Magnetism is the force by which magnetic materials attract and repel other magnetic materials. We can see a magnet pulling bits of metal towards itself but we can't see magnetic field lines or what happens at the atomic level when magnetism is present. This article explores those fundamentals before expanding into real-world applications for magnetic fields.
What to Know About Magnetic Fields
- Magnetic fields are 'fields' that denote the magnetic influence on electrical currents and charges, as well as magnetic materials.
- A magnetic field is a vector field: it has direction and magnitude at every point in space.
- Magnetic fields exert a force on moving charged particles (Lorentz force).
- Magnetic fields are sometimes called B-field.
What is a Magnetic Field?
Full disclosure: as I write this, I'm listening to the excellent Jean-Michel Jarre album, Magnetic Fields. Few English speakers get the joke. Translated from French, the actual title is Magnetic Chants. However, chants and fields (champs) are homophones in French.
In naming his album, this artist was expressing his love of magnetism while having a joke on us. In fact, his style of music, electronica, would be impossible to make without magnetic fields. Nor would we have any devices to listen to music with. Now, we discover the full power of magnetic fields, their study, and where we find them.
Magnetic Fields: Definition and Behaviours
Magnets attract and repel things that are susceptible to the force of magnetism. We know this from our lessons in magnetism and electromagnetism.
Yet, the important thing is what happens between the two materials that are magnetic. This is the magnetic field – an invisible field of force which is essentially the arrangement of the electrons in the surrounding area.
You may have seen diagrams of magnetic fields in your textbooks. They resemble this one, which traces a one-directional pattern of electron flow. These are magnetic field lines.
The lines that represent the magnetic flux seem to emerge from the magnet's north pole and enter the south, but they actually describe a closed loop. The closer together the lines, the stronger the magnetization. As you can see, these lines never cross.
Units of Measurement for Magnetic Field Strength
We determine magnetic field strength by the intensity it produces from electrical currents or magnetic sources. It is a vector quantity that does not consider the response of the material it is acting on. Your physics tutor may have drilled that into your head. To gauge magnetic field strength, we use two units of measurement.
The Gauss (G)
- a unit of magnetic flux density
- it measures the density of magnetic field lines through a given area
- magnetic flux density SI*: the tesla
- named after German physicist Carl Friedrich Gauss1
The Tesla (T)
- T quantifies magnetic strength in technological applications
- standard MRI machines operate at between 1.5 and 2T
- 1 T = 10 000 G
- Named after Serbian engineer Nikola Tesla
*SI: the International System of Units, the world's preeminent system of measurements.
The gauss operates like any other metric system of measurement, with prefixes (deca-, milli-, and kilo-) and abbreviations (dG, mG, and kG).
The magnetic field of the human brain measures around 10−9 G, or around 10-13 T.
In other words, our brains are magnetically very weak.
Magnetic Fields: Sources And Magnetic Field Lines
We know that magnetic fields are everywhere, that some are strong while others are weak, and that they have technological applications. The question is: what generates these fields? In very general terms, we can identify three main sources2.
Permanent magnet
- feature magnetic dipole alignment
- alignment is from electron spin within atoms
- magnetic field is consistent without any external current
Current-carrying conductors
- motion of electrical charge provokes magnetic field
- a straight wire can be a current-carrying conductor
- the right-hand rule determines the field's direction
Electromagnet
- a wire coil with a ferromagnetic core
- electric current flows through the wire, generating a magnetic field.
- the magnetic field disappears when power is removed.
Magnetic fields are integral to electromagnetism and its application across a broad spectrum of technologies3. Still, as vital as these magnetic fields are, there's one that supersedes them all.
Earth's Magnetic Field
You know that the Earth has a magnetic field, right? That’s why we assign magnets a ‘north’ pole and a ‘south’ one. In terms of magnetic field strength, the Earth's magnetic field at its core is about 25 gauss. For comparison, a standard refrigerator magnet is 50 gauss1.
The whole of the globe is magnetic – which is the reason why compasses work. If you were to pick a handful of mud, it wouldn’t be magnetic, but the size of the earth produces the biggest magnetic field on the planet.
Why does Earth's magnetic field do this? No-one is quite sure.
However, scientists think that it is because of convection currents in the Earth’s core – which are primarily made of iron and nickel. That's what produces the northern lights. This topic would make for a lively discussion with your Physics tutors Melbourne, wouldn't it?
Experimental Observation and Measurement Within Magnetic Field Lines
Iron filings make for one of the best ways to see a magnetic field in action. All you need is a bar magnet and some iron filings. Beware of getting a splinter, though. Safety first!
Lay the magnet on a clean, light-coloured surface. Alternately, you may cover your magnet with a clean white cloth or paper. Now, sprinkle your iron filings over and around it and watch the magnetic field appear.
Observe how these rod-shaped grains align. Their direction won't be haphazard; they will all point in the same direction.
This experiment is popular among students of all ages4. Even HSC Physics tutors enjoy engaging their pupils with this fun trial. Simple as it is, it can lead to a deeper understanding of magnetic properties and, more broadly, our physical world.
Science classrooms are full of such experiments to prove the existence of magnetic fields, even a few that demonstrate the magnetic field around a wire. We could continue writing them up but why not wow you with the simple experiments in this clip?
Magnetic Fields in Space
Magnetic fields are everywhere in space; they play a vital role in maintaining the universe's structure. Planets, including Earth, and some moons generate their own magnetic fields thanks to their internal cores. This phenomenon even has a name.
The region surrounding astronomical objects (planets) created by the object's magnetic field.
Galactic Magnetic Fields
Our galaxy's magnetic field is much weaker than Earth's but that's a good thing5. This allows us to detect and trace potentially harmful cosmic rays and detect the formation of new stars. Thanks to the Milky Way's magnetic field strength, astrophysicists can observe galactic processes as they happen.
With that said, the galaxy's magnetic field is not constant. Or, more specifically, it's not a field that planets 'interrupt'. Across the galaxy, magnetic field strength is stronger6 in its centre region (20-40 microgauss - μG) than around the sun (6μG). By contrast, our planet averages about 0.3 G.
Solar Magnetic Fields
For all of the sun's relative magnetic power (compared to Earth's), its magnetic fields are not evenly distributed. Sunspots and other active regions exhibit magnetic fields up to 4 000 G, far stronger than its average7.
These refer to the sun's reversal of magnetic polarity, which happens about every 11 years.
They happen because of the appearance and migration of sunspots.
The news often talks about solar winds and solar flares, suggesting they may be strong enough to take out transformers that regulate our electrical grids. That's not just hype. Influence from the sun's magnetic field extends far beyond its magnetosphere.
Magnetic Fields: Recent Research and Development
The information in the preceding chapter is available thanks to astrophysicists' observations on the behaviour of cosmic magnetic fields. In fact, researching this article delivered a deluge of information that quite nearly distracted from the task of organising and presenting it! What follows are a few fascinating advances across the scientific spectrum.
In 2024, this project captured an image that showed strong, organised magnetic fields spiralling near the black hole Sagittarius A*.
This astounding capture delivered direct proof of how magnetic fields behave near black holes. The implications and possibilities of this discovery are endless, as are all the new questions it provokes.
Where earthbound matters are concerned, the discoveries have been no less jaw-dropping. The National High Field Magnetic Laboratory (MagLab) recently engineered a high-temperature superconducting coil that could operate at 35.4T.
Such a device holds promise for further quantum materials research as well as medical diagnostic capabilities.
Ever concerned about sustainability and the environment, scientists are using electromagnetic induction and resultant magnetic fields to improve industrial production. Wastewater treatment is at the forefront of these experiments that use magnetic fields to enhance treatment catalysts' efficiency.
Scientists are even working on magnetic nanoparticles to improve the effectiveness of antibiotic medicines! Static magnetic fields - and cyclic ones - have demonstrated they have an enhancing effect on certain antibiotics. What with all the worry over medicine-resistant superbugs, that is welcome news, indeed!
Magnetic Fields Further Reading and Resources
- Apex Magnets: https://www.apexmagnets.com/news-how-tos/what-is-gauss/
- Transfer Multisort Elektronik: https://www.tme.com/us/en-us/news/library-articles/page/59558/magnetic-field-sources-and-properties/
- Geeks for Geeks: https://www.geeksforgeeks.org/physics/real-life-applications-of-electromagnetism/
- Eric Appelt: https://www.teachengineering.org/activities/view/van_mri_act_less_1
- FreshScience: https://phys.org/news/2019-11-galaxy-magnetic-field.html
- Christopher S. Baird: https://www.wtamu.edu/~cbaird/sq/2014/09/03/why-dont-galaxies-have-a-natural-magnetic-field-like-the-earth-does/
- Randy Russell: https://windows2universe.org/sun/sun_magnetic_field.html
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