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Too much politics - time for Physics Pop Quiz #5(14 posts)

Too much politics - time for Physics Pop Quiz #5jose_Tex_mex
Feb 21, 2003 2:45 PM
All this war talk and politics is just too much. Therefore it's time to relax with another Physics Quiz. This one comes in two parts. The second I only mention because I just found out about it myself and thought I would share it with you.

Part 1
You have an iceberg in your tub (no jokes please :-) which is floating. What happens to the level of water in the tub after the iceberg melts?

Part 2
Why is it that when a car or airplane is hit by lightning the passengers are "usually" not hurt?

I will try and post the answers tomorrow. It's 18:00 ET and time to get ready to go out...
This seems too easy:TJeanloz
Feb 21, 2003 2:50 PM
1. Nothing happens to the water level.

2. A car or airplane effectively act as a Farraday cages, making the path of least resistence for the electricity through the steel structure of the vehicle rather than through the (mostly water) occupants.
This seems too easy:DougSloan
Feb 21, 2003 3:14 PM
I agree. But *steel* airplanes? :-)
My mistake - but they must have some steel in them (nm)TJeanloz
Feb 21, 2003 3:32 PM
Al works fine without the steel. (nm)53T
Feb 21, 2003 5:32 PM
re: Too much politics - time for Physics Pop Quiz #5Skip
Feb 21, 2003 3:34 PM
Part 2:

Only works for metal A/C, not for fiberglass, CF, Kevlar, etc., unless expanded aluminum has been embedded into the entire surface layup.
Part 1aeon
Feb 21, 2003 4:42 PM
Do I have this right?

-ice is less dense than water
-the ice below the water line has less mass than an equivalent volume of water would
-the ice above water is the same volume as the missing mass below the water
-so the two balance and the water level would not change

humour me here, been a few sememsters since physics =)
re: Too much politics - time for Physics Pop Quiz #5asphalt assault
Feb 21, 2003 5:45 PM
1. The water level should remain the same.

2. The car/plane passengers should be riding a bike, you never hear of lightning hitting a bike do you?
Part 1aeon
Feb 21, 2003 10:26 PM
Do I have this right?

-ice is less dense than water
-the ice below the water line has less mass than an equivalent volume of water would
-the ice above water is the same volume as the missing mass below the water
-so the two balance and the water level would not change

humour me here, been a few sememsters since physics =)
Engineer's responseAlexx
Feb 22, 2003 7:39 AM
1) level remain the same, for reasons already stated.

2) Electrons, being all the same type of charge (in this case, negative) will repel each other, to achieve the gretest distance from each other. On any metal conductor, such as a plane, car, corrugated barn, etc, this will mean that the elecrtons will travel along the outer skin.
re: Too much politics - time for Physics Pop Quiz #5rwbadley
Feb 22, 2003 9:57 AM
1. If the water in the tub starts out warm, as the ice melts and brings the water temp down the level of the water will become very slightly lower, this being due to contraction of water due to the colder temperature. After the ice has melted and the water temperature starts to rise, the water level will raise slightly due to expansion.

2. Passengers are usually not hurt because they are sitting on the relatively high insulating value of the seats. If they have a grasp on metal and are further grounded they will be fried, (just like the electronics will be, causing the plane to go down in a giant fireball heh heh)

RW
Long Winded Answer Timejose_Tex_mex
Feb 23, 2003 7:05 PM
I see just about everyone did well on this quiz. I'll try and make it a more difficult next time. However, in these quizzes the fun normally starts after the "answers" are given.

Part 1
Yes, the water will remain the same. A "floating" iceberg will displace its weight. If, however, the iceberg were touching the bottom of the tub the situation would be different.

The density of water is about 1g/cm^3. That is one centimeter cubed of water (call this an ice cube) has a mass of one gram. Suppose you forced the cube under water - what would happen? The amount of water displaced would have a mass of more than one gram. Let the cube float and the volume of the cube under the water displaces one gram of water.

I guess I should have tried to be more tricky with the question.

Part 2
First of all, your tires do not save you - they are made of synthetics and probably have little to no rubber in them. Thus, we'll disgard that old idea.

What helps save us is called the skin effect. I believe this is a totally different process than the Faraday shielding, although I could be wrong. Here's my rationale ...

Simply stated, a current can travel along the surface of a metal (skinning) and in the case of an automobile is conducted around the driver. If the driver is in bare feet, touching wet carpets or metal objects - watch out, you're probably dead.

Whether a metal allows skinning depends on a lot of factors one of which is the application of the current - continuous or changing. The skinning effect applies to changing current. Once you have a direct current the charge will flow through the entire medium, such as in wires.

Normally, lightning is an impulse current and will promote skinning. Thus, most of the current will stay on the outer shell of an auto during a lightning strike.

However, the lightning that causes forest fires can often last more than a second. This sets up a direct current application, yet if we stay in our cars we remain safe. Even though the metal may be conducting charge throughout, air is not a very good conductor and the charge again is conducted around us.

Again, although I could be wrong, I believe Faraday cages do not deal with flow of current but an electrostatic field. Since it's current through our bodies that kills us, I would have to say nay to the Faraday Cage idea since the Faraday shielding effect is not what saves your life.

As for Airplanes, I think I will allow the Aviation people to explain this one better. I know Al is a very good conductor and that we do not want electricity being conducted throughout the fuselage. The spikes (static wicks) seen in wings allow planes to dump any charge built up during flight in to the air. Additionally, when lightning does strike a plane the charge will be dissipated here and minimize any skinning effect.

That's my story and I am sticking to it.
:-)
of course, you could be dead ifDougSloan
Feb 24, 2003 9:47 AM
The heat or electricity ignites fuel...

Doug
however...wClydeTri
Feb 25, 2003 6:32 AM
with regards to your ice berg question...you must have the assumption it is a homogenous mass, and more explicitly, you must consider if you have air susspended in it. Yes, you can get air into ice. Air bubbles/pockets in the ice would effect your answer, eh? Its all about the assumptions you make.