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P3 Quick run-through

P3 Quick run-through. X-Rays. Absorbed strongly by bone or dense tissue. Can penetrate easily through low density tissue. CCD’s can digitise image. CT scans Wavelength ~ size of atom. Ionising radiation – Risks to health X-Ray therapy. Ultrasound.

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P3 Quick run-through

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  1. P3 Quick run-through

  2. X-Rays • Absorbed strongly by bone or dense tissue. Can penetrate easily through low density tissue. • CCD’s can digitise image. • CT scans • Wavelength ~ size of atom. • Ionising radiation – Risks to health • X-Ray therapy

  3. Ultrasound • Above 20KHz. For imaging, typically 1-5MHz. • Transducer sends out short pulses and detects echoes. • Reflected at a density boundary. • S=d/t applies to the echo. • A-scans and B-scans. • Ultrasound therapy/kidney stones/cleaners.

  4. Refraction • n = Refractive index • Snells Law – nisinθi= nrsinθr • n for air/free space ~1 • Ifni> nr (from a more to less dense material), then the critical angle sin c = nr/ni. • If the angle of incidence is greater that c, then total internal reflection will occur. • TIR used in endoscopes/reflectors/optical fibres.

  5. Lenses • Use curved surfaces to refract light. • Can be converging(convex) or diverging(concave). • Have a focal point. The focal length is measured from the lens to the focal point. • Power = 1/focal length. +ve for converging lens, -ve for diverging lens. • Parallel rays imply the object is at optical infinity. • Image is produced where rays converge or appear to be diverging from.

  6. Image formation • Principal rays • In parallel emerges through/from focal point. • Through the centre undeviated. • In through/towards focal point emerges parallel. • Image can be real or virtual. • Image can be upright or inverted • Image can be magnified or reduced. • Magnification = Image size/Object size.

  7. The eye • Retina, Lens, Cornea, Pupil/Iris, Ciliary Muscle, Suspensory Ligaments. • The cornea and lens cause convergence of rays on the retina. • Accomodation – Lens becomes more curved to focus on near object. • Far point is infinity and near point is ~25cm for ‘normal’ eye. • The eye is similar to a camera with the film/CCD representing the Retina.

  8. Vision defects • Long sight – The eye is too short or not powerful enough. Rays converge behind the retina. A Converging lens is used to correct. • Short sight – The eye is too long or too powerful. Rays converge in front of retina. A Diverging lens is used to correct. • The focal length(and therefore power) of a lens is determined by the refractive index and the curvature of its surfaces.

  9. Moments • Moment = Force x Perpendicular distance between pivot and line of action of force. • For a system in equilibrium, the total clockwise moment must equal the total anticlockwise moment. • Levers. Small force x large distance to produce a large force at small distance.

  10. Centre of Mass • The Com is “that point where the entire mass of the object may be though to be concentrated.” • The object will behave as if its entire mass resides at the COM. • A freely suspended object will come to rest with its COM directly below the point of suspension (otherwise the COM will have a moment around the pivot). • T = 1/f – For a simple pendulum and depends upon its length.

  11. Hydraulics • Pressure = Force/Area • Liquids transmit pressure in all directions as they are virtually incompressible. • Hydraulic systems use cylinders with different cross-sectional areas to turn a small force into a large one (same pressure in each cylinder).

  12. Circular Motion • Acceleration towards the centre of curvature as velocity is dependent upon direction. • The resultant force causing acceleration is called a centripetal force. • Centripetal force acts towards the centre of curvature. • Centripetal force required increases as mass and speed increase and as radius decreases.

  13. Electromagnetism • When a current flows through a wire, there is a magnetic field around the wire (right hand screw rule). • A solenoid will have a N and S pole (Right hand screw rule) and a magnetic field around it that grows and shrinks as the current in the coil changes. • The Motor Effect uses Flemmings left hand rule. • If current and field are parallel, there is no force.

  14. Induction • If an electrical conductor cuts across lines of magnetic flux, a P.D. Is induced across its ends. The same is true if the magnet moves with respect to the conductor. • If the conductor is part of a complete circuit, an induced current will flow. • This is used in the transformer.

  15. Transformers • a.c. current needed to create a fluctuating magnetic field around the primary coil. • The core links to the secondary side and so the magnetic field is transferred there. • A P.D. Is induced across the secondary coil. • Vp/Vs = Np/Ns • Transformers can be step up or step down. • Switch mode tranformers – High frequency 50KHz-200KHz. Lighter as they have ferrite cores.

  16. Transformer Efficiency • Assumed to be 100% for the purposes of calculations. • Pin = Pout • IpVp= IsVs

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