Nuclear Weapons (and Energy)

Transcription

1 Nuclear Weapons (and Energy) the how, the what and why? Phys 1020, Day 25: Questions? Finish buoyancy Nuclear Weapons Blmfld 16.1 Reminders: work on projects 1

2 Air vs. Helium Balloon F buoyancy SAME VOLUMES AIR HELIUM Weight = mg Same volume so same F buoyancy = upwards net force from pressure of air surrounding balloon. How does the pressure inside each balloon compare? a. Pressure in Air > Pressure in He b. b. Pressure in Air < Pressure in He c. Pressure in Air = Pressure in Helium How do the number of He atoms compare to number of Air molecules in each balloon? a. # Air Molecules > # He atoms b. # Air Molecules < # He atoms c. # Air Molecules = # He atoms 2

3 Review: Air vs. Helium Balloon AIR HELIUM How do the number of He atoms compare to number of Air molecules in each balloon? P = k * (# molecules) * Temperature Volume 3

4 Another useful sim 4

5 So we could use He Balloon to lift stuff! (Pressures balance) fat air atoms, N- 14 neutrons & protons O- 16 neutrons and protons little helium atoms, only 2 protons and two neutrons each. Much less mass. Same number of gas particles in each balloon. But weight of each He particle is less 5

6 As the Helium balloon rises a. the volume of the balloon increases b. the F buoyancy increases c. the pressure inside the balloon increases d. a and b e. a, b, and c 6

7 Summary thoughts 1. Displaced stuff determines F bouyancy 2. Inside stuff determines weight F grav = W = mg 3. F net = F bouyancy - F grav 4. Difference in Pressure =k (N/V) T inside / outside --> force on walls ? Weight of gas Pocket of gas F buoyancy 7

8 A look at hot air What will happen if we heat a beaker of air, and then remove the stopper? a. nothing, the air will just stay there but get hotter b. there will be fewer molecules inside because some will be pushed out into the room c. the molecules inside will become lighter because they are hotter d. there will be fewer molecules inside because heating destroys some of them. 8

9 293 K 293 K 1.25 kg/m kg/m 3 Air at 20 Celcius Air at 70 Celcius If air inside balloon is heated so that it is 50 C hotter than before, how does the number of air molecules inside balloon change? What is ratio of number of air molecules of hotter balloon to number of air molecules of colder balloon? a. # air hot / # air cold = 70 C / 20 C b. # air hot / # air cold = 20 C / 70 C c. # air hot / # air cold = 293 K / 343 K d. # air hot / # air cold = 343 K / 293 K e. I do not really understand how to reason through this. P = k * (# molecules) * Temperature Volume 9

10 293 K 1.25 kg/m 3 F buoy So how do the numbers work out for making a real hot air balloon (at + 50 degrees)? Net force upwards = F buoy Weight of air inside (neglect material) F buoyancy = (density of air) (volume) (g) = (1.25 kg/m 3 ) * (5 m 3 ) (9.8 m/s 2 ) = 61.25N weight = mg Density of hot air: # hot air molecules = # air molecules cold x (293K /343 K) Volume Volume Weight of air inside = density of hot air x volume x g = (1.25 kg/m 3 * 293 K/343K) * 5 m 3 * 9.8 m/s 2 = 52.3 N Lift = F bouy - W = 61.25N N = 8.95N So to lift LESS than 1 kg would need a volume of 5 m 3! 10

11 If I stick a balloon full of air into liquid nitrogen and wait for some time, then. (Liquid Nitrogen temp = 77 K) a. number of molecules inside balloon will be less b. pressure inside the balloon will be lower c. volume will decrease d. b and c e. a and b 11

12 Bottle filled with hot liquid, sealed and then allowed to cool Bottle filled with hot liquid, sealed and then allowed to cool Why does this happen as it cools? a. Walls of bottle collapsed due to heat b. Pressure of the air outside is higher than pressure of air inside c. Liquid and air inside is pulling sides in from the inside 12

13 Suction Cup Strength? A B The diameter of Suction Cup B is twice as big as the diameter of Suction Cup A. How much weight can each suction cup hold? a. A and B can support the same amount of weight b. A can support about 2 x more weight than B c. A can support about 4 x more weight than B d. B can support about 2 x more weight than A e. B can support about 4 x more weight than A 13

14 Reading quiz. 1. What keeps a suction cup against the wall? a. sticky rubber, b. electrical attraction between oppositely charged particles, c. the force of air pressure, d. the buoyancy force, e. magic 2. You remove a partially filled sealed container of food from the refrigerator and let it warm up. You notice that the lid bows out as it warms up. This is because: a. as the food warms it produces gases that increase the pressure inside. b. the pressure on the outside of the container decreases as the container warms c. the air pressure in the room is higher than in the cold refrigerator d. as the gas inside the container warms up it increases the pressure. 3. A hot air balloon can lift more on a. a hot day, b. a cold day, c. there is no difference 14

15 Group Buoyancy Question related to lab Ship loaded with gold in canal lock (water can t move in or out), there s a mutiny and the gold is thrown overboard. What happens to the water level in the lock? (Assume no water splashes out) a. Goes up b. Goes down c. Stays the same

16 Nuclear Weapons* release of ENORMOUS amounts of energy stored in the nuclei at center of atoms. I. Atomic bomb (actually fission bomb) today a. how nuclei are held together, why so much energy involved. b. how they come apart and release LOTS of energy. alpha decay, neutron-induced fission c. how to make a whole bunch of them do it at once = LOTS x whole bunch= bomb II. Radioactivity- what is it and why bad for living cells. half-life III. Fusion bomb (little nuclei stick together). * don t try this at home 16

17 Recipe- how to make an atom: Ingredients: 1 pinch of protons 1 pinch ofneutrons 1 cup of electrons Proton (positive charge) Neutron (no charge) Electron (negative charge) 1. Mix protons and neutrons thoroughly. 2. Bake at 100 million degrees until sticks together to form solid dense nucleus (about s). 3. Frost with lightly with fluffy layer of negative electrons. 4. Chill before serving! atom size: Radius of nucleus is 10,000 times smaller than nucleus-electron distance 17

18 Each element has different number of protons. Atom ingredients: Proton (positive charge) charge = 1.6 x Coulombs mass = 1.66 x kg. Neutron (no charge) no charge mass = 1.66 x kg. Electron (negative charge) charge = -1.6 x Coulombs mass = 9.10 x kg. hydrogen 1 p deuterium 1 p, 1n helium 2 p, 2 n Uranium p, 146 n 18

19 Oxygen has 8 protons, 8 neutrons: Consider a nuclei with 7 protons, 7 neutrons (nitrogen atom) what if we want to add another proton to make oxygen (8 protons): What will we need to do to get proton stuck to nucleus: a. just give it a little push so it will hit nucleus dead on and it will drift towards nucleus and stick. b. the closer it gets, the harder you have to push, will take lots of work c. you ll need to push really hard at first and then less as you get closer d. you ll have to push the proton towards the nucleus with a fixed amount of force (constant force). 19

20 What if threw proton so starts out going towards nitrogen nucleus with a lot of speed (lots of kinetic energy)? Starts with lots of kinetic energy Repelling force from nucleus slows down proton Proton s kinetic energy converted into electrostatic potential energy, as it gets closer to nucleus r, separation distance Potential energy curvesrepresent energy to bring particles together. Potential energy Kinetic energy Gravity energy analogy. separation distance, r if at center, want to roll down hill/fly apart lots of electrostatic potential energy 20

21 Potential energy curves- represent energy to bring particles together. Gravity energy analogy. r, separation distance Potential energy Kinetic energy separation distance, r Potential energy = k x charge of 7 proton nucl. x charge of single proton at separation (separation distance) distance of r Charge of 1 proton = 1.6 x Coulombs; Charge of 7 protons=11.2 x C So at m away (~ radius of nucleus), Potential energy = 8.99 x 10 9 N m 2 /C 2 x 11.2 x C x 1.6 x C (10-15 m) = 1.61 x Joules = 10 million electron Volts. (1 electron volt = energy gained by electron moving through 1 Volt diff.= 1.6 x J) 21

22 Potential energy curve (Energy vs. separation distance) for bringing electron in to proton a. This curve would be flat, not going up or down b. look like bringing a proton into a proton except upside down so going down instead of up. c. would look same as proton on proton r 22

23 Analyzing shape of potential energy curves: most potential energy pushed apart very little, potential energy not changing along top E 1 2 distance pushed apart hardest, Potential energy changing rapidly! 23

24 How can a bunch of repulsive positively charged protons stick together? Like hopper toy- really strong nuclear force between protons and neutrons overwhelms electrical force if really close together. Spring legs- like electrical force, pushes over distance. suction cup- like nuclear force, only strong when very close like double sticky tape only works if in close contact. Nuclear force- Why like this? Just different! That is the way nature is! 24

25 Potential energy curve for proton approaching nucleus electrostatic repulsion attractive nuclear separation distance, r Energy scale gigantic compared to chemical energy. Why? Simple coulomb s law. F= k (charge of #1)(charge of #2) r 2 Chemistry- forces between electrons and protons on distance scale of atomic size (> m). combination--real nucleus separation distance, r Nuclear forces- forces between protons on distance scale ,000 times smaller. 10,000 times closer means forces 100,000,000 times bigger because of 1/r 2. Lots more potential energy stored!!! 25

26 Atomic nuclei Gets deeper until iron (26 P, 30 N) less deep if bigger. helium beryllium harder to push together, but bigger drop when do. Really stable: Have to add a whole bunch of energy to break up! Really big nucleus, >100 P, >100 N, like Uranium or plutonium 26

27 Nuclear decay- one kind of nucleus changes into another. alpha decay, (beta decay), induced fission a. alpha decay- alpha particle = 2p and 2n. They escape together. Most radioactivity is this type (e.g. radon). tunnel out. Not real tunnel. Quantum physics says jumping around all the time. Very small fraction of time appear outsidewhen happens-- run for it!! 2protons-2neutrons stuck together. (chained together prisoners escaping!) If alpha particle, finds itself outside, lots of PE, zooms away PE KE 2P-2N 27

28 A useful simulation alpha decay 28

29 tunneling difficulty = width x depth of tunnel E nucleus1 3 2 hard- takes long time, billions of years! How much energy released? a. 1 most, 2 second, 3 least b. 2 most, 1, 3 least c. 3 most, 2, 1 least d. 3 most, 1, 2 least medium easy!, happens in millionths of a second! 29