Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jezza on 05/05/2006 11:53:02
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well I have my first physics test on Tuesday, part of my engineering degree, and I was looking over the mock and there are some things I’ve got no idea on how to answer:
Q1
A uniform tube, 3 meters long is closed at on end and open at the other, what are the first three resonant frequencies of the tube?
Q2
A charge of 0.5 coulombs is placed in an electric field of 500v per meter, what force does the charge experience?
Q3
An aluminium rod has a diameter of 40mm and is 750mm long if the temperature difference between the ends of the rod is 100'C
a) what is the heat flow thro the rod?
b) How much energy flows in 5 mins?
take the thermal conductivity to be 235 Wm^-1K^-1
anything would be great, I’m totally lost with this stuff. thanks
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cmon people im diein here
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I don't know about (2) and (3), and I don't have time to work this through properly, but for (1), you'll have a node at the closed end and an antinode at the open end so the wavelengths of the first 3 resonances will be 12m, 4m and (I think) 2.4m.
(Since the 3m is 1/4 wavelengths, 3/4 wavelengths and 5/4 wavelengths)
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2) A volt is defined as a joule per coulomb, so 1 coulomb will gain 1J of energy if it falls through 1V.
now:
energy = force * distance
so if .5 coulombs gain .5 *500 =250J of energy in 1m
with a touch of rearrangement the force should be 250N
does that make sense?
3) If you don't know what thermal conductivity means, try and think what will affect the amount of heat flowing through a rod:
the temperature difference - the bigger this is the more heat should flow
the area - also the bigger this is the more heat will flow
the length - the longer it is the less heat will flow
so if we make the simplest possible equation from this information
Power ∝ Temp difference * Area / Length
so we can turn a proporionality into an equality by adding an arbitrary constant
Power = Temp difference * Area / Length * Constant
Now it would be nice if this constant was the thermal conductivity, we can do a simple check on this by looking at the units:
W = K * m^2 /m * (Wm^-1K^-1)
which reassuringly cancels out to:
W=W
It is generally a good idea to check the units in case someone defined thermal conductivity the other way up.
so the equation comes out as:
Power = Temp difference * Area / Length * Conductivity
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cool, thanks people thats reeeally helped me, thanks