# Physics

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• Created by: Emma
• Created on: 19-12-11 21:02

## Motion

• The steeper the line on a distance-time graph, the greater the speed it represents.
• Speed (metre/second, m/s)= distance travelled (metre, m)

time taken (second, s)

• Velocity is speed in a given direction.
• Acceleration is change of velocity per second.
• A body travelling at a steady speed is accelerating if its direction is changing.
• The slope of the line on a velocity-time graph represents acceleration.
• The area under the line on a velocity-time graph represents distance travelled
• To carry out calculations involving:
• The slope on a distance-time graph represents speed.
• The slope on a velocity-time graph represents acceleration.
• The are under the line on a velocity-time graph represents the distance travelled.
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## Speeding up and slowing down 1

• When two objects interact, they always exert equal and opposite forces on each other.
• The unit of force is the newton.

Remember that if a body is accelerating it can be speeding up, slowing down or changing direction.  If a body is accelerating there must be a resultant force acting on it.

Rusultant force = mass x acceleration

(newtons, N) = (kilograms) x (metres/second)2

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## Speeding up and slowing down 2

• The thinking distance is the distance travelled by the vehicle in the time it takes the driver to react.
• The braking distance is the distance the vehicle travels under the braking force.
• The stopping distance = the thinking distance + the braking distance.
• The weight of an object is the force of gravity on it.
• An object falling freely accerlerates on about 10m/s2.
• An object falling in a fluid reaches a terminal velocity.
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## Work, energy and momentum 1

• Work done = energy transferred.
• Work done (joules) = force (newtons) x distance moved in the direction of the force (metres).
• Elastic potential energy is the energy stored in an elastic object when work is done on the object.
• The kinetic energy of a moving object depends on its mass and its speed.
• Kinetic energy ) J = 1/2 mass x speed2.

(kg)         (m/s)2

• Momentum (kg m/s) = mass (kg) x velocity (m/s).
• Momentum is conserved whenever objects interact, provided no external forces act on them.
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## Work, energy and momentum 2

• Momentumhas size and direction.
• When two objects push each other, they move apart with equal and opposite momentum.
• The more time an impact takes, the less the force exerted.
• Force (newtons) = change of momentum (kilogram metre/second)

time taken (seconds).

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## Static electicity

• Like charges repel; unlike charges attract.
• Insulating materials that lose electrons when rubbed become positively charged.
• Insulating materials that gain electrons when rubbed become negatively charged.
• Electrical current is the rate of flow of charge.
• A metal object can only hold charge if it is isolated from the ground.
• A metal object is earthed by connecting it to the ground.
• If a metal object gains too much charge, it will produce sparks.
• A spark from a charged object can make powder grains or certain gases explode.
• To eliminate staic electricity,

a. use antistatic materials, and b. earth metal pipes and objects.

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## Currect electricty 1

• Every component has its own agreed symbol.
• A circuit diagram shows how components are connected together.
• A battery consists of two or more cells connected together.
• Resistance (ohms) = potential difference (volts)

current (amperes)

• The current through is a resistor at constant temperature is directly proportional to the potential difference across the resistor.
• Filament lamp: 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.
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## Current electricity 2

• For components in series circuits:
• a. the current is the same in each component.
• b. the potential differences add to give the total potential difference.
• c. the resistances add to give the total resistance.
• For components in parrallel circuits:
• a. the potential difference is the same across each component.
• b. the total current is the sum of the currents through each component.
• c. the bigger the resistance of a component, the smaller its current is.
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## Mains electricity 1

• Alternating current repeatedly reverses its direction.
• Mains electricity is an alternating current supply.
• A mains circuit has a live wire which is alternately positive and negative every sycle and a neutral wire at zero volts.
• Cables consist of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.
• Sockets and plugs are made of stiff plastic materials which enclose the electrical connections.
• In a three-pin plug or a three-core cable, the live wire is brown, the neutral wire is blue, the earth wire is yellow/green.  The earth wire is used to earth the metal case of  mains appliance.
• A fuse contains a thin wire that heats up and melts, cutting the current off, if too much current passes through it.
• A circuit breaker is an electromagnetic switch that opens (i.e. 'trips') and cuts the current off if too much current passes through it.
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## Mains electricity 2

• The power supplied to a device is the energy transfer to it each second.
• Electrical power supplied (warrior) = current (amperes) x potential difference (volts).
• An electrical current is the rate of flow of charge.
• When charge flows through a resistor, electrical energy is transferred as heat.
• Charge (coulombs) = current (amperes) x time (seconds).
• Energy transferred (joules) = potential difference (volts) x charge flow (coulombs).
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## Nuclear physics 1

• alpha decay- nucleus loses 2 protons and 2 neutrons and 2 protons and 2 neutrons are emitted as an alpha particle.
• beta decay- a nuetron in the nucleus changes into a proton and an electron is created in the nucleus and is instantly emitted.
• Alpha particles in a beam are sometimes scattered through large angles when they are directed at a thin metal foil.
• Rutherford used the measurements from alpha scattering experiments to prove that an atom has a small positively charged central nucleus where most of the mass of the atom is located.
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## Nuclear physics 2

• Nuclear fission occurs when a uranium-225 nucleus or plutonium-239 nucleus splits.
• A chain reaction occurs in a nuclear reactor when each fission event causes further fission events.
• In a nuclear reactor, one neutron per fission on average goes on to produce further fission.
• Nuclear fusion occurs when two nuclei are forced close enough together so they form a single larger nucleus.
• Energy is released when two light nuclei are fused together.
• A fusion reactor needs to be at a very high temperature before nuclear fusion can take place.
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