• Question: Why is there in certain places that there are strong magnetic fields and in other places there weaker?

    Asked by anon-258065 on 29 Jun 2020.
    • Photo: Ry Cutter

      Ry Cutter answered on 29 Jun 2020:

      Magnetic fields are generated by having a charge flux (Something that has an electric field that is moving). In a lab they are made by pushing a current through super cooled wires. In stars and planets it is to do with plasma rotating at different speeds which generates the field.
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      The strongest magnets in the universe are called Magnetars. These are neutron stars with magnetic fields so strong, they can rip entire planets apart. They are over a trillion times stronger than the Earth’s magnetic field.
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      Magnetic fields are like gravitational fields in a lot of regards. The main one, is that the further away you are from the source of the magnet, the weaker the field feels.
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      Earth has a magnetic field. We often call the surface of this field the magnetosphere. It’s made by the rotation of the molten Iron core. There’s a lot of complicated dynamics going on there, which can result in small changes to the magnetic fields we feel. As the field has a shape, called a dipole, the force you feel can vary depending how close to the centre of the Earth you are, where you are in the dipole, and how the dipole is moving/changing.
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      I hope that helps, great question 🙂
      Ry

    • Photo: Sam Carr

      Sam Carr answered on 29 Jun 2020:

      Is this question about the earth’s magnetic field?

      If so, then the main variation is because the earth’s magnetic field mostly looks like a “magnetic dipole”, which is not spherically symmetric. A magnetic dipole is what you would learn about as a normal magnet – there is a north and south pole, and magnetic field lines run in big curves from the North to the South poles. The magnetic poles of earth are quite close to the rotational poles, although there is still quite some distance (hundreds of miles, I don’t remember exactly) between the North pole and the magnetic North pole. At the magnetic poles, the earth’s magnetic field points vertically and is quite strong, while closer to the equator, they are horizontal and somewhat weaker. This really is just due to the dipole nature of the magnetic field — at the poles, all the field lines come together to get to the pole (which is why it is stronger), while near the equator they are more spread out. Although the dipole pattern is a bit different, the reason is very similar to why lines of longitude are closer together at the poles than at the equator. I’ve glossed over certain details here, such as the difference between the magnetic pole and the geomagnetic pole, which mostly arise because the dipole originates deep inside the core of the earth and not at its surface, but I hope it gives the overall picture.

      The dipole shape gives most of the picture of the magnetic field variations around the earth, but there are still smaller (although easily measurable) more local variations. Some of these are due to the way the magnetic field is created in the earths core – it is not a perfect dipole so there are variations on a medium (compared to the size of earth) length scale. Other variations on a more local level are due to e.g. the type of rock the hill is made of. For example, if the rock is iron rich, then the magnetic field could be enhanced around that hill. You can use small local variations in magnetic field to learn a lot about what is under the ground, going down a few miles. I still remember a practical I took as part of a geophysics course as an undergrad where we did this – measured local magnetic field as we walked around Edinburgh, and used it to deduce something about the layers of rock under us.

      On a still more local level, variations in magnetic field are caused by things around us — for example, the magnetic field will be much stronger if we are standing next to a big magnet than if we were standing in a Faraday cage.

      Also interesting is how the magnetic field around the earth changes with time – but that is a story for another day.

    • Photo: Krishna Mooroogen

      Krishna Mooroogen answered on 29 Jun 2020:

      Both my colleagues here have given excellent detailed answers, i’ll try to add to their answers with a simple idea. Magnetic fields have a ‘topology’ which you can think of the shape or ‘map’ of the field. Certain environmental factors causes the shape to be close together in some places and far apart in others. When the field is bunched together the field is stronger and the further away from the shape you are the weaker you feel magnetic field.

    • Photo: James Smallcombe

      James Smallcombe answered on 30 Jun 2020:

      One aspect I think the other answers have failed to address is the basics of magnetic fields. According to Gauss’s law for magnetism, because there are no “magnetic charges” , the total magnetic field going through the surface of any closed volume (the earths surface for example) must sum to zero. This mean magnetic field lines must form loops. So any magnetic fields cannot be the same strength in all placed, it must loop around to go back to where it started.

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