lightning protection

[T37 Phil]

Yes, that is one interpretation of it. The boat is "in the dielectric". Now take the boat and look at it in isolation. We don't care how it got polarized just that it is. So lots of electrons at the top and not so many at the keel. SURE does look like a capacitor to me. If I turn off the external field it will discharge JUST like a capacitor.

No, not just like a capacitor. A capacitor has a non-conducting region between the separated charge surfaces. If you disconnect a capacitor and turn it off it does not discharge. You have to short out (or otherwise load) the terminals. Your isolated mast-capacitor would not hold charge.

I don't buy your argument about the mast potential, but I'm not yet prepared to offer an alternative in any detail.

 

[T37 Phil]

Ok walt, my answer is - it depends. Given our uniform field from sea to clouds:

Case 1 - mast grounded through keel. Entire mast is at ground potential.

Case 2 - mast not grounded through keel. Mast is at equipotential with the potential being determined strictly by the charge in the mast. If the mast had most recently been grounded, it would still be at ground potential. If someone had brought it up to 1000kV somehow, then it would be at 1000kV. It has nothing to do with the average potential in the field unless there is current flowing between the mast and the atmosphere.

In other words,

V = Q a

where a is a geometric factor and Q is the net charge in the conductor (ie the mast).

 

[Bill Roosa]

You are correct Phil, however

All matter has capacatenance, it also has inductance. While not a capacitor proper it does have a charge concentration on it.
The classic example is two ping-pong balls. Rub one on wool and it gets a positive charge by loosing electrons to the wool. ground the other ball to make it neutral then unground it. bring it close to the positive ball and it will develope a negative charge on the side nearest to the other positive ball. \

The cloud and ground do the same thing, buildings to it too! I don't see what you are not getting. the fact that it is a conductor only makes moving the electrons around easier so what is the rub to think they move when subject to an e-field. that is also what happens as current is increasing/decreasing in a wire.

For the record the reason there is no e-field in a wire that has current flowing is because the e-field has been replaced with a magnetic field. the H-field actually keeps the electrons moving (in a perfect conductor)(gota have a little e-field in a normal conductor to over come resistance)

Let me make a counter argument.
if the mast is entirely at one potential then I can find a protion of it that is in an e-field that is at a different potential. How come the electrons do not move under the influence of the field? They are not locked to the material as in an insulator, they are free to move anywhere inside the body of the conductor.

Riddly me that Batman

 

[Bill Roosa]

another way to look at it Phil

think of the mast to cloud "circuit" as one capacitor and the keel to water surface "circuit" as another capacitor and then connect the mast part tot he keel part with a wire and call the wire a mast. So you have a capacitor with air between it. The top plate is the cloud and the bottom plate is the mast head. a wire connects this bottom plate to the top plate of another capacitor below it. the second capacitor has the top pate as the mast base and its bottom plate as the water surface.

They are just really bad capacitors with not very flat plates.

 

[T37 Phil]

... For the record the reason there is no e-field in a wire that has current flowing is because the e-field has been replaced with a magnetic field. the H-field actually keeps the electrons moving (in a perfect conductor)(gota have a little e-field in a normal conductor to over come resistance)

There's no point in arguing about the definition of capacitor. Material properties allow charge to be stored in material objects. When two surfaces of charge storing material are separated by a dielectric, you have a capacitor. I think we have no arguments there.

However, your picture from a few posts back was worth much more than your words. It was absolutely a correct picture and helped me get my neurons back in a row. The field generated between cloud and sea does in fact induce a separation of charge in the mast/conductor. It is that separation of charge that generates a field within the conductor, that combined with the cloud/sea field yields a zero field within the conductor. The geometry of the problem is strange, but correct. The very top of the mast and the very bottom of the mast are charged surfaces. However, it remains true that the potential in a conductor is the same everywhere. Since there is no field inside the conductor, the potential must be a constant.

E = del V and 0 = del <constant>

The question is what is that constant potential and the answer is it depends on the overall charge contained within the conductor. If the net charge is the same as it is at ground (say if the mast were grounded to sea through the keel), then the mast would stay at ground potential. If that connection were broken, the mast would remain at ground potential until some other event caused the net charge in the mast to change.

Riddly me that Batman

Good line of thought Riddler. The mast is an equipotential conductor at static equilibrium. No portion of the mast is at a different potential. No current flows. The surrounding air may be at a different potential, but there is no path to conduct your free electrons off the mast and down that gradient. On the other hand, that led me back to consider walt's original proposal. If a device could measure the difference in potential between the top of the mast and a point say one meter away from the top of the mast, then the lightning warning system might be a practical and useful device.

 

[T37 Phil]

Bill, I should add that potential is a theoretical quantity. Potential is related to what would happen to a test charge placed in a field. The potential difference between poles on a boat battery, on the other hand, is a measurable value, say 12V. If you connected a wire to one post and moved the end a mile away from the other post (perfect conductor, etc...), it would still be 12V. If you moved it right next to but not touching, still 12V. No current is flowing through the air, unless - you move too close. What is too close? Too close is when separation distance d is such that 12/d is a number big enough to make the air conductive (just like lightning) and allow a spark to jump.

 

[zeehag]

NOW, if y'all could figure out a way to USE the energy , instead of waste it and try to prevent it, mebbe the sea gods and weather gods might allow ye to learn how to predict and harness it?? goodness knows ye cannot protect from nor prevent it.....