooler objects emits more red lights, hence looks blue - Hotter objects also emits more total energy than cooler - Human body has temperature of 310 K, emits blackbody radiation in infrared part of spectrum – have wavelength of maximum intensity in infra-red part of spectrum Astronomy ch-5.4 Light, Matter and Motion o Electrons and other particles produce light when they change their speed or direction of motion Ion – an atom that has lost or gained one or more electrons - +ve ion mean fewer electrons than protons Ionization – process by which atom lose or gai
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