Physics P3

1. Physics P3 Topic 1 through to Topic 5

2. Radiation in treatment and medicine Topic One

3. Topic 1- Radiation in treatment and medicine • Diagnosis methods: • Endoscopes, which use visible light • CAT scans, which use x-rays • Ultrasound, which use sound waves • Treatment methods: • Ionising radiation, usually in the form of gamma rays and x-rays • Non-ionising radiation in the form of light (lasers) and ultrasound

4. Topic 1 Continued • Radiation is energy carried by waves or particles from a source • The intensity of radiation will decrease with distance from a source according to the nature of the medium through which it is travelling • This is calculated through the equation: • intensity=power of incident radiation/area

5. Topic 1 Continued Light is focused on the retina by the actions of the cornea and the lens. The average adult human eye has a near point of about 25cm and a far point of infinity. • Long sighted people can focus on distant objects but not near ones. It is caused by the eyeball being too short and therefore is fixed with a converging lens. • Short sighted people can focus on close objects but not far ones. It is caused by the eyeball being too long and therefore is fixed with a diverging lens.

6. Topic 1 Continued • Contact lenses are also available to correct vision- although they can cause infection and irritation to the eyes. • Laser correction is also an option where a laser is used to permanently alter the shape of the cornea to make it act as a converging/diverging lens dependant on the patients requirements

7. Topic 1 Continued Here we have reflection. This is refraction. This occurs because the light ray slows as it enters the glass due to the density. It therefore bends towards the normal. This is total internal reflection, this occurs when the angle of incidence is greater than the critical angle. Snell’s law explains refraction: Sin i/sin r = nr/ni (with n being the refractive index) To calculate the critical angle: Sin C=1/n (n being the refractive index)

8. Topic 1 Continued • TIR in fibre optics: • Light enters a fibre and reflects continually off the wall. Some of the optical fibres carry light inside them which reflects off the inside of the body and is focussed by the lens. • This can be used in an endoscope in order to examine a patient, but also allow keyhole surgery and therefore a faster recovery time. • Ultrasound in treatment: • High intensity ultrasound can break up kidney stones, and absorption of energy can be used to treat muscle damage. • Ultrasound in diagnosis: • It’s able to locate hard deposits such as kidney stones, accurately with a ‘real time’ image for doctors to see.

9. END OF TOPIC 1 QUESTIONS • How is light focussed on the retina? • What is long sight, what is it’s cause and how is it fixed? • How does total internal reflection work? • How is TIR used in fibre optics in an endoscope? • Name two uses of ultrasound in medicine.

10. ANSWERS • Light is focused on the retina by the action of both the cornea and lens, as they refract the light. • Long sight means you can’t focus on things close to you, caused by a short eyeball and fixed by a converging lens. • Total internal reflection occurs when the angle of incidence is larger than the critical angle. • It is used in fibre optics as it reflects all the way throughout the fibre, it is used in an endoscope as some fibres carry light and then the reflected image is focussed by a lens. • Ultrasound can be used in diagnosis, through a scan or can be used in treatment in high intensity to break up hard deposits, e.g. Kidney stones

11. X-Rays and ECGs Topic Two

12. Topic 2- X-rays and ECGs • The ability of X-rays to ionise materials depends on their energy: • The more energy means they are more ionising, the energy is related to the frequency. • Therefore, the more frequent, means more energy, which in turn means they are more ionising. • The movement of charged particles is equivalent to an electric current as charged particles or electrons can move from the cathode to the anode to complete a circuit.

13. Topic 2 Continued • X-ray machines consist of: • Thermionic emission of electrons from an electron gun (heated cathode) • A large potential difference between the cathode and the anode to accelerate the electrons • A Vacuum to stop the electrons from colliding in the tunnel • It works to produce x-rays because most of the kinetic energy from the collisions is transferred as thermal energy, yet some is converted into x-rays.

14. Topic 2 Continued • Current = no of particles per sec X charge on each particle • Kinetic energy = 1/2mv2 or electronic charge X accelerating potential difference • The inverse square law- when you double the distance from the source, the strength decreases to a quarter.

15. Topic 2 Continued • The denser a material is, the more x-rays it absorbs, hence becoming white on an x-ray. • X-rays in CAT scans- several cross section scans are taken to build up a 3D image on a computer • X-rays in fluoroscopes- used to show a patients organs working (a patient is placed between a source and a detector attached to a video camera) • Benefits- painless, non invasive and eliminates need for biopsy • Drawbacks- damages other tissues and not suitable for pregnant women/children

16. Topic 2 Continued • An action potential is an electrical signal sent to a muscle to tell it when to contract. These can be shown in the heart via an ECG which is a picture of the hearts action potentials. • For people who’s hearts do not work properly, a pacemaker can be fitted to correct the action potentials in order to make them spread across the heart. • Pulse oximetry works as it consists of two LEDs, one of red light and the other of infra red. Oxygenated blood surges after each heartbeat, and this absorbs more infra red light, therefore the pulse is worked out by how frequent the surges in absorption are. Then, it works out the percentage maximum of oxygen in the blood by comparing the absorbance between the two LEDs

17. Topic 2 Continued • P= action potential spreads across both atria. • QRS= action potential spreads up ventricle walls. • T= contraction spreads up from base of ventricles Frequency of heartbeat- work out average time period per beat, then divide 1 by it. Frequency(hertz)=1/time period

18. END OF TOPIC 2 QUESTIONS • How does an x-ray machine work? • Why is the flow of electrons the same as an electric current? • How does a fluoroscope work? • What is the inverse square law? • In an ECG, what is QRS? • How does a pulse oximeter show oxygenated blood? • Name a pro and con of x-rays.

19. ANSWERS • An x-ray machine fires electrons from a cathode to a rotating anode repeatedly in order to transfer some of the kinetic energy into x-ray energy. • The flow of electrons is the same as an electric current as they both carry charge and flow from the cathode to the anode to close the circuit. • A fluoroscope uses x-rays to show a real time view of a patients organs working. • The inverse square law says by doubling the distance from a source, the intensity decreases to a quarter. • QRS is the action potential spreading up the ventricle walls. • A pulse oximeter can measure the oxygen content as oxygenated blood absorbs more infrared light. • PRO= painless, non-invasive, no need for biopsy, CON= damages other tissues, not good for pregnant women/kids

20. Production, uses and risks of ionising radiation from radioactive sources Topic Three

21. Topic 3- Production, uses and risks of ionising radiation from radioactive sources In an atom, the number of protons equals the number of electrons. B- decay is where a neutron becomes a proton plus an electron; B+ decay is where a proton becomes a neutron plus a positron

22. Topic 3 Continued Nuclear equations: Alpha: 4/2 He Electron: 0/-1 e Positron: 0/+1 e Gamma: 0/0 y The N-Z curve for stable isotopes curves above the N=Z line, between the b+ and b- lines. Nuclei with high values of z (above 82) usually undergo alpha decay. An isotope above the curve has too many neutrons to be stable and will undergo b- decay. Whereas, an isotope below the curve has too many protons to be stable and therefore undergoes b+ decay.

23. Topic 3 Continued • Protons and neutrons each contain three quarks. • In a proton there are two up quarks and one down quark. • In a neutron there are two down quarks and one up quark. • In B- decay, a down quark changes into an up quark causing the neutron to become a proton. • In B+ decay, an up quark changes into a down quark causing the proton to become a neutron. Nuclei that have undergone radioactive decay often undergo nuclear rearrangement with a loss of energy as gamma radiation.

24. Topic 3 Continued • Ionising radiation causes mutations in the structure of DNA, it can also cause burns. Some precautions to prevent this are: • Increasing the distance from the source, • Shielding, • Containing the source, • Minimising time spent exposed to radiation. • Internal radiation is usually a beta emitter, whereas external radiation is usually gamma rays or x-rays.

25. Topic 3 Continued • Palliative care is used to improve health, not cure the issue. E.G. Shrinking tumours. • Radioactive tracers are used with a PET scan to locate areas of abnormality. The isotopes used in the tracer have to be made nearby as the isotope has to have a short half life, so it doesn’t remain in the body and cause excess tissue damage.

26. END OF TOPIC 3 QUESTIONS • What is the charge of an alpha particle? • How ionising is beta radiation? • Where on the stability curve would you see b- decay, and why? • What happens in b+ decay? • Name a precaution for radiation? • What is palliative care?

27. ANSWERS • +2 • Weakly ionising, but quite penetrative (stopped by aluminium) • B- decay would be above the curve as it has too many neutrons. • In b+ decay, the proton turns into a neutron plus a positron- an up quark changes into a down quark. • Limiting time spend with a source, distancing, shielding. • Palliative care is improving the condition of a patients life, not curing their issue. E.g. Shrinking a tumour.

28. Motion of particles Topic Four

29. Topic 4- Motion of particles • By using particle accelerators, scientists can attempt to make new particles to understand how the earth was created. Scientists also collaborate in big groups to have larger funds and bring together large amounts of expertise. • For motion in a circle, there must be a resultant force known as a centripetal force, that acts towards the centre of the circle.

30. Topic 4 Continued • Particle accelerators called cyclotrons cause charged particles to move in a circular or spiral path due to the magnetic field. • Certain stable elements can be bombarded with proton radiation to change them into radioactive isotopes. • Small cyclotrons are used in hospitals to produce isotopes with short half lives needed in PET scanners.

31. Topic 4 Continued • In elastic collisions, both kinetic energy and momentum are conserved. Whereas in inelastic collisions, only momentum is conserved. Momentum of an object=massXvelocity • Gamma rays can be produced by the annihilation of an electron and a positron. • In positron electron annihilation, charge and momentum are conserved. • The masses of the annihilated electron and positron are converted into an equivalent amount of energy. Mass-energy is conserved. • E=mc2 – E is the energy of a system, m is the mass and c is the speed of light (3x10^8ms)

32. Topic 4 Continued • Radio isotopes are used in PET scanners to produce gamma rays. This is because the radio isotopes the emit positrons are injected into the blood in a tracer which accumulates in various tissues. The body contains naturally occurring electrons which then annihilate, creating gamma rays which are then picked up by the detectors around the patient.

33. END OF TOPIC 4 QUESTIONS • What is E=mc2 for? • What is the centripetal force? • Why are cyclotrons used in hospitals? • What happens in an elastic collision? • What happens in positron electron annihilation? • What is the speed of light? • Why are radio isotopes used in PET scans?

34. ANSWERS • It shows how during annihilation, the masses are transferred into energy. • The centripetal force is the resultant force acting towards the centre of a circle to create motion in a circle. • Cyclotrons are used in hospitals to make radioactive isotopes for use in scanners. • In an elastic collision, kinetic energy and momentum are conserved. • In positron electron annihilation, charge and momentum are conserved. Mass is converted into mass energy- which is conserved. Gamma rays are made. • Speed of light is (3x10^8)2 • Radio isotopes that emit positrons are used as they collect in tissues and they encounter electrons in the body and annihilate producing gamma rays, which are picked up by the PET scanner.

35. Kinetic theory and gases Topic Five

36. Topic 5- Kinetic Theory and gases • Solids- Particles are held tightly together and cannot move, but can vibrate. • Liquids- Particles are still held together but are able to move over each other, but are uncompressible. • Gases- Particles are far apart and can move around quickly, also compressible. • The pressure of gases are caused by the particles hitting the walls of a container. The faster the particles move, the more collisions, therefore a higher pressure. • The higher the temperature, the more kinetic energy there is, so there is more collisions causing a higher pressure.

37. Topic 5 Continued • At ‘absolute zero’ (-273c) it is theorised that there is NO movement in any particles. • Converting from Kelvin to Celsius, -273 • Converting from Celsius to Kelvin, +273 • The average kinetic energy of the particles in a gas is directly proportional to the Kelvin temperature of the gas. • In medicine, gases e.g. Oxygen, are stored at high pressure in order to store a higher volume.

38. Topic 5 Continued • If the volume of a gas increases at a constant temperature- pressure decreases V1P1=V2P2 (v is volume m3) (p is pressure Pa) • If the temperature of a gas is increased at a constant pressure- the volume increases V1=V2T1/T2 (t is temperature K) • These two equations above can be combined: P1V1/T1 = P2V2/T2 (initial pressure X initial volume/initial temperature = final pressure X final volume/final temperature)

39. END OF TOPIC 5 QUESTIONS • What happens at ‘absolute zero’? • How do you convert from Celsius to Kelvin? • What happens if there is a gas at a high temperature? • If the volume of gas increases at a constant temperature, what happens? • If the temperature of a gas increases at a constant pressure, what happens?

40. ANSWERS • At absolute zero, it is theorised that no particles can move. • Add 273. • The gas will move quicker, causing a higher pressure. • The pressure decreases. • The volume increases.