# Confused about applying the Right Hand Rule (RHR)

• bluesteels
I get that the force point toward meFrom the direction of the current in each wire, you should be able to find the direction of the force.f

#### bluesteels

Homework Statement
You are given two infinite, parallel wires each carrying current I. The wires are separated by a distance d, and the current in the two wires is flowing in the same direction. This problem concerns the force per unit length between the wires.

Is the force between the wires attractive or repulsive?
Relevant Equations
none the answer is attractive. but i don't see how this is how I draw mine and I use the RHR on the magnetic field between the two wire and I got the force is opposite

Homework Statement:: You are given two infinite, parallel wires each carrying current I. The wires are separated by a distance d, and the current in the two wires is flowing in the same direction. This problem concerns the force per unit length between the wires.

Is the force between the wires attractive or repulsive?
Relevant Equations:: none

and I use the RHR on the magnetic field between the two wire and I got the force is opposite
Please be more specific. Describe what you did in detail.

• Delta2
Homework Statement:: You are given two infinite, parallel wires each carrying current I. The wires are separated by a distance d, and the current in the two wires is flowing in the same direction. This problem concerns the force per unit length between the wires.

Is the force between the wires attractive or repulsive?
Relevant Equations:: none

View attachment 299157

the answer is attractive. but i don't see how this is how I draw mine and I use the RHR on the magnetic field between the two wire and I got the force is opposite
Not sure how you got to "the force is opposite". Do you mean the field is opposite? A field is not a force.
Try considering the field due to one wire and the force that results in on the other.
The way I think of it, just as a memory aid, is that the total field wraps around both wires, and field lines like to shrink in length, spread in width. So that pulls the wires together.

@haruspex The lines have width?
If you pick some arbitrary relationship between line separation and field intensity then, yes, they effectively have width.
E.g. draw field lines between two opposite and equal charges. As you move laterally away from the line joining them, the lines get longer but more widely spaced. I think of this as a balance between tension along the lines, tending to keep them short, and repulsion between the lines.

Oh, but what you describe is the distance between the lines. I missread your post to mean that the width of the lines increases.

but i don't see how this is how I draw mine and I use the RHR on the magnetic field between the two wire and I got the force is opposite
There are actually two laws (or RHR, as you call them) that have to be used. First, you find the direction of the magnetic field, which you did correctly. Next, you have to apply another law to find the forces. Can you tell us how you found the directions of the forces from directions of the current and the magnetic field?

• nasu
There are actually two laws (or RHR, as you call them) that have to be used. First, you find the direction of the magnetic field, which you did correctly. Next, you have to apply another law to find the forces. Can you tell us how you found the directions of the forces from directions of the current and the magnetic field?
sorry this is late response so look at I1. I point my hand to the right and i curl my hand down cause the magnetic field is going down between the two rod. Then i get that my thumb is pointing away.

Same with I2 i point my hand to the right and curl up cause the magnetic field is going up between the rod then i get the force point toward me

Please be more specific. Describe what you did in detail.
ok so for the first rod the top magnetic field is out and bottom is into the page.

I point my right hand to the right and curl them down because the magnetic between the 2 rod for the first rod is down. Then I get that the position is going up

Same with rod 2 I did the same but I curled up cause the magnetic field between the 2 is going up then I get that the force is going down.

So that how I got that drawing

Last edited:
sorry this is late response so look at I1. I point my hand to the right and i curl my hand down cause the magnetic field is going down between the two rod. Then i get that my thumb is pointing away.

Same with I2 i point my hand to the right and curl up cause the magnetic field is going up between the rod
Alright, that explains how you got the direction of the magnetic field created by each current-carrying wire.
then i get the force point toward me
How do you get the direction of the force? You should be using a different law (a different RHR) to get the direction of the force. ##\vec{F}=I \vec{L}\times\vec{B}##.

Alright, that explains how you got the direction of the magnetic field created by each current-carrying wire.

How do you get the direction of the force? You should be using a different law (a different RHR) to get the direction of the force. ##\vec{F}=I \vec{L}\times\vec{B}##.
yeah that is how i get the direction of the force. Maybe my teacher teaches it different. i got the direction of the magnetic field by pointing my thumb in the direction current and curl my finger.

For the direction of the force I point my hand (Like index to pinky) to the L then I curl down in the first case the magnetic is down so the force is like point toward the screen. that like +y direction depending how you do your x,y,z coordinated.

yeah that is how i get the direction of the force. Maybe my teacher teaches it different. i got the direction of the magnetic field by pointing my thumb in the direction current and curl my finger.

For the direction of the force I point my hand (Like index to pinky) to the L then I curl down in the first case the magnetic is down so the force is like point toward the screen. that like +y direction depending how you do your x,y,z coordinated.
If you have two current carrying wires like you have, wires, 1 and 2, the force on wire 1 is due to the field cause by which wire?

in the first case the magnetic is down
When I look at your drawing I see that you have (correctly) labelled the directions of the magnetic field ##\vec{B}## as either into or out of the paper in the vicinity of each of the current-carrying wires. I'm not sure what you mean by the "first case" but let's assume you're speaking of the wire that carries the current ##I_1##. At the location of this wire (see your diagram), what is the direction of magnetic field due to the wire carrying current ##I_2##?

Do you know how to find the direction of the magnetic force given the directions of the magnetic field and the electric current?