Physics Unit 3

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P3.1 Medical physics part1

  • properties of X-rays: affect a photographic film in the same way as light, absorb metal and bone, transmitted by healthy tissue. used to check fractures and dental problems
  • charge-coupled devices (CCDs) can be used to form electronic images of X-rays. CT scanners use X-rays to produce digital images of a cross-section through the body
  • X-rays cause ionisation and can damage living tissue when they pass through it therefore workers wear film badges and use lead screens to shield them from the X-rays
  • X-rays are used to treat cancerous tumours at or near the body surface
  • ultrasound is non-ionising and can be used to scan unborn babies and soft tissue. it is used in therapy to shatter kidney stones into small pieces
  • refraction is the change of direction of light as it passes from one transparent substance into another. the refractive index of a substance is a measure of how much the substance can refract a light ray. if the object is denser then it will refract more to the normal line
  • the critical angle is the angle of incidence of a light ray in a transparent substance which produces refraction along the boundary
  • total internal reflection occurs when the angle of incidence of a light ray in a transparent substance is greater than the critical angle
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P3.1 Medical physics part2

  • converging lens: parallel rays of light pass through and are refracted so that they converge to a point which is called the principal focus, the distance from the centre is called the focal length. if the object is further away from the lens than the principal focus, an inverted, real image is formed. the nearer the object is to the lens, the larger the image. if the object is nearer to the lens than the principal focus, an upright, virtual image is formed behind the object. the image is magnified- the lens acts as a magnifying glass
  • diverging lens: parallel rays of light pass through and are refracted so they diverge away from a point called the principle point. the image produced is always virtual
  • a camera uses a converging lens to form a real image of an object on a film or an array of CCDs
  • light enters the eye through the cornea. the cornea and the eye lens focus the light on to the retina. the iris adjusts the size of the pupil to control the amount of light entering the eye.
  • the ciliary muscles alter the thickness of the lens to control the fine focusing of the eye. they are attached to the lens by the suspensory ligaments
  • the normal human eye has a near point of 25cm and a far point of infinity so its range of vision is 25cm to infinty
  • the power of a lens is given by P = 1/f  P is power in dioptres (D), f is focal length, metres
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P3.1 Medical physics part3

  • a short-sighted eye is an eye that can only see near objects clearly. we use a diverging (concave) lens to correct it
  • a long-sighted eye is an eye that can only see distant objects clearly. we use a converging (convex) lens to correct it
  • the camera has a lens of fixed shape but variable position, the eye has a lens of variable shape but fixed position.
  • the higher the refractive index of the glass used to make a corrective lens, the flatter and thinner the lens can be
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P3.2 Make things work part1

  • the turning effect of a force is called its moment
  • M = F x d   M is moment (Nm), F is force (N), d is the perpendicular distance from the line of action of the force to the pivot in metres (m)
  • to increase the moment: either the force must increase, or the distance to the pivot increases
  • when using a lever, the force we are trying to move is called the load and the force applied to the lever is the effort. a lever acts as a force multiplier
  • centre of mass with card test: suspend a thin sheet from a pin held by a clamp, when it becomes stationary hang the plumb line from the same pin, mark the position of the plumb line against the sheet, hang the sheet with a pin at another point and repeat procedure, the centre of mass is where the marked positions cross
  • if the line of action of the weight lies outside the base of an object, there will be a resultant moment and the object will tend to topple over
  • a hydraulic system uses the pressure in a fluid to exert a force
  • the force exerted by a hydraulic pressure system depends on: force exerted on the system, the area of the cylinder on which this force acts on, the area of the cylinder that exerts the force
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P3.2 Make things work part2

  • centripetal acceleration always acts towards the centre of the circle
  • the centripetal force on an object depends on its mass, speed and the radius of the circle
  • centripetal force- the resultant force towards the centre of a circle acting on an object moving in a circular path
  • a pendulum moves to and fro along the same line. this is an example of oscillating motion
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P3.3 magnetic fields part1

  • a magnetic field line is the line along which a plotting compass points
  • electromagnet: a current flows through a wire, a magnetic field is produced around the wire, it's made by wrapping insulated wire around a piece of iron called the core, when a current flows through the wire the iron becomes strongly magnetised, when the current is switched off the iron loses its magnetism, this temporary magnetism makes electromagnets very useful in scrapyard cranes, circuit breakers, electric bells and relays
  • flemings left hand: First finger= magnetic Field, seCond finger= Current, thuMb= Movement-direction of force
  • electric motor: speed of motor is increased by increasing the size of the current. direction of motor can be reversed by reversing the direction of current. when a current passes through the coil the coil spins because: a force acts on each side of the coil due to the motor effect and the force on one side of the coil is in the opposite direction to the force on the other side. size of force can be increased by- increasing the strength of the magnetic field, increasing the size of the current. the split-ring commutator reverses the direction of the current around the coil every half-turn because the sides swap over each half-turn and the coil is always pushed in the same direction
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P3.3 magnetic fields part2

  • electromagnetic induction: the process of creating a potential difference using a magnetic field. if a magnet is moved into a coil of wire a pd is induced across the ends of the coil. if the wire or coil is part of a complete circuit then a current will pass through it. a pd is only induced while there is movement. the size of the induced pd is increased by increasing: speed of movement, strength of magnetic field and the number of turns on the coil
  • transformers: a transformer only works on ac because a changing magnetic field is necessary to induce ac in the secondary coil. it has an iron core (easily magnetised) unless it is a switch mode transformer which has a ferrite core. a switch mode transformer operates at much higher frequency, it's lighter and smaller and it uses very little power when there is no device connected across its output terminals
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