Physics GRE Discussion Forums

# 69 in GR9677.............>link: http://www.physicsgrad.com/pgre/9677-69
why the answer is not (E)...i.e: no mag. field since they cancel each other in the region A
also i used the Right hand rule and they cancel each other not add up
what has went wrong???

Maybe you're using both hands in the process? (in which case one of your hands held the wrong direction)

Izaac wrote:Maybe you're using both hands in the process? (in which case one of your hands held the wrong direction)

actually iam pretty sure iam using my right hand which i eat with
u try it and u will find exactly as i say

johnhero2010 wrote:

Izaac wrote:Maybe you're using both hands in the process? (in which case one of your hands held the wrong direction)

actually iam pretty sure iam using my right hand which i eat with
u try it and u will find exactly as i say

No you won't.

When I apply the right hand rule, I find that the direction of the magnetic field is in the +y direction, NOT zero. So maybe try doing this again? If you are sitting at a desk, imagine the desk is the x-z plane: +x is to your right, +z is towards the computer, and +y is up towards you. For the wire on the right, use your right hand with your thumb pointing towards the computer. Your fingers curl up towards you, showing that the B field is in the +y direction. Now rotate your hand 180 degrees (for the wire on the left) and you will see that once again, your fingers point up towards you. So both wires contribute in the +y direction.

Current flows towards you in one wire and away in the other, brah.

Sometimes I wonder why people score poorly on the PGRE, but things like this make it less of a mystery.

TakeruK wrote:When I apply the right hand rule, I find that the direction of the magnetic field is in the +y direction, NOT zero. So maybe try doing this again? If you are sitting at a desk, imagine the desk is the x-z plane: +x is to your right, +z is towards the computer, and +y is up towards you. For the wire on the right, use your right hand with your thumb pointing towards the computer. Your fingers curl up towards you, showing that the B field is in the +y direction. Now rotate your hand 180 degrees (for the wire on the left) and you will see that once again, your fingers point up towards you. So both wires contribute in the +y direction.

thx alot...now i see what u mean but i have two problems
first: I was using the curled right hand as used in ampere's law to determine the mag field enclosing the current and all i got is a clock-wise mag field on the right shaded area and anti-clockwise mag. field on the left shaded area
2nd: what u say work fine but if the currents are in the same direction(which is not the case for this problem) i get also both mag. fields in the +Y direction which does not make any sense at all
so What is really wrong in my application?

bfollinprm wrote:Current flows towards you in one wire and away in the other, brah.

yeah sure now I believe u

kangaroo wrote:Sometimes I wonder why people score poorly on the PGRE, but things like this make it less of a mystery.

u have no idea!

johnhero2010 wrote: 2nd: what u say work fine but if the currents are in the same direction(which is not the case for this problem) i get also both mag. fields in the +Y direction which does not make any sense at all
so What is really wrong in my application?

It's more than just "clockwise" or "counter clockwise". I think in this case, you are not keeping the location of where you "measure" the B field constant. Try what I said above (desk = x-z plane etc.) but this time, draw a (imaginary?) line down in front of you, from your chest towards your computer. This is the "vacuum" area in the question, where we will measure the B field. Let's say both wires are going "into the page", i.e. +z direction. For the right wire, make sure your hand is to the RIGHT of the line and see what your fingers are showing where the line is. You should see your fingers curled up towards you, in the +y direction. Now, do the left wire -- move your RIGHT hand to the LEFT side of the line (since in the question, the wire is to the left of the measured area). Of course, your fingers are still curling the same way, but when you "sample" the direction of your fingers over the line, your fingers should be pointing DOWN, i.e. -y direction.

TakeruK wrote:

johnhero2010 wrote: 2nd: what u say work fine but if the currents are in the same direction(which is not the case for this problem) i get also both mag. fields in the +Y direction which does not make any sense at all
so What is really wrong in my application?

It's more than just "clockwise" or "counter clockwise". I think in this case, you are not keeping the location of where you "measure" the B field constant. Try what I said above (desk = x-z plane etc.) but this time, draw a (imaginary?) line down in front of you, from your chest towards your computer. This is the "vacuum" area in the question, where we will measure the B field. Let's say both wires are going "into the page", i.e. +z direction. For the right wire, make sure your hand is to the RIGHT of the line and see what your fingers are showing where the line is. You should see your fingers curled up towards you, in the +y direction. Now, do the left wire -- move your RIGHT hand to the LEFT side of the line (since in the question, the wire is to the left of the measured area). Of course, your fingers are still curling the same way, but when you "sample" the direction of your fingers over the line, your fingers should be pointing DOWN, i.e. -y direction.

thx alot Taker.....I do see now what was wrong with my application...thx