Gases & Plasmas

1. Gases& Plasmas

2. The Atmosphere

3. Atmosphere • Unlike water – Density of the atmosphere is depth-dependent • If you have a 30 km tall bamboo pole of cross section 1 sq cm – the mass of air in it would be about 1 kg. • This weight of the air is source of atmospheric pressure which is due to 1 kg/sq cm at sea level • A 30 km tall sewer pipe of cross section 1 sq meter has about 105 Newtons of weight.

4. Atmosphere • Dry air at 20o C has a density of 1.21 kg/m3 which is about 2 ¾ lbs. • What weighs more, the air in a frig or a grapefruit?

5. Barometers • Need 10.3 meters of water in a pipe to balance atmosphere (depends on atmospheric pressure). • Turn long pipe of water into a bucket of water and water level in pipe will be 10.3 M with vacuum above water in pipe.

6. Barometers • What is max height you can drink water through a straw? • What is the deepest well that you can pump water with a hand pump? • Mercury is 13.6 times as dense as water and needs 76 cm to balance atmosphere in a pipe.

7. Boyle’s Law P1V1 = P2V2 • Holds for constant temperature • Pressure is proportional to density – when volume is decreased, density and pressure increase • Double pressure by decreasing volume by half

8. Boyle’s Law P1V1 = P2V2 • Atmospheric pressure halves every 6 Km you go up. • What will happen to the volume of a balloon that rises 6 Km. • What happens to the density? • What happens if a Scuba Diver 10.3 M deep holds their breath and swims to the surface?

9. Archimedes’ Principle • An immersed body is buoyed up by a force equal to the weight of the fluid it displaces. • Is valid for gasses as well as liquids! • Objects weigh more in a vacuum than in air!

10. Archimedes’ Principle • What does a helium balloon do in an accelerating car? Why?

11. Archimedes’ Principle • What does a helium balloon do in an accelerating car? Why?

12. Bernoulli’s Principle Water speeds up in narrower pipes! Where Speed of fluid increases, pressure in fluid decreases

13. Bernoulli’s Principle

14. Bernoulli’s Principle

15. Plasma – the Fourth State of Matter • Solid Liquid Gas Plasma Increasing Temperature  Plasmas are generally greater than 10,000 degrees

16. Plasma – the Fourth State of Matter • A plasma is an ionized gas that responds to electric and magnetic fields. • The visible universe is made of 99% plasma. • The sun is a giant ball of plasma • High temperature Plasma must be contained in magnetic bottles. If the plasma were to come into contact with a physical container, it would vaporize the container. • Lightning is a plasma • Fusion power research uses a plasma at ~ 10M oC

17. Plasmas – Fluorescent Lamps • Fluorescent lamp – argon and mercury atoms ionize creating plasma where current flows. Current activates mercury atoms to emit ultraviolet radiation causing phosphor coating on inner surface of tubes to glow visible light. • Other gasses glow colors: Neon – red, argon – blue, sodium – yellow, helium – pink.

18. Plasmas – Fluorescent Lamps • What keeps fluorescent lights from getting extremely hot?? • Each of the ions in the plasma does have a high temperature, but each ion can only carry a small amount of energy. Since there are very few total ions and electrons in the tube, not much energy can be transferred.

19. Class Problem Compared to the mass of a dozen eggs, the mass of air in an "empty refrigerator" is1) negligible2) about a tenth as much3) about the same4) more

20. Class Problem • Compared to the mass of a dozen eggs, the mass of air in an "empty refrigerator" isa) negligibleb) about a tenth as muchc) about the samed) more • One cubic meter of air at 0°C and normal atmospheric pressure has a mass of about 1.3 kilograms. A medium-sized refrigerator has a volume of about 0.6 cubic meter and contains about 0.8 kilograms of air—more than the 0.75 kilograms of a dozen large eggs!We don't notice the weight of air because we are submerged in air. If someone handed you a bag of water while you were submerged in water, you wouldn't notice its weight either.

21. Class Problem • In the presence of air, the small iron ball and large plastic ball balance each other. When air is evacuated from the container, the larger ball1) rises2) falls3) remains in place

22. Class Problem • In the presence of air, the small iron ball and large plastic ball balance each other. When air is evacuated from the container, the larger balla) risesb) fallsc) remains in place • The answer is b:Before evacuation, the forces acting on each ball are the gravitational force, the force exerted by the balance beam and the upward buoyant force exerted by the surrounding air. Evacuating the container removes the buoyant force on each ball. Since buoyant force equals the weight if air displaced, and the larger ball displaces the greater weight of air, the loss of buoyant force is greater for the larger ball, which falls.

23. Class Problem • Consider a Ping-Pong ball floating in a glass of water that is enclosed in an air-tight chamber. When air pressure is increased in the chamber, does the ball float 1) lower, 2) higher, or 3) as before?

24. Class Problem • Consider a Ping-Pong ball floating in a glass of water that is enclosed in an air-tight chamber. When air pressure is increased in the chamber, does the ball float lower, higher, or as before? • The ball will float higher. The buoyancy that accounts for its floatation is due to the weight of the displaced fluid—both air and water. Higher-pressure air is denser air, and the greater weight of displaced denser air by the ball contributes to greater buoyancy by the air. This lifts the ball upward and the ball floats higher in the water.

25. Class Problem • If you release a ball inside a freely-falling elevator, it stays in front of you instead of "falling to the floor" because you, the ball, and the elevator are all accelerating downward at the same acceleration, g. If you similarly release a helium-filled balloon, the balloon will1) also stay in front of you2) press against the ceiling3) press against the floor

26. Class Problem • If you release a ball inside a freely-falling elevator, it stays in front of you instead of "falling to the floor" because you, the ball, and the elevator are all accelerating downward at the same acceleration, g. If you similarly release a helium-filled balloon, the balloon will: a) also stay in front of youb) press against the ceilingc) press against the floor The answer is a:Like the falling ball, the balloon will also stay in front of you because it loses its buoyancy. The buoyancy of a balloon is the result of a difference in air pressure against it—usually a greater pressure acting up against the bottom than down against the top. But in the freely-falling environment there is no difference in air pressure. Air in the elevator, like everything else inside, is in a state of free fall. Air in the top part does not press against air in the bottom part to give a greater pressure there. No pressure difference means no buoyancy, so the balloon freely falls like everything else inside the elevator.

27. Class Problem A block of wood and a block of iron on weighing scales each weigh 1 ton. Strictly speaking, which has the greater mass? 1) wood 2) iron 3) same

28. Class Problem A block of wood and a block of iron on weighing scales each weigh 1 ton. Strictly speaking, which has the greater mass? • Answer: • The wood has the greater mass. Why? Because the scale reading is weight minus the buoyant force of the surrounding air. The wood has a greater volume, displaces more air, and therefore has a greater buoyant force. To yield the same scale reading it must therefore have a greater mass than the iron.

29. Class Problem • A birthday candle burns in a deep drinking glass. When the glass is whirled around in a circular path, say held at arm's length while one is spinning like an ice skater, which way does the candle flame point? • straight up • inwards towards the center of motion • outward away from the center of motion

30. Class Problem • A birthday candle burns in a deep drinking glass. When the glass is whirled around in a circular path, say held at arm's length while one is spinning like an ice skater, which way does the candle flame point? • The candle flame points inward, toward the center of the circular motion. This is because the air in the glass is more dense than the flame and "sloshes" to the farther part of the glass. The greater air pressure at the farther part of the inner glass then buoys the flame to the region of lesser pressure—inward.

31. Class Problem • A candle will stay lit inside the space shuttle when it is on the launch pad, but when not when it is in orbit. Why?

32. Class Problem • When a candle ordinarily burns, the warmed carbon dioxide produced in the flame rises by convection, and oxygen comes in from below to keep the process going. But when in orbit, there is no effect of gravity inside the cabin and convection cannot occur. The warmed exhaust gasses do not "rise," and instead suffocate the flame.

33. Class Problem • 1. Atmospheric pressure is caused by the • A) density of the atmosphere. • B) weight of the atmosphere. • C) temperature of the atmosphere. • D) effect of the sun's energy on the atmosphere • 2. A balloon is buoyed up with a force equal to the • A) weight of air it displaces. • B) density of surrounding air. • C) atmospheric pressure. • D) weight of the balloon and contents. • E) all of these • 3. As a helium-filled balloon rises in the air, it becomes • A) bigger. • B) more dense. • C) heavier. • D) all of these • E) none of these

34. Class Problem • 4. In drinking soda or water through a straw, we make use of • A) capillary action. • B) surface tension. • C) atmospheric pressure. • D) Bernoulli's principle. • E) none of these • 5. Airplane flight best illustrates • A) Archimedes' principle. • B) Pascal's principle. • C) Bernoulli's principle. • D) Boyle's law. • 6. A suction cup sticks to a wall. It is • A) pulled to the wall by the vacuum. • B) pushed to the wall by the atmosphere. • C) both of these • D) neither of these

35. Class Problem • 7. As a balloon rises higher and higher into the atmosphere, its • A) volume decreases. • B) density increases. • C) weight increases. • D) mass decreases. • E) none of these • 8. Compared to the buoyant force of the atmosphere on a 1-liter helium-filled balloon, the buoyant force of the atmosphere on a nearby 1-liter solid iron block is • A) considerably less. • B) considerably more. • C) the same. • 9. When gas in a container is squeezed to half its volume, its density • A) halves. • B) doubles. • C) quadruples. • D) remains the same.

36. Class Problem • 10. In a vacuum, an object has no • A) buoyant force. • B) mass. • C) weight. • D) temperature. • E) all of these • 11. Suspend a pair of Ping-Pong balls from two strings so there is a small space between them. If you blow air between the balls, they will swing • A) toward each other. • B) apart from each other. • C) away from the air stream, but not necessarily toward or apart from each other. • 12. The main difference between gases and plasmas has to do with • A) the kinds of elements involved. • B) interatomic spacing. • C) electrical conduction. • D) fluid pressure. • E) the proportion of matter to antimatter in the universe.