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Magnetic Fields



Magnetic fields are fundamental to electromagnetism and underlie many practical applications ranging from compasses to magnetic resonance imaging.

Magnetic fields or B-fields can be generated by moving charges, such as an electrical current, or objects such as bar magnets due to the microscopic dynamics of charges inside the magnetic material.

This video will illustrate how to visualize magnetic fields produced by a current-carrying conductor and a permanent bar magnet. Moreover, this video will also demonstrate the force exerted by the magnetic fields produced by a current on another current-carrying wire.

Magnetic fields can be visualized using magnetic field lines. These are fictitious lines that help understand the distribution and direction of magnetic fields.

The tangent from a magnetic field line reflects the local direction of the magnetic field, and the density of the lines mirrors the strength of the local magnetic field, which in case of a bar magnet decreases as we move away from its surface. Different current conductor configurations produce different variations in magnetic field distributions.

For example, a long straight wire carrying an electrical current produces a magnetic field, whose direction, as represented by "magnetic field lines", is along the circular tangent direction around the wire.

In the case of a bar magnet, the magnetic field lines leave the north pole of the magnet and enter the south pole of the magnet. This is similar to the magnetic field pattern produced by a solenoid, which is a cylindrical coil of wire that is carrying current.

The direction of the magnetic field produced by a current can be determined by the "right-hand rule". The rule states that if the thumb points along the direction of the current, the fingers curling around the conductor indicate the direction of the magnetic field. Thus, a bar magnet, when brought close to the conductor, aligns with the generated local magnetic field.

Now we know that magnetic fields, produced by any conductor or magnet, interact with nearby magnetic materials. Additionally, the generated magnetic fields also interact with moving electrical charges, like those found in a second current carrying conductor.

When a moving charge 'q' is introduced in a magnetic field 'B', the field exerts a force 'F' on the charge. This is called the Lorentz force. The force is proportional to the magnetic field 'B', the charge 'q' and its velocity 'v', and is determined by the vector product of the velocity of the charge and the magnetic field, times the charge. The force therefore points in a direction perpendicular to both the motion of the charge and the magnetic field determined by the "right hand thumb rule".

Having reviewed the basics of magnetic fields, let us perform a simple experiment to visualize these magnetic field lines and demonstrate how the Lorentz force exerted by a generated magnetic field affects a parallel current-carrying wire.

Gather the necessary materials and instruments, namely a DC current source, a plastic board mounted with several compass needles and a straight conducting wire passing through its center, and a permanent bar magnet.

Observe the plastic board with a hole in its center. It is mounted with several compass needles around the center hole using pins, such that the needles are free to rotate.

Also note that the conducting wire is fed through the center hole of the board. Make sure that the wire is perpendicular to the board. Connect the wire to the DC current supply using cables with clamps.

Turn ON the current source and set the current supply to +5 amperes. Observe the behavior of the compass needles.

Next, turn OFF the power supply and switch the positive and negative cables. Then, turn ON the power supply to reverse the direction of the current flowing through the wire and observe the compass needles again.

Now turn OFF and disconnect the current supply and obtain a similar plastic board mounted with magnetic needles, but without the conducting wire fed through it. Next, identify the north pole of the bar magnet.

With the long-axis of the bar magnet parallel to the board, bring the north pole closer to the board from the side. Observe the compass needles for any change in orientation.

Now flip the bar magnet such that the south pole is closer to the board. Once again, observe the compass needles for any change in orientation.

First assemble a frame with two bars, one of them is horizontal running along the top of the frame and the other is vertical that connects the base to the first bar. Next, anchor or tape the mid section of the two long conducting wires to the frame. Dangle one end of both wires from the frame such that the two wires are parallel to each other.

Now, connect the end of the two wires to the switch and the terminals. Then connect the setup to a battery.

Make sure that the wires are connected such that the current flows in the same direction in both wires. Then, flip the switch to connect the battery to the conducting wires.

Observe the two wires when the current is passing through them. Next, turn OFF the switch to stop the flow of current through the wires.

Reverse the direction of the switch in order to change the direction of current flow through the wires. Observe the two wires when the current is ON.

Now having reviewed the protocols, let us review the results of the experiments performed.

In the experiment with the compass needles, initially, the needles are randomly oriented. On application of the current, the compass needles align themselves with the local magnetic field in a circular pattern.

On reversing the direction of the current, the local magnetic field reverses, which in turn reverses the orientation of the compass needles.

Similarly, when the north pole of the bar magnet is brought close to the compass needles, it creates a local magnetic field, and the compass needles align along these local magnetic field lines.

And when the bar magnet is flipped, the direction of the magnetic field also reverses, which reverses the orientation of the compass needles.

In the experiment with the two long wires, the wires are attracted to each other when the current flowing in them has the same direction. This is because of the Lorentz force generated by the magnetic field.

According to the right-hand rule, the left wire produces a magnetic field, which points in the direction perpendicular to the flow of current at the site of the right wire. Now, use the other right hand rule, and place the fingers along the direction of the current and the magnetic fields. Then the extended thumb gives the direction of the Lorentz force. In this case, the force is towards the left wire and thus attractive.

On the other hand, when the flow of current in the two wires is in opposite directions to each other, the right hand rule shows that the direction of Lorentz force at the right wire site is away from the left wire, making the force repulsive. Therefore, the two wires are pushed apart.

Magnetic fields are found everywhere around us, and are currently used in applications ranging from navigation to the clinical environment. Let us now look at a couple of common applications of magnetic fields.

Centuries ago, the Song dynasty of China invented the first magnetic compass that was used for navigation. Since then we have relied on the compass, which works in tandem with the earth's own magnetic field, for direction.

The magnetic south pole of the earth is located near its geographic north pole. Thus, the magnetic north pole of a compass needle aligns to the earth's magnetic field and points towards the earth's geographic north.

Magnetic fields also have a multitude of applications in the field of medicine and medical diagnostics. The most common use of magnetic fields is in magnetic resonance imaging or MRI. MRI scanners use strong magnetic fields and field gradients to generate images of the inside of the body. 

You've just watched JoVE's introduction to magnetic fields. You should now know how to visualize magnetic fields using compass needles and understand how the Lorentz force of a magnetic field produced by a current affects another nearly parallel current. Thanks for watching!

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