EdExcel Unit P3 Applications of Physics

1. EdExcel Unit P3Applications of Physics N Smith St. Aidan’s

2. Topic 1 – Radiation in Medicine

3. Medical Physics CAT scans Ultrasound Endoscopes Radiotherapy “Medical Physics” is a big branch of science and refers to using physics to investigate medical issues. Some examples:

4. Introduction to Radiation “Radiation” refers to any form of energy originating from a source, and usually falls into two types: Radiation A wave, such as light coming from the sun A particle, such as this alpha particle coming from the nucleus

5. Intensity Clearly, the intensity of radiation received by an object decreases the further out the object is. This is due to two things: • The radiation “spreads out” in a circle • It is also absorbed by the medium it travels through

6. Intensity 05/01/2020 Intensity = (in Wm-2) Power (in W) P 4πr2 I = Area (in m2) An “inverse square law” Definition: “Intensity” means the strength of light arriving at a certain point, and can also be called “Radiation flux density” Energy dissipation Clearly, a wave will get weaker the further it travels. Assuming the wave comes from a point source and travels out equally in all directions we can say:

7. The Law of Reflection Angle of incidence = Angle of reflection Normal Reflected ray Incident ray Angle of reflection Angle of incidence Mirror

8. Refraction

9. Refraction through a glass block Light slows down and bends towards the normal due to entering a more dense medium Light speeds up and bends away from the normal due to entering a less dense medium Light slows down but is not bent, due to entering along the normal

10. Lenses When light enters a MORE DENSE medium it slows down… A prism uses this idea to split light. This happens because purple light is refracted more than red light Lenses use the idea of refraction:

11. Converging and diverging lenses CONVERGING (Convex) Thickest at the centre. Thicker lenses will cause more refraction DIVERGING (Concave) Thinnest at the centre. Lenses that are thicker on the outside will cause more refraction

12. Ray Diagrams for Lenses The rays of light are refracted INWARDS and meet at the focus, F. The image formed is REAL – in other words, it can be seen on a screen F The rays of light are refracted OUTWARDS. A VIRTUAL image is formed – in other words, the image doesn’t actually exist F

13. Ray diagrams for Lenses This image is REAL, UPSIDE DOWN and SMALLER than the object To draw ray diagrams follow these two rules: 1) Draw a ray from the TOP of the object PARALLEL to the axis and then going through F: F 2) Draw a ray from the TOP of the object going through the CENTRE of the lens (which will be undeviated)

14. F F

15. F F

16. F F

17. F F

18. The Lens Equation 1 1 1 = + f u v Image distance (+ sign is a real image) Focal length Object distance For the ray diagrams you have just drawn verify that the lens equation works:

19. The Eye

20. The Eye Iris Lens Retina Pupil Optic nerve Optic nerve Cornea Suspensory ligaments Suspensory ligaments Ciliary muscles

21. Seeing things Rays of light are refracted (bent) first by the cornea and then by the lens. They focus on the retina which acts like the film in a camera or the CCD in a digital camera. The “range of vision” for an eye is around 25cm up to infinity.

22. Seeing things at different distances For distant objects the ciliary muscles relax and the suspensory ligaments pull tight making the lens pull thin – the light doesn’t bend as much. For close objects the ciliary muscles contract allowing the lens to go fat, thus bending the light more.

23. Using lenses to correct vision Short Long

24. More about lenses Notice that these glasses have got a large curvature. How would you make strong glasses but also make them thinner and with less curvature? Power in dioptres = 1 focal length in m Compare thin and thick lenses: Lenses are measured in units called “dioptres”: …where converging lenses (for long sighted people) have positive values and diverging lenses (for short sighted people) have negative values.

25. Using Lasers in Surgery Lasers are being increasingly used in laser eye surgery: What are the advantages and disadvantages of this treatment compared to using glasses or contact lenses?

26. Refractive Index 05/01/2020 sin i sinr Willebrord Snellius, 1580-1626 Refractive index = The Refractive Index of a material is a measure of the factor by which the material will bend light: Snell’s Law:

27. Example questions 05/01/2020 sin i sinr Refractive index = Here’s my law again: 1) Light passes from air into crystal with a refractive index of 1.5. Calculate the angles of refraction for light incident at 20O, 30O, 40O and 50O. 2) A ray of light travels through a vacuum and is incident upon a glass block (of refractive index 1.5) at an angle of 30O. The ray then passes into water. Draw an accurate diagram to show the path of this light as it travels from the vacuum through the glass and into the water.

28. Finding the Critical Angle… 05/01/2020 THE CRITICAL ANGLE 1) Ray gets refracted 2) Ray still gets refracted 4) Ray gets internally reflected 3) Ray still gets refracted (just!)

29. Calculating the Critical Angle 05/01/2020 sin i sinr 1 sinc Refractive index Refractive index = = Here’s my law again: Notice that, for the critical angle, this formula becomes: • What is the refractive index of a material with a critical angle of 40O? • What is the critical angle for water, which has a refractive index of roughly 1.3? 1.56 50.3O

30. Uses of Total Internal Reflection 05/01/2020 Optical fibres: An optical fibre is a long, thin, _______ rod made of glass or plastic. Light is _______ reflected from one end to the other, making it possible to send ____ chunks of information Optical fibres can be used for _________ by sending electrical signals through the cable. The main advantage of this is a reduced ______ loss and endoscopes use this principle: Words – communications, internally, large, transparent, signal

31. Ultrasound Humans can hear sounds between 20 and 20,000Hz. “Ultrasound” is the region of sound above 20,000Hz – it can’t be heard by humans. There are a number of medical uses for ultrasound: • Pre-natal scanning • Medical treatment e.g. treating kidney stones

32. Calculating distances with ultrasound 05/01/2020 25 50 75 100 125 150 175 200 t/μs The echo takes 0.8 seconds to return and the speed of sound in water is 1500ms-1. How deep is the water? Hard question! Use the ultrasound scan to determine the width of the amniotic sac and the width of the baby’s body. The speed of sound in the fluid is 1500ms-1 and in soft tissue the speed is 1560ms-1.

33. Using an oscilloscope with ultrasound 20ms/div Consider a block of metal with a flaw: Q. If the speed of the ultrasonic wave is 3,000m/s how far away is the flaw from the detector?

34. Topic 2 – X-rays and ECGs

35. Charge (Q) 05/01/2020 I n Q The beam of electrons through the tube was basically an electric current because current, by definition, is a flow of charge. Each electron has a negative CHARGE. Charge is measured in Coulombs (C). We can work out how much current flows in a circuit using the equation: Current = no. of charged particles per s x charge

36. Example questions 05/01/2020 • An electron has a charge of 1.6x10-19 C. If a million of these flow through a circuit every second what is the current? • In a different circuit 5,000,000,000 electrons travel from a battery to a bulb per second. What is the current? • Another circuit has a current of 0.01A. How many electrons are passing by every second? 1.6x10-13 per s 8x10-10 per s 6.25x1016 per s

37. Electron Beams in evacuated tubes 25KV Cathode Anode Fluorescent screen e e e e This beam of charged particles is basically an electric current e “Thermionic emission” Explain the role of each of these parts: • The heater • The potential difference between the anode and cathode • Why a vacuum is necessary

38. Calculating the speed of the electrons 05/01/2020 25KV Cathode Anode Fluorescent screen e e e e e Basically, the “electrical work done” on the electrons causes them to speed up, so we can say: Charge x voltage = ½mv2 How fast will the electrons go in the above example?

39. X-Rays X-ray images are possible because they are absorbed by thick, dense tissue like bone but transmitted by soft tissue. X-rays are formed through collisions with metal targets. X-rays are part of the electromagnetic spectrum with a high frequency and therefore high energy. They cause ionisation (which could cause cancer).

40. Using X-rays X-rays can be used to diagnose and treat some medical conditions like the tumours in this body: In CT scanners the X-rays are used to build up an image by “photographing” each layer of the body at a time. In fluoroscopes the patient is placed between an x-ray device and a fluorescent screen:

41. Comparing X-rays, ultrasound and CT scans X-rays Ultrasound CT scans Ionising Generally safe Can cause cancer Can cause cancer Non-ionising Bad quality image Medium quality image Good quality image Ionising

42. Monitoring the Heart Rate P – this electrical activity is small and corresponds to the atrium pushing blood through to the ventricle (1 and 2) The QRS complex – this shows electrical activity that triggers the ventricles to contract (ventricles are made of thicker muscle so a bigger action potential is needed) and push the blood through the heart to the rest of the body (3). The electrical impulses from the heart can be monitored using an electrocardiogram (ECG). T – this wave shows the repolarization of the ventricles so that the heart returns to its normal active potential, ready for the next heart beat.

43. Heart rate and Pacemakers The heart beat is normally controlled by a group of _____ on the _____ hand side of the heart called the “pacemaker”. These cells control the heart rate by producing small ______ impulses that cause the heart to _______. If the pacemaker fails, an _______ pacemaker can be fitted. Heart rates can be measured using the equation Frequency = 1/time period (In Hz) (in s) Words – artificial, right, cells, electrical, contract

44. Pulse Oximetry A pulse oximeter is a device used to measure the blood oxygen level and heart rate through non-invasive methods. It works by basically sending two different _______ of light through the _____ and the ______ of each of these wavelengths is measured by a _________. The absorbance of each wavelength is then turned into a blood-oxygen level by the attached ________. Words – photodetector, wavelengths, computer, finger, absorbance

45. Topic 3 – Ionising Radiation

46. The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”) Atoms have a neutral charge overall as they have the same number of electrons as protons

47. The structure of the atom MASS (Nucleon) NUMBER = number of protons + number of neutrons 4 He SYMBOL 2 PROTON (Atomic) NUMBER = number of protons (obviously)

48. Alpha and Beta decay New nucleus New nucleus 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) Unstable nucleus New nucleus Alpha particle 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle. Beta particle Unstable nucleus 3) + decay – similar to beta decay, this decay is when a proton turns into a neutron and emits a positively-charged beta particle called a _______ (+) Beta + particle Words – positron, proton, neutrons, helium, beta Unstable nucleus

49. Changes in Mass and Proton Number 241 237 4 α Am Np + 95 93 2 90 90 0 β Sr Y + -1 38 39 “positron” 11 11 0 β C B + 6 5 +1 Alpha decay: Beta - decay: Beta + decay:

50. N-Z Curve 120 Alpha emitters would be up here (above 82 protons): Beta – emitters would be this side of the line: 100 80 No. of neutrons (N) 60 40 Beta + emitters would be this side of the line: 20 0 20 40 60 80 100 Proton number (Z) An N-Z graph plots neutron number (N) against proton number (Z). The graph looks like this: