# Physics 2

Forces in Motion and Electricity and the Atom

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## Velocity and Acceleration

• Speed = How fast your going (m/s or mph)
• Velocity = How fast your going in a specific direction (m/s or mph)
• Speed (m/s) = Distance (m) / Time (s)
• Distance = Speed x Time
• Time = Distance / Speed
• Acceleration = How quickly your velocity is changing (m/s2 (squared))
• Acceleration (m/s2) = Change in Velocity (m/s) / Time Taken (s)
• Change in velocity = Acceleration x Time
• Time = Change in Velocity / Acceleration
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## D-T and V-T Graphs

• Distance-Time Graphs:
• Gradient = Speed (Verticle/Horozontal = ?m/s)
• Flat Sections = Stopped
• Steeper Graph = Faster
• Downhill = Going back towards starting point
• Curves = Acceleration or Deceleration
• Steppening curve = Speeding up
• Levelling off Curve = Slowing down
• Velocity-Time Graph:
• Gradient = Acceleration (Verticle/Horozontal = ?m/s2 (squared)
• Flat = Steady Speed
• Steeper Graph = Greater acceleration
• Up Hill sections (/) = Acceleration
• Down  Hill Section (\) = Decelleration
• Curve = Changing Accelleration
• Distance travlled in that time interval = Area under the graph
• Speed = read of Velocity Axis
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## Mass, Weight and Gravity

• Gravity:
• Attracts all masses with a force of 10N/kg
• On the surface of a planet makes thing accelerate towards the ground
• Gives everything a weight
• Keeps planets, moons, and satallites in their orbits
• Orbit is a balance bewteen forward motion and the force of gravity pullin it inwards
• Weight and Mass:
• Mass = The amount of matter in an object (kg)
• Weight = The force of gravity on an object (N) - Measure with a newton meter
• Weight (N) = Mass (kg) x Gravity (N/Kg)
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## The Three Laws of Motion

• 1) Balanced forcs mean no change in velocity
• So long as the force on a object are all balanced, then it will stay still or if its already moving it will carry on at the same velocity.
• Constant Velocity = Forces on object are balanced
• Steady Speed = Zero resultant force
• 2) A resultant force means acceleration
• If there is an unbalanced force, then the object will accelerate in that direction.
• Acceleration = Starting, Stopping, Speeding up, Slowing down, Changing Direction.
• The bigger the force the greater the acceleration and decelleration
• The bigger the mass, the smaller the acceleration
• To get a big mass to accelerate as fast as a small mass you need a big force.
• Force (N) = Mass (kg) x Acceleration (m/s2)
• Acceleration = Force / Mass
• Mass = Force / Acceleration
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## The Three Laws of Motion continued...

• 3) Reaction Forces
• If object A exerts a force on object B then objeect B exerts the exact opposite force on object A
• Dependant on the mass of each object the force felt will be different
• E.g. When skater A pushes on skater B (action force) she feels an equal and opposite force from skater B's hand (reaction force) Both skaters feel the same sized force in opposite directions and so accelerate away from each other. However skater A will be accelerated more than skater B because she has a smaller mass. As F = ma
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## Drag and Terminal Velocity

• Friction and Drag:
• Acts in the opposite direction to movement
• for a steady speed: Driving force = Friction
• In friction 2 surfaces need to be in contact or a object passes through a fluid e.g. water or air
• Drag is decreased by streamlined objects e.g. boat hulls or increased by large surface ares e.g. parachutes
• Resistance form fluids increases with speed as you are going through more fluid per second
• Terminal Velocity:
• When the resistance force is equal to the the accelerating force and wont accelerate any more.
• Accelerating force acting on all free falling objects = gravity
• Air resistance causes objects to fall at different speeds
• Terminal Velocity of an object id determined by its drag
• Example = Parachutist
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## Stopping Distances

• Stopping Distance (m) = Thinking Distance (m) + Braking Distance (m)
• 1) Thinking Distance is affected by:
• How fast your going - the faster you are the longer the thinking distance
• How aware you are - Tirdness, Drugs + Alcohol, Old age, Carelesness
• 2) Braking Distanceis affected by:
• How fast your going - the faster you are the longer it takes to stop
• The mass of the vehicle - Heavier = Longer to stop
• The quality of your brakes - check them regulary
• How good the grip is - Road Surface, Weather Conditions, Tyres
• Bad visibiliy can also be a major factor in accidents tooe.g. rain and fog.
• Stats:
• 30mph = TD - 9m + BD - 14m ---> 6 Car Lenghts
• 50mph = TD - 15m + BD - 38m ---> 13 Car Lenghts
• 70mph = TD - 21m + BD - 75m ---> 24 Car Lenghts
• Stick to the speed limit to avoid accidents.
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## Work Done

• Work Done (J) = Force (N) x Distance (m)
• Force = Work done / Distance
• Distance = Work Done / Force
• When a force moves an object energy is transfered and work is done
• The thing putting hte effort in needs a supply of energy e.g. fuel, electricity or food
• When work is done some of the energy transferred is wasted as heat or sound
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## Kinetic and Potential Energy

• Kinetic Energy (J) = 1/2 x Mass (kg) x Velocity(2 squared)
• Anything that is moving has potenital energy
• Stopping distance increases with extra speed to stop a car the kinetic energy needs to be converted into heat energy at the brakes and tyres. So if the velocity doubles then becuse velocity is squared the kinetic energy increases by a factor of four. So you need four times the distance to stop when applying the maximum possible braking force.
• Energy can be stored as potenital energy:
• Elastic Potential Energy e.g. springs or elastic bands when stretched and let go, go back to their oirgional shape, elastic potential energy is sotored when work is done on an elastic object to change its shape. It is released as kinetic energy and a bit of heat when it spring back into shape
• Gravitational Potential Energy is stored in an object when toy raise it to a height against gravity. Potential energy is a way of storing kinetic energy that is released when the object falls.
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## Momentum and Collisions

• Momentum (kgm/s) = Mass (kg) x Velocity (m/s)
• The greater he mass of an object the greater its velovity so more momentum.
• Momentum is a vector quality - it has size and direction but not speed.
• Momentum Before = Momentum After
• Momentum is conserved when no external forces act on it.
• Momentum before = ?kgm/s + ?kgm/s
• Momentum after = ?kgm/s and ?kgm/s
• Force Acting (N) = Change in Momentum (Kgm/s) / Time taken for change to happen (s)
• The larger the force the faster the cahnge in momentum so < acceleration
• Momentum cahges very quickly e.g. car crash teh forces on bodys will be large so more likly to cause injury:
• Crumple Zones - crumple on impact increasing time taken to stop car
• Seat Belts - Streach incrasing time taken so reducing force on chest
• Air Bags - Slow you down more gradually
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## Static Electricity

• A build up of static electricity is caused by friction
• When 2 insulating materials are rubbed together electrons are scraped off one and dumped onto the other.
• 1 material will be positive, 1 negative - depending in what materials
• Electrically charged objects attract small objects near them
• Polythene Rods lose electrons when a duster is rubbed against them
• Acetate Rods gain electrons when a duster is rubbed against hem
• Only the electrons move - not the positive charges
• Earthing
• A charged conductor can be discharged safely by connecting it to the earth with a metal strap.
• Electrons flow down the strap into the ground if the charge is negative
• Electrons flow up the strap from the ground if the charge is positive
• The rate of flow of electrical charge is called electric current
• Like charges repel and opposite charges attract.
• forces get weaker the further apart the two objects are
• As the charge builds up so dies the voltage - causing a spark
• If the voltage gets big enough a spark can jump across the gap.
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## Helpful Examples of Static Electricity

• 1) Photocopier:
• You put the original into the photocopier
• The copying plate is charged with positive static charge
• An image of the original is projected onto the plate
• The charge leaks away when the light falls on the plate
• The parts of the plate that are still charged pick up a black powder
• The powder is transferred onto a piece of paper
• It is heated so it sticks
• You now have the perfect copy
• 2) Smoke Precipitator:
• At the bottom of the chimney the smoke particles meet a wire grid with a high negative charge which charges the smoke negatively
• The charged smoke particles are attracted to the positively charge metal plates.
• The smoke sticks together to form larger particles
• When heavy enough the particles fall of the plate/knocked off with a hammer. The dust that falls to the bottom of the chimney is removed
• The gasses coming from the chimney have very little smoke in them.
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## Dangerous (and annoying) Examples of Static Electr

• 1) Lightning:
• Rain drops and ice bump together inside storm clouds
• They knock off electrons
• This leaves the top of the clouds positively charged and the bottom negatively charged.
• This creates a high voltage which causes a big spark.
• 2) Grain Chutes, Paper Rollers, Fuel Filling:
• As fuel flows through pipes, paper drag through rollers or gain shoots out of pipes static can build up
• This can easily lead to spark which could lead to an explosion in a dusty or fumy place
• The Solution: Make the nozzles or rollers out of metal so so that the charge can be conducted away instead of building up
• It is especially important to have earthing straps between the fuel tank and the fuel pipe.
• Annoying: When synthetic clothes are dragged over each other e.g. in a tubule dryer or over your head electrons are scraped off leaving static charges on both parts - could lead to little electric shocks on your part.
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## Circuits - The Basics

• Current = The flow of electrons around the circuit (Amperes, A)
• Voltage (Potential Difference) = The driving force that pushes the current around (Volts, V)
• Resistance = Anything in the circuit that slows the current down (ohms)
• Increased Voltage = Increased Current
• Increased Resistance = Less Current (or more voltage needed)
• The Standard Test Circuit:
• Battery, Variable Resistor, Ammeter, Component with a Voltmeter
• The Ammeter = Measures the current in Amps, must be placed in series
• Voltmeter = Measures voltage in Volts, must be placed in parallel around the component under test (not a variable resistor or the battery)
• Important Points:
• 1) Very basic circuit used for testing components
• 2) Voltmeter can only be placed in parallel, component, ammeter and veritable resistor are in series and can go in any order
• 3) The variable resistor it alter the current flowing through the circuit
• 4) This means you can take several reading from the ammeter + voltmeter
• 5) You can plot the current on for the values on a Voltage-Current graph
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## Resistance

• Resistance = Voltage (Potential Difference)
•  / Current
• Voltage = Current x Resistance
• Current = Voltage / Resistance
• Voltage-Current Graphs:
• Resistors = Straight line through the centre of the graph (The current is proportional to the voltage, different resistors have different slopes)
• Filament Lamps = A curve (As the temp increases the resistance increases)
• Diode = A curve through the positive part only as the current will only flow one direction through a diode.

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## Circuit Symbols and Devices

• Battery =
• Fuse =
• LED =
• Fixed Resistor =
• Variable Reistor =
• Ammeter =
• Voltmeter
• Diode =
• LDR =
• Thermistor =
• 1) Variable Resistor = Resistance can be changed
• Turn up resistance current drop, Down current increases.
• 2) Diode = Only allows current to flow in one direction (high resistance in  opposite direction)
• 3) Light-Dependant Resistor (L.D.R) = Bright Light - resistance falls, No light - resistance increases e.g. automatic night lights, street lights
• 4) Thermistor = Hot - resistance drops, Cool - resistance increases e.g. thermostats, temperature detectors, car engine temperature sensors
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## Series Circuits

• Components are connected in a line end to end
• Remove one component and the circuit is broken and it all stops working e.g. christmas tree lights
• The Potential Difference (Voltage) is shared:
•  V = V1 + V2 + V3
• The Current is the same everywhere: A1 = A2
• The Resistance adds up (total resistance is the sum of all the resistances): R = R1 + R2 + R3
• The bigger the resistance of a component the bigger its share of the total potential difference
• Cell Voltages add up: There is a larger voltage with more cells connected all in the same way.
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## Parallel Circuits

• Each component is separately connected to the supply of energy,
• Removing or disconnecting one will hardly affect the current
• This is how most things are connected e.g. cars and household electrics
• Advantage for real life = everything gets the full voltage of the battery.
• Potential Difference is the same across all components: V1 = V2 = V3
• Identical bulbs will have the same brightness
• Current is shared between branches: A = A1 + A2 + A3
• There are junctions where the current splits and rejoins.
• Resistance is tricky: The total resistance is hard to work out but it is always less than than that of the branch with the smallest resistance
• Ammeters are always connected in series even in a parallel circuit
• Voltmeters are always connected in a parallel even in a series
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## Mains Electricity

• UK Mains Supply is approximately 230 volts
• It is an A.C. supply - constantly changing direction
• Frequency of A.C. Mains Supply = 50Hz (50 cycles per second)
• Battery's run on D.C. supply - only flows in one direction
• A.C. can be shown on a Cathode Ray Oscilloscope (snazzy voltmeter)
• Plug into a A.C. supply you get a trace on the screen in a regular pattern
• The Vertical Height = input of voltage at that point
• You can change the scale of an oscilloscope using the timebase and gain dial which change the horizontal and vertical axes on the display.
• Frequency (Hz) = 1 / Time Period (s)
• 1 wave is from one point and at the same point after going up and down once
• D.C. Supply always gives you a straight line
• Time is measured in Milliseconds or ms/div
• Gain is measured in volts or volt/div
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## More on Mains Electricity

• Hazards:
• Long or frayed cables, Cables in contact with hot or wet things, Water near sockets, Shoving things in sockets, Damaged plugs, Too many plugs per socket, Lighting sockets without bulbs, appliances without covers.
• Plugs and Cables:
• The right wiring:
• Right coloured wire to each pin
• No bare wires showing
• Cable grip tight over cables outer layer
• Plug Features - keeps electricity in the right places:
• Metal Parts - copper and brass - good conductors
• Case, cable grip + insulation = plastic/rubber - good insulators/flexible
• Earth Wire = Green and yellow stripes
• Neutral Wire = Blue
• Live Wire = Brown
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## Fuses and Earthing

• Main Cables have three separate wires:
• Live Wire (Brown) in a mains supply alternates between positive and negative voltage.
• Netral Wire (Blue) is always at zero voltage
• Earth Wire (Green and Yellow Stripes) is just for safety and works together with the fuse to prevent electric shocks.
• Fuses prevent electric shocks:
• To prevent surges of current In electrical shocks s fuse is placed in the circuit (could be a circuit breaker/resettable fuse)
• If the current get too big the fuse wire heats up and blows breaking the circuit and preventing electric shocks
• Fuses should be rated just higher than the normal operating current
• Earthing prevent fires and shocks (earth wire works together with fuse):
• Earth pin connected to case via the earth wire.
• A fault develops live wire touches the metal case.
• Case is earthed so a big current flows through the live wire, case and out down the earth wire.
• The surge in the current blows the fuse and cut of the supply
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## Energy and Power in Circuits

• Cells, batteries + generator all transform energy to components in the circuit
• e.g. Motion = Motors, Light = Bulbs, Heat = Kettles, Sound = Speaker
• All resistors produce heat when a current flows through them.
• Increased current = increased heat
• Increased voltage means increased current = increased heat
• Measure it by putting resistor in water and measuring temp increase
• Electrical power is used to measure fuse ratings
• Energy (J) = Power (W) x Time (s)
• Power (W) = Voltage (V) x Current (A)
• Voltage = Power / Current
• Current = Power / Voltage
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## Charge, Voltage and Energy Change

• Total Charge (c) = Current (A) x Time (s)
• Current = Charge / Time
• Time = Charge / Current
• Energy Transformed (J) = Charge (c) x Potential Difference (or voltage, V)
• Charge = Energy transformed / Potential Difference
• Potential Difference = Energy Transformed / Charge
• c = coulombs
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## Atomic Structure

• The Plum Pudding Model:
• Atoms are a sphere of positive charge with tiny negative charged electrons stuck in them like plums in a plum pudding
• 1909 - Rutherford:
• Fired alpha particles at thin gold foil.
• Most went straight through but some rebounded
• Realised most of the mass of the atom was concentrated in the centre in a tiny nucleus with a positive charge --> most = empty space
• Nuclear Model of the Atom:
• Proton = mass of 1 ---> +1 Charge
• Neutron = mass of 1 ---> No Charge
• Electron = mass of 1/2000 ---> -1 Charge
• Isotopes:
• Same number of protons but different number of neutrons
• Different isotopes of the same element = same atomic number
• Unstable isotopes are radioactive = they decay into other elements and give out radiation.
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## THe Radioactive Decay Process

• Alpha Particles:
• Big, Heavy, Slow moving. Don't penetrate far - stopped quickly
• Strongly ionising - bash into a lot of atoms knocking off electrons
• Helium Nucleus = Mass of 4, Charge +2, 2 Protons and 2 Neutrons
• When emitted the mass no. and the atomic/proton no. decrease
• Beta Particles:
• Quite fast moving, Quite small
• Moderately penetrating and moderately ionising
• An electron = No mass, Charge -1
• A beta particle is emitted from nucleus: a neutron becomes a proton.
• When emitted the elements the mass no. stays the same and the atomic/proton no. increases by 1 (neutron ---> proton)
• Gamma Rays:
• penetrate a long way without being stopped - weakly ionising
• Is a photon with no mass and no charge
• Emitted after an alpha or beta emission when the nucleus has extra energy to get rid of.
• Never changes the mass no. or the atomic/proton no.
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• Background radiation come in many sources:
• Naturally occurring unstable isotopes found in the air, food, rocks and building materials
• From space known as cosmic rays - mostly from the sun
• Due to human activity: nuclear explosions, dumped nuclear waste
• Amount of background radiation depends on where you are:
• High Altitudes = increase due to more exposure (commercial pilots)
• Underground in mines = increase due to rocks all around
• Underground rocks  e.g. granite can cause higher levels at the surface especially if they release radioactive radon gas which get trapped in peoples houses.
• Radon Gas ---> Scientific Debate:
• Concentration varies wildly in the UK depending on rocks built on
• Exposure to high dose can cause lung cancer
• Scientific community divide on the effects of low concentrations
• 1 in 20 deaths from lung cancer caused by Radon bass (2000 /year)
• New houses in high concentration areas need good ventilation
• In existing houses government recommends one is put in.
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## Nuclear Fission and Fusion

• Nuclear Fission - The splitting up of large atomic nuclei:
• In nuclear reactor a controlled chain reaction takes place.
• Atomic nuclei split up and release energy in the form of heat
• The heat produced used to heat water to drive a steam turbine
• Fuel = Uranium-235 or Plutonium-239
• Chain Reactions:
• Slow moving neutron absorbed by Pu or U nucleus can split
• Each time a Pu or U nucleus split 2 or 3 neutrons are released
• When larger atom splits form 2 new lighter elements - radioactive which hard and expensive to dispose of safely
• Nuclear Fusion - The joining of small atomic nuclei:
• 2 light nuclei e.g. hydrogen can combine to create a larger nucleus
• Releases a lot of energy - Occurs naturally in stars like the sun
• Doesn't leave any radioactive date + spare H can be used as fuel
• Can only happen an really high temperatures- 10000000 *C
• Have to contain hydrogen in a magnetic field rather than a mateiral as no material can stand that temperature
• A few experimental reactors - need more energy than they produce
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