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### Course: Physics archive>Unit 10

Lesson 1: Magnets and Magnetic Force

# Magnetic force on a current carrying wire

Sal derives the formula F=ILB to determine the force on a current carrying wire. Created by Sal Khan.

## Want to join the conversation?

• what if in f=BIL,length of the conductor is infinite .
• Ah, good question

If you look at diagrams of this situation you will see that the conductor may be infinite in length but the only part of the conductor that we are interested in is the length L which is only equal to the length of the conductor which is INSIDE the magnetic field.

OK?
• will there be deflection in the wire due to the force applied on it.
• I'm confused. Wouldn't the force point downward because current is the movement of elections... So we would have a negative current move right magnetic field going in to page and force down. OR in cases like this do we just always assume that the direction of current is positive charges moving in the currents direction?
• Current is defined as the direction positive charges are moving. So if you have electrons moving in some direction, the current will be moving in the opposite direction. The direction of the current in all formulas is always the direction of positive charge or opposite direction of negative charge.
• What exactly denotes time as a scalar quantity? How do you look at an equation and know exactly what is a vector quantity and what is scalar, other than memory?
• Scalar quantity is the quantity which has only magnitude but no direction. Vectors are those which has both magnitude as well as direction.
• If the formula (ilb) applies, then a particle never experiences a force because its length is zero! if its a formula for a conductor of finite length, then how could we derive a formula for a conductor carrying current from a formula that applies for a particle?
• F = IL x B right?
But I = q/t
and L = v*t where v is the velocity of the particle
Substitute those in and you will get
F = qv x B for a particle
• I am a little confused about the right hand rule as well. On video "magnetism 3", Sal drew a hand with the palm facing in while on this video the hand's palm was facing out. If Sal had drawn a hand with the palm facing in for this video, the direction would be different. How do I know which way to face my hand palm when applying the right hand rule?
• When applying the right hand rule to a F = q(v x B) equation, start by pointing your fingers in the direction of the particle's path (v), and have your palm face the direction of the magnetic field (B). This orientation should give you the proper direction of the Force when you give the thumbs-up sign.

Does this help?
• If you were to move a wire through an electric field, would it produce a greater force than a stationary wire?
• It should, because even if we are dealing with current here there'll be opposing forces relative to the velocity, I don't know it we can somehow relate it to the electric flux or something.
(1 vote)
• what is the direction of drift velocity in current carryin wire?