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### Course: Physics archive>Unit 13

Lesson 2: Magnetic field created by a current

# Magnetic force between two currents going in opposite directions

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

## Want to join the conversation?

• How can the magnetic fields be going the same direction if the currents are going opposite directions? For current 1 your thumb points upward, but for current 2 it points downward. Should the magnetic field of the area between the wires be into the page and the area outside's come out?
• He did the first equation wrong it should be pointing the direction towards the left
(1 vote)
• so, if there're two wires which has current, the net force for a wire is only affected by the other wire's magnetic field? I mean, a wire is not affected by the magnetic field created by itself?
• Right. Just like you can't lift yourself up by your own belt with your own gravity. A wire can't make itself move with its own magnetic field.
• Among audiophiles, it is recommended that high voltage cables should intersect low voltage cables at 90 degrees. What is the scientific basis for this?
• Imagine a line running north/south, the field induced by the current is in rings perpendicular to the length of wire. If a wire runs through this field in the plane of the field (aka, perpendicular to our original wire) there will be no induced current from the magnetic field. There may be future videos that will accurately explain why, or you could look up Walter Lewin's lectures from MIT (excellent series of videos) for more detailed explanation why. Electromagnetic Flux = B dot product A (B for mag field, A for area), and dot product involves the cos of the angle between the field and length, and as cos(90) = 0, we can see there is no induced current.

This of course is using perfectly ideal straight wires, which you probably don't have, but getting them perpendicular will at least minimize the resultant interference.
(1 vote)
• you did not calculate F12. Does it always equal to F21?
• Yes, F12 is equal to F21 in terms of just values, but directions are opposites .
• In the question posed in this video shouldn't we take account of Newton's third law . If wire 1 is exerting a force on wire2 then wire2 must be exerting an equal and opposite force wire1 ?
• Yes, both wires pull on the other one equally
Just like the earth pulls on the moon just as hard as the moon pulls on earth.
(1 vote)
• This video only made me more confused. he drew his left hand, not his right hand. If he drew his right hand the force should be pointing to the left, right?
(1 vote)
• He drew a right hand, not a left hand. He said the top of the hand (not the palm) is pointing toward you. See the fingernail?
• Is there any DIRECT formula to calculate the force exerted by a current carrying wire on another current carrying wire?
(1 vote)
• Of course. You should be able to figure it out from watching the video.
You know that the force is IL x B, and you know what B from the other wire is, so plug it in.
• when you say mu, permeability of a material, why air, vacuum or the space around it? i know that we assume our space is air but shouldn't it be the wire, copper or the material it self? its it the material or the space around it that affect the magnetic field?
(1 vote)
• mu is the ability of the 'space around' the thing that produces the field to 'carry' or support that field.

ok??