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Energy, Distance and Force

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Jim Thompson

Jan 1, 1970
0
I'm drawing a blank trying to remember how to use (IIRC) Hamilton's
Principle.

Isn't there some simple-minded way to take the energy at point A and
energy at point B, and calculate the force required to get from point
A to point B?

...Jim Thompson
 
T

Tim Wescott

Jan 1, 1970
0
Jim said:
I'm drawing a blank trying to remember how to use (IIRC) Hamilton's
Principle.

Isn't there some simple-minded way to take the energy at point A and
energy at point B, and calculate the force required to get from point
A to point B?

Not the force, but possibly the work.

Energy at point B = energy at point A + work to get to point B.

In a perfect physics world that means you could either go one inch while
exerting 10 tons, or you could go 20 000 inches while exerting one pound.
 
J

Jim Thompson

Jan 1, 1970
0
Not the force, but possibly the work.

Energy at point B = energy at point A + work to get to point B.

In a perfect physics world that means you could either go one inch while
exerting 10 tons, or you could go 20 000 inches while exerting one pound.

Defining the problem more simplistically... what is the force between
the two plates of a capacitor, spaced "d", area "A", voltage "V"?

...Jim Thompson
 
T

Tim Williams

Jan 1, 1970
0
Jim Thompson said:
Defining the problem more simplistically... what is the force between
the two plates of a capacitor, spaced "d", area "A", voltage "V"?

Oh, electrostatic force?

Don't know why you'd need to know this...this is first year textbook stuff,
Jim... ;-)

Lesse, one definition for electric field is the force divided by the charge,
E = F/q where E and F are vectors.

q = VC and C = e0*A / d (e0 = epsilon naught, permittivity of space), so:
E = F*d / e0*V*A > F = E*e0*V*A / d
E is also defined in volts per meter, so E = V/d. Thus:
F = e0 * V^2 * A / d^2

Force goes down inverse square with distance, and linear with area, I'd
think voltage would be linear as well though. Eh, what do you expect for a
usenet reply mmmh?

Tim
 
T

Terry Given

Jan 1, 1970
0
Jim said:
Defining the problem more simplistically... what is the force between
the two plates of a capacitor, spaced "d", area "A", voltage "V"?

...Jim Thompson


Woodson & Melcher (part 1) gives:


F = dW/dx (where the d's have melted and leaned to the left a bit)

W = energy = .5CV^2

C = Ae/x (3.1.54)

F = AeV^2 (3.1.55)
-----
2x

which is about what I guessed.

Cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
Terry said:
Woodson & Melcher (part 1) gives:


F = dW/dx (where the d's have melted and leaned to the left a bit)

W = energy = .5CV^2

C = Ae/x (3.1.54)

F = AeV^2 (3.1.55)
-----
2x

which is about what I guessed.

Cheers
Terry

and agrees with Tim, so must be right :)

Cheers
Terry
 
J

Jim Thompson

Jan 1, 1970
0
Oh, electrostatic force?

Don't know why you'd need to know this...this is first year textbook stuff,
Jim... ;-)

Of course it is. It's just that I'm coming up on 50 years since high
school physics and, you know, use it or lose it ;-)
[snip]

Tim


...Jim Thompson
 
J

Jim Thompson

Jan 1, 1970
0
Woodson & Melcher (part 1) gives:
[snip]

Wow! Old times. I should buy that book as a keepsake if nothing
else.

I have White & Woodson, the predecessor.

I worked as a technician in Professor Woodson's lab in MIT Building 20
while I was a student at MIT.

Melcher was a graduate student studying MHD under Woodson.

Melcher also taught some EE courses that I attended.

Melcher went on to become head of the EE Department, but died of colon
cancer, IIRC, mid 80's.

...Jim Thompson
 
T

Terry Given

Jan 1, 1970
0
Jim said:
Woodson & Melcher (part 1) gives:

[snip]

Wow! Old times. I should buy that book as a keepsake if nothing
else.

I have White & Woodson, the predecessor.

I worked as a technician in Professor Woodson's lab in MIT Building 20
while I was a student at MIT.

Melcher was a graduate student studying MHD under Woodson.

Melcher also taught some EE courses that I attended.

Melcher went on to become head of the EE Department, but died of colon
cancer, IIRC, mid 80's.

...Jim Thompson

Hi Jim,

I bought vol. 2 & 3, brand new, for $1 each while I lived in
Taxachusetts - from a book clearing place, Hamiltons IIRC. Vol. 1
arrived about a month ago, is older than I am, and cost about $20. They
are an excellent treatment of electromechanical systems. I knew you
would recognise them, which is why I used them ;)

My copy of Zverev turned up this morning, and I eagerly await
Motchenbacher (its in the mail....).

Cheers
Terry
 
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