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## Physics library

### Unit 13: Lesson 2

Magnetic field created by a current

# Magnetic force between two currents going in the same direction

Sal shows how to determine the magnetic force between two currents going in the same direction. Created by Sal Khan.

## Want to join the conversation?

• I don't disagree with this as it correlates exaclty with whats in my text books.
However from my limited understanding of the law of superposition - the magnetic fields between the wires should cancel each other out - so why do the wires still experience a net attractive force. The mathematical analysis seems to take consider one wire at a time. I think I have may have my logic in a twist. Please help :) •   It depends on where you evaluate the field. It's true that along a line exactly between the (infinitely long) wires, the magnetic field will cancel. [Recall that the field on wire 1 due to wire 2 is s B=mu0*I2/(2pi*r) and corresponding for the other wire, so if I1=I2 they will cancel where the distance between the two is equal.] But along wire 1, say, only the field from wire 2 will contribute to the force on w1. (It may seem like we'd have an infinete field at the conductor, but actually, the field within the conductor is always zero! The force on each wire is in this classical view due to the fields at their surfaces.)
• I am really confused cause in my textbook it's soppose to be fleming's left hand rule ......left hand , but you r doing it on th right hand ,!
so is my text book wrong or or have u made an error •   I had exactly the same problem. Wikipedia told me that.. when you are using the left hand rule, you are using the direction in which the electrons are moving. This direction is exactly the opposite of the current direction... I mean, electrons move from - to + and current (by convention) goes from + to -.
So, when you use your left hand, you are thinking about how are the electrons moving in reality With your right hand, you follow the direction of current, which would bethe exact opposite.
Hope this helps :)
• Since the magnetic field is pointing both in and out of the screen,depending on which side of the wire you reference, how do I determine the correct side to use as the direction of B? • After some reading in my physics text, I'll answer my own question.

To decide which direction of the magnetic field to use, hold the wire in your right hand, thumbing pointing towards direction of conventional current, and curl your fingers. The direction that "hits" the other wire in question is the one to use.
• Is he using the right or left hand rule to determine the direction of the force on the wire? I had tried using both and was able to get the required force but am not sure if that was by fluke or what:P • You can use the right hand rule or Fleming's Left hand rule. It's usually considered less confusing to use the right hand rule and change your signs and directions as needed. Fleming's left hand rule is usually for motors or current carrying conductors (which this wire is). I was taught to just use the right hand rule and change signs to avoid that confusion. Alternatively you could use the i,j,k wheel to determine the cross product direction. :)
• At , why is Sal saying that L goes in the same direction as current. My intuition tells me that the current is going through the L of the conductor and how can length have a direction? • In addition to the comment below...F=I*LxB and LxB is ILI*IBI*sin<LB, since the wires are parallel, the angle <LB = 0, and the sin(0) equals 0 so why doesnt become the whole equation 0?
(1 vote) • at , why do we use the magnetic field strength 'B' of wire 1? shouldn't we use the 'B' of wire 2 since we use I2 and L2? • A constant current produces a constant magnetic field, but a constant magnetic field produces no current at all. The magnetic field has to be changing with time to induce a current.
Have I got this right?   