Physics (additional)

Distance-time graph:

• Distance-time graph tells you how an object's distance changes over time
• The gradient represents the speed
• The steeper the gradient the greater the speed of the object 
• Flat-line = no moment 
• Downwards line = returning to starting position 
• Upwards curve = deceleration 

Speed (m/s)=Distance(m)/Time taken(s) 

Velocity and acceleration:

• Velocity is the speed in a given direction
• Acceleration is the change of velocity per second

a = v-u / t 

• a = acceleration, m/s2
• v = final velocity, m/s
• u = initial velocity, m/s
• t = time taken, s

Velocity-time graphs;

• Gradient represents acceleration
• Steeper gradient = bigger acceleration 
• Flat (horizontal) line = no acceleration 
• The area under the line in the graph = distance travelled (Total distance = total area) [H]

Using graphs;

• If you calculate the gradient of the line on the distance-time graph for an object, the answer will be the speed of an object
• If you calculate the gradient of the line on a velocity-time graph for an object, the answer will be the acceleration of the object
• Calculating the area under the line between two times on a velocity-time graph gives the distance travelled between those times 
• Always use the numbers from the graph scales in any calculations

Forces between objects;

• A force can change the shape of an object or change its motion or state of rest
• The unit of force  = newton (N)
• When two objects interact they always exert equal and opposite forces on each other

Resultant forces;

• The resultant forces = a single force that has the same effect as all the forces acting on an object 
• If an object is accelerating there must be a resultant forces acting on it 
• If the resultant force is:
• Zero (balanced) - the object will continue to move at the same speed and same direction 
• Not Zero (unbalanced) - the object will accelerate in the direction of the resultant (single) force 

On the road;

• Friction and air resistance oppose the driving force of a car 
• The stopping distance depends on the thinking distance and the braking distance 
• The stopping distance of a vehicle is the distance it travels during the drivers reaction time (thinking distance) plus the distance it travels under the braking force (the braking distance)
• The thinking distance is increased if the driver is tired or under the influence of drugs or alcohol 
• The braking distance can be increased by: 1) - Poorly maintained roads or bad weather conditions. 2) - The condition of the car (e.g. Worn tyres or worn brakes)

Stretching and squashing 

• The extension is the difference between the length of the spring and its original length
  
• Hooke's law - The extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded
• The spring constant of a spring is the force per unit extension needed to stretch it
• F = k x e 

Force and speed issues;

• Fuel economy of road vehicles can be improved by reducing the speed or fitting a wind deflector 
• Average speed cameras are linked in pairs and they measure the average speed of a vehicle 
• Anti-skid surfaces increase the friction between a car tyre and the road surface. This reduces skids, or even prevents skids altogether 

Energy and work;

• Work is done on an object when a force makes the object move 
• Energy transferred = work done 
• W = F x d 
• Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings

Gravitational potential energy;

• The gravitational potential energy of an object depends on its weight and how far it moves vertically 
• Ep = m x g x h 

Kinetic energy;

• The kenetic energy of a moving object depends in its mass and its speed 
• Ek = 1/2 x m x V2
• Elastic potential energy is the energy stored in an elastic object when work is done on the object 

Momentum;

• p = m x v 
• The unit of momentum is kg m/s
• Momentum is conserved whenever objects interact, provided no external forces act on them

Explosion;

• Momentum = mass x velocity
• When two objects push each other apart, they move apart: 1) With different speeds if they have unequal masses, 2) With equal and opposite momentum so their total momentum is zero.

Impact force;

• When vehicles collide the force of the impact depends on; mass, change of velocity and the duration of the impact
  
• When two vehicles collide: 1) They exert equal and opposite forces on each other, 2) Their total momentum is unchanged. 
  

Car safety;

• Seat belts and air bags spread the force across the chest and they also increase the impact time 
  
• Side impact bars and crumple zones 'give away' in an impact so increasing the impact time 
• We can use the conservation of momentum to find the speed of a car before an impact 

Electrical charges;

• Certain insulating materials become charged when rubbed together
• Electrons are transferred when objects become charged 
• Like charges repel; unlike charges attract 

Electric circuit;

• I = Q / t 

Resistance;

• V = W/Q = E/Q
• R = V/I
• Ohm's law statues that the current through a resistpr at contant temperature is directly proportional to the protential difference across the resistor
• Reversing the current through a component reverses the protential difference across it 

More current - protential difference graphs;

• Filament bulb: resistance increases with increase of the filament temperature 
• Diode: 'forward' resistance low; 'reverse' resistance high
• Thermistor: resistance decreases if it temperature increases
• LDR: resistance decreases if the light intensitiy on it increases 

Series circuit;

• For components in series; 1) The current is the same in each component, 2) adding the potential differences gives the total protential difference, 3) adding the resistances gives the total resistance  

Parallel circuits;

• For components in parallel; 1) The total current in the sum of the currents through the separate component, 2) The bigger the resistance of a component the smaller the current is 
• In a parallel circuit the potential difference is the same across each component 
• To calculate the current through a resistor in a parallel circuit use I = V/R

Alternating current;

• Direct current is in one direction only - Alternative current repeatedly reverses its direction
• The peak voltage of an alternative potential difference is the maximum voltage measured from zero volts 
• A mains circuit has a live wire that is alternatively positive and negative every cycle and a neutral wire at zero volts. f = 1/T

Fuses;

• A fuse contains a thin wire that heats up and melts if too much current passes through it. This cuts off the current 
• A circuit breaker is an electromagnetic switch that opens (i.e. 'trips') and cuts the current pff if too much current passes through it 

Cables and plugs;

• Sockets and plugs are made of stiff plastic materials, which enclose the electrical connections 
  
• Cables consist of two or three insulated copper wires surrounded by an outer layer of flexible plastic material
• In a three-pin plug or a three-core cable: 1) The live wire is brown, 2) The neutral wire is blue, 3) The eath wire is green and yellow
• The earth wire is used to earth the metal case of a mains appliance 

Electrical power and protential difference;

• The power supplied to a device is the energy transferred to it each second 
• P = I x V
• Correct rating (in amperes) for a fuse = electrical power (watts) / protential difference (volts)

Electrical energy and charge;

• An electric current is the rate of flow of charge
• Q = I x t 
• When charge flows through a resistor, energy transferred to the resistor makes it hot 
• E = V x Q

Electrical issues;

• Electrical faults are dangerous because they can cause electric shocks and fires
• Never touch a mains appliance (or plug or socket) with wet hands. Never touch a bare wire or a terminal at a potential of more than 30 V
• Check cables, plugs and sockets for damage regularly 

Observing nuclear radiation;

• A radioactive substance contains unstable nuclei that become stable by emitting radiation
• There are three main types of radiation from radioactive substances - alpha, beta and gamma radiation
• Radioactive decay is a random event - we cannot predict or influence when it happens
• Background radiation is from radioactive substances in the enviroment, or from space, or from devices such as X-ray machines

The discovery of the nucleus;

• Rutherford used the measurements from alpha particle scattering experiments as evidence that an atom has a small, positively charged, central necleus where most of the mass of the atom is located
• Most alpha particles passed straight through = most of the atom is atom 
• Some alpha particles defected through small angles = nucleus has a positive charge
• Few reflected back = nucleus has a large mass, very positivley charged 
• The nuclear model of the atom correctly explained why the alpha particles are scattered and why some are scattered through a large angles 

Nuclear reactions;

• Isotopes of an element are atoms with the same number of protons but different number of neutrons. Therefore, they have the same atomic numbers but different mass numbers

More about alpha, beta and gamma radiation;

• a-radiation is stopped by paper or a few centimetres of air 
• B-radiation is stopped by a thin metal or about a metre of air
  
• y-radiation is stopped by a thick lead and has an unlimited range in air 
• A magnetic or an electric field can be used to separate a beam of alpha, beta and gamma radiation 
• Alpha, beta and gamma radiation ionise substances they pass through

Half-life

• The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve
• The activity of a radioactive source is the number of nuclei that decay per second
• The number of atoms of a radioactive isotope and the activity both decrease by half every half-life

Radioactivity at work;

• The use we can make of a radioactive isotope depends on its half-life, and the type of radiation it gives out 
• For radioactive dating of a sample, we need a radioactive isotope that is present in the sample which has a half-life about the same as the age of the sample

Nuclear fission;

• Nuclear fission is the splitting of a nucleus into two approximately equal fragments and the release of two or three neutrons 
• Nuclear fission occurs when a neutron hits a uranium-235 nucleus or a plutonium-239 nucleus and the nucleus splits
• A chain reaction occurs when neutrons from the fission go on to cause other fission events
  
• In a nuclear reactor control rods absorb fission neutrons to ensure that, on average, only one neutron per fission goes on to produce further fission 
  

Nuclear fusion;

• Nuclear fusion in the process of forcing two nuclei close enough together so they form a single larger nucleus 
  
• Energy is released when two light nuclei are fused together 

Nuclear issues;

• Radon gas is an a-emitting isotope that seeps into houses in certain areas through the ground
  
• There are hundreds of fission reactors safely in use throughout the world. None of them are the same type as the Chernobyl reactors that exploded
• Nuclear waste is stored in safe and secure conditions for many years after unused uranium and plutonium (to be used in the future) is removed from it 

The early universe;

• A galaxy is a collection of billions of stars held together by their own gravity
• Before galaxies are stars formed, the universe was formed of hydrogen and helium 
• The force of gravity pulled matter into galaxies and stars

The life history of a star;

• A protostar is a gas and dust cloud in space that can go on to form a star
• Low mass star (sun):
  • Protostar > main sequence star > red giant > white dwarf > black dwarf
• High mass star:
  • Protostar > main sequence star > red supergiant > supernova > black hole (if sufficient mass)
    
• The son will eventually become a black dwarf
• A supernova is the explosion of a supergiant after its collapses

How the chemical elements formed;

• Elements as heavy as iron are formed inside stars as a result of nuclear fusion 
• Elements heavier than iron are formed in supernovas, along with lighter elements
• The sun and the rest of the solar system were formed from the debris of a supernova

Force and acceleration;

• The bigger the resultant force on an object, the greater its acceleration 
• The greater the mass of an object, the smaller its acceleration for a given force
• F = m x a

Falling object;

• The weight of an object = force of gravity on it 
• The mass of an object = the quantity of matter in it 
• An object acted on only by gravity accelerates at about 10m/s2
• The terminal velocity of a falling object = the velocity it reaches when falling through a fluid. The weight is then equal to the drag force on the object
• The drag force may also be called air resistance or fluid resistance