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### Course: 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?

• 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.
• 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)
Where is the B-field pointing?
This is a haiku.
• 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?

Moving electric charges create magnetic fields. (Another way to say "moving electric charges" is current. )

Moving magnetic fields create current.
• at , how shall we decide that we should consider the direction of our middle finger because it seems as though we can take either the field to be going in the screen or coming out of the screen?
thanks
• I would like to continue this question. Why for the left wire did he stick his middle finger out of the plane and not in?

in other words, how do we know when to decide when to stick the finger inwards towards the screen or out?
• Why can't we see that attract of wires in real life?
(1 vote)
• You mean in wires used in home appliances ? they're coated with insulating material (plastic/rubber) so that they don't cause electrical mishaps. This material also cancels forces between parallel wires
• Can (suppose 2) Magnetic Fields exist in a space without doing anything to each other?
• The magnetic field is a vector, so it has only one value at any point in space. If you are imagining the fields created by two magnets, you have to add them up to find what the actual field around those magnets looks like. At every point in space, the field will be the sum of the field from magnet A plus the field from magnet B.
• When it is said "the current is moving to the right" does that mean that protons are moving to the right in the wire?
(1 vote)
• No, protons don't move because they are in the nucleus.
Current moving to the right means electrons moving to the left.
• I don't really understand the thing about the direction that your fingers are meant to go. Is it a right hand rule or a left hand rule? I am actually very confused.
(1 vote)
• Forget every hand rule you learnt , and do this
Learn the equations in terms of cross product.
Use your right hand , your four fingers point to the direction of first vector and curl them into the direction of second vector , the thumb points towards the cross product.

It can be used for all equations with cross product
For example: Force due to magnetic field on current carrying wire. F=i(l x b) (cross product form)
place the four fingers (except thumb) in the direction of length of conductor( direction of current) and curl them up into the direction of magnetic field.
Hence you get the force direction