Still no confident with these answers.

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In summary, the conversation focused on various problems involving magnetic forces and fields. The first problem asked to rank the magnitude of magnetic forces on three equally spaced wires with different currents. The correct answer was 1 and 3 tie, with an unknown force on 2. The next problem involved the dependence of magnetic field on distance from a current-carrying wire, with the correct answer being 1/R. Another problem asked for a reasonable explanation for a charged particle following a clockwise spiral in a uniform magnetic field, with the correct answer being a positive charge speeding up. It was also discussed that electrons moving in a circle produce a magnetic field that is out of the page. The conversation then moved on to finding the force per unit length between two
  • #1
nautica
I appreciate all of the help I have received on these review problems. I feel pretty confident going into the test. Here are a few problems, I am still having trouble with. (some have been asked before, but I still don't understand)

11. The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank theses wires according to the magnitude of the magnetic forces, from least to greatest.


. x .
1 2 3

a) 1,2,3
b) 2, 1 & 3 tie
c) 2 and 3 tie, then 1
d) 1 and 3 tie, then 2 (I believe it is d, but am not sure)
e) 3,2,1

18) The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:
a) R
b) 1/R
c) 1/R^2
d) 1/R^3
e) 1/R^3/2

I choose 1/R b/c of this formula B=uI/2piR

20) A uniform magnetic field is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius. A reasonable explanation is:

a) the charge is positive and slowing down
b) the charge is negative and slowing down
c) the charge is positive and speeding up (I choose this one)
d) the charge is negative and speeding up
e) none of these

21. Electrons are going around in a circle counterclockwise direction. At the center of the circle they produce a magnetic field that is:

a) into the page
b) out of the page (I believe it is this one)
c) to the left
d) to the right
e) zero

24) Two parralle wires, 4 cm apart, carry currents of 2 A and 4 A resp;ectively, in opposite directions. The force per unit Lenght in N/m of one wire on the other is:

a) 1 x 10^-3 repulsive
b) 1 x 10^-3 attractive
c) 4 x 10^-5 repulsive
d) 4 x 10^-5 attractive
e) none

I choose c used this forumula F = uI1I2/2pid

25) An electron (q = -1.6 x 10^-19)nis moving at 3 x 10^5 m/s in the positive x direction. A magnetic field of 0.8 Tesla is in the positive z direction. The magnetic force on the electron is:

A) zero
b) 4 x 10^-14 in the positive z direction
c) 4 x 10^-14 in the negative z direction
d) 4 x 10^-14 in the positive y direction
e) 4 x 10^-14 in the negative y direction

I don't even know which formula to use on this one.

thanks
nautica
 
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  • #2
Originally posted by nautica
11. The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank theses wires according to the magnitude of the magnetic forces, from least to greatest.


. x .
1 2 3

a) 1,2,3
b) 2, 1 & 3 tie
c) 2 and 3 tie, then 1
d) 1 and 3 tie, then 2 (I believe it is d, but am not sure)
e) 3,2,1
No. 1 and 3 are tie, but what is the force on 2? Figure it out this way: What is the B field at 2 caused by 1? caused by 3?
18) The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to:
a) R
b) 1/R
c) 1/R^2
d) 1/R^3
e) 1/R^3/2

I choose 1/R b/c of this formula B=uI/2piR
Right.
20) A uniform magnetic field is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius. A reasonable explanation is:

a) the charge is positive and slowing down
b) the charge is negative and slowing down
c) the charge is positive and speeding up (I choose this one)
d) the charge is negative and speeding up
e) none of these
No. There are two parts to consider. First, what is the direction of the force: that is what determines whether it will spiral clockwise or counterclockwise. That depends on the charge. (F = q V X B) A positive charge will spiral counterclockwise. To find the size of the spiral, consider centripetal acceleration.
21. Electrons are going around in a circle counterclockwise direction. At the center of the circle they produce a magnetic field that is:

a) into the page
b) out of the page (I believe it is this one)
c) to the left
d) to the right
e) zero

No. Once again, you must know how a moving charge creates a magnetic field: magnitude and direction. Learn the right hand rule.
24) Two parralle wires, 4 cm apart, carry currents of 2 A and 4 A resp;ectively, in opposite directions. The force per unit Lenght in N/m of one wire on the other is:

a) 1 x 10^-3 repulsive
b) 1 x 10^-3 attractive
c) 4 x 10^-5 repulsive
d) 4 x 10^-5 attractive
e) none

I choose c used this forumula F = uI1I2/2pid
Right.
25) An electron (q = -1.6 x 10^-19)nis moving at 3 x 10^5 m/s in the positive x direction. A magnetic field of 0.8 Tesla is in the positive z direction. The magnetic force on the electron is:

A) zero
b) 4 x 10^-14 in the positive z direction
c) 4 x 10^-14 in the negative z direction
d) 4 x 10^-14 in the positive y direction
e) 4 x 10^-14 in the negative y direction

I don't even know which formula to use on this one.
The force on a moving charge in a magnetic field is:
F = q V X B
 
  • #3


Originally posted by Doc Al
#11 No. 1 and 3 are tie, but what is the force on 2? Figure it out this way: What is the B field at 2 caused by 1? caused by 3?
Right.

That is the way I looked at it, so I figured that 1 and 3 would have the least and 2 would have the most.


#20 No. There are two parts to consider. First, what is the direction of the force: that is what determines whether it will spiral clockwise or counterclockwise. That depends on the charge. (F = q V X B) A positive charge will spiral counterclockwise. To find the size of the spiral, consider centripetal acceleration.

As it moves closer to the center the acceleration will be faster

#21 No. Once again, you must know how a moving charge creates a magnetic field: magnitude and direction. Learn the right hand rule.

Thumb goes in the direction of the electrons which would make the palm face in, so if I curl my fingers the B field will come out of the page - what am I doing wrong.

25) The force on a moving charge in a magnetic field is:
F = q V X B


I am coming up with the right number using this formula, but the direction is confusing.
 
  • #4
25) I am coming up with the correct #, but I am confused about the direction of the force.
 
  • #5


Originally posted by nautica
#11 ...
That is the way I looked at it, so I figured that 1 and 3 would have the least and 2 would have the most.

Answer my question. What it the net magnetic field at 2?
#20 ...

As it moves closer to the center the acceleration will be faster
F=ma = qVxB=mV2/r
so... V/r is constant. If the radius is getting smaller, the speed must be getting smaller.
#21 ...

Thumb goes in the direction of the electrons which would make the palm face in, so if I curl my fingers the B field will come out of the page - what am I doing wrong.
Here's how I use the right hand rule: Hand spread out, fingers point in direction of V, palm in direction of B, thumb points towards force on a positive charge. Reverse the direction of force for an electron.
25) ...


I am coming up with the right number using this formula, but the direction is confusing.
Keep practicing the right hand rule until you get it straight.
 
  • #6
Right hand rules

This site may help: http://www.physics.brocku.ca/faculty/sternin/120/slides/rh-rule.html

There are several versions; pick one and stick with it. (I suspect you are just mixing up signs: positive and negative charges feel opposite forces.)
 
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  • #7
I think I got it. should be in the negative y.
 
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