Physics Unit 2

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  • Created by: YOIMO
  • Created on: 11-06-17 12:02

P2.2 Forces

  • distance is y-axis and time taken is the x-axis
  • acceleration- is the change of velocity per second (m/s2)
  • the stopping distance of a car depends on the thinking distance and breaking distance
  • thinking distance is increased by drugs and alcohol and the braking distance is increased by weather conditions, road surface or the condition of the car
  • when an object falls through a liquid, the liquid exerts a drag force on the object, resisting its motion. the faster the object falls, the bigger the drag force becomes, until eventually, it will be equal to the weight of the object. the resultant force is now zero, so the body stops accelerating. it moves at a constant velocity called the terminal velocity.
  • Hooke's law- the extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded
  • limit of proportionality- the limit of Hooke's law applied to the extension of a stretched spring
  • the stiffer a spring is, the greater its spring constant
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P2.3 Work, energy and momentum

  • both work and energy have the same unit J
  • the greater the mass & the faster the speed of an object, the more kinetic energy it has
  • the greater the mass & velocity of an object, the greater its momentum (Kgm/s)
  • conservation of momentum- the total momentum before the interaction of objects and after is equal OR another way to say it is that the change in momentum is zero
  • the interaction could be a collision or explosion where they may either move together or apart
  • an example of momentum is when a gun shoots a bullet and the gun 'recoils' with equal momentum in the opposite direction as the bullet
  • when vehicles collide, the force of the impact depends on mass, change of velocity and the duration of the impact
  • when 2 vehicles collide: they exert equal and opposite forces on each other, their total momentum is unchanged
  • crumple zones and side impact bars in cars are designed to fold in a collision. this increases the impact time and so reduces the force on the car and people in it
  • seat belts and air bags spread the forces on the body across a larger area
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P2.4 Current electricity part1

  • when 2 insulating materials are rubbed together, electrons are rubbed off one material and deposited on the other. electrons have a negative charge so the material that has gained electrons becomes negatively charged. the one that has lost electrons is left with a positive charge. this process is called charging by friction. a polythene rod and a dry cloth are used for this experiment where the cloth loses electrons to form a positive charge
  • ammeters are always connected in series
  • potential difference (V) is measured with a volt metre and is always connected in parallel
  • resistance is measured in ohm's
  • any component that obeys Ohm's law is called an ohmic conductor
  • Ohm's law- the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor
  • filament bulb: resistance increases with increase of the filament temperature 
  • diode: forward resistance low, reverse resistance high
  • thermistor: resistance decreases if its temperature increases
  • LDR: resistance decreases if the light intensity on it increases
  • total resistance= all the resistance of each component added together
  • the current through each component depends on its resistance. the bigger the resistance of a component, the smaller the current through it
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P2.4 Current electricity part2

  • in a parallel circuit the potential difference is the same across each component
  • (http://static.aqa.org.uk/assets/image/0003/47505/Electrical-circuit-symbols.jpg)(http://www.bbc.co.uk/staticarchive/e930b98b99a1eb05c36dd6ecc7b7883fba393e2f.gif)
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P2.5 Mains electricity

  • the frequency of the mains in the UK is 50Hz and the voltage is 230V
  • the live wire alternates between a positive and negative potential. neutral wire has zero Volts
  • the live wire alternates between a peak voltage of +325V and -325V
  • plug wires: earth (green & yellow), neutral (blue), live (brown)
  • appliances with metal cases must be earthed so the case is attached to the earth wire cable
  • plastic cased appliances do not need to be earthed as they are said to be double insulated so they only have a live and neutral wire
  • the pins in a plug are made from brass as it's a good conductor, hard and it will not rust or oxidise
  • the circuit breaker is an electromagnetic switch which does the same job as a fuse
  • a RCCB (Residual current circuit breaker) does the same job as both the fuse and circuit breaker but it works faster and notices the difference between the live and neutral by cutting off the live
  • filament bulbs and halogen bulbs are much less efficient than low-energy bulbs and do not last as long
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P2.6 Radioactivity part1

  • radioactive substances contain unstable nuclei that become stable by emitting radiation
  • 3 types of radiation: alpha, beta and gamma
  • radioactive decay is a random event which we can't predict or influence when it happens
  • background radiation is radioactive substances in the environment, or from space, or from devices such as X-ray machines
  • scientists thought that atoms consisted of spheres of positive charge with electrons stuck into them which looked like a plum pudding model
  • Rutherford used the measurements from the alpha particle scattering experiments as evidence that an atom has a small, positively charged, central nucleus where most of the mass of the atom is located
  • alpha particles: 2 protons & 2 neutrons, positive charge, large with lots of collisions, strongly ionising, don't penetrate far into a material, stopped by thin sheet of paper & human skin, deflected by electric & magnetic fields
  • beta particles: 1 electron, negative charge, smaller, faster, less ionising, penetrate through paper, stopped by aluminium, deflected by electric and magnetic fields
  • gamma radiation: no charge as it's an electromagnetic wave released from the nucleus, no mass, weakly ionising, very penetrating, several cm of concrete to absorb most of the radiation, NOT deflected by electric & magnetic fields
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P2.6 Radioactivity part2

  • alpha sources are used in smoke alarms, the alpha particles are not dangerous because they are poorly penetrating with a half-life of several years
  • beta sources are used for thickness monitoring in the manufacture of things like paper or metal foil. alpha particles would be stopped by a thin sheet of paper and all gamma rays would pass through it. the half-life is many years
  • gamma and beta sources are used as tracers in medicine. the source is injected or swallowed by the patient. its progress around the body is monitored by a detector outside the patient. the half-life is a few hours so that the patient is not exposed to unnecessary radioactivity
  • radioactive dating is used to find the age of ancient material
  • carbon dating is used to find the age of wood and other organic material
  • uranium dating is used to find the age of igneous rocks
  • the half-life is the same for any sample of a particular isotope
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P2.7 Energy from the nucleus part1

  • nuclear fission is the splitting of an atomic nucleus. normally it's uranium and plutonium
  • for fission to occur the nucleus must absorb a neutron. the nucleus then splits into 2 smaller nuclei. in this process 2 or 3 neutrons are emitted and energy is released. more energy is released here than in a chemical process such as burning
  • nuclear fusion is the process of forcing 2 nuclei close enough together so they form a single larger nucleus. fusion occurs in stars and requires a lot of energy to join together in a magnetic field.
  • radon gas is an alpha emitting isotope that seeps into houses in certain areas through the ground which is a health hazard if breathed in
  • radiation sources are from food, drink, travel, X-rays (ionising) and nuclear weapons testing
  • uranium and plutonium are chemically removed from used fuel rods from nuclear reactors as these substances can be used again. the remaining radioactive waste must be stored in secure conditions for many years
  • to reduce exposure workers should: keep far away from the source, spend little time being exposed, shield themselves with materials such as concrete and lead
  • the universe was created 13 billion years ago
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P2.7 Energy from the nucleus part2

  • before galaxies and stars formed, the universe was a dark patchy cloud of hydrogen and helium. eventually dust and gas were pulled together by gravitational attraction to form stars. the resulting intense heat started off nuclear fusion reactions in the stars, so they began to emit visible light and other radiation
  • dust and gas pulled together by gravitational forces form protostar
  • the protostar becomes denser and the nuclei of hydrogen atoms and other light elements start to fuse together. energy is released so the core gets hotter and brighter
  • stars radiate energy because of hydrogen fusion in the core. the star is stable because the inward force of gravity is balanced by the outward force of radiation from the core and is called the main sequence star
  • eventually, a star runs out of hydrogen nuclei, swells, cools down and turns red
  • low mass star: protostar - main sequence star - red giant - white dwarf - black dwarf
  • high mass star: protostar - main sequence star - red supergiant - supernova - neutron star or black hole if sufficient mass
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P2.7 Energy from the nucleus part3

  • the sun will eventually become a black dwarf
  • a supernova is the explosion of a supergiant after it collapses
  • elements as heavy as iron are formed inside stars as a result of nuclear fusion. heavier and lighter elements are formed in supernovas. the sun and the rest of the solar system were formed from the debris of a supernova
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