AQA Physics A

1. AQA Physics A Unit 1: Particles, Quantum Phenomena and Electricity Dr K. A. Newson Head of Physics TGSG 2008

2. Overview of New AS Physics Course Internally assessed practical (½) 75 minute exam. 75 minute exam. 40 40 20 10 20 20

3. Physics A Grading System

4. New A* Grade • New A* grade is for complete A level, it is not awarded for the AS course alone. • To gain an A* grade students must both: • Gain at least 270 (90%) Uniform marks on the A2 • Gain at least 480 (80%) Uniform marks overall on the A level course.

5. Overview of Unit 1 • 1 Matter and Radiation • 2 Quarks and Leptons • 3 Quantum Phenomena • 4 Electric Current • 5 Direct Current Circuits • 6 Alternating Currents

6. Matter and Radiation 1.1 Inside the atom

7. 1.1 Inside the atom Aims of lesson • Understand the structure of the atom and nucleus, • Develop your knowledge of the constituent particles that make up the atom/nucleus • Know what is meant by an Isotope and how to represent them Prior knowledge • Every atom consists of a small positively charged nucleus surrounded by negatively charged electrons; • The nucleus consists of neutrons and protons which are roughly equal in mass; • Protons and electrons have equal but opposite charges, neutrons are uncharged.

8. Large and small • Physics deals with the smallest sub-nuclear particle to the entirety of the Universe. Physicists need to use a vast range of values, to make this easier prefixes are used: • For example, a high energy particle collider may accelerate protons to energies of 3000000000eV, its far simpler and less prone to error to simply say 3GeV. In this case G means Giga which means 109 eV. • The full list of prefixes is given to you as a handout.

9. 1.1 Inside the atom

10. The basicparticles... Proton positive charge Neutron no charge Electron negative charge

11. 3 + 4 = 7 Describing the atom Atomic Number - the number of protons Neutron Number - the number of neutrons Mass (nucleon) Number - the number of nucleons

12. Properties of protons, neutrons and electrons

13. Isotopes Isotopes are atoms of the same element i.e. that have the same number of protons, but have different numbers of neutron.

14. Examples of Isotopes For example, there are 3 isotopes of hydrogen: • H (hydrogen) • H (AKA deuterium) • H (AKA tritium) In the case of Tin (Sn) there are 13 isotopes from Sn to Sn 1 1 2 1 3 1 125 50 112 50

15. Nuclear structure Key words and terminology • Atomic number (Z): the number of protons contained in the nucleus (AKA proton number). • Mass/Nucleon number (A): the number of protons and neutrons contained in the nucleus. • Neutron number (N): the number of neutrons contained in the nucleus. N = A - Z

16. Isotope Notation A quick way of representing isotopes is to use the following system: Mass number (A) Element’ s chemical symbol Atomic number (Z)

17. Specific Charge • The word ‘Specific’ in Physics means ‘per unit mass’, • The specific charge therefore is given by: • E.g. the specific charge on a proton is found by: specific charge = 1.60 x 10-19 ÷ 1.67 x 10-27 = 9.58 x 107 Ckg-1

18. Example • Find the specific charge of the following: • Electron of mass 9.11 x 10-31 kg • An Aluminium ion (Al3+) of mass 4.51 x 10-26 kg Answers a) 1.76 x 1011 Ckg-1 b) 1.06 x 107 Ckg-1

19. Nuclear Stability Aims of lesson • Understand that some isotopes are unstable and examine the causes of this. • Know about the three types of radioactive decay and what happens to the nucleus in each case Prior knowledge • Radioactive substances emit radiation because they are unstable. • Know about the 3 types of ionising radiation and their properties e.g. their penetrating abilities.

20. Nuclear Stability • Protons and neutrons make up the nucleus. However, the protons are all positively charged; so how can they form a nucleus when they should repel one another? • The reason why nuclei do not fall apart arises from a force which will be new to you….. • This new type of force is called the STRONGNUCLEAR FORCE (for obvious reasons), it is also called the Strong interaction. • It is one of the four Fundamental forces.

21. Fundamental Interactions (forces) There are four fundamental interactions, and all forces in nature can be attributed to these four: • Gravitational • Electromagnetic • Weak nuclear • Strong nuclear

22. Fundamental forces

23. Forces in the nucleus The strong nuclear force holds protons and neutrons together in the nucleus. • The strong nuclear force has a very short range typically about 4 fm (1 fm = 10-15m). • At distances larger than this it is insignificant and the repulsive electromagnetic force dominates. • At very short distances ~0.5 fm the strong nuclear force becomes repulsive to stop protons and neutrons being pushed into each other.

24. The heaviest unstable nuclides can be alpha emitters “Strong” force holdsthe nucleonstogether Larger stable nuclides N > Z “Electromagnetic (coulomb) force”forces protonsapart Smaller stable nuclides N = Z Strong forcemustbe greater thanelectromagneticforcefor stability Excess of neutrons…therefore Beta minus emitters Excess of protons…therefore Beta plus emitters

25. Beta Gamma Alpha Three types of ionising radiationall come from the nucleus Electrons(or positrons) Electromagneticwave 2 protons, 2 neutrons

26. Useful websites • http://durpdg.dur.ac.uk/lbl/particleadventure/index.html

27. Electromagnetic waves and Photons

28. Electromagnetic Waves

29. Electromagnetic spectrum

30. The Electromagnetic spectrum • The Electromagnetic (EM) spectrum is the entire range of EM waves (radiations). Properties of EM radiation • All types consist of vibrating magnetic and electric waves (handout), the two waves are in phase, that is they vibrate together. • All EM waves are transverse, that is the vibration is at right angles to the direction of the wave’s motion. • All travel with the same speed (3.00 x 108 ms-1) in a vacuum.

31. Radio waves

32. Microwaves

33. Infra-red

34. Visible light

35. Ultra-violet

36. X-rays

37. Gamma-rays

38. The EM Spectrum

39. Photons • EM waves are emitted in packets called

40. Photons and energy • Max Planck (1901) concluded that the energy carried by light and the other types of electromagnetic radiation existed in discrete packets called Quanta. The energy E carried by each quantum is given by: E = hf • Where f is the frequency (in Hz), • h is a constant called the Planck constant. The value of Planck’s constant is 6.63×10-34 Js.

41. Photons and energy • In 1905 Einstein proposed that light radiation consists of a stream of quanta called Photons.

42. Example: Q) What is the energy of a photon of red light that has a wavelength of 650nm? Answer: As frequency (f) = speed of light (c) ÷ wavelength (λ) • f = c/λ, therefore the energy E = hf = hc/λ Therefore E = (6.63×10-34 × 3×108) ÷ (6.5×10-7) E = 3.1×10-19 J Q) What is the energy of violet light (λ = 400nm) Answer E = 5×10-19 J

43. Example • What are the photon energies of the following EM radiations: • X-ray photon of frequency of 2.5 x 1019 Hz • A photon of blue light having a wavelength of 475nm. Answers • E = hf = (6.63 x 10-34 x 2.5 x 1019) = E = 1.66 x 10-14 J • E = hf = hc/λ = (6.63 x 10-34 x 3.00 x 108/4.75 x 10-7) E = 4.19 x 10-19 J

44. Laser power • A laser beam consists of a stream of monochromatic (same f and λ) photons. • The power of a laser = energy transferred each second by the photons. • If the energy of a single photon = hf • Then the laser power = nhf • Where n = number of photons emitted per second

45. The electron volt • The electron volt (eV) is defined as the energy gained by an electron when it is moved through a potential difference of 1Volt. 1eV = 1.6 x 10-19J • Sometimes the mass of particles is given in terms of their mass-energy in units of eV. This arises because of the relationship between mass and energy (E = mc2). For example, the mass of an electron (me) is given by: • Me = 9.1 x 10-31kg = 8.19 x 10-14 J = 0.511 MeV • More about this later.

46. Rest energy • Einstein said that the mass of a particle when stationary, its rest mass (m0), has an energy called its rest energy which is locked up as mass. • The rest energy is given by E = m0c2

47. 11 11 0 C B β 6 5 +1 Particles and antiparticles • Consider the following decay: • This is called beta+ decay, the beta+ particle is usually known as a positron. It is virtually identical to an electron other than its charge which is positive. The positron is the antimatter equivalent of an electron. We call it the antiparticle of the electron. + +

48. Particles and antiparticles Carl Anderson 1932 Tracks of positrons in a cloud chamber, identical to normal beta (β-) particles but bent in the opposite direction indicating the charge on a positron is opposite to beta particles.

49. Particles and antiparticles • The existence of antimatter was predicted by the English Physicist Paul Dirac in 1928 • According to Dirac’s theory for every particle there exists a corresponding antiparticle. • Such antiparticle has exactly the same rest mass but the opposite charge (if the particle is charged). • The particle and antiparticle pair annihilate each other if they meet converting their rest-mass into photons of gamma radiation.

50. Annihilation of positron and electron pair • An electron and positron meet and they release two gamma rays.