# Thread: Need help with science/math schoolwork? Post here!

1. ## Re: Need help with science/math schoolwork? Post here!

Originally Posted by Nikitn
Could somebody help me with my electricity & magnetism homework?

Question 1:

Two condensers connected in series will be charged such that one gets +Q charge on one of its plates, and the other -Q.

Schematic: ------(-Q)| |(a)------(b)| |(+Q)-------

Can somebody prove (or at least show) intuitively to me why the plate "a" gets a +Q charge, and the plate "b" -Q? Ie, why do the inner plates have charges equivalent to the outer ones?
First of all, don't you mean capacitors instead of condensers? Secondly, consider the first free electron in the middle to be set in motion. Because it is attracted by the outermost right hand plate charge (+Q) and pushed away by the outermost left hand plate charge (-Q) due to the respective Coulomb forces, it will move to (b). Because of the fact that this electron was detached a positive ion emerges, so a +e will move to (a) for the analogous reasons. The second electron will move to (b), but the force driving it is now somewhat weaker because there is one -e in (b) pushing it away and one +e in (a) attracting it. The third electron experiences an even weaker force because there are 2(-e) in (b) and 2e in (a) now, and so on, until (b) gets a sufficient number of electrons to add up to -Q and (a) +Q, when the total force dies out.

Originally Posted by Nikitn
Question 2:

A dialectic of length d is inserted into a condenser made up of two plates a distance D apart (d<D). Afterwards, the condenser acts like it is two condensers connected in series. Why?
Because the dielectric will polarise due to the mechanism described above and a +Q -Q +Q -Q concentration will form, so it's like two capacitors in series.

2. ## Re: Need help with science/math schoolwork? Post here!

yeah, thanks for Q1.

Why does the polarized material get Q charged on its sides? Doesn't this require that no electric field passes through the polarized material? Usually, the dialectic material isn't able to get polarized enough to completely cancel the electric-field from the capacitor plates, so why should it be different when its height d is shorter than the distance D between the capacitor plates?

edit: it is no longer a problem.

3. ## Re: Need help with science/math schoolwork? Post here!

A and B can fill a cistern in 4 hours. A and C can fill the same cistern in 5 hours. B can fill twice as fast as C. Find how long C would take to fill the cistern working alone.

Please help. This is supposedly easy but I can't for the life of me figure it out.

4. ## Re: Need help with science/math schoolwork? Post here!

Originally Posted by VALIS
A and B can fill a cistern in 4 hours. A and C can fill the same cistern in 5 hours. B can fill twice as fast as C. Find how long C would take to fill the cistern working alone.

Please help. This is supposedly easy but I can't for the life of me figure it out.
It's a rate. So Rate*Time = Volume.

i) (A + B)*4 = V.
ii) (A + C)*5 = V.
iii) B = 2C.
iv) (A + 2C) = V/4 (from i) and iii))
v) (A + C) = V/5 (from ii))
vi) A + C + C = V/4 = (V/5) + C (from iv) and v))
C = V/4 - V/5 = V/20
C*20 hours = V

20 hours

5. ## Re: Need help with science/math schoolwork? Post here!

That is a system of equations just itching to made. The resulting matrix and vector looks like this

1 1 0 | 1/4
1 0 1 | 1/5
0 1-2 | 0

Use some internet magic to invert the matrix ...

|-1 2 1|
|2-2 -1|
|1 -1-1|

Multiply by the vector and you get ....

A = 3/20 Cisterns/hr
B = 2/20 Cisterns/hr
C = 1/20 Cisterns/hr

So

1 Cistern / C = 1/(1/20) = 20 hrs

(**** you Jack, **** you for stealing the Linear Algebra glory that was rightfully mine. One night, I'm gonna come to you, inside of your house, wherever you're sleeping, and I'm going to cut your throat.)

7. ## Re: Need help with science/math schoolwork? Post here!

We have three nodes forming a triangle and place one additional node in the centre. Connect the central node to the other three using three equal inductors and use three equal capacitors to connect the corners to each other.

Does anyone know if this circuit has a special name or how the frequency should be calculated? I got f=1/(2pi*scrt(2CL)) but that seemed far to simple.

8. ## Re: Need help with science/math schoolwork? Post here!

Hey, I have another physics question which goes as follows: Decide whether fusion or fission can release energy from the following options:

a) Led
b) Gold
c) Iron

and some others, but if someone can explain how to figure it out on 1 of the questions I will be able to do the rest myself. How can I know if they(fusion or fission) can release energy from the tasks or not? Cant seem to find anything about it in the book. Can anyone give me the explanation or a hint?

The answers if you are wondering:

a) Fission
b) Fission
c) None of them

9. ## Re: Need help with science/math schoolwork? Post here!

You should have a picture like this one in your book
From it you can see what happens to different elements.

10. ## Re: Need help with science/math schoolwork? Post here!

If you tried to fuse two iron nuclei (the most common isotope = 30 neutrons) you'd get tellurium except you'd be short on neutrons (stable isotopes of 72 to 74 neutrons). To be stable this new nucleus would need to draw in neutrons

If you split an iron nuclei in half you'd get two Aluminium nuclei (=14 neutrons x 2 = 28) + 2 extra neutrons which would get expelled to make the nuclei stable.

So I would say fission for iron. But I am just a laymen when it comes to nuclear physics.

11. ## Re: Need help with science/math schoolwork? Post here!

Originally Posted by Sphere
If you tried to fuse two iron nuclei (the most common isotope = 30 neutrons) you'd get tellurium except you'd be short on neutrons (stable isotopes of 72 to 74 neutrons). To be stable this new nucleus would need to draw in neutrons

If you split an iron nuclei in half you'd get two Aluminium nuclei (=14 neutrons x 2 = 28) + 2 extra neutrons which would get expelled to make the nuclei stable.

So I would say fission for iron. But I am just a laymen when it comes to nuclear physics.
It might be possible to fission/fusion iron but you won't get energy from it.

For example two Al 27 nuclei have 0.02347u greater mass than a Fe 54 nucleus. So you needed E=mc^2 to split it.

12. ## Re: Need help with science/math schoolwork? Post here!

Finally, a nuclear physics question...

It's a poorly written question to be honest. The important factor is the binding energy per nucleon, as edse already posted. Nuclei generally want to tend to their most stable configuration, which means the state in which their binding energy per nucleon is largest. So in general, fusion is exothermal below iron, whilst fission is exothermal above it.

However. Firstly, one should really specify an isotope. Iron is not actually the element which contains the nucleus with the highest binding energy per nucleon. That claim to fame belongs to nickel (a common misconception). So strictly speaking, one could fuse an isotope of iron with another nucleon to create a more well bound nucleus, which would therefore release energy.

You simply cannot deal with elements when it comes to these things, the isotope is important. For example, 4He is incredibly well bound, whilst 5He isn't.

[Fun fact] In fact, the only reason stellar nucleosynthesis ever makes it past the 4He stage is because of a resonance in 12C (basically 3 4He nuclei) which allows the 4He to fuse. 4He is so well bound that there are no stable mass 8 nuclei, because they all just fission back into 4He nuclei at the first opportunity.[/Fun fact]

13. ## Re: Need help with science/math schoolwork? Post here!

Originally Posted by The Norseman
Hey, I have another physics question which goes as follows: Decide whether fusion or fission can release energy from the following options:

a) Led
b) Gold
c) Iron

and some others, but if someone can explain how to figure it out on 1 of the questions I will be able to do the rest myself. How can I know if they(fusion or fission) can release energy from the tasks or not? Cant seem to find anything about it in the book. Can anyone give me the explanation or a hint?

The answers if you are wondering:

a) Fission
b) Fission
c) None of them
You need to look at the average mass per nucleon of each atom. If the mass is lower after fusion or fission, it means mass has turned into energy, and if the mass is higher, energy has turned into mass. So just compare the mass of the product to that of the reactants and you will figure it out.

14. Is anyone using mathematica or some similar program? I would need help to find a code that sums all the elements diagonally in a matrix. For example

1 2 3
4 5 6
7 8 9

would give the result {1,6,15,14,9}

15. ## Re: Need help with science/math schoolwork? Post here!

Would Matlab be okay?

Does it need to work for any size matrix, or just 3x3?

16. ## Re: Need help with science/math schoolwork? Post here!

Matlab is okay. And the actual matrix I want to do this with is 19x19 (that's why I don't want to do it by hand ) so it would need to work for any matrix.

17. ## Re: Need help with science/math schoolwork? Post here!

It is ugly but it works. Just edit the starting matrix

M = [1 2 3 4; 5 6 7 8; 9 10 11 12; 13 14 15 16]
D = size(M)
S = D(1,1)
E = S - 1
T = S + E

A = zeros(1,T)

for i = 1:S
for j = 1:i
K = j - 1
row = K + 1
col = i - K
A(1,i) = A(1,i) + M(row,col)
end
end

for i = 1:E
for j = 1:i
K = j - 1
row = S - K
col = S - K
B = T+1-i
A(1,B) = A(1,B) + M(row,col)
end
end

display(A)

18. ## Re: Need help with science/math schoolwork? Post here!

Originally Posted by Sphere
It is ugly but it works. Just edit the starting matrix

M = [1 2 3 4; 5 6 7 8; 9 10 11 12; 13 14 15 16]
D = size(M)
S = D(1,1)
E = S - 1
T = S + E

A = zeros(1,T)

for i = 1:S
for j = 1:i
K = j - 1
row = K + 1
col = i - K
A(1,i) = A(1,i) + M(row,col)
end
end

for i = 1:E
for j = 1:i
K = j - 1
row = S - K
col = S - K
B = T+1-i
A(1,B) = A(1,B) + M(row,col)
end
end

display(A)
Thank you. It didn't work perfectly for me ( maybe I wrote something wrong) but I got what I wanted from my matrix.

19. ## Re: Need help with science/math schoolwork? Post here!

A lead weight is disk shaped, and has a hollow center filled with ball bearings. It is dropped down a rod with grooves much like that of a screw. What formulas are relevant to finding the relationship between the force of gravity, the downward velocity of the disk, friction, and the torque (Wrong term?) of the disk?

If it's relevant, I need to find out how to maximize spin and minimize downward velocity.

20. ## Re: Need help with science/math schoolwork? Post here!

I need to write a paper about a digital alarm clock display.
It isnt a long paper ,6 pages at max, but truth be told . Though I have a deep interest in exact scineces , I lack any practical knowhow of how stuff works.
ATM for example I am trying to find out how the LED display works with this as serial name : LTC 637C1P
I alrdy found the datasheet here ,but I absolutly have no idea on how to continue on this. It doesnt tell me anything ,except giving me numbers. Could somebody point me to a specific document or book that contains the answer in unridling this?

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