Physics P3

Physics P3

1. Medical Applications

   • X-Rays
     • Process
       • High energy photons are projected onto the body part which is being x-rayed
         • The photons cause a chemical reaction in the exposed areas of film to reveal the bone
     • An ionising type of radiation
       • This can cause cancer or death
     • Safety precautions
       • Warning lights to show x-ray production
       • Shielding the x-ray source with lead
       • Wearing lead aprons
       • X-ray film badge to show high levels of radiation
   • Ultrasound
     • Human hearing range 20Hz - 20,000Hz
     • The reflection
       • When ultrasound is transmitted from one medium to another. Some of the sound is reflected and some is absorbed. Gel is used to avoid the air medium as the Ultrasound will travel at a different speed through the air
     • Distance (S) = Speed of Sound (V) x Time taken (T)
     • Uses
       • Foetus imaging
       • Shattering kidney stones
       • Sonar
   • CT Scans (Computerised Tomography)
     • X-Ray machine spins around
     • The body is moved through the machine
     • Advantages
       • Produces a 3D image
       • Distinguishes between different tissues
     • Disadvantages
       • High exposure to X-ray radiation
       • Equipment is expensive

2. Lenses

   • Refraction
     • Refractive Index = Sin i / Sin r
     • When light changes direction whilst passing from one medium to another
     • Critical Angle - The angle of incidence where the refracted ray makes 90° with the normal line
   • Convex Lens
     • Convex Ray Diagram
       • Real image
       • Inverted
       • Bigger
   • Concave Lens
     • Concave Ray Diagram (Magnifying Glass)
       • Virtual image
       • Upright
       • Smaller
   • The Eye
     • Range of Vision
       • The ciliary muscles change the shape of the less to allow the eye to focus on object at various distances
       • Close Objects - The muscles contract and the lens becomes rounder
       • Distant Objects - The muscles relax and the lens becomes thinner
     • Correcting Eyesight Problems
       • Convex lens is used to correct long sight
       • Concave lens is used to correct short sight
   • Lens Power (Dioptres) = 1 / Focal Length (Metres)

3. Mass

   • Centre of Mass - The point at which the whole mass of an object is concentrated
   • Pendulum
     • Period (Seconds) = 1 / Frequency (Hertz)
     • The period is increased by the length of string only
     • A cycle is from one position and returning to it
   • Turning Effect and Moments
     • Moment = Force (Newtons) x Distance to pivot (Metres)
     • Levers - Force multipliers
   • Stability
     • An object is stable as long as the line of action of its weight passes through the base
   • Hydraulics
     • Pressure (Pa) = Force (N) / Area (M^2)
     • Pressure in a liquid acts equally in all directions
     • Liquids are incompressible
   • Circular Motion
     • When moving in a circle an object is constantly changing direction thus it is accelerating
     • The acceleration acts towards the centre of the circle
     • The force which cause the acceleration is Centripetal Force

4. Electromagnetic Fields

   • Electromagnets
     • The wire is usually wound around a bar made of soft iron
     • To increase the force
       • Increase the current
       • Increase the number of turns in the wire
   • The Motor Effect
     • To Increase the force
       • Increase the current
       • Increase the strength of the magnetic field
     • There is no force if the current is parallel to the field direction
     • The Motor
       • The split ring commutator swaps the battery connections every half turn to enable the coil to keep rotating in the same direction
   • Electromagnetic Induction
     • 1. A conductor cuts through a magnetic field
       • 2. If the wire is part of a complete circuit, a potential difference is induced in the wire
     • 1. Magnet enters coil - Potential difference induced
       • 2. Magnet stops - No potential difference
         • 3. Magnet removed - Potential difference induced in the opposite direction
   • Transformers
     • Primary Voltage/Secondary Voltage = Primary Turns/Secondary Turns
     • Made of lamintaed(layered) iron core
     • Used to increase or decrease voltage
     • Switch-Mode Transformers
       • Used to convert AC - DC
       • Almost 100% efficient

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