Electric Currents Generate Magnetic Fields (36.5)

Electromagnetism — An electric current in a wire creates a magnetic field around the wire. Image adapted from Wikimedia Commons User:Stannard.

Moving charges create magnetic fields. Currents moving through a wire are moving electric charges (electrons). Therefore, current-carrying wires generate a magnetic field around them.

Bending wires into a loop will create will create a magnetic field through the loop.

The more loops you have the stronger the magnetic field.
The magnets created by pushing a current through loops of wire is called an electromagnet.
If you use superconducting wire, you can create an extremely powerful superconducting magnet that can be used in magnetic levitation (maglev) trains.

VFPt_dipole_magnetic3 — Magnetic field through a coil of wire (with a current running through it). The more loops of the wire, the stronger the electric field.

Magnets Deflect the Movement of Charged Particles (36.6)

Moving charged particles create magnetic fields. So if a moving charged particle encounters a magnetic field the two magnetic fields will interact and the motion of the particle will be deflected.

particle-deflection — A charged particle is deflected from it's forward motion when a magnetic field is turned on.

Note that the deflection only occurs when the particle is not traveling parallel to the magnetic field lines.

The big, old styled TV’s use this to shoot electrons at the screen to make the picture.
The Earth’s magnetic field deflects the charged particles ejected by the Sun, protecting the planet.

Interaction Between Magnets and Currents in Wires

Just like charged particles are deflected when they run into a magnetic field, charges running through a wire will create a magnetic field that will interact with external magnets to cause the wire to move.

The force on the wire is perpendicular both to the direction of the current and the lines of the magnetic field.
If you reverse the current in the wire (send it the other way) the force will be in the opposite direction.

Right_hand_rule_cross_product — The force resulting from a current is at right angles to the magnetic field and the current; the "right hand rule" is an easy way to remember this. Image adapted from User:Acdx on Wikimedia Commons.

You can use this principle to create a galvanometer, which is a device that detects electric currents, or to build motors.

Earth’s Magnetic Field

Convection currents in the Earth’s molten, metalic outer core create the Earth’s magnetic field.

Because the pattern of convection changes over time, the Earth’s magnetic field:

Is not located at the north pole (axis of rotation).
Wanders: it moves a little each year.
Flips so its poles reverse every 800,000 years or so.

GPS-MagneticNorthPole — The location of the magnetic north pole changes with time. Image via the National Forest Service.

As lava cools, the magnetic minerals in it orient themselves with the Earth’s magnetic field. One way of telling how old basalt rocks on the seafloor are is by looking at the direction of their magnetic field. Since the Africa and the Americas are moving apart, slowly over millions of years, there is a suture in the Earth’s crust in the middle of the Atlantic ocean where new seafloor is made from erupting, under-sea volcanos. As a result, there are magnetic stripes all along the Atlantic Ocean (and all the other oceans too) that have recorded each time the Earth’s magnetic polarity has reversed.

Fig7 — Magnetic striping in oceanic crust. Image from the USGS.

The USGS has high-resolution geomagnetic maps.

WDMAM_NGDC_V1.1 — Map of the remnent magnetism in the crust (focus on North America and North Atlantic). Via the USGS.

Notes:

Magnet0873 — Magnetic field of a bar magnet; shown by the alignment of iron filings. Image by Black and Davis (1913) via Wikipedia.

What creates magnetic fields? (36.3)

Magnetic fields are created by moving electric charges. At the atomic level:

In atoms, most of the magnetic field comes from the spinning of electrons (remember electrons have a negative charge).
In most elements, the magnetic fields of electrons pair up and cancel each other out
Only certain elements, which have a few electrons that don’t pair up, can form magnets:

Iron (Fe), Nickel (Ni) and Cobalt (Co) are the common magnetic elements.
Iron is the most powerful magnetic element. It has 4 electrons whose magnetism are not canceled out (because of their arrangement in their electron shells)
Some rare earth elements are also naturally magnetic.

Magnetic Domains (36.4)

Each iron atom has a very small magnetic field, but when a bunch of them line up they add to each other to create a stronger field.

A region with a bunch of lined up atoms is called a magnetic domain.

NdFeB-Domains — Microscopic view of the grains that make up a magnet. Each grain has a magnetic field that, if oriented in the same direction, makes for a strong magnet. Each magnetic grain is called a magnetic domain. (Image by Gorchy (2005) via Wikipedia).

If all the magnetic domains line up you have a strong magnet. If they’re all randomly arranged, you don’t have a magnet at all; they cancel each other out and it’s unmagnetized).

Magnetic Poles (36.1)

Magnets have two poles: a north-seeking pole, and a south seeking pole; they align with the Earth’s magnetic field.

Like poles repel and opposite poles attract.

The force between the poles depends on the strength of the poles (p) and the distance (d) between them:

$F \propto \frac{p_1 p_2}{d^2}$

Note how similar this equation is to the force between two charges (Coloumb’s Law; F_c), and the force between two masses (gravitational force; F_g).

Electric fields come from charged particles, which can be separated, but north and south magnetic poles belong to each domain (and even each atom) so they cannot be separated.

If you break a bar magnet in half you don’t get a separate north and south poles, you just get two magnets, each with it’s own north and south pole.

Magnetic Fields (36.2)

magnetic-lines-of-force — Magnets that are free to move will align themselves with the magnetic fields around them.

The region around the magnet that is affected by the magnetic force is filled with a magnetic field. In theory the magnetic force goes on forever, but is only strong in a relatively small region.

Compasses, which have magnets that are free to move, will align themselves with the magnetic fields around them. When you’re away from other magnets and electronic devices, compasses align with the Earth’s magnetic field.

So how do you create a magnet?

In an unmagnetized piece of iron, the magnetic domains are arranged randomly. If you place it in a strong magnetic field, the domains will align with the strong magnetic field and the iron will become magnetic.

Softer iron alloys will align easier, and stay aligned to make strong, permanent magnets.
Any metal with iron in it (like steel cans or filing cabinets) will gradually align their magnetic domains with the Earth’s magnetic field if they not moved for a long time, but the Earth’s field is not strong enough to make a permanent magnet.
Stroking a piece of iron with a magnet will also align the domains.
Touching a magnet to a paperclip or iron nail will align its magnetic domains and create a temporary magnet, which is why you can use a magnet to hold up a chain of nails. This induced magnetism will only last a short time and eventually (within seconds or minutes) of removing the magnet, the nails will lose their magnetism.
Dropping or heating a permanent magnet will shift some of the domains out of alignment, reducing the strength of the magnet.

Tag: geomagnetism

Electromagnets