# P2

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• Created by: jacktd98
• Created on: 16-05-15 12:23

## Distance-Time Graphs

• Speed = metres per second = m/s = distance travelled in metres
•                                                        time taken in seconds
• Flat = stopped
• Straight = constant speed
• Curves = acceleration/deceleration
• Line going down = travelling backwards towards start
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## Acceleration and Velocity

Velocity is speed in a given direction = m/s

Acceleration is a measure of how quickly the velocity is changing, this can be a change of speed or a change of direction

• ACCELERATION = change in velocity
•                                   time taken

Acceleration = Metres per second squared =

• Horizontal = constant speed
• Up = acceleration
• Down = deceleration
• Curve = changing acceleration

The distance travelled is equal to the area under the graph

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## Resultant forces

Whenever two objects interact, the forces they exert on each other are equal and opposite. The unit of force is Newtons (N). In most situations there are at least two forces acting on an object at any point in time. The overall effect of these forces will decide the motion of the object - Acceleration, Deceleration or Steady speed. These forces can be replaced by a single force that causes the same motion this is the Resultant force. A resultant force means a change in velocity

A stationary object has the force of gravity pulling it down and a reaction force pushing upwards these are both equal so the resultant force is 0N.

Laws of Motion

• An object needs a force to start moving - If the resultant force on a stationary object is zero the object will remain stationary
• No resultant force means no change in velocity - If there is no resultant force on a moving object it'll just carry on moving at the same velocity
• A resultant force means acceleration - If there is a non-zero resultant force then the object will accelerate in the direction of the force
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## Forces and acceleration

The acceleration of an object is determined by the resultant force acting on the object and the mass of the object.

F = ma     or      a = F/m

• m = mass in kilograms (Kg)
• a = acceleration in metres per second squared (m/s )
• F = the resultant force in newtons (N)

Reaction forces are equal and opposite - when two objects interact, the forces they exert on each other are equal and opposite. This means that if you push something for example a shopping trolley the trolley will push back with exactly the same force. The reason things move then is because they have different masses, the thing with the smaller mass gets accelerated more than the the larger mass.

e.g.     Skater A                                  Skater B                      Skater A will accelerate faster

<-------------- ---------------->  equal forces            as she has a smaller mass

55kg                                        65kg                        a = F/m

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## Stopping distances

When a car travels at a constant velocity the resultant force on the car is 0 as the driving force of the engine is balanced by the resistive forces (friction, air resistance). The braking force needed to sop a vehicle in a certain distance depends on:The speed of the vehicle, the mass of the vehicle

The Stopping distance -  is the distance covered from first identifying a hazard to coming to a complete stop. The stopping distance is the sum of the thinking distance and the breaking distance.

Thinking distance - The distance travelled by the vehicle in the time it takes the driver to react - It is affected by:

- How fast you're going                         - Tiredness, drugs, alcohol, carelessnes

Braking distance - The distance travelled by the vehicle under the braking force. This is affected by:

- How fast you're going                            - The quality of the brakes                                    - The grip which depends on - Road surface - Weather conditions - tyres

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## Forces and terminal velocity

The weight of an object is the force of gravity on it, this is measured in Newtons

The mass of an object is the quantity of matter in it, this is measured in Kilograms

1kg  =  10N

• Weight  =   Mass  x    Gravitational field strength
•     W     =      m    x                   g

Objects without a force propelling them will always come to a stop because of friction, it always acts in the opposite dirrection to movement. Once friction and the driving force meet a balance the object will travel at a steady speed.

In fluids air resistance or drag acts to slow an object down. Drag increases as speed increasesAn object falling through a fluid will initially accelerate due to the force of gravity. The friction builds as the speed does, reducing the acceleration until the forces are equal and the resultant force is zero and the object will move at its terminal velocity (steady speed). The terminal velocity depends on the shape and surface area of the falling object, the larger the surface area the slower its terminal velocity.

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## Forces and elasticity

An elastic object regains its origional shape when the forces deforming it are removed. When a force is applied to an elastic object to change the shape, the energy is not lost but is stored as elastic potential energy. When the force is removed the elastic potential energy is converted to kinetic energy (and some heat/sound) springing the object back to its origional shape.

Extension of an Elastic object is directly propotional to force

F = k  x e

• e = the extension measured in Metres (m)
• k = the spring constant - this depends on the material. measured in newtons per metre (n/m)
• F = the load or force applied, measured in newtons (N)

This proportionality has a limit, once you apply too much force to an object, the object stretches more that predidcted. This is because the object has gone beynd its limit of proportionality. This can be plotted on a graph with a straight line to begin then when a curve starts to appear a mark P can be used to show the limit of proportionality

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## Energy and Work done

When a force moves an object through a distance energy is transferred and work is done. Work done is just energy transferred - they are both given in Joules. Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings.

Work Done  =   Force  x   Distance

Gravitational Potential energy - in Joules

Gravitational potential energy is the energy that is stored in an object because of its position in the Earths gravitational field.

When an object moves up it gains gravitational potential energy, this gain is equal to the work done on it by the lifting force.

Gravitational Potential energy  =   Mass   x    Gravity     x    Height

Ep = m x g x h

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## Kinetic energy

Kinetic energy is energy of movement. Anything that is moving has kinetic energy.

Kinetic energy  =   1/2    x    mass    x    speed squared

Ek  =   1/2  x   m   x   v

• e.g.   A car of mass 500kg is moving at 12m/s what's its kinetic energy
•         0.5    x    500   x   (12x12)    =   36 000J

A moving car has a lot of Ek, to slow it down this energy has to be converted into other forms. To stop the car, the kinetic energy is converted to heat energy through friction.

Kinetic energy transfered   =   Work Done by brakes

When an objects falls, its gravitational potential energy is transferred into kinetic energy therefore the further it falls the faster it goes.

Kinetic energy gained  =  potential energy lost

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## Power

Power is the rate that work is done at, how much is done per second. A powerful machine is not necessarily a strong one but is one that transfers a lot of energy in a short space of time.

• Power  =  work done (energy transfered)
•                              time taken

power is measured in Watts (W) or Joules per second (J/s)

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## Momentum

Momentum is a property of moving objects. The greater the mass of an object the greater its velocity and the more momentum it will have. Momentum is a vector quantity, it has size and direction.

Momentum (kg m/s) =  mass (kg)   x    velocity (m/s)

In a closed system the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.

When two objects push each other apart, they move apart:

• With different speeds if they have unequal masses
• With equal and opposite momentum so their total momentum is 0

When vehicles colide, the force of the impact depends on mass, change in velocity and the duration of the impact. The longer the impact time, the more the impact force is reduced. When two vehicles collide:

• They exert equal and opposite forces on each other       -     their total momentum is unchanged
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## Static electricity

When certain insulating materials are rubbed together, negatively charged electrons will be scraped off of one and dumped on the other. This leaves a positive charge on one and a negative charge on the other.

examples

• polyethene rod and a duster - the electrons move from the duster to the rod
• acethate rod and a duster - the electrons move from the rod to the duster

Only the electrons move never the protons

When two electrically charged objects are brought together they exert a force on each other. Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract. Electrical charges can move easily through some substances, eg metals

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## Current and potential difference

Electric current is the flow of charge round a circuit. Current will only flow through a component if there is a potential difference across that component. The size of the electric current is the rate of flow of electric charge.

I   =   Q/t

• I = current in amperes (A)
• Q = charge in coulombs (C)
• t = time in seconds (s)

The potential difference (voltage) between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points.

P.D  =  W/Q

• P.D = potential diference in volts (V)
• W = work done in Joules (J)
• Q = charge in coulombs (C)
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## Resistance

Resistance is anything in a circuit that slows the current. The resistance of a component can be found by measuring the current through, and potential difference across, the component.

When an electric current flows through a resistor, some of the electrical energy is transferred to heat energy and the resistor gets hot, this causes the ions to vibrate more which in turn makes it more difficult for the electrons to get through the resistor. For most resistors there is a limit to the amount of current that can go through them as increased current means increased resistance which then decreases current again.

R  =  V/I

• R = resistance in ohms
• V = Potential difference in volts (V)
• I = current in amps  (A)

The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor.

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## Ammeters and Voltmeters

The Ammeter -                                                                 The voltmeter

• Measures the current through the component             - Measures the P.D across the component
• must be placed in series                                          -  Must be placed in parallel around the
• can be put anywhere in the main circuit                        component under test
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## Circuit devices

Diode - Current only flows in one direction through the diode, it is a semiconductor

LED - It emits light when a current flows through it in the forward direction

LDR - It has the highest resistance in the dark and less resistance in bright light

Thermistor - The resistance increases as the temperature decreases. highest resistance in the cold

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## Series circuits

In a series circuit all the components are connected in a line between the positive and negative power supply. If you disconnect one component, the circuit is broken and they all stop.

Voltage/ Potential difference - The total voltage from the supply is shared between the components and these always add up to equal the voltage from the supply

V = V1 + V2 + ...

Current - The current is the same all the way round the circuit each component has the same current

A1 = A2 = A3 = ...

Resistance - The total resistance is just the sum of all the resistances

R = R1 + R2 +R3

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## Parallel circuits

In parallel circuits, each component is connected seperately to the positive and negative supply. If you disconnect a component it will hardly affect the others at all.

Voltage/ Potential difference - The voltage is the same across the circuit, all the components get the full source potential difference so the voltage is the same everywhere

V1 = V2 =V3 = ...

Current - The current is shared between the components, the total current is equal to the total current of all the seperate components

A = A1 + A2 + ...

Resistance -  Each component has a different resistance causing the different currents through the components

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## Mains electricity

Mains supply is AC, battery supply is DC. The UK mains supply is 230 volts and is alternating current meaning the current is constantly changing direction. The frequency of the mains supply is 50 hert (Hz)

Electrical supplies can be shown on an oscilloscope screen.

The vertical height - shows the input voltage

Each horizontal square - This is the timebase, how many milliseconds each division represents. e.g 5ms/div

Time period - The space between two peaks, measure gap then multiply by timebase

Frequency (Hz)  =  1 /  Time period

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## Cables and Plugs

Sockets and plug cases are often made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good insulator

Mains cables consist of two or three insulated copper wires surrounded by an outer layer of flexible plastic material. In a three pin plug:

Live = Brown

Neutral = Blue

Earth = Green and Yellow

The earth wire is connected to the longest pin and is used to earth the metal case of an appliance, so if the live is broken the current is carried to earth and not the person touching the appliance

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## Fuses and Circuit breakers

Fuses - contain a thin wire that heats up and eventually melts when too much current flows through it, this then breaks the circuit and stops current flowing through the circuit. They are always fit in series with the live wire

A circuit breaker - An electromagnetic switch the opens and cuts off the current if too much current passes through it. Once the switch is open current can't flow through the circuit and the appliance is shut off

RCCB - Normally exactly the same amount of current flows through the live and neutral wire, an RCCB detects any change between the live and neutral wire and cuts the circuit ny opening a switch

Advantages - They work much faster, they are more sensitive, they are easier to reset.

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## Power and potential difference

When you use an appliance it transfers electrical energy into other forms, The power of the appliace in Watts is the energy it transfers per second

Power (Watts)  =  energy transferred (Joules) / time (seconds)

P = E / t

Power supplied (Watts) = Current (amps)  x  Potential difference (volts)

P = I x V

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## energy and potential difference

Charge (coulombs) = current (amps) x time (seconds)

Energy transferred (Joules) = Potential difference (volts) x charge (coulombs)

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## Atomic structure

Rutherford disscovered the nuclear model of an atom by firing a beam of alpha particles (Helium nuclei) at a plate of gold foil. The results showed

protons = +1 charge

neutrons = 0

electrons = -1 charge

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Isotopes are different forms of the same element, they have the same number of protons but different numbers of neutrons. Most elements have different isotopes but on one or two stable ones, the other isotopes tend to be radioactive, which means they decay into other elements and give out radiation.

Radioactivity is a totally random process - radioactive substances give out radiation from the nuclei of their atoms, this process is entirely random, it's completely unaffected by physical conditions like temperature or by any sort of chemical bonding

Background radiation comes from many sources, it is present at all times, all around us.

- Radioactivity in the environment - in the air (radon), in food, building materials, rock in the ground

- Radioactivity from space - cosmic rays mainly from the sun

- Radioactivity from man-made sources - Nuclear weapon tests, X rays, dumped waste

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There are three types of radiation

Alpha particles - An alpha particle is two neutrons and two protons (the same as a helium nucleus) they are big, heavy and slow moving - they don't penetrate far into materials and are stopped quickly even through air. They are strongly ionising, they knock a lot of electrons out of place from other atoms creating ions

Beta particles - A beta particle is an electron and move quite fast and penetrate moderately into materials before colliding, they have a long range in air and are moderately ionising. For every Beta particle emitted, a nuetron turns to a proton in the nucleus.

Gamma rays - gamma rays are very short wavelength EM waves, they penetrate far into materials without being stopped and pass straight through air. they are weakly ionising as they pass through things rather than colliding with them . They have no mass and no charge

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## Half Life

Half life is the average time it takes for the number of nuclei in radioactive isotope sample to halv eor the time it takes for the count rate from a sample containing the isotope to fall to half its initial level. The radioactivity of a sample decreases over time as it gets more stable every time radiation is given off. The activity of a radioactive source is the number of nuclei that decay per second.

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Smoke detectors use alpha radiation to detect smoke

Medical trackers - swallowed or injected

Radiotherapy - gamma rays kill the cancer cells

Sterilisation of food

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## Nuclear fission

Nuclear fission is the splitting up of big atomic nuclei. Nuclear power stations use a controlled chain reaction which takes place when atomic nuclei split up and release energy which is then used to heat water and a steam turbine. Either Uranium 235 or plutonium 239 is used.

To start the reaction a nuetron is fired at the uranium, this then causes the atom to and two or three neutrons are spit out too.

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## Nuclear fusion

Two light nuclei can join together to create a larger nucleus - this is nuclear fussion, this releases lots of energy and dont leave radioactive waste. However fusion can only happen at really high temperatures and pressures. At the moment the reactors use more energy than they can create.

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