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## Distance-time graphs

• The steeper the line the greater the speed
• Speed = distance/time
• The unit for speed is meters per second
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## Velocity and acceleration

• Velocity is speed in a given direction
• Acceleration is change in velocity per second
• Acceleration = chnage in velocity/time taken for change
• A body travelling at a steady speed is accelerating is the direction is changing
• The slope of the line on a velocity-time graph represents acceleration
• The area under the line represents the distance travelled
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## Forces between objects

• Fource is measured in zero
• When two objects interact they always exert equal and opposite forces on each other
• The resultant force is the single force that would have the same effect on the objects as all the original forces
• When the resultant force is zero the object will continue how it was before
• When the resultant force is not zero it will accelerate in the direction of the resultant force
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## Force and accleration

• Resultant force = mass x acceleration
• The greater the resultant force the greater its acceleration
• The bigger the mass of the object the bigger the force needed to give it a particular acceleration
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• The thinking distance is the distance travelled before the driver reacts
• The braking distance is the distance travelled whilst the brakes are on
• The stopping distance = thinking + braking
• Thinking distance gets increased by drugs and alcohol
• Braking distance gets increased by the condition of the road, wheels and the weather
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## Falling objects

• Weight = mass x acceleration due to gravity
• If an objects is on the earth and falling you use the equation:
• weight = mass x gravitational field strength
• The weight of an objects is the force of gravity acting on it
• An object falling freely towards the earth accelerates at about ten meters per second squared
• When the resultant force becomes equal to the weight of the objects the resultant force is zero. The objects stops accelerating and moves at a constant velocity, its terminal velocity
• You can only have a terminal veolicty when the object is falling through a fluid
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## Energy and work

• When a force moves an object energy is transferred and work is done
• The work done is equal to the energy transferred
• Wrok done = force x distance moved in the direction of the force
• When work is done against frictional forces the energy supplied is mainly transferred in to heat
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## Kinetic energy

• Elastic potential energy is the energy stored in an elastic object when work is done on the object
• Kinetic energy is the energy of momentum
• The kinetic energy of an object depends on its mass and speed
• Kinetic energy = 1/2 mass x (speed squared)
• The unit for kinetic energy is joules
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## Momentum

• All moving objects have momentum
• Momentum = mass x velocity
• The unit for momentum is kgm/s
• Momentum is conserved whenever objects interact, as long as no external forces are acting on them
• The interaction could be caused be a collision or explosion
• The objects may move together or apart
• Momentum has size and direction
• When objects interact one is positive and one is negative
• When two objects push each other apart they move apart with equal and opposite momentum
• The total momentum before an explosion is usualy zero so the momentum after is zero aswell
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## Changing momentum

• force = change in momentum/time taken for change
• The longer the time taken for the change of momentum the smaller the impact
• As impact time increases the force becomes less
• Crumple zones are designed to fold in a collision to increase impact time
• An air bag is another saftey feature of a car that is designed to do the same thing
• Impluse = force x time taken to change momentum
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## Electrical charges

• When two electrically insulating materials are rubbed together electrons are rubbed off one material and deposited on the other
• The way electrons are transferred depends on the particular materials used
• Like charges repel
• Unlike charges attract
• Insulating materials that loose electrons when rubbed become positively charged
• The bigger the distance between the objects the weaker the force
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## Charge on the move

• Electric current is the rate of flow of charge
• In a solid conductor the charge carries the electrons
• Metals are good conductors because they contain free electrons that are not confined to a single atom
• A metal objects can only hold charge is it is isolated from the ground
• Otherwise electronss will flow to or from the Earth and discharge it
• A metal object has ot be earthed by connecting it to the ground
• The bigger the charge the higher the potential differnce between the object and the Earth
• If the potential difference becomes high enough a spark may jump across the gap between the object and the earthed conductor brought near it
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## Uses and dangers of static electricty

• A spark from a charged object can make powder grains or certain gases explode
• Static electricty is used in an electrostatic paint sprayer by:
• The spray nozzel is connected to a positive terminal
• As the paint droplets pass through it they pick up a positive charge
• The paint droplets repel each other so the spread out
• The item that is going to be painted is connected to a negative terminal so the positively charged droplets are attracted
• The filler pipes on fuel tanks are earthed to prevent a static charge building up in the pipe which could lead to a spark
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## Electric circuits

• Cell: pushes electrons around a complete circuit
• Battery: consists of two or more cells
• Switch: enables the current to be switched on or off
• Light bulb: emits light as a signal when a current passes through it
• Diode: allows current through in only one direction
• Ammeter: measures an electric current
• Fixed resistor: limits the current in a circuit
• Variable resistor: allows the current to be varied
• Fuse: is designed to melt and therefore break the circuit if the current is greater than a certain amount
• Heater: designed to transform electrical energy in to heat
• Voltmeter: used to measure the potential difference (voltage)
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## Resistance

• Resistance = potential difference/current
• Resistance is measured in ohms
• The current through a resistor at a constant tempreture os directly proportional to the potential differnence across the reistor
• Resistance is the oppostion to current flow
• In a filament lamp the reistnace increases as the tempreture of the filament lamp increases
• In a diode the forward resistance is low and the reverse resistance is high
• In a thermister the resitance decreases as the tempreture increases
• In an LDR the resistance decreases if the light intensity on it increases
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## Series circuit

• The components are connected one after the other
• If there is a break in the circuit the charge will stop
• The current is the same in each component
• The potential difference across each component add up to give the total potential difference
• The resistors add up to give the total resistance
• The bigger the resistance of a component the bigger its share of the supply potential difference
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## Parallel circuits

• Each component is connected across the supply
• If there is a break in one part the chrage can still flow in other parts
• The potential difference is the same across each component
• The total current is the sum of the currents through each component
• The bigger the resistance of a component the smaller its currrent is
• The current through each component depends on its resistance
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## Alternating current

• Cells and batteries supply current that passes round the circuit in one direction, the direct current
• The mains supplys a current that changes its direction, the alternating current
• The UK mains supply is fifty hertz which means it alternates fifty times a second
• The voltage of the mains in 230 volts
• The mains circuit has a live wire which alternates between postive and negative potential
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## Cables and plugs

• The outer cover is made out of plastic or rubber as these are good insulators
• The pins are made out of brass becasue it is a good conductor and it is hard and will not rust or oxidise
• The nuetral wire is blue and it is connected to the neutral pin
• The live wire is brown and it is connected to the live pin
• The earth wire is green and yellow and is connected to the earth pin
• Appliances with a metal case must be earthed to prevent someone being given a shock if it becomes live
• Appliances with plastic cases do not need to be earthed because they are insulators
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## Fuses

• Appliances must be earthed becuase otherwise if a fault develops and the live wire touches the metal case it will become live
• If a fault develops on a earthed appliance a large amount of current flows to the earth and melts the fuse, disconecting the supply
• A fuse contains a thin wire that heats up and melts cutting of the current
• A fuse must be put near the live wire so that if it melts it cuts of the current
• The rating of the fuse must be slightly higher that the normal current
• A circuit breaker is an electromagnectic switch that opens and cuts off the supply is the current is bigger than a certain value
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## Electrical powere and potential differernce

• An electrical device tranforms electrical energy in to other forms
• An electrical deices transfers energy from one place to another
• This is called the power
• Power = energy transformed/time
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## Electrical energy and charge

• An electric current is the rate of flow of charge
• When charge flows through a resistor electrical energy is transformed in to heat
• Energy transformed = potential difference x charge
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## Nuclear reactions

• Background radiation is the radiation that is around us at all times
• When a nucleus emits a alpha particle the atomuc number goes down by two and the mass number goes down by four
• When a nucleus emits a beta particle the atomic number goes up by one and the mass number is unchanged
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## The discovery of the necleus

• Rutherford divised an alpha particle scattering experiment in whcih he fired alpha particles at a piece of thin gold fire
• Most of the alph particles passed straight through which showed most of the atom was an empty space
• Some were deflected through small angles showing the nucleus had a postive charge
• A few rebounded trhough very large angles suggesting the nucleus has a large mass and a very large positive charge
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## Nuclear fission

• Is the splitting of atomic particles
• The two fissionable isotopes are uranium-235 and plutonium- 239
• Naturally occuring uranium-238 is non-fissionable so they use enriched unranium that contains 2-3% uranium-235
• For fission to occur the uranium and plutonium must absorb a neutron
• The necleus then splits in to two smaller nuclei and two or three nuetrons and energy are released
• The nuetrons go on to produce futher fissions causing a chain reaction
• In a nuclear reactor the process is controlled
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## Nuclear fussion

• It is the joining of two atomic nuclei to form a single, larger nucleus
• During the process fussion energy is released
• Nuclei approaching each other will repel one another due to their positive charge
• To overcome this the nuclei must be heated to very high tempretures
• This gives them enough energy to overcome the repulsion and fure
• The reaction has to take place in a contained magnetic field
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Hey Nina,

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Hey,

This is very useful stuff, thanks!

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gud stuff, will come handy

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