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Science P3 Topic 2

GCSE edexcel further additional physics: X-rays and ECGs

What is ionising radiation? Radiation that has enough energy to knock electrons out of their shells in an atom, causing it to become a positively-charged particle
What is the strength of an X-ray dependent on? Its frequency and its energy
How does the strength of ionising radiation change over distance? It decreases according to the inverse square law, i.e. if the distance is doubled, the intensity is reduced by four times (two squared)
What are CAT scans and how are they done? CAT scans are X-ray images that show a slice through the body. An X-ray source is moved round a patient in a circle and X-rays are detected by a detector opposite the source to build up many cross-sectional views of the body. image built up on computer
What are CAT scans used for? They are used for diagnosing cancers- unusual areas of brightness or darkness can indicate tumours or small areas of dead tissue
What determines how many X-rays are absorbed by a medium? Its density - on an X-ray photo, dark areas are where lots of X-rays have been detected because not many have been absorbed, and light areas are where not many X-rays have been detected because more have been absorbed
What are fluoroscopes and how do they work? Fluoroscopes show the patients organs working - they consist of an X-ray source and a detector attached to a digital video camera with the patient placed in between
What are the advantages and disadvantages of using CAT scans and fluoroscopes for diagnosis? Advantages: painless for the patient, non-invasive, can eliminate the need for a biopsy. Disadvantages: they give a dose of radiation that is equivalent to up to 10 years of background radiation- increased risk of developing cancer
How does an X-ray machine work? • Thermonic emission of electrons from a heated wire filament (cathode) • Electrons accelerate towards the anode when there is a big potential difference • Electrons collide with metal anode and some energy is converted into X-rays
What is the tube evacuated? The tube is evacuated so that the electrons don’t collide with other particles as they cross the tube
Why is the cathode at a high temperature? A cathode with a higher temperature emits more electrons so more X-rays are produced
Why is the beam of charged particles in an X-ray machine equivalent to an electric current? When electrons flow form the cathode to the anode, they complete the circuit so the movement of charged particles is equivalent to an electric current
How do you calculate the current in the X-ray machine? Current in the X-ray machine (amperes) = number of particles flowing each second x charge on each particle (coulombs)
How do you calculate the kinetic energy of the electrons in an X-ray machine? The kinetic energy of an electron (joules) = charge on an electron (1.6 x 10-19 coulombs) x potential difference (volts)
What is an action potential? An action potential is an electrical signal sent to each muscle cell in the heart to tell it when to contract
Why does the human body conduct electricity? Because it contains a high proportion of water with salts dissolved in
What is an ECG? An ECG is an electrocardiogram, a picture of the heart electrical signals, built up through the detection of action potentials
What is the characteristic shape of an ECG? P wave = action potential spreads across both atria (small bump). QRS wave = action potential spreads up ventricle walls (large peak). T wave = contraction spreads up from base of ventricles (small bump)
What are ECGs used for? ECGs can be used to work out how fast the heart is beating and how well it is working
What type of graph paper are ECGs printed on? ECGs are printed on graph paper with a horizontal scale of 0.2 seconds for each larger square
How do you work out the frequency of the heartbeat in beats per minute? 1. Work out the time period of a heartbeat on the ECG 2. Use the equation: frequency (Hz) = 1 / time period (s) 3. Multiply the frequency by 30
What is pulse oximetry? Pulse oximetry is used to measure a person’s pulse rate
What does a pulse oximeter consist of? Consists of: LED emitting red light, LED emitting infrared radiation & detector.
How does a pulse oximeter work? It works out the pulse by looking at time interval between the peak points of infrared absorbance, which occur after each heart beat, because a heart beat releases more oxygenated blood into the arteries which absorbs more infrared radiation.
What is the percentage oxygen saturation? The amount of oxygen that the blood is carrying - this is calculated by a pulse oximeter by comparing the absorbance of the two LEDs
Created by: 11043
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