P3

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  • P3
    • Medical Applications of Physics
      • X-Rays
        • Short wavelength
        • Can cause ionisation
        • Affect a photographic film in the same way as light
        • Can be detected using charge-coupled devices (CCDs) to form an image electronically
        • CT Scans
          • More detailed 2d and 3d images
            • Image can be observed from any angle
          • Increased levels of radiation for patient
          • Higher resolution images
      • Ultrasound
        • Sound greater than 20,000 Hz
          • Inaudible to humans
        • Non-ionising
        • Partially reflected at a boundary between two different media
        • Uses
          • Pre-natal scanning
          • Imaging of damaged ligaments and muscles
          • Destruction of kidney stones
    • Optics
      • Lenses
        • Converging (Convex)
          • Image is real and inverted
          • Image is smaller for distant objects
          • Image is magnified for objects between F and 2F
          • Light is refracted inwards and rays meet at F
          • Can be used as magnifying glasses
        • Diverging (Concave)
          • Image is virtual
          • Image is upright
          • Rays of light diverge (separate) as they leave the lens
      • Virtual
        • Image is the same side as the object
      • Real
        • Image is on the opposite side to the object
      • Refraction
        • When light crosses a boundary between two different transparent media of varying densities, it changes direction
        • No refraction occurs when light enters along the normal
        • Angle of incidence, i
          • Angle between ray entering a boundary and the normal
        • Angle of refraction, r
          • Angle between ray leaving a boundary and the normal
      • Power
        • One over focal length
        • Converging lens
          • Positive
            • Real focal point
        • Diverging lens
          • Negative
            • Virtual focal point
      • Critical Angle
        • Occurs when the angle of refraction equals 90 degrees.
        • The light ray travels on the boundary between the media e.g. glass and air
        • Diagram
        • Total internal reflection
          • Occurs when the angle of incidence is greater than the critical angle
          • No refraction occurs so no light leaves the media
          • Used in medical endoscopes to view the inside of the body
            • Visible light directed along optical fibres to an eye piece or camera
      • Magnifying glasses
        • Convex lens
        • Magnifies between F and the centre of the lens
      • Lasers
        • A device that amplifies light to produce a very narrow and intense beam
        • Can be made from solids, liquids or gases
        • Uses in eye surgery
          • Repairing damaged retinas
          • Remove damaged cells by cutting, cauterising or burning tissue
          • Optical fibres guide the laser beam to the correct area of the body
      • The Eye
        • Optic nerve
          • Sends information to brain to interpret image
        • Cornea
          • Refracts most of light
        • Pupil
          • (Iris opening) Adjusts light intensity
        • Lens
          • Further refration onto retina
        • Ciliary muscles
          • Control shape of lens and therefore focus
        • Normal vision
          • Near point 25cm
          • Far point infinity
        • Eye defects
          • Long sight (Hyperopia)
            • Eyeball too short
            • Image forms behind retina
            • Unable to focus on near objects
          • Short sight (Myopia)
            • Eyeball too long
            • Image forms in front of retina
            • Unable to focus on distant objects
          • Glasses
            • Adjust light before entering the eye to allow it to focus correctly
            • Made from convex lenses, concave lenses, or a combination of both
    • Keeping Things Moving
      • Electromagnets
        • Uses
          • On cranes for lifting iron and steel
          • In circuit breakers
          • In loudspeakers
          • In electric bells
        • When a current flows through a wire a magnetic field is produced around the wire
      • The motor effect
        • When a wire (conductor) carrying a current is placed in an external magnetic field, the magnetic field formed around the wire interacts with this permanent magnetic field. This causes the wire to experience a force that makes it move.
          • The wire will experience no force if it is parallel to the direction of the magnetic field
        • Increase size of force on wire
          • Increase size of current
            • e.g. have more cells
          • Increase strength of magnetic field
            • e.g. stronger magnets
        • Reverse direction of force on wire
          • Reverse direction of the flow of current
            • e.g. turn the cell around
          • Reverse direction of magnetic field
            • e.g. swap magnets around
        • Split ring commutator
          • Reverses the flow of the current every half turn
            • Allows the wire to keep spinning rather than just 'wobble' in the same position
        • Flemming's left-hand rule
          • First Finger = magnetic Field
          • seCond finger = Current
          • thuMb = Movement
      • Electromagnetic Induction
        • Used by bicycle dynamos and generators to produce electricity
        • If a conducting wire/coil of wires is moved through a magnetic field, a potential difference is induced across the ends of the wire
          • An electrical current will be induced if the coil of wire forms a complete circuit
          • The same effect occurs if the coil is stationary and the magnetic field moves
        • Increase potential difference:
          • Increase speed of movement of magnet/coils
          • Increase strength of magnetic field
          • Increase number of turns on the coil
      • Transformers
        • An alternating current is supplied to the primary coil
          • This produces an alternating magnetic field in the iron core
            • This magnetic field links with the secondary coil
              • An alternating potential difference is induced across the ends of the secondary couil
        • Switch mode transformers
          • Smaller and lighter
          • Much higher frequency
            • 50-200 kHz
          • Operate on 50 Hz mains supply
          • Use very little power when switched on and no load is applied
          • e.g.
            • Mobile phones
            • Digital cameras
            • Laptops
        • Never truly efficient as energy is lost as heat to the surroundings
        • Step up
          • More turns on secondary coil
          • Greater p.d on secondary coil
        • Step down
          • Less turns on secondary coil
          • Lesser p.d across secondary coil
          • Used in national grid to ensure efficent transmission of electricity
            • Step up
              • More turns on secondary coil
              • Greater p.d on secondary coil
    • Using Physics to Make Things Work
      • Hydraulic Systems
        • Pressure  in a liquid is transmitted equally in all directions
        • Liquids are virtually incompressible
          • Gases compress, so are not suitable for hydraulics
        • Different cross-sectional areas of pistons allow them to be used as force multipliers in hydraulics
        • e.g. hydraulic jacks, car breaking, mechanical diggers
        • Pressure
          • The force acting over a particular surface area
          • A small area results in a larger pressure than a big area when the force is the same
          • 1 N/m^2 = 1 Pa
      • Pendulums
        • A mass at the end of a string that swings back and forth
        • Frequency = number of times a pendulum swings backwards and forwards in 1 second
        • Time period = time for one complete swing
          • The time period only depends on the length of the pendulum
      • Stability
        • An object will topple if the line of action of the force, e.g. its weight lies outside its base
          • The weight of the object causes a turning effect which makes the object topple
        • Increasing stability
          • Lower centre of mass
          • Increase the width of the base
      • Centre of Mass
        • The point of an object where the whole mass of the object is considered to be concentrated
        • Found where lines of symmetry meet on regular shapes
        • Can be found with a plumb line for irregular shapes
        • A suspended object will come to rest with its centre of mass directly below the point of suspension
      • Moments
        • The turning effect of a force around a pivot point
        • D= perpendicular distance between the line of action of the force and the pivot
        • Increase size of moment
          • Increase force
          • Increase distance
        • When an object isn't turning, the clockwise and anticlockwise moments are balanced.
          • f1 x d1 = f2 x d2
        • Levers use moments to act as force multipliers
      • Centripetal Force
        • An object moving at a constant speed in a circular path is continuously accelerating towards the centre of the circle
          • Constantly changing direction when moving in a circular path means a changing velocity, and acceleration must be changing as acceleration is the rate of change of velocity
            • Resultant foce causing this acceleration is the centripetal force
              • This acts towards the centre of the circle
        • Increase centripetal force
          • Increase mass of object
          • Increase speed of object
          • Decrease radius of circle

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