# Physics

1)A wire is carrying a current, i, in the positive y-direction. The wire is located in a uniform magnetic field, B, oriented in such a way that the magnetic force on the wire is maximized. The magnetic force acting on the wire, FB, is in the negative x direction. What is the direction of the magnetic field?
a) positive x-direction
b)negative x-direction
c)negative y-direction
d) positive z-direction
e) negative z-direction

I'm not really sure how the right hand rule 3 works for this but I just put my thumb in the direction of i and my fingers were curving downwards, so I pick e) but I'm not sure.

2. Two solenoids have the same length, but solenoid 1 has 15 times the number of turn, 1/9 the radius and 7 times the current of solednoid 2. Calculate the ratio of the magnetic field inside solenoid 1 to the magnetic field inside solenoid 2.
a) 105
b) 123
c) 144
d) 168
e) 197

I'm really not sure how to do this, I started by just plugging random numbers in for the equation B=(mu_naught*i)/(2pi*r)
I just plugged 2 for N, 3 for r, and 5 for i, and calculated the magnetic field for both and divided B of s2 by B of s1 and got 1.05E-3, I thought that that was right since I looked on the back of the book and the answer is in fact a) but I later noticed that I was using the equation for a toroid not a solenoid, since radius doesn't even affect the B of solenoids. I did the same thing with the equation B=(mu_naught*i)*(N/L) and (with L=15) got a number that wasn't on the choices, so I really don't know how to do this. Help please.

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1. 1. First of all, assume the z direction is what you get as the cross product of the x and y unit vector directions, using the right hand rule. The magnetic force will be in the i X B direction, which is the direction of the cross product of y and -x unit vectors. That will be the +z direction, according to the right-hand rule

2. See http://plasma.kulgun.net/sol_page/
for the B field of a solenoid. it does not depend upon the radius. For two solenoids of the same length, the B field is proportional to the number of turns and the current. Thus solenoid 1 has a field 15 x 7 = 105 times stronger than solenoid 2.

Thank you for showing your work. Almost no one does here anymore, although we keep encouraging students to do so. When tutors are in short supply here, students who show their work or thought process are given priority.

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posted by drwls

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