PHYSICS_AQA GCSE Physics Unit 3 Mindmap

PHYSICS_AQA GCSE Physics Unit 3 Mindmap

1. Medical applications

   • X-rays
     • High frequency and short wavelength
     • Properties
       • Affect photographic film in the same way as light
       • Absorbed by metal and bone
         • Why lead aprons are worn
           • Absorb the rays and reduce exposure to radiation
           • Can cause cancer
             • Aswell as kill cancerous cells
               • At the surface
               • Near the surface of the body
         • Film badges
           • Show amount of exposure
       • Transmitted by healthy tissue
     • CCD
       • form electronic images
       • CT scanners
         • Digital cross-section
         • Soft tissue organs filled with contrast medium
           • Absorb X-rays to be seen
   • Ultrasound waves
     • Sound waves
       • With a frequency above 20,000Hz
       • Human ear
         • Detect sound waves between 20Hz and 20,000Hz
     • Produced by electronic systems
       • wave meets a boundary between 2 different materials
         • Part of the wave is reflected
         • Travels back through material to detector
           • Time taken to reach the detector can calculate how far away the boundary is
             • Results may be processed by a computer to produce an image
           • May need to half the time as it has travelled there and back
     • Non ionising
       • So its safer than X-rays
     • For scanning
       • Unborn babies (not unicorn)
         • And soft tissue
           • E.g. the eye
           • Not kleenex
       • Therapy
         • Shatter kidney stones
   • Refractive index
     • Refraction
       • The change of direction of light from one substance to another
       • Takes place because waves change speed when they cross a boundary
         • Because of the change in speed
     • How much a substance can refract a light ray
       • Sin i divided by Sin r
         • Incidence
         • Refraction
     • TAGAGA
       • Air to glass refracted towards the normal
       • Glass to air away from the normal
   • The endoscope
     • Critical angle
       • angle of incidence of a light ray in a transparent substance
         • Produces refraction along the boundary
       • If angle of incidence is higher than the critical angle the light ray under goes total internal reflection
         • When it occurs the angle of reflection is equal to the angle of incidence
       • Refractive index = 1 divided by sin c (critical angle)
     • Used to look inside a patients body
       • without cutting it open (bleh)
       • contain bundles of optical fibres
         • Thin, flexible glass fibres
         • Visible light sent along
           • By total internal reflection
       • Laser light may be used for cutting
         • Colour is matched to tissue type
           • To produce maximum absorption
   • Lenses
     • Converging (convex)
       • Used in magnifying glasses
       • If the object is NEARER to the lens than the principal focus, an upright virtual image is formed behind the object
         • If the object is FURTHER away from the lens than the principal focus an inverted real image is formed
       • Parallel light rays pass through and are refracted so they converge at a point called the principal focus
     • Diverging (concave)
       • If you go in a cave and there is a mean bear you will die (so a concave lens is diverging)
       • Light can pass through the lens in either direction there is a principal focus soon either side of the lens
       • Image is always virtual
       • Distance from the centre of the lens to the principal focus is the focal length
       • parallel light rays pass through and are refracted so they diverge away from a point, the principal focus
     • Magnification = image height divided by object height
     • Cameras use converging lenses to form a real image of an object on a film
     • Ray diagrams
       • Parallel to axis and through f
         • Through centre
           • Through F then parallel to axis
   • The eye
     • Iris - coloured ring of muscle that controls the amount of light entering the eye
     • Cornea - Transparent layer that protects the eye and helps focus light onto the retina (like corn which is yellow like light so it focuses light)
     • Retina the light sensitive cells around the inside of the eye
     • Pupil - The central hole formed by the iris and where light enters the eye through the pupil
     • Blind spot - region where the retina isn't sensitive to light
       • No light sensitive cells are present
     • Ciliary muscles - Attatched to the lens by suspensory ligaments to change the thickness of the eye lens
     • Optic nerve - carries nerve impulses to the brain
     • Eye lens - focuses light onto the retina
     • Conjuctiva membrane (its just there!)
     • Aqueous humour - transparent watery liquid that supports the front of the eye
     • Vitreous humour - transparent jelly like substance that supports the back of the eye
     • Eye muscles - move the eye in the socket
     • Power of a lens
       • = 1 divided by f (focal length)
         • Power of a lens is measured in deportees (D)
         • Focal length is measured in metres (m)
       • Human eye
         • Near point 25cm and a far point of infinity! (and beyond)
         • Range of vision
         • Eyeball defects may cause long or short sightedness (if thats a word)
           • Short sighted
             • See close objects clearly
               • distant objects are blurred
                 • uncorrected image is formed in front of the retina
             • Eyeball being too long
             • eye lens too powerful
             • Corrected with a diverging lens
           • Long sighted
             • see distant objects clearly
               • the uncorrected image is formed behind the retina
             • Eye lens is too weak
             • Eyeball being to short
             • Corrected using a converging lens

2. Making things work

   • Hydraulics
     • Pressure
       • P = F divided by A
       • Measured in pascals, (not the chameleon from tangled) Pa
       • liquids are virtually incompressible
         • pressure in a liquid is transmitted in all directions
           • Made use of in hydraulic systems
     • Hydraulic pressure system
       • Force exerted depends on...
         • Force exerted on the system
         • Area of the cylinder on which the force acts
         • Area of the cylinder that the force is exerted on
     • Use of different cross sectional areas on the effort and load mean the system can be used as a force multiplier
   • Stability
     • The line of action of the weight of an object acts through its centre of mass
     • Line of action
       • lies outside the base of an object
         • topple over
         • resultant moment
       • Inside the base
         • Won't topple
     • The wider the base of an object
       • lower centre of mass
         • Has to tilt further to topple
       • Increased stability
     • Bowling pins narrow base so high centre of mass
       • Easily knocked over
   • Centre of mass
     • Where the mass of an object is though to be concentrated
     • When a suspended object is in equilibrium, its centre of mass is directly beneath its point of suspension
     • Finding the centre of mass
       • Symmetrical sheets where the axis of symmetry cross
       • Irregular sheets
         • suspend the sheet from a pin held in a clamp stand
           • Because it is freely suspended it is able to turn
           • Hang a plumb line from the same pin
             • Mark the position of the plumb line
               • Hang the sheet with the pin at another point and repeat the procedure
                 • The centre of mass is where the lines cross
             • When it comes to rest
     • The position depends on the shape of the object which sometimes may lie outside the object
   • Pendulums
     • Moves to and fro along the same line
       • Oscillating motion
     • A simple pendulum consists of..
       • A mass (bob) suspended on the end of a string
       • when bob is displaced to one side and let go the pendulum oscillates
         • Through the equilibrium position
           • When it stops moving
     • Amplitude is the distance from the equilibrium position to the highest position on each side
     • Time period of an oscillation is the time taken for one complete cycle
       • To measure the time period of a pendulum
         • measure the average time fro 20 oscillations and divide the time by 20
           • Park swing is an example
       • depends on length, it increases as length increases
       • Time period = 1 divided by f (frequency (number of complete oscillations in one second))
   • Circular motion
     • Centripetal force increases when..
       • Mass increases
       • Radius decreases
       • Speed of the object increases
     • If centripetal force stops the object will continue to move in a straight line at a tangent to the circle
     • An object only accelerates when a resultant force acts on it
       • This force is called centripetal force and always acts towards the centre of a circle
     • Continuously changing direction
       • Continuously changing velocity
         • This acceleration is called centripetal force
   • Moments
     • Balance
       • If an object is in equilibrium the anticlockwise moment will be equal to its clockwise moment
         • This is the principal of moments
       • We use the principal of moments to calculate the force needed to stop an object turning
     • The turning effect of a force
     • M = F x d
       • d = perpendicular distance from the line of action of the force to the pivot in metres
     • To increase the moment of a force...
       • Increase the force
       • Increase the distance from the pivot
         • for example using a spanner
         • Using a lever
           • Force we are trying to move is called the load
           • The force applied to the lever is called the effort
     • Opening a door
     • Using a crowbar

3. Keeping things moving

   • Transformers
     • Two coils
       • Around wrapped around a laminated iron core
         • When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core
           • This field continually expands and collapses
         • Easily magnetised
     • The alternating magnetic field passes through the secondary coil and induces an alternating potential difference along its ends
       • If the secondary coil is part of a complete circuit an alternating current is produced
     • coils of wire are insulated
       • doesn't short circuit on any adjacent turns of wire or the iron core
     • Switch mode transformer
       • Has a ferrite core (not ferret thats a cute fluffy animal)
         • Lighter and smaller
       • Operates at a higher frequency than a normal transformer
         • uses very little power when there is mo device connected across its output terminals
     • Step up or down
       • If pd across the secondary coil is higher than across the primary coil it is step up and the opposite for step down
         • Number of turns
       • Like a snowy mountain
   • The motor effect
     • when we place a wire carrying an electric current in a magnetic field it may experience a force
     • Maximum force if the wire is at 90 degrees to the magnetic field
       • Zero if parallel to the magnetic field
     • Flemings left hand rule
       • First finger is the direction of the magnetic field (north to south)
       • Second finger is the direction of the current
       • The thumb shows the motion the wire
     • Increase the force
       • Increase the current
       • Increase the strength of the magnetic field
     • To change the motion of the wire
       • reverse the direction of the magnetic field
       • reverse the direction of the current
     • electric motor
       • speed of the motor is increased by increasing the size of the current
         • direction reversed by changing current direction
       • coil spins when current goes through
         • force acts on each side due to the motor effect
     • Split ring commutator
       • Reverses coil current every half turn
       • Sides swap over each half turn the coil is always pushed in the same direction
   • Transformers in action
     • the national grid uses transformers to step-up the pd from power stations
       • The higher the pd at which electrical energy is transmitted across the grid the smaller the energy wasted in the cables
     • Step down transformers are used to reduce the pd so that it is safe to be used by consumers
     • Transformers are almost 100% efficient
       • 1
       • 2
     • There are two equations for transformers
   • Electromagnetic induction
     • If an electrical conductor cuts through magnetic field lines a potential difference is induced across the ends of the conductor
     • Increase the size of the induced pd...
       • Increase the speed of movement
       • Increase the strength of the magnetic field
       • Increasing the number of turns on the coil
         • so many turns!!
     • Reversing the direction of the movement or the magnetic field the direction of the pd is also reversed
       • pd is potential difference by the way
     • If a magnet is moved into a coil of wire a pd is induced across the ends of the coil, this process is called electromagnetic induction
   • Electromagnets
     • Magnets
       • Ends called magnetic poles
       • Region around the magnet in which pieces of iron will be attracted is called its magnetic field
         • Iron filings placed near a magnet will produce a pattern of lines that loop from one pole to the other, these are lines of force or magnetic lines
         • A plotting compass placed in the magnetic field will always point along the field line
       • Like poles will repel
       • Unlike poles will attract
     • When a current flows through a wire a magnetic field is produced around the wire
       • Made by wrapping an insulated wire around a piece of iron, iron core
         • When a current flows through the wire it becomes strongly magnetised
           • This temporary magnetism makes electromagnets very useful
             • If current is off it loses its magnetism

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