P3

P3

1. Unit 1

   • Medical applications of physics
   • 1.1
     • X-rays
     • X- ray
       • X-rays used to: - make image and CT scans - destroy tumours at body surface X- ray photograph = radiograph Flat panel detector placed on the other side I'm of the patient X-rays pass through soft tissue but are absorbed by teeth and bones, showing brighter negative images.
       • X-radiation is dangerous as it ionises and kills living cells when they pass through tissue. Lower doses can cause mutation and cancerous growth. Workers must wear a film badge and stand behind glass so not overexposed to radiation. Lead absorber plates between tube and patient stop rays from reaching other parts of the body.
       • CCD sensors in flat panel detector are covered by a substance that converts X-rays to light. They create electronic signals in the sensors that are sent to computer which displays a digital X-ray image.
       • Image distinguishes between bone and soft tissue but not between different types of tissues. Produces 2D images. Lower radiation dose and cost than CT scanner.
     • CT scanner
       • Produces a digital image of any cross section through the body. Can also produce a 3D image of an organ - X-ray tube automatically moves round inside the ring in small steps - at each position X-rays pass from tube through patient and reach the detector ring - electronic signals from the detector are recorded by a computer until the tube has moved round the ring - computer displays a digital Image of the scanned area
       • Each detector receives X-rays that have travelled through different types of tissue. Detector signal depends on : - the different types of tissue along ray path - how far the X-rays pass through each type of tissue
       • Distinguishes between bone and soft tissue as well as different types of soft tissues. Produces 3D image. CT scanner gives a much higher dose and has a greater cost than an ordinary X-ray machine.
   • 1.2
     • Ultrasound
     • Ultrasound waves = 20kHz +
       • Human hearing range = 20 - 20 000 Hz Testing ultrasound = loudspeaker + generator produce frequency detected by microphone + oscilloscope to: - Measure Hz - test materials for absorbing - show they can be partially reflected
     • Medical applications
       • Used for prenatal scans of baby. Transducer, which produces and detects pulses of ultrasound waves, placed on body surface. Each pulse is partially reflected from different tissue boundaries and returns to the transducer as a sequence of reflected pulses arriving at different times. This is used to build an image on a screen of internal tissue boundaries.
       • Advantages: - non ionising - used to scan organs and other soft tissues Can measure length of eyeball using : Distance travelled by pulse = speed of wave x transit time Ultrasound therapy can be used to break kidney stones using powerful frequency waves. Used as an A - scan system so waves aimed exactly.
   • 1.3
     • Refractive index
     • Refractive index = n
       • Measure of how much a substance can refract a light ray. N= sin i \ sin r The greater the angle of incidence the greater the angle of refraction.
       • When a light ray travels from a transparent substance into the air at a non zero angle of incidence : - ray is refracted away from the normal - larger Angle of i larger angle of r
   • 1.4
     • The endoscope
     • Critical angle = c
       • The angle of incidence of a light ray in a transparent substance that produces refraction along the boundary ( r = 90). N = 1 / sin c Total internal reflection : angle I = angle r
     • Endoscope
       • Used to see inside a body cavity (stomach ulcer) without cutting the body open. Inserted via throat and contains bundles of optical fibres to shine light onto the cavity and see the internal surfaces of the cavity. The other bundle is used to form an image on the ends of the fibres and can be seen on a camera at the other end. Each light ray entering an optical fibre is totally internally reflected each time it reached a boundary.
   • 1.5
     • Lenses
     • Lenses
       • Convex (converging) lens focuses parallel lines to a point called principle focus. As a magnifying glass. Concave (diverging) lens makes parallel rays spread out as if they came from a principle focus. Distance from centre of lens to principle focus = focal length
       • Real image - formed by converging lens if object is further away than the principle focus - can be put on a screen - where light rays meet - Inverted image Virtual image: - formed when object is nearer than the principle focus - magnified image Magnification = image height / object height
   • 1.6
     • Using lenses
     • Ray diagrams
       • Can be drawn to find the position and nature of an image formed by a lens. When object placed between a converging lens and F, the image is formed virtual, upright, magnified and on the same side of the lens as the object Magnifying glass. Image formed by a diverging lens is always virtual, upright and smaller than the object. Shown as a line with inward arrows.
     • Camera
       • Converging lens used to produce real image on film (array of pixels-digital). Position of lens adjusted to focus image on film. - far away object, distance from lens to film must equal focal length - nearer object, greater distance from lens to film
   • 1.7
     • The eye
     • Inside eye
       • Aqueous humour - transparent watery liquid that supports the front of the eye Blind spot - region where the retina is not sensitive to light (no light sensitive cells present) Cornea - transparent layer that protects the eye and helps focus light onto retina Ciliary muscles - (attached to lens by suspensory ligaments) change thickness of the eye lens Conjunctiva membrane - outer layer of pupil
       • Eye lens - focuses light onto the retina Eye muscles - move the eye in the socket Iris - coloured ring of muscle that controls the amount of light entering the eye
       • Pupil - the central hole formed by the iris where light enters the eye Retina - the light sensitive cells around the inside of the eye Optic nerve - carries nerve impulses from the retina to the brain Vitreous humour- transparent jelly like substance that supports the back of the eye
       • Normal human eye has range of vision from 25cm to infinity. Near point = 25cm. Far point = infinity Lens power (dioptre) = 1 / focal length (meters)
   • 1.8
     • More about the eye
     • Sight defetcts
       • Short sight means you can only see near objects clearly. Use a diverging lens to correct it. Long sight means you can only see distant objects clearly. Use a converging lens to correct it. Higher the refractive index of the glass used to make a spectacle lens the flatter and thinner the lens can be

2. Unit 2

   • Using physics to make things work
   • 2.1
     • Moments
     • Moment = turning effect of force
       • Moment (Nm) = force(N) x perpendicular distance from line of action to pivot (m) Increased by: - Increasing size of force - using a spanner with a longer handle Weight of object = load Person applying force = effort Point which crowbar turns = pivot
   • 2.2
     • Centre of mass
     • COM = point where mass thought to be concentrated
       • Releasing a suspended object means it will eventually rest with COM directly below point of suspension = equilibrium. Weight or height where it's dropped doesn't effect the moment, only the length of the string. COM of an object is where the lines of symmetry meet. If object is irregular use a plum line and suspend the object at different points to find its centre of mass.
   • 2.3
     • Moments in balance
     • W1d1 = W2d2
       • When balanced: - the anti-clockwise moment due to W1 about pivot = W1d1 - the clockwise moment due to W2 about pivot = W2d2 For an object in equilibrium (principle of moments) Sum of all clockwise moments about any point = sum of all anti-clockwise moments about that point F1d1 = W0d0 - for wheelbarrow and hand fork lift F = effort
   • 2.4
     • Stability
     • Stability
       • Can be increased by widening base, will topple if line of action of its weight is outside its base and if the resultant about its turning point is not 0.
   • 2.5
     • Hydraulics
     • Pressure
       • Pressure = pascals (Pa) = 1N/m2 P= F / a The pressure in a liquid acts equally in all directions. The grab of a digger is operated by a hydraulic pressure system as its muscle power. Liquid is virtually incompressible so the volume doesn't change when under pressure. This is why a force exerted on one part of a liquid is transmitted to the other parts. The force of a hydraulic system is much greater than the force applied to it. Hydraulic car jack : F2 = (F1/a1) x a2
   • 2.6
     • Circular motion
     • Centripetal force
       • For any object moving in a circle at a constant speed at any instant: - the objects velocity is directed along a tangent to the circle - its velocity changes direction as it moves around - the change of velocity is towards the centre of the circle = centripetal acceleration -Centripetal force of vehicle moving round a roundabout due to friction - London eye constant speed due to resultant force of weight and support force from the floor - when gravity wheel spins fast enough it spins vertically, when a rider is at the top there's a downward push to keep them moving in a circle - c force due to weight and downward push
       • If car is too fast it will skid off in a straight line / c force increases if speed increases If circle is too small car will skid off / c force increases if radius decreases Greater mass = greater c force
   • 2.7
     • The pendulum
     • Pendulum
       • Example of an oscillating motion - amplitude of an oscillating object = distance it moves from equilibrium to highest point. Time period = time it takes to swing across and back from its highest position / time between successive passes in same direction through its equilibrium. Only dependant on length. Frequency of the oscillations = number of complete cycles of oscillations per second Time period = 1 / frequency of oscillations Can measure average time for 20 and divide time by 20 after. Friction at top of swing and air resistance will stop it oscillating if it isn't pushed repeatedly.

3. Unit 3

   • Using magnetic fields to keep things moving
   • 3.1
     • Electromagnets
       • Like poles repel; unlike poles attract Magnetic field line is the one along plotting points of a compass. Electromagnet made from: insulted wire wrapped around an iron bar (core). When current passed along no wire a magnetic field is created around it and magnetises iron bar strongly. Steel is unsuitable as it keeps its magnetism when current is switched off.
       • 1. Scrapyard Crane : steel frame of vehicle slicks to electromagnet when current passes through coil - when switched off it drops off the electromagnetic 2. Circuit breaker : switch in series with electromagnet and held in place by iron catch - when too much current passes through the switch is pulled open by electromagnet and stays till its resent manually 3. Electric bell : when bell is connected to the battery, iron armature is pulled onto electromagnet - opens a make and break switch and magnet is turned off - means that armature springs back and switch closes again so the cycle repeats itself 4. Relay : used to switch an electrical machine (motor) on or off - when current is passed through magnet, armature is pulled onto electromagnet so the armature turns about pivot and closes switch gap - this way a small current is used to switch on a larger current
   • 3.2
     • Motor effect
       • Force: - increased if current or strength of magnetic field is increased - at right angles to direction of magnetic field and to wire - reversed if direction of current or field is reversed Left hand rule : - first finger = direction of field - second finger = direction of current - thumb = direction of wire
       • Electric motor : the coil is connected via 2 metal or graphite brushes to the battery which press onto a metal split ring commutator fixed to the coil. When current is passed through: - force acts on each side due to motor effect - force on one side is in opposite directs to force on other side Split ring commutator reverses current round coil every half turn as coil is pushed in the same direction. Loudspeaker : designed to make diaphragm attached to coil vibrate when alternating current passes through the coil. - when current passes through force due to motor effect makes coil move - each time current changes direction the force reverses so it's repeatedly forced back and forth, makes the diaphragm vibrate to produce sound waves
   • 3.3
     • Electromagnetic induction
       • P.d is induced across ends of the wire where it cuts the magnetic field lines. When one end of a bar magnet is pushed into the coil, the ammeter pointer deflects because: - movement of magnet causes induced pd in coil - induced pd causes current as coil is part of complete circuit
   • 3.4
     • Transformers
       • Used in national grid to change size of the alternating pd. When a.c. Passes through primary coil alternating PD is induced in secondary coil because: - a.c. Causes alternating magnetic field - lines of alternating magnetic field pass through secondary coil - magnetic field is changing Step up = pd across secondary coil is greater and has more turns than primary coil Step down = of across secondary coil is less and has fewer turns than primary coil Has laminated (layered) core to cut out induced currents in iron layers, if not the efficiency would be reduced.
       • Switch mode transformer : - used in mobile phones - lighter and smaller than traditional transformer - uses very little power when no device is connected across its output terminals - has ferrite core = lighter and can work at higher frequencies - direct voltage output at low voltage
   • 3.5
     • Transformers in action
       • The higher the pd, the greater the efficiency of transferring electrical power through the grid. Much more current would be needed to deliver the same amount of power. The grid cables would therefore heat up and waste more energy. PD across primary (Vp) / PD across secondary (Vs) = no. turns on primary (Np) / no. turns on secondary (Ns) Transformers are almost 100% efficient: - power supplied = Ip x Vp - power delivered = Is x Vs Ip x Vp = Is x Vs
   • 3.6
     • In hospital
       • We use physics in hospitals when: - blood pressure or temp. is measured - an ECG recording is made - an endoscope is used -