5.3.1 Magnetism Overview

Watch the video below!

5.3.2 Representing Magnetic Fields

A magnetic field is defined by the force that a charged particle experiences moving in this field, after we account for the gravitational and any additional electric forces possible on the charge. Magnetic fields are represented by magnetic field lines. These lines are very useful in visualizing the strength and direction of the magnetic field. As shown in Figure 5.3, each of these lines forms a closed loop, even if not shown by the constraints of the space available for the figure. The field lines emerge from the north pole (N), loop around to the south pole (S), and continue through the bar magnet back to the north pole.

Magnetic field lines have several hard-and-fast rules:

1. The direction of the magnetic field is tangent to the field line at any point in space. A small compass will point in the direction of the field line.
2. The strength of the field is proportional to the closeness of the lines. It is exactly proportional to the number of lines per unit area perpendicular to the lines (called the areal density).
3. Magnetic field lines can never cross, meaning that the field is unique at any point in space.
4. Magnetic field lines are continuous, forming closed loops without a beginning or end. They are directed from the north pole to the south pole.

The last property is related to the fact that the north and south poles cannot be separated. It is a distinct difference from electric field lines, which generally begin on positive charges and end on negative charges or at infinity. If isolated magnetic charges (referred to as magnetic monopoles) existed, then magnetic field lines would begin and end on them.

5.3.3 Ferromagnetic Materials

Common magnets are made of a ferromagnetic material such as iron or one of its alloys. Experiments reveal that a ferromagnetic material consists of tiny regions known as magnetic domains. Their volumes typically range from 10⁻¹² to 10⁻⁸ m³, and they contain about 10¹⁷ to 10²¹ atoms. Within a domain, the magnetic dipoles are rigidly aligned in the same direction by coupling among the atoms. This coupling, which is due to quantum mechanical effects, is so strong that even thermal agitation at room temperature cannot break it. The result is that each domain has a net dipole moment. Some materials have weaker coupling and are ferromagnetic only at lower temperatures.

If the domains in a ferromagnetic sample are randomly oriented, as shown in Figure 5.4, the sample has no net magnetic dipole moment and is said to be unmagnetized. Suppose that we fill the volume of a solenoid with an unmagnetized ferromagnetic sample. When the magnetic field of the solenoid is turned on, the dipole moments of the domains rotate so that they align somewhat with the field, as depicted in Figure 5.4. In addition, the aligned domains tend to increase in size at the expense of unaligned ones. The net effect of these two processes is the creation of a net magnetic dipole moment for the ferromagnet that is directed along the applied magnetic field. These dipoles of these magnetic domains also become locked together so that a permanent magnetization results, even when the field is turned off or reversed. (This permanent magnetization happens in ferromagnetic materials but not paramagnetic materials.) Besides iron, only four elements contain the magnetic domains needed to exhibit ferromagnetic behavior: cobalt, nickel, gadolinium, and dysprosium. Many alloys of these elements are also ferromagnetic.

The partial alignment of the domains in a ferromagnet is equivalent to a current flowing around the surface. A bar magnet can therefore be pictured as a tightly wound solenoid with a large current circulating through its coils (the surface current). You can see in Figure 5.5 that this model fits quite well. The fields of the bar magnet and the finite solenoid are strikingly similar. The figure also shows how the poles of the bar magnet are identified. To form closed loops, the field lines outside the magnet leave the north (N) pole and enter the south (S) pole, whereas inside the magnet, they leave S and enter N.

Ferromagnetic materials are found in computer hard disk drives and permanent data storage devices. A material used in your hard disk drives is called a spin valve, which has alternating layers of ferromagnetic (aligning with the external magnetic field) and antiferromagnetic (each atom is aligned opposite to the next) metals. It was observed that a significant change in resistance was discovered based on whether an applied magnetic field was on the spin valve or not. This large change in resistance creates a quick and consistent way for recording or reading information by an applied current.

5.3.4 Origin of Magnetic Fields in Planets

Since motions of charged particles create magnetic fields, a world can have a magnetic field if charged particles are moving inside. To generate a magnetic field, a planet must meet three requirements: 1) A molten, electrically conducting interior; 2) Convection in the interior and 3) A moderately rapid rotation.

Of the four terrestrial planets, Earth has the strongest magnetic field (Figure 5.6). Geologists believe that Earth’s rotating molten core produces a dynamo effect that in turn, generates its magnetic field. Earth’s core comprises about 33% of the planet’s mass. As noted in the previous section, the core is molten because of the heat leftover from its formation and the presence of radioactive isotopes.

After Earth, Mercury has the second strongest magnetic field (Figure 5.7). It is also the most metallic and its core makes up about 60% of its mass, giving it the largest ratio of a planet’s core to its size in the Solar system. However, Mercury is also the smallest planet and therefore has the highest surface area to mass ratio. As a result, its core likely lost most of the heat from its formation. The presence of a relatively strong magnetic field raises the question as to whether it is still molten despite losing most of its heat of formation. One hypothesis posits that the core contains sulfur, which would lower the melting point of the iron core. Another possibility is that Mercury’s magnetic field is somehow produced by charged particles from the solar wind.

Venus is close to the Earth in size but does not have a strong magnetic field. This has puzzled planetary scientists. The lack of plate tectonics on Venus may indicate a lack of convective forces in the mantle. This would indicate that is core may not be molten. Also, its slow rotation may not be sufficient to generate a dynamo.

In contrast, Mars has a rotation period that is close to the Earth’s. Data indicates it has an iron core about half the planet’s radius in size. Like Venus, however, Mars lacks any convective currents in its interior. Data from the oldest rocks indicate that Mars once did have a strong magnetic field. So, what could have shut it down? Mars’ small size might have caused the core to cool down and solidify. Also, there might be hydrogen in the core, which could shut down convection. The Mars InSight lander landed on Mars on November 26, 2018 to explore the interior and may answer some of these questions.

Magnetospheres

The magnetosphere is the region in space around a planet where that planet’s magnetic field is the dominant magnetic field. It interacts with and deflects high-speed charged particles emitted by the sun (called the solar wind). Learn more about magnetospheres and the magnetic fields of the outer planets by watching the below video and following webpage:

• Wolf, Portia. “Giant Planets: Magnetospheres.” The Outer Planets. Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Aug 2007.

• Sections 11.2 and 12.7 of OpenStax’s University Physics Volume 2 (2006) by Samuel J. Ling, William Moebs, and Jeff Sanny. Licensed under CC BY 4.0. Access full book for free at this link.

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