What is a Magnetic Field? (Electromagnetism – Physics)

If you have studied or are studying physics, or even if you are just a physics fan, you must have already met magnetic fields, and more than once!

These show up a little everywhere: of course in electrical sciences, but also in planetology, astrophysics, medical physics, analytical chemistry, and many other subjects. At school and in Uni, you have been taught how to manipulate them mathematically, and how they are an intricately involved in today's technology (motors, generators, transformers etc…).

But do you actually know what they truly are physically? For example, could you explain it to a young student that encounters magnetic fields for the first time in physics class ?

That is the exercise I attempted with this video

First, we review what you learned in high school, starting with a simplified version of Ampere’s law: an electrical current generates a magnetic field that curls around it. We also discuss how a magnetic field affects the path of a moving charge.

In a second section, we take a brief turn in order to observe one of the consequences of Einstein theory of special relativity: Length contraction.

The third section puts everything together and draws a coherent picture of the nature of a magnetic field. And in a conclusion, we realise that a magnetic field emerges from the combination of electrical theories and special relativity.

And of course all is spiced up with reflections on how historically, magnetic fields took such a predominant place in modern science.

I hope you enjoy this video, I sure had fun producing it!

Structure of the video:

00:00 Introduction 00 :34 30K subs! 00:55 Content of the Video 01:32 Magnetic Effects – description of a charge moving parallel to a current carrying cable. 02:38 Magnetic Effects – magnetic field generated by an electrical current 03:09 Magnetic Effects – Magnetic force on a moving charge 04:14 The birth of electromagnetism, a historical reflections about magnetic fields 05:27 Length contraction 07:05 The nature of magnetic fields 09:55 What is a magnetic field ? 10:17 Magnetism is an emergent phenomena (a discussion) 11:41 End of video salutation to the Physics Made Easy community

Prior knowledge required:

In order to fully grasp the content of this video, the viewer needs to be familiar with the following concepts already presented in a PME video. _ What is a charge : https://youtu.be/8VN8MDn4CFA _ What is a field : https://youtu.be/_n_jRP8u1gI _What is an electric field : https://youtu.be/hrHCj09_dpc

Addendum: regarding conservation of charge…

You might wonder : because relativity changes the charge density experienced by a moving charge, could this violate the principle of conservation of charge ? No, it doesn’t. The reason is that for a current to flow in a circuit, the circuit needs to be closed, i.e. in a loop: relatively to the motion of the moving charge, the current needs to flow in the opposite direction in another section of the circuit : Imagine a moving charge q magnetically affected by a current going from left to right in section A of the circuit. The charge will also see a current flowing from right to left in another section of the circuit (let’s call it section B). For charge q, section B will have charged densities reversed compared to section A. On the other hand, it might not be magnetically affected that much, because that effect depends on spacial distance (inversely proportional). Overall, charge is conserved wether the observer is moving or not.

Credits :

The script, the illustrations, the music and the video were produced and presented by Edouard Reny, Ph.D. in solid state chemistry and private tutor in Physical Sciences.

The galactic background at the end of the video : Traverse galaxy by F. Summers, G. Bacon (STScI). This background is more than an artistic view, it is a simulation : C Hummels, P. Hopkins (CalTech) and A. Wetzel (UC Davis)

Need help with your high school physics ?:

Edouard provides one-on-one private tuition in Physics (face to face or by Skype). If you wish to know more about this, or contact Edouard, visit his website: https://www.physics-tutor.nl/ or this one (for French speakers): https://profdephysique.fr/ ... https://www.youtube.com/watch?v=NOQK57fgu80