00:01
Number thirty eight is a fun little real world example that has us looking at some power lines with a current traveling west, which is over here.
00:13
And, ah, we want to find a couple of things.
00:18
But for part a, we want to find the magnitude and direction of the magnetic field produced by this wire at the ground directly below so down here.
00:28
And we're going to assume that this is the eight point five meters.
00:32
So where this green line is to the ground.
00:37
So the way that we can calculate magnetic field from a line is believe equation twenty dash six to have b l, which is just knew, not times the current and then two pi r.
00:57
Luckily, we have everything we need for this because this is a constant constant.
01:03
This is eight point five meters and this is ninety five amps, and that works out to be two point two roughly times ten to the minus six tesler.
01:18
Because we have to consider significant figures.
01:21
Well, i've got two of them, all right, and the direction off this magnetic field well, we can use our right hand rule, so if you take your hand.
01:36
There's no way i can draw this, really.
01:38
But if you imagine you're going to grab this wire first, hold your hand out flat in place your thumb in the direction of the current.
01:47
So this way, and then curl your fingers around the wire so they're going to curl around this way like that and at each point there, whichever way your fingertips are pointing, so your index and middle and ring and pinky that will tell you the direction of that magnetic field around that.
02:13
So for this one, you see that it's pointing out of the page, which, if we recall our cardinal directions north, east, east, south and west west is this way.
02:31
Thus, south is the direction that the magnetic field is paint pointing, and the problem also asked us to compare with right south here.
02:50
It wants us to compare it with the magnetic field from earth, and the magnetic field from earth is roughly five times ten to the minus...