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### Course: Physics library>Unit 13

Lesson 2: Magnetic field created by a current

# Magnetic field created by a current carrying wire

See how a wire carrying a current creates a magnetic field. Created by Sal Khan.

## Want to join the conversation?

• A magnetic field creates a force on a moving charge and a moving charge creates a magnetic field which would then create a force on a moving charge..., right? How does stationary charge create its own static electric charge like Sal is talking about at ? Did I misunderstand him? I am trying to grasp how electricity and magnetism are the same. Thanks for your help! :-)
• Electric fields originate from any charge (moving or not) and changing magnetic fields. Magnetic fields originate from moving charge (ie. current) and changing electric fields. A charge will not interact with the field it generates itself.
• Hello. How does the electromagnetic wave carries energy?
• An electromagnetic wave is a vibration in the electric and magnetic fields. Like vibrating a rubber band takes energy the vibration of the electric and magnetic fields requires energy.
• Why does inserting a iron rod inside a solenoid multiplies the magnetic strength
• The magnetic field from the solenoid aligns the magnetic domains inside the iron along the same direction. This domain alignment then creates a permanent net magnetic field inside the iron which supplements the external mag field from the solenoid since they are both aligned in the same direction.
• How does one get the net magnetic force generated by two of such current-carrying wires at a given point?
• Superposition. Solve for the field from each wire separately and add them together to get the net field.
• Hi, could someone please explain "popping out of the screen" and "into the screen"?

Thanks!
• "[P]opping out of the screen" means pointing toward the viewer, and "into the screen" means it points away from the viewer.
• Which right-hand rule governs a magnetic emitted from a wire? (first, second, Third?ect.)
(1 vote)
• I never heard of numbering them. The right-hand rule for a wire is point your thumb in the direction of the current, wrap your fingers around the wire, and they will point in the direction of the magnetic field.
• if the magnetic field due to current is produced in circular form...the why the magnetic field of bar magnet is not produced in circular form(it starts from north and ends on south) ?
• The shape of the magnetic fields depends on the geometry and characteristics of what is producing it. A magnetic field produced by an electric current traveling thru a straight cable will take a circular shape around the cable. A magnet, on the other hand, is a dipole, and produces a magnetic field that starts at the north pole, curves down and ends at the south pole.

There are many other shapes for magnetic fields, for example, if you have a helical metal coil (like a spring) with a current thru it, the magnetic field in the centre of it will be uniform and lineal following the centre of the spring.
• why do two magnetic lines of force never intersect each other ??
• Gravitational field lines never cross, electrical field lines never cross, and magnetic field lines never cross. It's all for the same reason: all these fields are defined by the force they apply to some object placed in the field (a mass, a charge, or a small test magnet). In all cases, the force will have one unique direction at any point in space, not two or more. The direction of that force defines the field line at that point. Balls always "know" which way to fall, charges always "know" which way to move, and test magnets always "know" which way to point. You will never find a point in an electric field where you can place a test charge and see it sometimes go one way and sometimes go another way. You will never see a compass undecided about which way to point.
• I have two doubts:

The first one is,
What is the difference between a regular magnet and a metal objects that creates a magnetic field when current passes through it? From reading Rayr's explanation, I can understand the electric field part. But, how is it that natural magnets already have that magnetic field with them? Is it because all their atom are lined up or something?

My second doubt is,
If a magnetic field only be created when current is made to pass through a metal object, can it also be created in exceptional conductor say, like Graphite?

Thank You
(1 vote)
• Natural magnets are created from lightning strikes. The large current from the lightning creates a powerful magnetic field which aligns the magnetic domains of any iron ore nearby.

Any current (movement of electrical charge) will create a magnetic field. Certain materials are capable of realigning the angular momentum of their electrons, and iron is one of them. When the angular momentum of electrons gets aligned, an external magnetic field is created.
• Can someone redirect me to a place where I can actually arrive at the formula ?
(1 vote)
• Just think about it a little bit. Pick a point in space near a wire that is carrying a current. Let's see if we can guess what the formula for the field should look like. We can guess that the field should be proportional to the current, right? No current, no field. Lots of current, more field.

We can also guess that as we move further away from the wire the field should get weaker. If we imagine the field radiating outward from the cylindrical wire, we can see that as we move further away from the wire, the same number of field lines are spread out over the CIRCUMFERENCE of circles whose radii increase as we go further away.

So we can guess that the field should also be proportional to 1/(2*pi*r). And we already said it should be proportional to I. So if it is proportional to both those things it must be proportional to
I / (2*pi*r). Now to go from proportionality to equality we need to add a constant of proportionality to the formula.

B = k*I / (2*pi*r)

We will call the constant permeability, and give it the symbol mu instead of k, and if we do the experiment in free space we will find a particular value for mu and that will give us our formula.