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22/05/2017 Mechanics W Richards The Weald School Uncertainty 22/05/2017 Consider a ruler: It has an uncertainty of ±0.5mm Now consider the time taken for a ball to drop: Drop no. Drop 1 Drop 2 Drop 3 Uncertainty Time taken to fall/s Percentage = uncertainty Uncertainty Average value X 100% Summary 22/05/2017 Take appropriate measurements and complete the table: Thing to measure Width of book Width of table leg Diam. of hair Depth of beaker What device? Meas. 1 Meas. 2 Meas. 3 Ave. Uncert- % ainty uncertainty Density Density = Mass Volume 22/05/2017 ρ= m V 1) What is the density of a piece of wood of volume 2m3 and mass 1200kg? 2) Air only has a density of 1.3kg/m3. What is the mass of 0.2m3 of air? 3) Carbon dioxide is more dense and the same volume would have a mass of 0.38kg. What is its density? 4) The mercury in a thermometer has a volume of 5x10-5m3. If the density of the mercury is 13600kg/m3 what mass of mercury is in the thermometer? Density Object Mass/kg Volume/m3 22/05/2017 Density/ kg/m3 Standard Form and prefixes 22/05/2017 Prefix Giga Symbol G Multiplier 109 Mega Kilo Milli M K m 106 103 10-3 Micro Nano Pico μ n p 10-6 10-9 10-12 Try these hard questions… • What is 1mm2 in m2? • • • What is 1μm2 in m2? What is 10mm3 in m3? How many pm3 fit in a cubic kilometre? International System of Units 22/05/2017 There are six basic quantities we need to know about. Their units are called S.I. units: Base quantity Base unit Symbol Length metre m Mass kilogram kg Time second s Current ampere A Temperature Kelvin K Amount of substance mole mol Derived Units 22/05/2017 Derived units are units that are made up out of base units. For example, the unit for speed (metre per second) comes from the base units for distance and time. The following units are derived. Use suitable equations to express each unit in terms of base units: 1) Newton (force) 2) Joule (energy) 3) Pascal (pressure) 4) Watt (power) 5) Coulomb (electric charge) Homogeneity Calculate the following: 10kg + 5m = ?? 22/05/2017 It doesn’t make sense! You can’t add kilograms to metres. That’s just silly. For an equation to be correct it has to be “homogenous”. In other words, it has to add and equal the same type of units. Prove, by considering base units or otherwise, that the following equations are homogenous with respect to units: 1) Volume of cylinder = πr2h 2) Acceleration = change in speed / time taken 3) Kinetic energy = ½mv2 4) Power = force x velocity 22/05/2017 Distance, Speed and Time revision Speed = distance (in metres) time (in seconds) D S T 1) Simon walks 200 metres in 40 seconds. What is his speed? 2) Howard covers 2km in 1,000 seconds. What is his speed? 3) How long would it take Ryan to run 100 metres if he could run at 12m/s? 4) Ben throws a book at Dan and it travels at 50m/s for 0.2s. How far away was Dan? 5) Chris is learning to drive. He drives his car at 85mph (about 40m/s). How long does it take him to drive 20km? Some subtle differences… 22/05/2017 “Distance” is how far you have gone, “displacement” is how far you are and can be positive or negative: Distance = Distance = Displacement = Displacement = Start -1 metre 1 metre Distance Distance = = Displacement Displacement = = Some subtle differences… 22/05/2017 “Distance” is how far you have gone, “displacement” is how far you are and can be positive or negative: Speed = Speed = Velocity = Velocity = Start -1 metre 1 metre Speed Speed = = Velocity Velocity = = “Speed” is how fast you go. “Velocity” is how fast in a given direction. Vector vs. scalar Scalar quantities have size only and no direction. Vector quantities have both size and direction. Scalar or vector??? Scalar Vector 8. Power 2. Distance12. Acceleration 1. Mass 6. Energy 7. Time 3. Displacement 4. Speed 11. Force 10. Current 5. Velocity 9. Momentum 22/05/2017 22/05/2017 40 Distance (metres) 30 20 10 0 Time/s 20 40 60 80 100 1) What is the velocity during the first 20 seconds? 2) What is the displacement after 60 seconds? 3) What is the velocity during the last 40 seconds? 4) What is the displacement after 100 seconds? 22/05/2017 20 10 Displacement (metres) 0 -10 -20 Time/s 20 40 60 80 100 1) What was the displacement after 20 seconds? 2) What was the velocity between 20 and 40 seconds? 3) When was this person travelling the fastest? 4) What was the average speed for the first 40 seconds? Understanding Velocity 1) Is this car travelling at constant speed? 2) Is this car travelling at constant velocity? 22/05/2017 Understanding Velocity 22/05/2017 40 30 Displacement (metres) 20 10 0 Time/s 20 40 1) What’s the average velocity? 2) What’s the velocity at 60s? 60 80 100 22/05/2017 Acceleration V-U Acceleration = change in velocity (in m/s) (in m/s2) time taken (in s) A T 1) Ryan accelerates on his bike from 0 to 10ms-1 in 5 seconds. What is his acceleration? 2) Harry drops a ball and it accelerates downwards at a rate of 10ms-2 for 12 seconds. What speed did it reach? 3) A car accelerates from 10 to 20ms-1 with an acceleration of 2ms-2. How long did this take? 4) A rocket accelerates from 1,000ms-1 at a rate of 20ms-2 for 2 minutes. What speed did it reach? 22/05/2017 80 60 Velocity m/s 40 20 0 T/s 10 20 30 40 50 1) How fast was the object going after 10 seconds? 2) What is the acceleration from 20 to 30 seconds? 3) What was the acceleration from 30 to 50s? 4) How far did the object travel altogether? 22/05/2017 20 10 Velocity (metres) 0 -10 -20 Time/s 20 40 60 80 100 1) When did the object have zero acceleration? 2) What is the average acceleration from 0 to 40s? 3) What was the acceleration from 40 to 60s? 4) How far did the object go between 50 and 100s? A closer look at motion graphs 22/05/2017 Consider a bouncing ball: Displacement Time A closer look at motion graphs 22/05/2017 Consider a bouncing ball: Velocity Time A closer look at motion graphs 22/05/2017 Consider a bouncing ball: Acceleration Time Equations of Motion 22/05/2017 If we’re talking about any object travelling in a straight line with constant acceleration then we can use these 4 “golden equations”… Golden equation equation #1 #1 Golden Goldenequation equation#2 #1 Golden Vel Vel v v Ave v-u u u T Ave. velocity = (u + v) / 2 Therefore x = u+v t 2 T Acc = (v – u) / t Therefore v = u + at Equations of Motion Goldenequation equation#3 #1 Golden 22/05/2017 Goldenequation equation#4 #1 Golden Vel From eqn #2 t = (v-u) / a v u v-u t(v-u)/2 ut T From equation #2 (v-u) = at Therefore x = ut + t/2 x at Therefore x = ut + ½at2 From eqn #1 x = t(u+v) / 2 So x = (v-u) (v+u) 2a 2ax = v2 – u2 Therefore v2 = u2 + 2ax Equations of Motion 22/05/2017 u+v x= 2 t v = u + at x = ut + ½at2 v2 = u2 + 2ax They’re golden! Example questions 22/05/2017 1) Ben drops a ball on Dan’s foot. How long does the ball take to fall 1m? 2m? Why is the second answer not twice the first? 2) Ryan flies to Belgium. His aeroplane has a maximum acceleration on the ground of 3.4ms-2. What is the minimum length of runway needed to reach its take off speed of 110ms-1 and how long will this take? 3) Luke likes watching kangaroos. A kangaroo jumps to a vertical height of 2.8m. For how long was it in the air? 4) Tom likes baseball. A baseball pitcher can release a ball at 40ms-1 after accelerating through a distance of 2.5m. Calculate the average acceleration of the ball. Example questions 22/05/2017 5) Andrew wants to play with the air track. The air track is slightly tilted. He pushes a trolley up the track with a speed of 1ms-1 and its acceleration due to the tilt is 0.5ms-2 down the track. How long does it take to drop 1m below the starting point? 6) Howard travels in a rocket powered sledge and accelerates from rest to 284ms-1 in 5s and then comes to a rest in 1.5s. Calculate his acceleration in both stages. 7) Harry has a good chance of surviving a car crash with a seatbelt on if his deceleration does not exceed 30g. Calculate the distance by which the front end of the car must collapse in if a crash occurs at 70mph. Vertical Projection 22/05/2017 If I throw this ball upwards with a speed of 40ms1 how high will it go? Use v2 = u2 + 2ax 0 = 402 + (2 x -9.81 x x) 0 = 1600 – 19.62x 1600 = 19.62x x = 1600/19.62 x = 81.5m Practice Questions 22/05/2017 1) How far will a cricket ball go if it is thrown upwards with an initial velocity of 10ms-1? 2) How far will a table tennis ball go if it is thrown upwards with an initial velocity of 5ms-1? 3) A human cannonball is projected vertically upwards and she reaches a vertical height of 20m before coming back down. How fast was she going when she left the cannon? 4) A test tube falls off the table. If the table is 1m high how fast was the test tube going when it hit the floor? Measuring g 22/05/2017 Consider the equation x = ut + ½at2… If we consider a ball being dropped then u=0, so x = ½at2 We also know that a = g, therefore… x = ½gt2 x x x = ½ g t2 x x y=mx+c Gradient = g x ½t2 Projectile Motion Aha! If I let go of the branch when he fires his gun I’ll be safe because the bullet will go above me… 22/05/2017 Projectile Motion 22/05/2017 Question – how long did this take and how fast was the bullet? 1.5m 50m 1) Use x = ut + ½at2 vertically to find the time 2) Then use speed = distance / time horizontally to get the speed Example questions 22/05/2017 1) Ben throws a bowling ball at Tom and it lands on his foot. If the ball started 1.2m above Tom’s foot and the distance between them was 2m calculate both the time taken and the initial velocity of the ball. 2) Rob fires a gun and the bullet leaves the barrel at a speed of 200ms-1. If it landed on the ground 500m away calculate how long the journey took and how high up Rob was holding the gun from ground level. 3) Andrew likes knocking test tubes off the table. If he hits one with an initial velocity of 2ms-1 and the table is 1m high calculate the time taken for the test tube to hit the floor and how far away from the table it landed. Recap questions 22/05/2017 1) Andrew Murray hits a tennis ball and it passes horizontally over the net and lands just inside the baseline of the court. The net has a height of 1.07m and is 11.9m from the baseline. Find the horizontal speed of the ball. 2) David Beckham takes a free kick and it flies into the top corner horizontally. If the corner is 2.4m above the ground and the goal is 18m away calculate the time taken for the ball to reach the goal. Newton’s st 1 Law of Motion 22/05/2017 Basically, a body will remain at rest or continue to move with constant velocity as long as the forces acting on it are balanced. …and an unbalanced Newton 1642-1727 backwards force will make me slow down… An unbalanced forwards force will make me accelerate… Newton’s nd 2 Law of Motion 22/05/2017 The acceleration of a body is proportional to the resultant force causing its acceleration and is in the same direction. Newton 1642-1727 In other words… F force = mass x acceleration M A Revision questions 1) A force of 1000N is applied to push a mass of 500kg. How quickly does it accelerate? F 2) A force of 3000N acts on a car to make it accelerate by 1.5ms-2. How heavy is the car? 3) A car accelerates at a rate of 5ms-2. If it weighs 500kg how much driving force is the engine applying? 4) A force of 10N is applied by a boy while lifting a 20kg mass. How much does it accelerate by? 22/05/2017 M A Testing Newton’s nd 2 Law 22/05/2017 For each version of the experiment: 1) Draw a diagram of how you set it up 2) Describe your method 3) Describe what equipment you used to get the results and how you analysed them (you only need to do this once as they’re both the same). Newton’s rd 3 Law of Motion 22/05/2017 When body A exerts a force on body B, body B exerts an equal and opposite force on body A. My third law says that if I push to the right I will move backwards as well. Newton 1642-1727 Newton’s rd 3 Law of Motion What will happen if I push this satellite away from me? 22/05/2017 Types of Forces Gravitational (W=mg) 22/05/2017 Electromagnetic/ electrostatic +++ + + Nuclear (2 types) +++ + + Describe each force, including a comment on the distance it works over, whether it repels or attracts and other important points Free body force diagrams 22/05/2017 The Earth pulls Newton down with a gravitational force of 700N. direction what on what type size Newton pulls the Earth up with a gravitational force of 700N. This is a Newton III pair of forces Free body force diagrams 2 22/05/2017 Consider a man standing on a table on the Earth: Newton I vs. Newton III 22/05/2017 These two forces are acting on the same body, they’re two different types of force and the man is in equilibrium as long as the forces balance – this is a “Newton I pair of forces”. These two forces are acting on different bodies, they’re both the same type and they are always equal and opposite – this is a “Newton III pair of forces”. Summary Newton I A law about the forces on _ _____ ____ 22/05/2017 Newton III A law about the forces on ____ _______ _____ Concerns any _____ of forces Always concerns ____ forces only The forces can be ______ types Both forces are ___ ______ type If there are two forces and the body is in equilibrium the forces are _____ and ________ The two forces are ALWAYS ______ and ________ Newton I only applies when the body is in ________ Newton III ______ applies Random recap questions 22/05/2017 1) Nick runs the last 100m of a 200m race over 15s. If he was accelerating at a rate of 1ms-2 during those 15s how fast was he running when he passed the 100m mark? 2) Ben throws a ball from the 1st floor at Ryan below. If the ball travels for 1.5s before hitting Ryan how far above Ryan is Ben? If Ryan is 20m away from the building how fast did Ben throw it? 3) Dan is swinging a conker around on a piece of string. Draw a free body force diagram for each object (you may find it easier to draw both on the same diagram). 4) For each of the forces in the previous two diagrams identify the Newton III pair and describe what the force is, what is acts on and its direction. Vectors 22/05/2017 10km 10km 14.1km 100ms-1 5ms-1 100.1ms-1 Resolving Vectors 22/05/2017 Consider a diagonal push: This force is given by: F1 = F sin θ θ F1 F2 This force is given by: F2 = F cos θ 22/05/2017 Resolving Vectors – example questions Calculate the horizontal and vertical components of the following: 1) 2) 10N 20N 35O 50O Work out the size and direction of the resultant force: 3) 4) 8N 10N 50O 80O 20N 15N 45O 30O Free body force diagrams 3 22/05/2017 Consider a man on a sloping table: Reaction (a contact force) is perpendicular to the surface. Friction (a tangential contact force) goes up the slope. Let’s combine the forces… Resultant force is zero, so no acceleration Free body force diagrams 22/05/2017 1) Draw a free body force diagram for a ladder against a wall. 2) A car pulls a caravan along the M25. Draw a free body force diagram for the caravan. 3) Draw a free body force diagram for a 4-wheel drive car driving up the M1. 4) Draw a free body force diagram for a 2-wheel drive (engine at the front) car driving up the M1 as well. Moments revision 22/05/2017 A moment is a “turning force”, e.g. trying to open or close a door or using a spanner. The size of the moment is given by: Moment (in Nm) = force (in N) x PERPENDICULAR distance from pivot (in m) Calculate the following turning moments: 5 metres 100 Newtons 2 metres 200 Newtons Turning Moments revision 22/05/2017 2 metres 200 Newtons Total ANTI-CLOCKWISE turning moment = 200x2 = 400Nm 2 metres 100 Newtons Total CLOCKWISE turning moment = 100x2 = 200Nm The anti-clockwise moment is bigger so the seesaw will turn anti-clockwise Balanced or unbalanced? 22/05/2017 Turning Moments 22/05/2017 Consider a man walking along a plank of wood on a cliff. How far can he walk over the cliff before the plank tips over? Aaarrgghh Man’s weight = 800N 1m 3m Plank’s weight = 200N Another example 22/05/2017 Consider a car on a suspension bridge: How much weight does each support take? 20m 3m Weight of car = 10,000N Weight of bridge = 500,000N A recap question 22/05/2017 1) State the principle of moments 2) Calculate the mass of man in the example given below: 30kg 0.4m 1.2m Momentum 22/05/2017 Any object that has both mass and velocity has MOMENTUM. Momentum (symbol “p”) is simply given by the formula: P Momentum = Mass x Velocity (in kgms-1) (in kg) (in ms-1) M What is the momentum of the following? 1) A 1kg football travelling at 10ms-1 2) A 1000kg Ford Capri travelling at 30ms-1 3) A 20g pen being thrown across the room at 5ms-1 4) A 70kg bungi-jumper falling at 40ms-1 V Conservation of Momentum 22/05/2017 In any collision or explosion momentum is conserved (provided that there are no external forces have an effect). Example question: Two cars are racing around the M25. Car A collides with the back of car B and the cars stick together. What speed do they move at after the collision? Speed = 50ms-1 Mass = 1000kg Speed = 20ms-1 Mass = 800kg Mass = 1800kg Speed = ??ms-1 Momentum before = momentum after… …so 1000 x 50 + 800 x 20 = 1800 x V… …V = 36.7ms-1 22/05/2017 Momentum in different directions What happens if the bodies are moving in opposite directions? Speed = 50ms-1 Mass = 1000kg Speed = 20ms-1 Mass = 800kg Momentum is a VECTOR quantity, so the momentum of the second car is negative… Total momentum = 1000 x 50 – 800 x 20 = 34000 kgms-1 Speed after collision = 34000 kgms-1 / 1800 = 18.9ms-1 Another example 22/05/2017 Consider the nuclear decay of Americium-241: 237 93 Np 241 95 Am If the new neptunium atom moves away at a speed of 5x105 ms-1 what was the speed of the alpha particle? 4 2 α More questions… 22/05/2017 1) A white snooker ball moving at 5m/s strikes a red ball and pots it. Both balls have a mass of 1kg. If the white ball continued in the same direction at 2m/s what was the velocity of the red ball? 2) A car of mass 1000kg heading up the M1 at 50m/s collides with a stationary truck of mass 8000kg and sticks to it. What velocity does the wreckage move forward at? 3) A defender running away from a goalkeeper at 5m/s is hit in the back of his head by the goal kick. The ball stops dead and the player’s speed increases to 5.5m/s. If the ball had a mass of 500g and the player had a mass of 70kg how fast was the ball moving? 4) A gun has a recoil speed of 2m/s when firing. If the gun has a mass of 2kg and the bullet has a mass of 10g what speed does the bullet come out at? Newton’s nd 2 Law and Impulse 22/05/2017 Instead of F=ma Newton actually said that the force acting on an object is that object’s rate of change of momentum. In other words… mv Force = Change in momentum (in kgm/s) (in N) Time (in s) Also called “impulse” F T For example, David Beckham takes a free kick by kicking a stationary football with a force of 40N. If the ball has a mass of 0.5kg and his foot is in contact with the ball for 0.1s calculate: 1) The change in momentum of the ball (its impulse), 2) The speed the ball moves away with Example questions 22/05/2017 1) Ben likes playing golf. He strikes a golf ball with a force of 80N. If the ball has a mass of 200g and the club is in contact with it for 0.2s calculate a) the change in momentum of the golf ball, b) its speed. 2) Nick thinks it’s funny to hit tennis balls at Tom. He strikes a serve with a force of 30N. If the ball has a mass of 250g and the racket is in contact with it for 0.15s calculate the ball’s change in momentum and its speed. 3) Dan takes a dropkick by kicking a 0.4kg rugby ball away at 10m/s. If his foot was in contact with the ball for 0.1 seconds calculate the force he applied to the ball. 4) Simon strikes a 200g golf ball away at 50m/s. If he applied a force of 50N calculate how long his club was in contact with the ball for. 22/05/2017 Another way to ask the same question… Here’s a situation we looked at earlier… Speed = 50ms-1 Mass = 1000kg Speed = 20ms-1 Mass = 800kg What’s the impulse of the car on the left if the cars stick together? Energy loss in collisions 22/05/2017 In the “Forces” module we looked at how to calculate an object’s kinetic energy: Kinetic energy = ½ x mass x velocity squared in J in kg in m/s We’ve also said that in a collision momentum is conserved (unless an external force acts). The same cannot usually be said for kinetic energy… For example, consider the following collision. How much kinetic energy is lost? Before Speed = 50m/s Speed = 20m/s Mass = 1000kg Mass = 800kg After Mass = 1000kg Speed = 20m/s Mass = 800kg Speed = 30m/s Energy loss in collisions 22/05/2017 Consider a head-on collision where the cars stick together. How much kinetic energy is lost in this example? Where does all the energy go? Before Speed = 50m/s m=800Kg Speed = 30m/s m=3000Kg After Speed = 10m/s In this example more kinetic energy was lost. We say it was a “less elastic collision”. An “elastic collision” is one where the kinetic energy is conserved. Work done 22/05/2017 Work done (in joules) is simply the force needed to move an object multiplied by the distance moved in the direction of the force: ΔW ΔW = FΔx F Δx Power 22/05/2017 Power (in watts) is “the rate of doing work”: ΔW P = ΔW Δt P Also, using our “work done” equation: P = ΔW = FΔx Δt Δt Δt ΔW = FΔx …therefore P = Fv Random questions on work and power 22/05/2017 1) Luke pushes Ben in the direction of a cliff. If he uses a force of 40N and he moves Ben 10m in 4s calculate the work done and Luke’s power rating. 2) Dan runs up some stairs and has a power rating of 600W while he does so. If he does it in 5 seconds and his weight is 750N calculate how high the stairs are. 3) A man pulls a block of wood at an angle of 200 to the horizontal and uses a force of 50N. If the distance travelled horizontally is 5m calculate the work done by the man and his power if the journey lasted 10 seconds. 200 50N Conservation of Energy 22/05/2017 Consider a bouncing ball: Gravitational Potential Energy Time Conservation of Energy 22/05/2017 Consider a bouncing ball: Kinetic Energy Time Conservation of Energy 22/05/2017 Now put these graphs together: Kinetic Energy Total energy of the ball Time 22/05/2017 Radioactivity W Richards The Weald School Structure of the atom A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it… Ernest Rutherford, British scientist: I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment” 22/05/2017 22/05/2017 The Rutherford Scattering Experiment Alpha particles (positive charge, part of helium atom) Most particles passed through, 1/8000 were deflected by more than 900 Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”. Thin gold foil The structure of the atom 22/05/2017 ELECTRON – negative, mass nearly nothing NEUTRON – neutral, same mass as proton (“1”) PROTON – positive, same mass as neutron (“1”) Atoms are roughly 10-10m in diameter, while the nucleus is 10-15 – 10-14m The structure of the atom 22/05/2017 Particle Relative Mass Relative Charge Proton 1u (1.7x10-27kg) +1.6x10-19C Neutron 1u (1.7x10-27kg) 0 Electron 0 -1.6x10-19C MASS NUMBER (A) = number of protons + number of neutrons SYMBOL PROTON NUMBER (Z) = number of protons (obviously) No. of neutrons N = A - Z Isotopes 22/05/2017 An isotope is an atom with a different number of neutrons: Notice that the mass number is different. How many neutrons does each isotope have? Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating. Quarks 22/05/2017 We can investigate the structure of protons by bombarding them with electrons: Low energy scattering e- P Elastic collision. Electrons and protons behave as expected. High energy scattering e- P Inelastic collision. Energy is “absorbed” by the proton and increases its internal energy. This is Deep Inelastic Scattering and suggests that the proton is made of smaller particles called quarks. Introduction to Radioactivity 22/05/2017 Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactive decay”. Ionisation 22/05/2017 Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by giving them enough energy to “knock off” electrons: Alpha radiation is the most ionising (although short range). Ionisation causes cells in living tissue to mutate, usually causing cancer. The Geiger-Muller Tube Metallic case (cathode) 22/05/2017 Mixture of argon and halogen Mica end window Central anode Types of radiation Unstable nucleus New nucleus Alpha particle 22/05/2017 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, Beta high energy electron is called a _____ particle particle. Unstable nucleus New nucleus Unstable nucleus New nucleus 3) Gamma – after or decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Gamma radiation Words – frequency, proton, energy, neutrons, helium, beta 22/05/2017 Changes in Mass and Proton Number Alpha decay: 241 Am 95 237 Np 93 + 4 + 0 2 α Beta - decay: 90 Sr 38 90 Y 39 “positron” Beta + decay: 11 6 C β -1 11 B 5 + 0 +1 β Blocking Radiation 22/05/2017 Each type of radiation can be blocked by different materials: Sheet of paper (or 6cm of air will do) Few mm of aluminium Few cm of lead Summary Property Charge Rest mass Penetration What is it? Ionising ability Alpha Beta - 22/05/2017 Beta + Gamma Deflection by Magnetic Fields 22/05/2017 Alpha and beta particles have a charge: + 2 protons, 2 neutrons, therefore charge = +2 + 1 electron, therefore charge = -1 - Because of this charge, they will be deflected by electric and magnetic fields: + - Background Radiation 22/05/2017 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power Nuclear fission 22/05/2017 More neutrons Neutron Uranium nucleus Unstable nucleus New nuclei (e.g. barium and krypton) Chain reactions Each fission reaction releases neutrons that are used in further reactions. 22/05/2017 Radioactive Decay 22/05/2017 Radioactivity is a random process. The number of radioisotopes that will decay clearly depends on the number of radioisotopes present at that point in time: Activity (in Bq) = λN λ = “The decay constant” and has units of s-1. It is constant for a particular radioisotope. Half Life 22/05/2017 The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… = radioisotope At start there are 16 radioisotopes After 1 half life half have decayed (that’s 8) = new atom formed After 2 half lives another half have decayed (12 altogether) After 3 half lives another 2 have decayed (14 altogether) A radioactive decay graph 22/05/2017 Count 1 half life 1 half life 1 half life Time Half Life To calculate half life there are a few methods: 1) Read from a graph 2) Calculate using an equation t½ = ln2 λ 22/05/2017 Half Life questions 22/05/2017 1) The graph shows the activity of a radioisotope. Determine the half life and decay constant. 2) If there are 106 atoms present right now calculate how many will decay over the next second. 100s 3) What percentage of a sample of radioactive material will exist after 200 years if the half life is 50 years? 4) Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.