# PHYSICS

Two thin, infinitely long, parallel wires are lying on the ground a distance d=3cm apart. They carry a current I_0=200A going into the page. A third thin, infinitely long wire with mass per unit length lambda=5g/m carries current I going out of the page. What is the value of the current I in amps in this third wire if it is levitated above the first two wires at height h=10cm above them and at a horizontal position midway between them?

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1. draw the forces, there's repulsion between the two equal wire against the third one

We have:
F = [uo.I1.I2/(2.pi.d)]*Ltotal

2*F*sin(a)= m*g = lamba*Ltotal*g

sin(a)=h/0.5d

Replace the values and you'll I.

By the way, have you done the one with the thick metal slab?

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2. Hi fima, In above formula, I did not find where is I. How to solve it? I_1and I_2 both are 200A right? But where is I? answer shoud be negative due to repulsion? Thank you very much for your help.
I did mistake continuously Q1 and Q2, and this is my third question. I am just afraid to try any other question and I don't get any clue to guess the answer.

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3. Hi Lora,

I1 and I2 are the currents, so one is 200A, and the second is the one you are looking for. So, you have to solve the equation.

And forget about the sign, the repulsion force created by the 2 wire are in equilibrium with he weight.

Well, and the question: which is the bigger B created by a cable?

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4. fima, I am confused in L_total..what is L_total?

I am still not doing bigger B created by cable one, but I just try average current one. I thought it is B*l*v but I got wrong. If I can solve bigger B, of course I will let you know.
Thanks

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5. L total is just reference, since you don't have the lenght of the wire, but you need it for the total mass, and the Force is by lenght unit, it will go, just write it down.

F/L=uo*I1*I2/(2*pi*d), that L will be Ltotal. and consdier that the three wire have the same lenght

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6. whats h? is that the height 10 cm?

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7. I just tried and L was cancelled and I got 2.812 but it is wrong..can you tell me answer please? I appreciate your help.

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8. I know asking answer is not good at all but I got only 20% in fina till now..just worried to pass

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9. That seems really small.

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10. hey guys can you help me in theta of hinge one question?

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11. This is the important eq.
F/Ltotal = [uo.I1.I2/(2.pi.r)]

r = sqrt(h^2+d1^2), d1=1.5 cm & h=10cm
sin(a)=h/r

2*F*sin(a)= m*g = lambda*Ltotal*g

in this last Eq replace all values, and be careful with the units...

2*uo*I1*I2/(2*pi*r)*h/r=lambda*g

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12. anyone with this q:
A straight copper wire that is 1 milimeter in diameter carries a current of 20 miliamps. Whats the magnitude of the largest magnetic field created by this wire in Tesla?

I think B=uo.I/(2*pi*r)=8*10^-6

B at the perimeter, but I used r^2 instead of r, so I don't want to waste my last chance, anyone has tried that value? or feel like try it out and let me know if that is the correct value?

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13. Hi flo,
by using this formula, I found B= 40*10^-7..How did you find 8*10^-6?
I used I=0.02A, r=0.0005m.

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14. Hi flo, yes it is right. I forgot to multiply by 2

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15. Hi Lora, so the answer 8*10^-6 is right? you got the green check?

And did you get the answer for current in the third cable?

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16. Yep, that's the value.
@Lora, did you get the answer? Yesterday I could get into edx, I=63.7

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17. yes flo I got green check, but for current in third cable,when I saw answer I was already tried different no. and got incorrect..:(

Can you guys tell me which one is correct answer for theta?

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18. I'm on that question too, but i'm not sure how to start that problem....

and the question with the moving rod???!!!

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