1
00:00:00,390 --> 00:00:04,330
‫Welcome back, I hope you tried this fun exercise yourself.

2
00:00:04,740 --> 00:00:05,850
‫Let's look at it now.

3
00:00:06,480 --> 00:00:13,530
‫First of all, what direction will the cross be give you?

4
00:00:14,040 --> 00:00:21,030
‫According to the right hand rule, the Krosby will give you an upward direction.

5
00:00:21,540 --> 00:00:31,620
‫Your fingers first go in the direction and then they go in the B direction into the screen.

6
00:00:31,740 --> 00:00:35,580
‫And that's why you don't see the fingers, because they go into the screen.

7
00:00:36,000 --> 00:00:39,750
‫These are knuckles here, so your thumb goes up.

8
00:00:40,350 --> 00:00:47,490
‫So you might think that the force goes up and therefore the electron as well.

9
00:00:47,640 --> 00:00:55,110
‫So you might think that the electron will go in this direction, that it will go upwards.

10
00:00:55,770 --> 00:01:01,310
‫Now, that would be true for a proton that has a positive charge, right?

11
00:01:01,620 --> 00:01:03,180
‫A positive cue.

12
00:01:03,780 --> 00:01:11,540
‫However, you have an electron and an electron has a negative charge and negative.

13
00:01:11,550 --> 00:01:18,900
‫Q So you will have an upward direction produced by this operation here.

14
00:01:19,200 --> 00:01:25,740
‫However, since your cue is negative, then your direction will be reversed like this.

15
00:01:26,430 --> 00:01:33,840
‫So since you have a force now that is in the downward direction, then the electron will actually start

16
00:01:33,840 --> 00:01:35,700
‫moving down as well.

17
00:01:36,390 --> 00:01:39,050
‫But it also has this forward velocity.

18
00:01:39,600 --> 00:01:42,800
‫So after some time it will end up here.

19
00:01:43,410 --> 00:01:51,420
‫But then you have a new velocity vector, which is in this direction like this, and now this formula

20
00:01:51,660 --> 00:01:55,250
‫will give you a force in this direction.

21
00:01:55,680 --> 00:02:03,840
‫First, your force was in this direction and now it's in this direction because your velocity vector

22
00:02:03,840 --> 00:02:05,270
‫is in this direction.

23
00:02:05,940 --> 00:02:13,590
‫And then again, you're here, your velocity is in this direction and your force is in this direction.

24
00:02:14,190 --> 00:02:15,720
‫And you see where it is going.

25
00:02:15,720 --> 00:02:16,060
‫Right.

26
00:02:16,950 --> 00:02:24,300
‫You can see that the force vector is a centripetal force that will make the electron move in a circle

27
00:02:25,140 --> 00:02:27,030
‫like this.

28
00:02:27,660 --> 00:02:34,650
‫OK, this doesn't look like a perfect circle, but actually it would be a perfect circle because what

29
00:02:34,650 --> 00:02:38,370
‫happens here happens on a continuous basis.

30
00:02:38,970 --> 00:02:47,850
‫Every infinitesimal second, the velocity vector of the particle changes and therefore the force vector

31
00:02:47,850 --> 00:02:50,500
‫changes its direction as well.

32
00:02:51,120 --> 00:02:57,720
‫So if that thing happens on a continuous basis, then in the end you see that you force vector is a

33
00:02:57,720 --> 00:03:02,610
‫centripetal force in your electron will move in a perfect circle.

34
00:03:03,330 --> 00:03:10,770
‫And unless the conditions change, for example, maybe you have a different velocity vector or a different

35
00:03:10,770 --> 00:03:12,360
‫magnetic field vector.

36
00:03:12,990 --> 00:03:20,020
‫Unless those conditions change, the electron will stay here and make this circle forever.

37
00:03:20,670 --> 00:03:24,440
‫That's how scientists control charged particles.

38
00:03:24,810 --> 00:03:26,710
‫They do it with magnetic fields.

39
00:03:27,300 --> 00:03:28,490
‫So there you go.

40
00:03:28,740 --> 00:03:33,540
‫Now you know the dot and cross products and some of their applications.

41
00:03:33,990 --> 00:03:39,830
‫In the next video, we will look at mass moment of inertia and inertia tensor.

42
00:03:40,230 --> 00:03:43,370
‫So thank you very much and I'll see you in the next video.