In order to get the G.C.S.E. grade you are capable of, you must make your own revision notes using your Physics notebook.

Transcription

1 In order to get the G.C.S.E. grade you are capable of, you must make your own revision notes using your Physics notebook. When summarising notes, use different colours and draw diagrams/pictures. If you do, you will find them easier to remember. Once you have made revision notes for a topic, re-visit these regularly (on the day of your examination you will not remember something you revised 4 weeks previously). Each time you re-visit a note tick the top of the page/card. This will allow you to identify any notes you have neglected. WARNING: DO NOT RELY SOLELY ON THE REVISION POWERPOINTS!

2 Plum Pudding Model Rutherford-Bohr Model Electrons randomly distributed through a sphere of positive charge. The was the accepted model before Rutherford s experiment Protons and neutrons located within the nucleus. Electrons in orbit around the nucleus. This model was developed after Rutherford s experiment.

3 To test the Plum pudding model positive alpha particles were fired at a thin piece of gold foil. If the model was correct the alpha particles, because of their high energy, should pass straight through. Apparatus Results detector gold Alpha source Observation Most of the alpha particles passed straight through the gold. Some of the alpha particles were deflected through large angles, with a few coming straight back. Conclusion The atom is composed of mostly empty space. There is something inside the nucleus capable of repelling an alpha particle. Rutherford called this structure the nucleus. Rutherford argued the following: Since most of the alpha particles missed the nucleus it had to be very small. The nucleus must be positively charged as it repelled the positively charged alpha particles. Most of the mass must be contained within the nucleus since the electron is so light.

4 Mass number: The number of protons plus the number of neutrons in a neutral atom. Atomic number: The number of protons (which is the same as number of electrons) in a neutral atom. A Z X

5 Mass number: The number of protons plus the number of neutrons in a neutral atom. Atomic number: The number of protons (which is the same as number of electrons) in a neutral atom. A Z X

6 Isotopes of an element are atoms which have the same number of protons (same atomic number) but different numbers of neutrons (different mass numbers). Most elements are mixtures of isotopes. Some isotopes are unstable. These isotopes are radioactive Can you identify the atom which isn t an isotope of hydrogen?

7 The nucleus emits an alpha particle (helium nucleus). It s mass number decreases by 4 and its atomic number by 2. As the number of protons inside the nucleus changes, a new element is formed. Example Unstable nucleus New nucleus Alpha particle A Z A - 4 X Y + α Z e.g. Pu U + He

8 A neutron inside the nucleus changes into a proton and an electron. The electron is ejected from the nucleus at high speed. This electron is known as a beta particle. As the number of protons inside the nucleus changes, a new element is formed. Example Unstable nucleus New nucleus Beta particle A Z A X W + Z e.g. Cs Ba + e

9 This normally follows alpha or beta decay. The excited nucleus loses energy in the form of a gamma ray. The composition of the nucleus does not change the element does not change. Example Unstable nucleus Stable nucleus Gamma radiation A Z A X X + γ Z 137 e.g. Cs Ba + γ

10 What are the missing mass and atomic numbers in the following decay equations. Remember: the mass numbers on the rhs add to give you the mass number on the lhs. The atomic numbers on the rhs add to give the atomic number on the lhs Click for solutions

11 Apparatus Method The detector is used to measure the background activity activity due to natural sources. Apparatus is set up as shown Different materials are placed at x. It is noted when the count rate drops to the background level when all the radiation is absorbed.

12 Properties of Radiation Property Alpha Beta Gamma Nature (what it is) 2 protons and 2 neutrons Fast moving electron Electromagnetic radiation Charge Positive- twice the charge on a proton. Negative- same as an electron None Ionising ability (how good is it at ionising substances) Penetrating power Strong Weak Very weak Small Average Large Stopped by A few cm of air A piece of paper 3mm of aluminium Several metres of air A thick block of lead Mass 4 x proton mass 1/1840 x proton mass None

13 The half-life of a radioactive isotope is the time it takes the activity of the source to halve. The half-life of an isotope is constant it doesn t matter what the activity is, the time for it to halve is always the same

14 Examples 1. Radium has a half life of 1622 years. If the initial activity of a sample is 1000 counts per second what is the activity after: a years? b years? Note: for each half life multiply the initial activity or mass by ½. a = 1 half life activity = 1000 x ½ = 500 counts per second = 500 becquerel = 500 Bq b = 1622 x 2 = 2 half lives activity = 1000 x ½ x ½ = 250 counts per second = 250 becquerel = 250 Bq Click for solutions

15 2. A radioactive isotope has a half life of 5 hrs. If a fresh sample has an activity of 1200 counts per second: a. What is its activity after 10 hours? b. What is its activity after 15 hours? a. 10 = 2 half lives activity = 1200 x ½ x ½ = 300 counts per second = 300 Bq b. 15 = 3 half lives activity = 1200 x ½ x ½ x ½ = 150 counts per second = 150 Bq Click for solutions

16 Locating leaks A short half life, gamma source is pumped through the pipe. Isotope accumulates at the leak. Leak is located above the surface with the use of a detector

17 Thickness Control Material placed between source and detector Source has a long half life. Beta is used if the material is paper and gamma if metal. Rollers are adjusted automatically to ensure correct thickness click opposite to see. Sterilisation Objects passed in front of a long half life gamma source. Bacteria killed, thus sterilising the objects

18 Cancer Treatment Gamma rays are directed at the tumour from different angles. Tumour absorbs energy and cancer cells are killed. Unfortunately, other cells are also killed. Medical Imaging A short half life gamma source is injected into the patient. The sources path through the patient is recorded using a gamma camera.

19 Nuclear fission is the splitting of a large nucleus into smaller nuclei. This normally also results in neutrons being released. It can be induced by the absorption of a neutron When the nucleus splits, mass is lost and so energy is released Fission of U-235 or Plutonium-239: neutron neutron Uranium-235 or plutonium-239 nucleus Uranium-235 or plutonium-239 nucleus absorbs a neutron Unstable nucleus (Uranium 235 or plutonium 239 nucleus with additional neutron) Unstable nucleus splits into two (or more) smaller nuclei and two or three neutrons.

20 The energy released during fission is used to turn water to steam. The steam then passes through the blades of a turbine which drives a generator.

21 Pros Building a nuclear power plant will create employment. Nuclear power plants do not release pollutants which cause acid rain or the greenhouse effect. There is enough fuel to produce energy for a long time. Cons Nuclear power plants are expensive to build. The waste material is radioactive. It is therefore dangerous and expensive to dispose of. Building nuclear power plants may reduce investment in renewables. If there is an accident then the people living around the power plant are at serious risk. Nuclear energy can be used in weapons of mass destruction.

22 Nuclear fusion is the joining of two light nuclei to form a larger nucleus. This reaction releases large amounts of energy. An example of a fusion reaction is shown below: 2 1 H 3 1 H 4 2 He 1 0 n energy Nuclear fusion powers the Sun. The goal is to build a power station on Earth that uses fusion, however it is very difficult to achieve the high temperatures required. Fusion is thought of as a solution to our growing energy needs: It doesn t produce any pollutants. It releases a lot of energy. There is almost a limitless amount of fuel.

23 Number of protons Number of neutrons 2-1 1/2000 zero Unstable nucleus wave Radiation that is all around us and which we cannot avoid

24 See graph 1.1 hours Activity in counts per minute Time in hours

25 gamma Gamma can penetrate 8 mm of aluminium, with only some of it being absorbed years Fusion is the joining of two light nuclei. You only want a change in the amount of radiation detected to be due to a change in thickness Fission Fusion

26 512 x ½ = 256; 256 x ½ = 128; 128 x ½ = 64; 64 x ½ = half lives = 120 seconds = 2 minutes It would remain active for too long, damaging cells It wouldn t be active for long enough it would have decayed too quickly