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Show Monopoles are (as yet) theoretical objects that have either a north, or a south pole. Another way to imagine these is as magnetic charges, analogous to protons and electrons. Their existence is disputed, although there is some evidence that they can be artificially synthesized. Nevertheless, they are useful tools for thinking about magnetic field lines and to otherwise conceptualize ideas about magnetism. Note that monopoles reduce magnetic phenomena to their electrostatic analogue. We can say that the magnetic field lines emerge out of north poles and converge at south poles. If monopoles were isolated in nature, they would be found to undergo similar interactions in the electric field as an electric charge undergoes in a magnetic field. For example, the magnetic field of monopoles and the electric field of charges would exhibit the same behavior, and a moving magnetic monopole would induce a circulating electric field (diagram below).
This idea brings us to a different definition for the field lines:
Whether magnetic monopoles are real or not, the elegance of this idea is evident. One can easily propose a definition of magnetic charge analogous to the electric charge and claim that the field follows a Coulomb-type law B=km1r2.B = k \frac{m_1}{r^2}. In fact, employing the Coulomb-style law to model a bar magnet as two magnetic charges of opposite "sign" separated by a short distance faithfully reproduces the shape and variation of the magnetic field strength.
Let us codify our observations from the case of the bar magnet:
A compass is a simple device that consists of a permanent magnetic dipole that can rotate freely on top of a pin, and this dipole is called the needle. When placed in a magnetic field, the north pole tends to move in the direction that a north monopole would, and likewise for the south pole. Clearly, this motion is highly constrained and stops once the needle is aligned with the local magnetic field.
The compass is therefore useful to find the local direction of the magnetic field. Since Earth has a simple field with the North pole roughly aligned with true north, the compass is a most reliable tool for navigating the globe. However, if a local magnetic field dwarfs the strength of Earth's magnetic field, the needle will no longer give information about the Earth's field. It can thus be used to map out the magnetic field lines of an electrical device in the lab.
Much like the compass, iron shavings can be used to map out the magnetic field of a device. Each particle in a jar of iron shavings is a small (ferro)magnetic dipole and will align to the local magnetic field. In this way, it is like deploying thousands of tiny compasses all around to get simultaneous global view of the field. Consider this photograph of iron filings settled around a bar magnet. As you can see, the filings allow us to quite literally see the field.
So far we've considered the magnetic field lines due to a single device. When we want to find out the magnetic field strength due to an object at a particular point, we consider that object only and ignore all other potential field generating objects. This is fine as long as the device under consideration produces a much stronger field than all other nearby sources, or if all other sources are located very far away. However, this will not do if we want to find the magnetic field resultant from several field generating devices of comparable strength located close to one another. In the case of several devices we carry out the same procedure that we did for a single device and then add them to find the resultant magnetic field at a point. That is, if device AA produces a magnetic field B⃗A \vec{B}_A at a point xx and device BB produces a magnetic field B⃗B \vec{B}_B at xx then the resultant magnetic field at x x will be B⃗(x)=B⃗A(x)+B⃗B(x)\vec{B}(x) = \vec{B}_A(x) + \vec{B}_B(x). This is known as the principle of superposition.
South Impossible to tell North It can be north or south The diagram above depicts two bar magnets suspended by string. If "N" labels one end of the magnet as magnetic north, then determine the magnetic pole labeled with "?". We now seek to understand how magnetic field lines align around current carrying conductors. To understand the direction of magnetic field lines, we need to be familiar with a mnemonic called the right hand rule. The origins of this rule come from early experiments with iron shavings placed around current carrying wires. The observations are mathematically encoded in one of Maxwell's equations known as Ampere's law.
Note: The current here refers to the conventional flow (i.e. positive charge), so do not confuse this to be the flow of electrons. The electrons flow in the opposite direction to the conventional flow.
After all, field lines are just a "crude" way to explain the magnetic field. However, there is more to it than just being a geometrical object. Here' is what Richard Feynman, the pioneer of Quantum Electrodynamics, says in The Feynman Lectures on Physics:
He further adds:
Why are magnetic field lines directed from north to south?If you place a compass near the north pole of a magnet, the north pole of the compass needle will be repelled and point away from the magnet. Thus, the magnetic field lines point away from the north pole of a magnet and toward its south pole.
Do magnetic field lines move from north to south?And so in short we see that outside the magnetic field lines run north to south. Inside they run from south to north. And as a result we'll always see that the magnetic field lines are always closed loops. Even this loop is a completely closed loop.
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