# GCSE Physics Edexcel P3 Topic 2

These are revision notes I made myself. I got A* in the Physics GCSE exam and 80 UMS (full marks) in the P3 exam using only these notes.

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P3 Topic 2: X-rays and ECGs
X-ray machines
To produce X-rays there are four key things that are needed: a cathode, an anode, an evacuated gas tube
and thermionic emission.
1. A filament acts as the cathode, which is very hot. The heat gives energy to its electrons. When they
have enough energy they "Boil off" i.e. they escape. This process is called thermionic emission.
2. The electrons then accelerate towards the anode as it is positively charged. This is due to the
potential difference between the cathode and anode.
3. When the electrons collide with the anode (metal target) some of their kinetic energy is converted
to X-rays.
4. The glass tube is a vacuum to ensure there are no other particles that the electrons could collide with.
If there were particles, these would knock them off target and decrease their energy.
5. A lead casing surrounds the X-ray machine to absorb some of the X-rays so they are only aimed at
the target.
Kinetic energy of each electron
kinetic energy (joules, j) = electronic charge (coulombs, C) x accelerating potential difference (volts, v)
KE = ½ mv2 = eV (electronic charge x potential difference)
Size of current produced
current (amperes, A) = number of particles per second (1/second, 1/s) x charge on each particle (coulombs,
C)
I=Nxq
X-rays are ionising
X-rays can harm body cells by the ionisation of their molecules. This can leave them damaged and destroyed,
so X-rays can be very harmful to humans.
Inverse square relationship
If you stand next to a source emitting radiation, the intensity of it depends on the total area its spread over.
If you move twice as far from a source, the same radiation is being spread over four times the area.
Therefore, you only receive a ¼ of the intensity. This is the inverse square relationship.
Thickness of materials and X-ray absorption
More dense the material the more radiation it absorbs.
The thicker the material the more radiation will be absorbed.
Lead and concrete are used in hospitals to reduce people's exposure to X-rays.
A graph can show you that the thicker the material the greater reduction in X-ray intensity passing out the
other side of the material i.e. more X-rays that are absorbed.
X-ray imaging
Fluroscopes
Moving images of the inside of a patient can be created by using X-ray fluoroscopy.
Basic fluoroscopy works by placing a patient between an X-ray source and a fluorescent screen. The intensity
of X-rays will vary depending on what they've passed through in the body.

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The X-rays then hit a fluorescent screen which absorbs them and fluoresces (gives off light) to show a live
image on the screen. The image we see is due to the difference in X-ray intensity ­ exposing the different
bits of the screen to different amounts of X-rays. The higher the intensity of X-rays the brighter the screen.
Modern fluoroscopy now generally involves using an image intensifier to increase the brightness of the
image seen (up to 5000 times brighter).…read more

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The heart rate can be worked out from an ECG using: frequency (Hz) = 1/time period (seconds)
Pacemakers
The heart has a natural pacemaker ­ a group of cells in the wall of the right atrium which produce electrical
signals that pulse about 70 times a minute.
These signals spread throughout the atria and make them contract via the action potential. The signals then
pass onto the ventricles and they contract too.
This natural pacemaker directly controls heart rate.…read more