X-Rays and ECGs

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Electron Beams

Thermionic emission can be used to produce X-rays.

  • Thermionic emission involves heating a filament (cathode) to give more energy to electrons. This enables the electrons to boil off.
  • A potential difference accelerates the electrons towards the metal anode.
  • When they collide, some of their kinetic energy is converted into X-rays which are released through a window in the casing.

Conditions Required:

  • A vacuum is needed to prevent the electrons colliding with air particles.
  • A lead casing is needed to absorb some X-rays so that they're only aimed at a certain point.
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Electron Equations

Calculating the kinetic energy of an electron:

  • Kinetic energy (J) = electronic charge (C) x accelerating potential difference (V)

KE (1/2 x m x v2) = e x v


Calculating the size of current produced:

  • Current (A) = number of particles per second (1/s) x charge on each particle (C)

I = N x q

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X-Ray Intensity and Absorption

Intensity

  • X-rays are ionising enough to be able to damage or destroy body cells.
  • X-ray intensity follows an inverse square relationship.
    • i.e. when you double the distance from the source, the strength decreases to a quarter.

Absorption 

  • Generally, the more dense the material is, the more radiation it absorbs.
  • A thicker material will absorb more X-rays.
  • Lead or concrete can be used to limit exposure to X-rays
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X-Ray Imaging

Computerised axial tomography (CAT) scans

  • An X-ray source is moved around a patient in a circle. X-ray detectors positioned opposite the source pick up the rays.
  • A computer can produce many cross sectional, high quality images of the body by working out how many X-rays are absorbed.
  • CAT scans can also produce 3-D images by stacking up the individual slices
  • Unusual darker or brighter areas in images can indicate tumours.

Fluoroscopes

  • A patient is placed between an X-ray source and a fluorescent screen.
  • Different amounts of X-rays are absorbed as they pass through the body.
  • X-rays hit the screen which absorbs them and fluoresces to show a live image.
  • The moving image can show whether an organ is working or not.
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Risks and Benefits of X-Ray Imaging

Risks

  • X-rays are ionising so they can damage or destroy unhealthy cells
  • Gives a very high dosage that can increase the risk of cancer.

Benefits

  • A painless, quick and non-invasive procedure.
  • Produced a better quality image than safer methods such as ultrasound.
  • It is better to diagnose an injury with X-ray imaging than to risk using the wrong treatment.
  • Risk of damaging other tissues is reduced by firing X-rays at different directions so that only the tumour gets a high dosage.
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ECGs

Electrocardiography

  • Action potentials produce weak electrical signals on the skin.
  • An ECG records the a.p. of the heart using electrodes

Electrocardiographs

  • An action potential passes through the atria, making them contract. (P)
  • Another a.p. passes through the ventricles, making them contract too. The atria relaxes. (QRS)
  • The ventricles relax once the a.p. has passed. (T)

Heart Rate

Frequency (hertz) = 1 / time period (seconds)

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Pulse Oximetry and Pacemakers

Pulse Oximetry

  • A pulse oximeter has a transmitter which emits red and infrared light. A photo detector measures light.
  • Beams of light pass through the finger tissue and some is absorbed by the blood.
  • Oxyhaemoglobin absorbs more infrared than reduced haemoglobin.
  • Looking at peaks of infrared absorption can be used to calculate a pulse.
  • Pulse oximetry can also be used to work out how much oxygen the blood is carrying.

Pacemaker

  • A pacemaker detects action potential and applies electrical signals to other parts of the heart so that the heart chambers contract correctly.
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Comments

Miss KHP

These are great notes for Edexcel Science GCSE unit P3. They are clear and consise and coloured to make it easier. If you have your own revision cards, you can print and add these to your own.

nicet

simple why 

pastel-princess

Thank you, Harry. I have my p3 exam this Friday. It is my last one.

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