Physics 2

Physics

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  • Created by: Chloe
  • Created on: 08-01-12 17:19

Distance Time Graphs

  • Gradient = speed
  • Flat sections are where it has stopped
  • The steeper the graph, the faster it's going.
  • Downhill sections means it is going back towards its starting point.
  • Curves represent acceleration or deceleration
  • A steepening curve means it is speeding up
  • A levelling off curve means it is slowing down
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(http://t2.gstatic.com/images?q=tbn:ANd9GcTKnKuAcszQyOcE_KfUNp23uz-TzSX85mGZ61gWnEjy19xdZON4)

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Velocity Time Graphs

  • Gradient = acceleration
  • flat sections represent steady speed
  • The steeper the graph the greater the acceleration or deceleration
  • Uphill section (/) are acceleration
  • Downhill section (\) are deceleration
  • The area under any section of the graph is equal to the distancxe travelled in that time interval
  • A curve means changing acceleration
  • The speed is simply found by reading the value of the velocity axis
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Mass, Weight and Gravity

  • Gravity is the force of attraction
  • Gravity gives everything a  weight
  • Gravity keeps everything in their orbits. The orbit is a balance between the forward motion of the object and the force of gravity.
  • Mass is the amount of stuff in an object and is the same anywhere in the universe,
  • Weight is caused by the pull of gravity.
  • On the moon you will weight less but you will have the same mass.
  • Weight = mass x gravitational field strength
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The Three Laws of Motion

  • First Law - balanced forces mean no change in velocity. 
  • If an object is stationary and the forces are balanced it will stay still but if the object is moving it will carry on at a constant speed.
  • This means there is a zero resultant force.
  • Second Law - A resultant force means acceleration. 
  • If there is an unbalanced force than the object will accelerate in that direction.
  • This acceleration can take 5 different forms: starting, stopping, speeding up, slowing down and changing direction.
  • Force = mass x acceleration
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The Three Laws of Motion (cont.)

  • In most situations there are at least two forces acting on an object in any direction. 
  • The overall effect of these forces will decide the motion of the object (acceleration, deceleration or a steady speed).
  • "The overall effect" is found by just adding or subtracting the forces. The answer is the resultant force.
  • Force = Mass x Acceleration
  • Third Law - Reaction forces. If object A exerts a force on object B then object B exerts the exact oppostie force on object A. 
  • That means if you push something eg. a shopping trolley then the trolley will push back against you just as hard
  • All the forces are always equal the important thing to remember is that the two forces are acting on different objects. 
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Drag and Terminal Velocity

  • Friction is always there to slow things down. 
  • If an object has no force pushing it along it will always slow down and stop because of friction. 
  • Friction always acts in the opposite direction to movement.
  • To travel at a steady speed the driving force needs to balance the frictional forces. 
  • Drag is friction through a fluid. 
  • Resistance from fluids always increases with speed. 
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Stopping Distances

  • Many factors effect your total stopping distance. 
  • The distance it takes to stop a car is dividing into the thinking distance and the braking distance. 
  • Thinking distance - The distance the car travels in the split second between the driver noticing the hazard and applying the brakes.
  • It is affected by how fast you are going and how dopey you are (tiredness, drugs, alcohol, old age and a careless attitude). 
  • Braking distance - The distance the car travels during its deceleration whilst the brakes are being applied. 
  • It is affected by how fast you're going, hwo heavily loaded the vehicle is, how good the brakes are and hwo good the grip is (grip depends on road surface, weather conditions are the tyres). 
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Work Done

  • Work done = energy transferred. 
  • When a force moves an object energy is transferred and work is done.
  • Whenever something moves, something else is providing effort to move it. 
  • The thing putting in the effort needs a supply of energy. 
  • It then does "work" by moving the object and one way or another it transfers the energy it receives into other forms. 
  • Whether this energy is transferred usefully (eg. lifting a load) or is wasted (eg. lost as heat) you can still say work is done. 
  • Work done = force x distance
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Kinetic and Potential Energy

  • Kinetic energy is energy of movement. It depedns both on its mass and speed. 
  • The greater the mass and fatser its going, the bigger its kinetic energy. 
  • Kinetic energy = 1/2 x mass x velocity 2
  • Energy can be stored as potential energy.
  • Elastic potential energy - springs and elastic bands can be stretched and then return to their original shape.
  • Eleastic potential energy is the energy stored when work is done to change its shape. 
  • Gravitational potential energy - is the nergy stored in an object when you raise to a height against gravity. 
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Momentum and Collisions

  • Momentum = mass x velocity.
  • The greater the mass of an object and the greater the velocity the more momentum it has. 
  • Momentum is like velocity and is about both size and direction, 
  • Momentum before = Momentum after. 
  • Forces cause changes in momentum (when a force acts on an object it cause a change in the momentum).
  • Force acting = change in momentum / time taken. 
  • A larger force means a faster change in momentum. 
  • If someone's momentum changes very  quickly (like in a car crash) the forces on the body will be very large and more likely to cause injury. 
  • Crumple zones increase time for the car to stop. 
  • Seat belts increase the time for the wearer to stop.
  • Air bags slow you down more gradually.
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Static Electricity

  • A build up of static is caused by friction.
  • When two insulating materials are rubbed together electrons will move from one to another. 
  • This will leave a positive static charge on one and a negative static charge on the other. 
  • Only electrons move, never the positive charges. 
  • A charged conductor can be discahrged safely by connecting it to an earth wire with a metal strap. The electrons flow down the strap to the ground if it's negative and up the strap if it's positive. 
  • A charge builds up, so does the voltage causing sparks.
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Basic Circuits

  • Current is the flow of electrons round the circuit. 
  • Voltage is the driving force that pushed the current round. 
  • Resistance is anything in the circuit which slows the flow down. 
  • If you increase the voltage more current will flow and if you increase resistance less current will flow (or more voltage will be needs to keep the same current).
  • Ammeter measures current is Amps flowing through the component.
  • It must be placed in series and it can be placed anywhere in the circuit. 
  • Voltmeter measures the voltage in volts accross a component.
  • The voltmeter must be placed in parallel to the component. 
  • The proper name for voltage is potential difference. 
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Resistance and V=IxR

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Circuit Symbols and Devices

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Series Circuits

  • In a series circuit all components are collected in a line between the positive and negative sides of the power supply. 
  • If you remove or disconnect one component the circuit will be broken. 
  • Potential Difference is shared between all the components so the voltage around a circuit will always add up to the source voltage. 
  • Current is the same everywhere.  
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Parallel Circuits

  • In parallel circuits each component is seperately connected to the supply. 
  • If you remove or disconnect one it will hardly affect the others. 
  • Potential difference is the same across all components. 
  • Current is shared between the branches. 
  • The lower the resistance of a component the bigger the current that will flow through. 
  • Voltmeters and ammeters are exceptions to the rule. 
  • Ammeters are always connected in series even in a parallel circuit. 
  • Voltmeters are always connected in parallel even in a series circuit. 
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Mains Electricity

  • The UK mains supply is approx 230volts.
  • It is an alternating current (AC) which means it is constantly changing direction. 
  • The frequency of the AC mains supply is 50 hertz. 
  • In contrast cells and batteries supply direct current (DC) this means that current keeps flowing in the same direction. 
  • If you plug an AC supply into an oscillo you get a trace on the screen that shows how the voltage of the uspply changes with time. 
  • The vertical height of the trace at any point shows the input voltage at that point. 
  • The two dials on the front called the Timebase and the Gain control the display.
  • The Gain controls how many volts each cm division represents on the voltage axis.
  • The Timebase controls how many miliseconds each division represents on the horizontal axis. 
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Mains Electricity (cont.)

  • Common hazards with mains electricity are long cables, frayed cables, cables in contact with something hot or wet, water near sockets, shoving things into sockets, damaged plugs, too many plugs in each socket, lighting sockets without bulbs in and appliances without their covers on. 
  • It is important to get the wiring right. the right coloured wire to each pin and firmly screwed in. 
  • No bare wires should be showing inside the plug. 
  • Cable grip should be tightly fastened over the cable outer layer.
  • The metal parts of the plug are made of copper or brass as these are the best conductors. 
  • The case, cable grip and cable insulation are made of plastic or rubber because these are the best insulators and are flexible too. 
  • This means the electricity flows where it should.  
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(http://www.bbc.co.uk/schools/gcsebitesize/science/images/68_wiring_a_plug.gif)

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Fuses and Earthing

  • The brown live wire in a mains supply aterates between a high +VE and -VE voltage. 
  • The blue neutral wire is always at OV. 
  • Electricity flows through the live and neutral wires only. 
  • The green and yellow earth wire is just for safety and works with the fuse to prevent fires and shocks.
  • To prevent surges of current or electric shocks a fuse is placed in the circuit.
  • If the current gets too big (bigger than the fuse rating) the fuse wire heats up and blows breaking the circuit and preventing electric shocks.
  • Fuses should be rated as near as possible but just higher than the normal operating current.
  • The earth pin is connected to the case via the earth wire.
  • If a fault develops in which the live touches the metal case a big current flows through the live thought the case and out down the earth wire because the case is earthed.
  • This surge in current breaks the fuse, cutting off the live supply and preventing electric shocks. 
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Earth Wire Diagram

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Energy and Power in Circuits

  • Anything which supplies electricity supplies energy.
  • Cells, batteries, generators etc. transform energy into the component's energy: Motion - motors, Light - light bulbs, Heat - hairdriers/kettles and Sound - speakers. 
  • All resistors produce heat when a current flows through them (electrical energy transfers to heat energy).
  • The more current that flows the more heat.
  • Bigger voltage means more heating as it pushes more current through.
  • You can measure heat produced by putting the resistor in a known amount of water and measuring the increase in temperature.
  • Total energy transformed by an appliance depends on how long the appliance is on and its power rating. 
  • Energy = power x time
  • The formula for electrical power is power = voltage x current.
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Charge, Voltage and Energy Change

  • Current is the flow of electrical charge (in Coulombs, C) around a circuit.
  • When current (I) flows past a point in a circuit for a length of time (t) the charge (Q) that's passed is shown by the formula: 
  • total charge = current x time
  • The bigger the current the more charge
  • When electrical charge goes through a change in voltage energy is transferred. 
  • Energy is supplied to the charge at the power source to raise it through a voltage.
  • The charge gives up this energy when it falls through any voltage drop in components.
  • Energy transferred = charge x potential difference
  • The bigger the charge the more energy transferred. 
  • A battery with a bigger voltage will supply more energy for every coulomb of charge which flows. 
  • Because the charge is raised up higher more energy will be dissipated in the circuit.
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Atomic Structure

  • J J Thompson instigated the plum pudding theory about the structure of atoms (spheres of positive charge with negative electrons stuck in them)
  • Ernest Rutherford fired alpha particles at thin gold foil: most of them went straight through but some bounced back. 
  • He realised that it was because most of the mass of the atom is concentrated in the centre (a tiny nucleus). Most of the atom is empty space.
  • The nucleus contains protons and neutrons so is positively charged.
  • The rest of the atom is mainly empty space but negative electrons orbit making the overall charge of the whole atom 0.
  • Mass: Protons 1, Neutrons 1 and Electrons 1/2000
  • Isotopes are different forms of the same element.
  • The atomic number is the total number of protons.
  • The mass number is the total number of protons and neutrons.
  • Isotopes have the same number of protons but different number of neutrons. They have the same atomic number but a different mass number.
  • Unstable isotopes are radioactive, meaning they decay into other elements giving out radiatio
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Radioactive Decay Processes

  • Alpha particles are helium nuclei (mass 4 - 2 neutrons, 2 protons, charge +2)
  • They are big, heavy and slow moving therefore they do not penetrate very far into materials but are stopped quickly.
  • Because they are big they are strongly ionising which means they bump into atoms and knock electrons off before they slow down, creating lots of ions
  • A typical alpha emission;
  • Beta particles are electrons with virtually no mass and a charge of -1. 
  • Everytime a beta particle is emitted from a nucleus a neutron is converted to a proton.
  • They move quite fast and are quite small, they pentrate moderately before colliding and are moderately ionising.
  • A typical beta emission:
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Radioactive Decay Processes (cont.)

  • Gamma rays are very short wavelength EM waves.
  • They penetrate long way into materials without being stopped. 
  • They are weakly ionising as they pass through rather then collide with atoms
  • A Gamma ray is a photon with no mass and no charge.
  • After an alpha or beta emission the nucleus usually has extra charge to get rid of so it emitts a gamma ray. 
  • Gamma radiation never changes the proton or mass numbers of the nucleus.
  • A typical combined alpha and gamma emission:
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Background Radiation

  • It comes from natural radiation like in the air, food and rocks.
  • We receive cosmic rays from space mostly from the Sun.
  • There is radiation due to humans eg. fallout from nuclear explosion or dumped nuclear waste.
  • At high altitudes background radiation increases because you are more exposed to cosmic rays.
  • It increases in underground mines because of the close rocks.
  • Certain underground rocks like granite can cause higher levels at the surface, especially if they release radon gas.
  • Studies show that exposure to radon gas can cause lung cancer.
  • We still don't know the effects of low doses of it.
  • New houses built nnear hgih levels of radon gas are designed with ventilation systems.
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Nuclear Fission

  • Fission is the splitting of big atomic nuclei. 
  • Nuclear power station and nuclear submarines are powered by nuclear reactors.
  • In which a controlled chain reaction occurs where atomic nuclei split up and release energy in the form of heat. 
  • This heat is used the heat up water driving a steam turbine. 
  • The fuel is usually plutonium - 239 or uranim - 235.
  • Neutrons are fired at the nuclei of the fuel which makes the fuel split.
  • It spits out 2/3 extra neutrons which then bump into other nuclei and so on.
  • When a large atom split it forms two lighter elements which are usually radioactive as they have the wrong number of neutrons in them.
  • Fission leaves us with lots of nuclear waste which is expenseive to dispose.
  • Each nucleus splitting gives a lot of energy, a lot more than you get with a chemical bond.
  • Nuclear processes release more energy than chemical processes do.
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Nuclear Fusion

  • Two ligth nuclei (eg. hydrogen) can fuse to create a large nucleus.
  • This releases a lot of energy, all the energy in stars comes from this.
  • Scientists are trying to develop fusion reactors to make electricity.
  • It doesn't leave a lot of radioactive wate and there is plenty of hydrogen to use in the world.
  • The problem is that this process only happens at 10,000,000 degrees.
  • No materials can stand that heat so the atoms would have to be held in a magnetic field. 
  • At the moment it takes more power to make the process happen than is being generated. 
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