Magnetfeld einfach erklärt!
Elektrotechnik fürs Studium by "Elektro-Nik"
https://www.youtube.com/watch?v=hsBk3XrIoTE

Transkript (automatisch erstellt)

0:00 What's up? Electronics here, and you are here to learn all about magnetic fields, so let's go. Okay, how can we
0:08 visualize this whole thing? Well, let's imagine a conductor with a current flowing through it. If I bring magnetic
0:17 needles near this conductor, they will align themselves in a specific direction. And if I plot the direction
0:28 for many of these needles, they follow a clear path, namely around the conductor. It has been established that
0:40 field lines outside a magnet always point from the north to the south pole. However, to determine the direction of
0:48 this magnetic field, we use the right-hand grip rule. In doing so, our thumb points in the direction of the
0:56 current, and the curve of our fingers indicates the direction the magnetic field curls around the conductor. If I
1:07 look at coils, I can imagine that a much stronger magnetic field is generated, because I have, in quotes,
1:15 many more conductors creating a field, even though it's practically just one single wound wire. The field strength
1:28 was determined by placing twisted magnetic needles into the field and seeing how much torque was needed to
1:35 align them with the field. When this was done in a toroidal coil, it was found that the entire torque was
1:45 proportional to the current and number of turns, and inversely proportional to the mean length of the coil L. Thus,
1:52 the following material-independent expression for field strength was obtained. H = I times n divided by L,
2:03 though the term field strength can be confusing here, as H does not directly indicate the strength of the field,
2:12 since H is material-independent and different materials result in a stronger or weaker field. Rather, H
2:22 simply indicates that a field is being generated by the current at all. How strong it is depends on the material
2:28 used inside the coil, and we'll learn about that shortly. What we can also note is that the numerator of H
2:38 corresponds to our magnetomotive force Theta, which is I times n. Yes, and as I already said, our field strength H
2:50 does not provide information about how strong the magnetic field is at a specific point. Therefore, the quantity
2:59 B, magnetic flux density, was introduced. And how to determine it was shown by the following experiment,
3:08 where a current-carrying conductor of length L was placed into this B-field, and it was discovered that a force acts
3:15 upon this conductor. My force was the product of flux density, current, and the length of the conductor, and the
3:27 directions were derived from the right-hand rule, also known as the CMI rule; where CMI stands for Cause,
3:34 Mediation, and Impact, with the cause being the current, the mediation our B-field, and our middle finger then
3:43 pointing in the direction of the impact force F. This is how we incorporated B into an equation that simultaneously
3:51 represents the field strength, because the higher my flux density, the greater my force. So, we now know how the force
4:01 depends on the B-field, and our B-field depends on the magnetic field, i.e., our field strength, as follows: B = µ*
4:11 H. And this mu is composed of µ0 times µr, where µr depends on the material and µ0 represents the permeability of
4:22 free space. It is also important to reiterate that all these quantities— the B-field and the H-field—are
4:28 directional, meaning they are vector quantities that possess not only a magnitude but also a direction. If flux
4:37 density indicates how much flux flows per area, then my magnetic flux is logically flux density times area. The
4:47 unit is the Weber. For magnetic tension , at least in a homogeneous field like this one, it can be stated that the
4:57 magnetic tension is our field strength times the distance between these two surfaces. So, H times s. Let’s take a
5:07 closer look at Ampere's circuital law and recall that the field strength equals NI/L, where our number of turns
5:16 is 1 here, since we are only considering one conductor, and our length L is naturally 2πr. If I want
5:25 to calculate my magnetic tension along an arbitrary integration path, I have to take the line integral of the
5:33 different field strengths all the way around this entire integration path. That is obviously super messy and ultra
5:41 complicated, so nobody actually does that. We make life easier for ourselves , of course. Someone discovered that
5:46 the integration path actually doesn't matter. We can therefore simply take a normal circle around our conductor. And
5:55 if I move along a circle, as you can see at the top left, then at every point on this circle our field strength
6:02 is constant, because logically our field is a circle, and our complicated line integral simply becomes the
6:08 product of the field strength and the path 2πr. And using the equation on the left, we can show that this is also
6:17 equal to I. This means nothing more than that the line integral of the magnetic field strength is equal to our
6:25 electrical current linkage, Theta. So, if I have a surface with perimeter L, my Theta simply tells me the sum of all
6:35 currents entering and exiting through this surface. Right, and now let’s quickly wrap up magnetic circuits with
6:45 what we’ve just learned. If we now look at our area bounded by L1, our Ampere's Law tells us that our
6:55 magnetomotive force here is simply the number of turns times the current, because our area is penetrated by this
7:05 current. The formula for magnetic reluctance is L over mu A, where L is naturally the mean length of the
7:15 material through which the magnetic field passes. A is the cross-sectional area and mu is the material-dependent
7:23 constant we already discussed. This leads us to our Ohm's Law for the magnetic circuit. Ohm's Law is U equals
7:33 R times I, and here it is translated into theta equals R sub m times phi. This means our theta is equal to the
7:44 magnetic voltage. We already learned that in Ampere's Law just now. Our R sub m is simply our magnetic reluctance
7:53 and our phi is analogous to the electric current. That means, just like in DC circuits, we can set up and
8:03 calculate circuit diagrams with this knowledge using the very simple rules we already know. And I hope that I have
8:14 been able to convey some basics about the magnetic field to you, with which you can now tackle my practice problems
8:19 . Have fun with them. Your Elektronic. Yes.