# crazy mcat physics question

#### synapse

##### Senior Member
10+ Year Member
7+ Year Member
I = + y direction
Flow = + y direction

Every current-carrying wire produces a magnetic field. Imagine that a wire is next to your skin and parallel to one your arteries. What is the direction of the force on the positively charged ions in your blood?
A. +X B. -X
C. +Y D. -Y
E. +Z F. -Z

10+ Year Member
Toward the wire.

#### Parscope

##### Junior Member
10+ Year Member
5+ Year Member
Shrike said:
Toward the wire.
I think this would be the case if a negative charge were placed next to the artery, but the question deals with the B-field created by a current flow.

------------I------------------->
==============artery======>

Using the right hand rule (thumb in direction of current, B-field follows curled fingers), we can determine that the B-field from the current carrying wire is pointed in to the page on the side of the wire with the artery, and out of the page above the artery.

Therefore, I think the force on a posative charge in the artery will be in the same direction as the B-field, and be directed out of the computer monitor.

##### Why so Serious?????
15+ Year Member
Parscope said:
I think this would be the case if a negative charge were placed next to the artery, but the question deals with the B-field created by a current flow.

------------I------------------->
==============artery======>

Using the right hand rule (thumb in direction of current, B-field follows curled fingers), we can determine that the B-field from the current carrying wire is pointed in to the page on the side of the wire with the artery, and out of the page above the artery.

Therefore, I think the force on a posative charge in the artery will be in the same direction as the B-field, and be directed out of the computer monitor.
According to the right hand rule:

thumb is the direction that the particle travels.
Fingers are the magnetic field
Palm is the force

##### Senior Member
10+ Year Member
5+ Year Member
Shrike said:
Toward the wire.
why towards the wire.

wouldn't you need to know if the wire was on the left or right side?

I do not get this question errr!

#### Shrike

##### Lanius examinatianus
10+ Year Member
Parscope said:
I think this would be the case if a negative charge were placed next to the artery, but the question deals with the B-field created by a current flow.

------------I------------------->
==============artery======>

Using the right hand rule (thumb in direction of current, B-field follows curled fingers), we can determine that the B-field from the current carrying wire is pointed in to the page on the side of the wire with the artery, and out of the page above the artery.

Therefore, I think the force on a posative charge in the artery will be in the same direction as the B-field, and be directed out of the computer monitor.
You were doing fine, at first. The paragraph in which you determine that the magnetic field is into the page at the artery (in the diagram you've drawn) is correct.

So now you are at RHR 1. Fingers = Field, so stick your fingers into the page. Thumb hitchhikes in the direction of motion, so your right thumb is facing to the right. Your palm now shows the direction in which the (positive, per the problem) charge is pushed. Up. Toward the wire.

It's all symmetrical relative to the wire, so it is the same everywhere, but convince yourself. Using RHR 2, the field above the wire is out of the page. Now apply RHR 1 at that spot -- fingers out of the page, thumb to the right. Your palm, showing the direction of the force, is facing down -- toward the wire.

#### Shrike

##### Lanius examinatianus
10+ Year Member
why towards the wire.

wouldn't you need to know if the wire was on the left or right side?

I do not get this question errr!
Key to these is to relax, and to practice. Pick an orientation for the wire. Pick a spot next to it. Any spot. Do right hand rule -- in this case twice, the first time to find the magnetic field, the second time to find its effect. (See above for details.) Now picture the current running the other direction, and try it again; convince yourself that it always works. After sufficient practice, it will be comfortable if probably not intuitive (magnetism just isn't intuitive).

##### Senior Member
10+ Year Member
5+ Year Member
Shrike said:
Toward the wire.
Ok my stupidity momentarily left me and I understand the rhr approach, but then bam it came back. Which answer would correspond to "toward the wire ?"

thanx!

#### Mediculous

##### The Verbal Herman Munster
10+ Year Member
15+ Year Member
Ok my stupidity momentarily left me and I understand the rhr approach, but then bam it came back. Which answer would correspond to "toward the wire ?"

thanx!
I think Shrike didn't give a cartesian direction because the question doesn't specify whether the artery is to the left or right of the wire, rather it is merely parallel. The force is towards the wire, but which x-direction can't be determined without knowing what side of the wire the artery is on.

##### Senior Member
10+ Year Member
5+ Year Member
Mediculous said:
I think Shrike didn't give a cartesian direction because the question doesn't specify whether the artery is to the left or right of the wire, rather it is merely parallel. The force is towards the wire, but which x-direction can't be determined without knowing what side of the wire the artery is on.
thanx, yeah thats what I thought in the first place!!! you need to know on which side the artery is.

#### Shrike

##### Lanius examinatianus
10+ Year Member
Mediculous said:
I think Shrike didn't give a cartesian direction because the question doesn't specify whether the artery is to the left or right of the wire, rather it is merely parallel.
Correct. Also, on the real test the answer choices would have been arrows, with much less likelihood of confusion.