If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Force on a current carrying conductor in a magnetic field

Current carrying wires can push magnets. Can magnets push current carrying wires too? Let's find out. Created by Mahesh Shenoy.

Want to join the conversation?

  • piceratops tree style avatar for user Naveen Sundar
    But why does force depend on angle of wire ?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • starky seed style avatar for user Dishita
      Hey!
      Before I try to answer your question(which is awesome!) I'd like to make a couple things clear-----My answer is just a logical approach configured by me and I'm in 10th grade (India) so it's not exactly technical but it helped me to understand why(more or less)---

      Now, here's my approach
      > Try actually drawing the diagram and incline the wire
      > In addition draw the concentric circles as well(magnetic field in a wire)-abt 2 for every point on the wire which touches the magnetic field lines produced by the 2 poles(parallel)
      (I know..fun ;))

      What can we observe
      (Try it out yourself and then check out the rest)

      > We can observe that when the wire is perpendicular, the magnetic field produced by the wire is literally lying on the magnetic field/s produced by the poles
      keeping in mind that this is actually in 3-Dimensions and not in 2-D we can conclude/imagine that all the points on the magnetic field of the wire would experience a force by the magnetic fields of the poles,

      > now, if we incline it, the area of the interaction between the magnetic fields decreases(remember there are gaps/spaces between 2 field lines

      that's it!

      by 'wire' I meant current carrying wire/conductor and force exerted/experienced depends on the region of interaction of the magnetic fields.

      Hope this helps,

      P.S;again I'm stressing on the point that I'm not sure if this is like 'The explanation', this is just how I was able to reason this out,
      hope this appealed to your imagination as well
      Onward!
      (8 votes)
  • old spice man green style avatar for user Dutch Man
    If we pass a current passing wire in between two magnets which obviously have magnetic fields , don't the magnetic field of current carrying wire and magnets field collide . If not how does the image look like ?
    (5 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user aradhyaraina15
    I have a doubt here
    .why force doesn't increase with decrease of current
    Let's take a setting

    The B is in direction perpendicular to current flowing in a wire (current north and the external B west)
    so shouldn't the force be in the same direction as the B?
    For this particular force on conductor situation In my textbook ,the force's direction is from north to south pole of the horseshoe magnet.
    " keep the things above in mind for now"

    The wire(from our setting) has its own magnetic field which interacts with the external field applied on it
    If the current will decrease, the field would be less stronger now and would lead to more domination by the external current causing it to feel more force. But instead, it feels lesser force.


    I know that there is equal and opposite force but we didn't change the external field nor position of our wire to cause lesser force b/w them
    .

    Would someone bother to help me get out of this confusion
    (2 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Isteak Ahamed Imon
    At , you mentioned Newton's third law. As far as I know, Newton's third law is only applicable for forces, not for other stuffs. So is it valid to explain these magnetic stuffs using that?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Abhi
      Yes newtons third law is applicable for all type of forces, and even the current carrying wire is applying force on magnetic needle, so there should be equal and opposite rxn for applied force acc. to newtons third law.
      (2 votes)
  • blobby green style avatar for user abhiram.7554
    What is the difference between the Right Hand Thumb Rule and the Flemings Left Hand Rule, because I felt that to, find the direction of the current and the direction of the magnetic field in a straight current carrying conductor we use the Right Hand Thumb Rule but can't we use the Left Hand Rule to find the direction of the current and the direction of the magnetic field ? I didn't understood the relation between these two things .
    (1 vote)
    Default Khan Academy avatar avatar for user
    • leaf blue style avatar for user Jyotiraditya Pradhan
      Hey Abhiram!
      The difference between the right-hand thumb rule & Fleming's left hand rule is that they are used for different purposes in different scenarios.

      Right Hand Thumb Rule is used when we know the direction of current and wish to find out the direction of the magnetic field.

      On the other hand, Left Hand Rule is used when we know the direction of current as well as magnetic field, and wish to find out the direction of the force exerted

      As you are mentioning, we can also use the Left Hand rule to find out the direction of current/magnetic field, provided we know the direction of other two factors (Force & Current/Magnetic field). But we would rather use the right hand thumb rule to find it as we need to know the direction ofonly one of the two factors here(Current/Magnetic field).

      Hope it helps. Feel free to comment if your doubt persists...Have fun learning :)
      (2 votes)
  • eggleston blue style avatar for user Mia  Rose
    At , how are the field lines almost parallel? We know that outside the magnet, the field lines are like semi-circles of sorts, are not equidistant and go from North to South. Why is it different in this case?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • leafers tree style avatar for user Kalpit Vishnoi
      They are in that way cause they want to go to the south pole and as the south pole of a bar magnet is below the magnet, they go that way but in this example, the south pole of another magnet is placed in front of another north pole so they go straight.
      (1 vote)
  • ohnoes default style avatar for user sudhanshu singh
    I think you have drawn the direction of magnetic field wrongly AFTER THE TIME OF 1'32"
    (1 vote)
    Default Khan Academy avatar avatar for user
  • male robot hal style avatar for user Akshat kumar Singh
    isnt the direction of magnetic field at is reversed?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • piceratops tree style avatar for user Naveen Sundar
    Isn't the magnetic field in closed loops? How does these just vanish?
    (0 votes)
    Default Khan Academy avatar avatar for user
    • leafers tree style avatar for user Roopjot Kaur
      According to me, the magnetic field is always in hoops. But in this part we are referring to an electromagnet. The magnetic field will disappear when the current is stopped or when the current carrying wire is parallel to the magnetic field.
      I hope this helped!
      (3 votes)
  • blobby green style avatar for user Yashvi Kudrigikar
    Does the magnetic field of the current generate any effect on the two bar magnets here at ?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

in a previous video we saw that if you run an electric current through a wire then it can deflect a magnetic needle kept close to it this means a current carrying wire can push on a magnet but Newton's third law says that for every action there must be an equal and opposite reaction so if a current carrying wire can push on a magnet then that means a magnet can also push on a current carrying wire so this wire must also experience a force due to this magnet and in this video we are going to explore the properties of this force well first let's think about how the why the magnet is pushing on the wire well remember how we said the wire was pushing on the magnet we said that whenever a wire carries current it produces its own magnetic field and we said it's that magnetic field that pushes this magnet now similarly how can we answer the question how does the magnet push on the wire we can say the magnet also produces its own magnetic fields let me get rid of the field of the wire and let me bring back that needle so the magnet also produces its own magnetic field which may look somewhat like this and it's that magnetic field that is pushing on this wire so this means in general we can say that whenever we have a current carrying wire or a current carrying conductor in a magnetic field it experiences a force so magnetic fields not only push magnets but they can also push current-carrying wires and that kind of makes sense because we've seen in previous videos that current-carrying wires sort of kind of behave like a magnet and so it makes sense that they would be affected by magnetic fields and this is awesome because now magnetic fields can push on current-carrying conductors and make them move this means we can now convert but you can now use electricity to make something move and this is the principle behind your electric fans or your electric cars or any electric machines where we use electricity to move something it is by using this principle you just place it in a magnetic field and the magnetic field will push that conductor but before we can do anything useful with this let's first learn the properties of this force like what does that force depend on what direction is that force acting over here things like that and so to figure this out people performed experiments in which they place this wire in a uniform magnetic field not like this a field where the field strength and the field Direction is same everywhere over the wire so that we can understand exactly how the field affects the wire you can see over here the field on this wire is not uniform its Direction is also continuously changing over the wire right so let's place the wire in a uniform magnetic field and see how it affects the wire so to produce a uniform magnetic field all we will need are two giant poles of a magnet and so now if we look at the field in between these two poles you may recall the field always starts from north and ends into the south outside the magnet the field will look somewhat like this and you can see that in this region the field is pretty uniform you can see the direction of the field is same everywhere and the field the crowdedness of the field lines is also same pretty much they are pretty much equidistant everywhere right so the field strength is also same everywhere over here and so now we can place a current carrying wire over here and for former experiment and if you're wondering how do we get to polls like this well then you can do two things either get to giant bar magnets one for this and one for that and keep it over here or we can use something called a horseshoe magnet which looks like a horseshoe which is shown over here and you can see the poles of a horseshoe magnet are apart just like what we need all right so now let's place our current-carrying wire in this magnetic field you can pass the current through it by attaching it to a battery which have not shown over here and then we can perform experiments and see what does that force depend on the first thing people wanted to check is how does the force depend on the strength of the current and I want you to sort of guess this what do you think will happen to the force acting on this wire if the strength of the current was to increase what do you expect well we know that a current carrying wire is sort of acting like a magnet and that's why it's being affected by the magnetic field right now if you increase the current then it will become sort of like a stronger magnet isn't it so you would expect the force to increase I think that's exactly what we found out we found out so this is a result number one from what experiment is that if you put more current through that wire it'll automatically experience more force so you can increase the force just by increasing the current if you decrease the current the force will decrease if you make the current zero the force will vanish the second result is not so obvious what they found is that the force acting on this wire not only depends on the current but it also depends upon the angle between the wire and the magnetic field what they found is that when that angle is perpendicular when the wire is perpendicular to the field like we have kept over here that's when the force is maximum so right now the force on this wire is max but if you were to keep our wire this way place the wire this way then you don't know the current is the same and everything else is the same turns out that the force will reduce because the angle has decreased this is very important and this is not at all obvious but experiments show us that and if you were to decrease this angle further the force will decrease just look at this angle angle between the wire and the field the force will keep on decreasing and eventually if the angle becomes zero if we make our wire parallel to the magnetic field the angle becomes zero the force vanishes the force becomes zero I've not written it down but we can remember this this is super important to remember so the force depends on the angle if it's zero the force is zero as the angle increases the force increases and when the angle reaches 90 degrees that's when the force turns out to be maximum the last thing we'll talk about is the direction of the force acting on this wire now we will figure out the direction of the force in a separate video all right we'll do that in complete detail in a separate video but the question over here is what do you think the direction of the force depends on again pause the video and think about this for a while all right well experiment shows us the direction of the force acting on this wire depends on two things it depends on the direction of the current in which direction the current is going so if the current is going upwards the direction of the force would be in one direction if the current is going downwards the direction of the force will reverse and it also depends upon the direction of the field if you reverse the field turns out again the direction of the force will reverse but don't worry too much about this Third Point as I said we will discuss about this in great detail in another video so what did we learn in this video we learned the current-carrying wire placed in a magnetic field experiences a force and using this we can now convert electricity to motion and we learned a couple of properties of this force we saw that more the force more more the current more the force and we also saw that the force is maximum when the wire is kept perpendicular to the direction of the field