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PHYS 1470 (2)
Chapter 4

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Physics and Astronomy
PHYS 1470
Paul Delaney

Chapter 4 Matter and Energy in the Universe Rutherford and Atomic Structure  Radioactivity  Placed a radioactive source to send a stream of tiny particles towards a thin sheet of gold foil  Used a Geiger counter to detect the particles and see what happened when they hit the foil  The particle acted as if the foil was not there  Bounced back in next experiment Early Greek Physics  Democritus thought it unlikely that we could divide and subdivide matter endlessly  Indivisible unit of matter which he called an atom  Moving constantly in space  Primary and secondary properties of matter  Secondary properties like color, smell, and taste result from combinations of atoms  Atoms themselves have no color, odor or flavor  Empedocles proposed that nature contained only 4 substances – earth, water, fire and air  All other materials are made of different combinations of these 4 substances which he called elements Dalton and Atoms  Published a theory about the aurora borealis, or northern lights – caused in part by Earth’s magnetism  How atoms mixed in the air, absorbed into water, or combined together  Air is not uniform but a mixture of gases including nitrogen and oxygen  A single gas is a chemical element  Gases combine only in certain proportions  Molecule is a linked set of two or more atoms  First to describe composition of water  Burning wood produces carbon dioxide, water, and carbon in the form of charcoal  Water can be broken down with an electric current  Carbon can not be broken down further  Atomic theory o All elements are composed of atoms o Each element has unique chemical properties and each element is composed of atoms of a unique weight o Compounds are formed when atoms are linked in certain proportion into molecules. Each compound has unique chemical properties o Elements and compounds combine and separate called chemical reactions o Elements are fundamental and cannot be broken down further o One element cannot be changed into a different element by chemical reactions The Periodic Table  Soft metals have planes of atoms that easily slide over each other  Tin atoms provide the grit in the copper lattice that stops the planes from moving feely and give bronze its stiffness  Elements at the far right share the properties of being colorless and odorless gases that do not react with other elements – the noble gases  Elements at the fat left are highly active, but combine with elements in the seventh column to form stable compounds The Structure of the Atom  Electron has a negative electrical charge and is lighter than the lightest atom  Atoms have structure, not tiny hard spheres  Alpha particles are emitted at high speed by some radioactive materials  They have positive electric charge and they are much more massive than the electron  Rutherford added the gold foil with alpha particles and saw these things o Most of alpha particles passed through but were deflected by a small angle o Smaller fraction recoiled to reverse direction  Empty space  Center of atom had positive electrical charge  Almost the entire mass of the atom was concentrated in a tiny nucleus at the center  Neutron is similar in mass to the proton but neutral  The number of protons in the nucleus defines the element  Periodic table is sequence of atoms with increasing number of protons  Number of protons and electrons are always equal  Neutrons affect the weight but not the chemical properties of an element  If two atoms bump together, only the electrons interact  Atomic theory states that elements are fundamental and the on element cannot be changed into another by chemical means  Molecules form when atoms share their electrons  110 different elements or different sized building blocks  90 of these occur in nature the rest are made from a lab  chemical reactions occur when atoms or molecules give up or receive electrons  nanotechnology – fabricate devices from single atoms and molecules Energy  Radiation flowing through space from he sun was intercepted by earth  That sunlight allowed plants to grow  After plants died and were buried deep underground the stores energy was turned into petroleum  Energy = ability to do work  Energy is anything that can cause a change  Potential energy – stored energy or energy waiting to be released  When paper or wood burns, the oxygen in the atmosphere reacts with the carbon to release energy  Gravitational energy – form of potential energy that depends on an objects response to gravity  Dropping a brick – release its gravitational energy as motion  The higher the body is raised against gravity. The more gravitational energy it has because it has more potential to release energy as it crashed down  Mechanical energy – type of potential energy, stored in coiled spring or stretched rubber band  Radiation – changing electric and magnetic forces that can travel through space or a thin gas  Light is an example of electromagnetic energy  Energy is also stored in mass – when the frozen energy stored as mass is released  Kinetic energy – the energy of motion  The more massive or faster the object is, the more kinetic energy is has  Energy is measured in a single set of units – called calories or Joules Heat and Temperature  Heat is the result of friction  The energy of motion when two surfaces rub against each other  Kinetic energy or energy or motion can be converted directly into heat  Moving particles have kinetic energy  Temperature is a measurement of the motion of molecules and atoms  The higher the temperature the faster molecules are moving  The cooler the temperature the slower they are moving  Thermal energy and temperature are not the same thing  A drop of boiling water and a cup of boiling water have the same temperature but one has more heat energy  Temperature does not measure the total amount of heat in an object  It is the measure of the microscopic motions of the molecules or atoms  Temperature where zero corresponds to no motion of atoms or molecules – kelvin scale  Water boils was 373 K and freezes at 273 K and room temperature is 295 K  Absolut zero is 0 K or -273 degrees  Heat is measured of the amount of thermal energy contained  Temperature is a measure of the kinetic energy Potential and Kinetic Energy  Kinetic Energy = ½ mv2  M = mass (kg)  V = velocity (m/s)  Kinetic energy = Joules  Any object that is subject to the force of gravity will have an amount of gravitational potential energy  Gravitational potential energy = m g h  M = mass (g)  G = Earth’s gravitational acceleration (9.8m/s2)  H = height about the ground (m)  Gravitational potential energy = joules  Gravitational potential energy is converted into kinetic energy as the weight falls  Power is the rate at which energy is expended  Power = Energy/ Time  Energy = joules  Time = seconds  Power = joules/second or watts Conservation of Energy  All the different forms of energy are interchangeable  Energy changes form all the time  Transformation of energy  Can be stored  Mechanical energy can be stored in the wind up spring  Battery stores electrical energy  Kepler’s second law – the speed of a plants orbit to its distance from the sun  Plant has most gravitational potential energy and the least kinetic energy when it is farthest from the sun  It has the least gravitation potential energy and the most kinetic energy when it is closest to the sun  after the impact, the total system would have the same amount of energy, but the asteroid's initial energy would now be present as extra heat in the planetary material.  In any closed system, the total amount of energy is constant  This is the of conservation of energy  Energy can change but the total amount is conserved  Total energy in a closed orbit is constant  An elliptical orbit has a continual exchange between kinetic and potential energy but the sum does not change  In circular orbit, neither kinetic nor potential energy changes  Calories – the amount of energy needed to heat 1 kg if water by 1 degrees Celsius Velocity of Gas Particles  All gas particles have kinetic energy due to their motons as they bonce around  Temperature is a measure of this energy  1/2 mv = 3/2 kT -23  K = Boltzmann constant (1.38 × 10 J/K)  V = √(3 kT / m)  V = m/s  The velocity of a gas particle depends on the type of particle and the temperature  Particle velocity is proportional to the inverse square root of the
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