PHYSICS

PHYSICS

1. Medical Physics

   • X-rays
     • Part of the electromagnet spectrum
     • Type of ionising radiation
     • Very short wave length - about the same as an atom
     • Can cause cancers or death
       • Safety Precautions
         • Warning lights to show when X-rays are being produced
         • Keeping X-ray operators away from X-ray source when it's operating (usually behind a shield or into a separate room
         • Wearing goggles and protective clothing such as a lead apron
     • X-ray photographs used to detect broken bones or cavities in teeth
     • Used to kill cancer cels
   • Ultrasound
     • Frequency higher than upper hearing for humans (20-20000 Hz)
     • Produced by electronic systems
     • Partially reflected off the boundary between two different materials
       • A probe sends out a pulse of ultrasound and detects the echos
         • A computer converts the time taken for the echoes to arrive into an image
           • Image can be used to check the growth and development of the fetus
     • Equation - s = v x t
       • s is the distance travelled by the sound in metres, m
       • v is the velocity of the sound in metres per second, m/s
       • t is the time taken in seconds, s
     • Used to shake kidney stones into tiny fragments that can be excreted
   • CT Scans
     • Computerised Tomography
     • Expose patients to more X-rays than single picture, giving higher risk of cancer

2. Lenses

   • Refraction in lenses
     • Light changes direction when it passes from one medium to another
     • The size of the change of direction is determined by the Refractive Index
       • Refractive Index = sin i/ sin r
         • i is the angle of incidence
         • r is the angle of refraction
   • Real Images
     • A converging lens produces a real image when the object is placed more than one focal length away from the lens
   • Virtual Images
     • Converging lens form virtual images when the object is closer than one focal length
   • Power of Lens
     • Power = 1/focal length

3. The Eye

   • Lens - Can change shape to help focus light to form image on the retina
   • Pupil - Hole in the centre of the iris through which light passes
   • Cornea - Transparent so lets light enter the eye and helps to focus it onto the retina
   • Iris - Adjusts size of the pupil to control the amount of light entering the eye
   • Suspensory Ligaments - Hold the ciliary muscles and lens in place
   • Ciliary \Muscles - Contract and relax to change the shape of the lens
   • Retina - Made up of cells sensitive to light. Sends impulses to the brain

4. Range of vision

   • The ciliary muscle change the shape of the lens to allow the eye to focus on objects at various distances
     • Viewing distant objects
       • Light from distant objects don't need to bent very much to form an image on the retina
         • The lens is thin
       • Correction of Sight problems
         • Long sight is corrected using converging lenses
           • These bend the light more so that the rays meet to form an image on the retina
     • Viewing close objects
       • Light from close objects needs to converge more, so the lens needs to be fatter
       • Correction of sight problems
         • Short sight is corrected using diverging lenses
           • These spread the light before it enters the eye and is converged by the cornea and lens onto the retina

5. Centre of Mass

   • The centre of mass of an object is the point at which the whole mass of the object may be thought to be concentrated

6. The Pendulum

   • The period of a simple pendulum is the time taken for a complete swng
     • The frequency of a pendulum is the number of swings completed in one second
       • Period = 1/frequency

7. Turning Effect and Levers

   • Moments
     • The size of the moment, or turning effect, of a force is given by the equation
       • Moment = Force x Perpendicular distance to pivot

8. Electromagnets

   • Used in cranes, relays and circuit breakers
   • When an electric current travels along a wire it produces a magnetic field
     • If the wire is wound into a coil, the shape of the magnetic field is similar to that of a bar magnet
       • The wire is usually wound around a bar made of soft iron
         • The force can be increased by increasing the current and the number of turns of wire
   • Electromagnetic Induction
     • A potential difference is induced in a wire if the wire is moved through a magnetic field
       • When an electrical conductor moves in a magnetic field it 'cuts' across the lines of force
         • This induces a potential difference in the wire and an electrical current flows through the circuit

9. Transformers

   • A transformer can be used to change the potential difference of an alternating electricity supply
   • Step-up Transformers
     • The potential difference across the secondary coil is greater than the potential difference across the primary coil
   • Step-down Transformers
     • The potential difference across the secondary coil is less than the potential difference across the primary coil
   • The potential difference between the primary and secondary coils is determined by the ratio of the number of turns of wire on the primary and secondary coils
   • Switch Mode Transformers
     • Use electronic components to convert alternating current to direct current
   • Traditional Transformers
     • Mainly used to step up or down mains alternating voltages with a frequency of 50 Hz
     • Use electrical energy whenever they are connected to the supply even if there is no load
       • The energy is dissipated as heat energy

10. Other uses of light

    • Optical fibres and lasers make use of properties of lith
      • Optical Fibres
        • Made from special glass
        • Bend because fibres are very thin
        • As a ray of light travels down the glass fibre it is reflected every time it hits the edge of the fibre, even if the fibre is bent
        • The light does not escape the fibre
      • Lasers
        • Produces a narrow, intense beam of light of a single wavelength
        • Can concentrate a lot of energy into a small area
          • This means it can burn or cut through materials very precisely

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