Physics P3 Topic 2

Physics P3 Topic 2

1. X-Rays

   • Electromagnetic waves with a very short wavelength
     • Very ionising because they transfer more energy
   • Use them to produce images of bones and organs
     • Able to penetrate soft body tissue
     • When X-Rays pass through body can be absorbed by molecules
       • These become ionised
       • X-Rays lose energy and eventually cannot ionise any more tissue
   • Producing X-Rays
     • In X-Ray machine electrons produced by cathode, which is heated by filament in an evacuated tube (a vacuum tube)
       • Electrons accelerate towards an anode because high potential difference between anode and cathode
         • Embedded in anode is tungsten target
           • When very energetic electrons strike the target some of energy converted into x-rays
             • Beam in an x-ray machine consists of large numbers of electrons travelling from the cathode to tungsten target
               • Since electrons carry charge beam provides current through vacuum in x-ray tube
       • Vacuum means no gas molecules to scatter and absorb energy of electrons striking target
         • Anode positive
         • Cathode negative

2. X- Ray calculations

   • Current in beams of charged particles
     • Size of current in x-ray tube is given by the formula: I = N x q
       • I = Current (amps, A)
       • N = Number of particles per second (seconds, s)
       • q = Charge carried by each electron (coulomb ,C)
   • The kinetic energy of particles in beams
     • In an x-ray electron beam the electrons gain KE as they are accelerated though the vacuum by the potential difference
       • The unit of potential difference is the volt
         • One coulomb of electric charge gains one joule of energy when it is accelerated through a potential difference of one volt
     • KE = e x V
       • KE = Kinetic energy (joules, J)
       • e = Charge on one electron (coulomb, C)
       • V = Accelerating potential difference (volts, V)
     • KE = 1/2 mv²
       • m = Mass of electron (kg)
       • v = Velocity of electron (m/s)
     • e x v = 1/2 mv²

3. Using X-rays

   • When electromagnetic radiation (such as light) spreads from a source its energy is spread out over an area when increases with distance
   • Inverse square law
     • Area increases with the square of distance so intensity decreases with the square of distance
       • Example: at twice the distance from the source the intensity is one quarter as much as area is four times greater
     • Applies to any energy source
   • Absorption of X-Rays
     • Increases the thickness of the material increases the x-rays absorbed
   • CAT scans and fluoroscopes
     • Conventional x-rays produce a 2-dimensional image of a patient
     • In a CAT scanner an x-ray source is used to produce a 3D image of internal structure of patient
     • Fluoroscopes use x-ray images on a screen to see real time images of patients internet condition
   • Advantages
     • Help to give a reliable diagnosis and make sure treatment is fast and accurate
     • X-rays are non-invasive (no surgery)
   • Disadvantages
     • X-rays can damage living cells
     • Risks are greater for babies and children so exposure kept to a minimum

4. ECGs and pacemakers

   • Action potentials
     • Electrical signals that control the heart muscles
       • Generated by the hearts natural pacemaker to make heart beat at correct rate for body to be supplied with Oxygen
     • ECGS (electricalcardiogram) measures changing electrical signals in heart
       • An ECG shows possible problems with the heart as it may have an abnormal rhythm or be irregular
         • Labelled PQRST = time for one complete heart beat
           • Heart/pulse rate
             • A normal ECG shows heart beating regularly
               • Average = 72 Beats/Minute
               • Pacemaker monitors heart and detects irregular heartbeats, if irregular heartbeat pacemaker produces electrical signals to correct heart rate
             • f = I/T
               • f = Frequency (hertz, Hz)
               • T = time period (seconds, s)
               • Pulse rate usually given in beats per minute so frequency given by this formula multiplied by 60 to give pulse in beats/minute
   • Pulse oximetry
     • Measure amount of oxygen in the blood and pulse rate
     • Two light sources = one infrared and one visible red light
       • Blood with a lot of oxygen absorbs move infrared light as it is a different colour from deoxgyenated blood
         • The difference allows amount of oxygen in the blood to be calculated

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