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Nuclear Weapons (and Energy)

Phys 1020, Day 25:

Questions?

Finish buoyancy

Nuclear Weapons Blmfld 16.1

Reminders:

work on projects

the how, the what … and why?

Air vs. Helium Balloon

AIR HELIUM

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 c. Pressure in Air = Pressure in Helium…. Both are equal to air pressure in the room! Pressure pushing in = pressure

pushing out or else bag will collapse. 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

SAME VOLUMES

Same volume so same Fbuoyancy = upwards net force from pressure of air surrounding balloon.

Fbuoyancy

Weight = mg

Review: Air vs. Helium Balloon

AIR HELIUM

How do the number of He atoms compare to number of Air molecules in each balloon?

c. # Air Molecules = # He atoms…. Ideal Gas Law:

P = k * (# molecules) * Temperature Volume

Balloons have same volumes … same pressure … same temperature of gas … so have same # of gas particles.

Air molecules heavier, but slower He atoms lighter, but faster

See simulation for diff molecules at same temperature

Another useful sim

http://www.colorado.edu/physics/phet/simulations/idealgas/idealgas.jnlp

So we could use He Balloon to lift stuff! (Pressures balance)

little helium atoms, only

2 protons and

two neutrons each.

Much less mass.

fat air atoms,

N- 14 neutrons & protons

O- 16 neutrons and protons

Same number of gas particles in each balloon. But weight of each He particle is less…

As the Helium balloon rises…

a. the volume of the balloon increases b. the Fbuoyancy increases c. the pressure inside the balloon increases d. a and b e. a, b, and c

BALLOON AT 42 KM ALTITUDE!

a. The volume of the balloon increases!!! Pressure of surrounding air decreases Balloon expands until pressure inside = pressure outside

Same # He molecules inside, to decrease P, volume increases!

Fbuoyancy does not increase. Volume is much bigger, but density of surrounding air is less Fbuoyancy is weight of displaced air: = Volume of balloon x density of air displaced by balloon x g

Volume increases but air density decreases!

Summary thoughts

1. Displaced stuff determines Fbouyancy

2. Inside stuff determines weight

Fgrav = W = mg

3. Fnet = Fbouyancy - Fgrav

4. Difference in Pressure =k (N/V) T inside

/ outside --> force on walls

F buoyancy

Weight of gas

Pocket of gas 3

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.

A look at hot air

Lower T, lower P outside

Higher T, higher P inside

Answer is b. Atoms push out into room until

pressure inside and out is the same

P= k (# molecules/Volume) T

lower higher

Hot air has fewer atoms in same volume, but same pressure.

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.

293 K 1.25 kg/m3

Air at 20 Celcius Air at 70 Celcius

Answer is c: # air hot / # air cold = 293 K / 343 K

293 K 1.25 kg/m3

Air at 20 Celcius Air at 70 Celcius

When heating: NOT CHANGING: volume, pressure CHANGING: temperature, # of molecules

P * V = (# molecules) * T k At 293 K: At 343 K: (# cold) * Tcold = (# hot) * Thot (# hot)/(#cold) = Tcold/Thot

all constant during heating

weight = mg

293 K 1.25 kg/m3

So how do the numbers work out for making a real hot air balloon (at + 50 degrees)? Net force upwards = Fbuoy – Weight of air inside (neglect material)

Fbuoyancy= (density of air) (volume) (g) = (1.25 kg/m3) * (5 m3 ) (9.8 m/s2 ) = 61.25N 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/m3 * 293 K/343K) * 5 m3 * 9.8 m/s2 = 52.3 N Lift = Fbouy- W = 61.25N - 52.3N = 8.95N

So to lift LESS than 1 kg would need a volume of 5 m3!

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

P= k gas T

T down, P down initially so gets squished in by air around it

until (# air molecules / Volume) or density of particles (gas)

so big that P inside balloon = P in room.

With particle (number density) is so large,

mass density is large also:

Mass = 1.25 kg/m3 * (293 K / 77 K) ~ 3 times higher than air in room

Fbuoy < < weight, drops like rock!

Weight

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

b. Air inside cools.. Lowers pressure inside, so force of air pressure pushing out less than force of air pressure pushing in. Walls cannot sustain pressure difference, Volume decreases until pressure inside pushing out equals pressure outside, or until walls can sustain pressure difference.

Suction Cup Strength?

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

Suction Cup Strength?

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? b. B can support about 4 x more weight than A

Area of suction cup = pi * r2

Area of B is 4 times Area of A Force upwards from air molecules colliding with suction cup = Pressure * Area

Pushed many air molecules out of here

Force up = Air pressure X Area

Force down = Mass of weight x gravity

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

ans. 1 c, 2. d, 3. b.

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

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).

Nuclear Weapons* release of ENORMOUS amounts of energy stored

in the nuclei at center of atoms.

* don’t try this at home

http://kuroiso.org/a005.html

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 .0000001 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

hydrogen 1 p

deuterium 1 p, 1n

helium 2 p, 2 n Uranium 238

92 p, 146 n

Proton (positive charge) – charge = 1.6 x 10-19 Coulombs mass = 1.66 x 10-27 kg.

Neutron (no charge) – no charge mass = 1.66 x 10-27 kg.

Electron (negative charge) – charge = -1.6 x 10-19 Coulombs mass = 9.10 x 10-31 kg.

Atom ingredients:

Each element has different number of protons.

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