1 00:00:00,150 --> 00:00:06,030 Now that we have learned about the early theories of light, we also want to learn about the early theories 2 00:00:06,030 --> 00:00:09,740 of two other very important quantities of electrodynamics. 3 00:00:10,230 --> 00:00:13,230 These are the charge and the electric field. 4 00:00:14,910 --> 00:00:22,920 It was already 550 B.C. that Thalia's conducted a very interesting experiment or just a general observation. 5 00:00:23,640 --> 00:00:27,120 So he was rubbing a piece of ember with a thorough. 6 00:00:29,150 --> 00:00:38,180 And what he observed is that when he took a feather, that this feather was attracted to the piece of 7 00:00:38,180 --> 00:00:38,570 amber. 8 00:00:40,340 --> 00:00:48,350 So what was happening here by today's understanding is that initially the amber, the feather and the 9 00:00:48,350 --> 00:00:50,600 fur are all electrically neutral. 10 00:00:51,200 --> 00:00:57,710 So they have protons and electrons and on average, all of these charges will cancel out. 11 00:00:58,550 --> 00:01:05,510 However, when we wrapped this Amber with the piece of fur, the fur transferred some of its electrons 12 00:01:05,510 --> 00:01:06,470 to the amber. 13 00:01:07,010 --> 00:01:14,090 So this is why now we have an excess amount of electrons here, which is why this piece of amber is 14 00:01:14,270 --> 00:01:18,140 electrically charged and therefore it attracts this feather. 15 00:01:19,640 --> 00:01:26,150 And some fun facts that you may not know, or at least I didn't know before preparing this course is 16 00:01:26,150 --> 00:01:30,750 that the Greek word for Amber is actually electron. 17 00:01:31,490 --> 00:01:34,530 So the word electron or electronic comes from Amber. 18 00:01:36,500 --> 00:01:43,940 So the theory for this experiment was derived in in the 70s and 80s, but actually already Rubinson 19 00:01:43,940 --> 00:01:52,880 and 1769 analyzed his experiments where he took two charges or two objects that were charged and investigated. 20 00:01:52,880 --> 00:01:57,760 The force between these two objects and its dependence on the distance are. 21 00:01:58,280 --> 00:02:04,460 And he found out that it's scaled with one over R to the power of two point 006. 22 00:02:04,580 --> 00:02:09,410 So this already gave rise to that's probably of one over our square law. 23 00:02:10,010 --> 00:02:17,390 And then Coulomb was the first really famous person to formulate this law, that the force is characterized 24 00:02:17,390 --> 00:02:23,060 by the product of the two charges and one over distance square dependence. 25 00:02:23,510 --> 00:02:28,040 And that's the orientation for the forces is always oppositely. 26 00:02:28,580 --> 00:02:36,110 If you if you have two charges and that these two forces would lead to an attractive interaction if 27 00:02:36,110 --> 00:02:46,400 the charges are oppositely and that they will have a repulsive shape if the charges are of equal sine 28 00:02:47,600 --> 00:02:54,620 and the, prefectly here is this number and it can be calculated as one over four PI times Epsilon zero. 29 00:02:55,520 --> 00:03:02,060 Now, more elaborate way of understanding what is happening here was developed in the eighteen thirties 30 00:03:02,060 --> 00:03:06,040 by Faraday, so he considered lines of force. 31 00:03:06,710 --> 00:03:13,160 So he again considered to charges or two charged objects and investigated on the force. 32 00:03:13,730 --> 00:03:21,740 And on the previous slide we had to Cullum's law, which basically analyzed the force independence of 33 00:03:21,740 --> 00:03:22,370 the distance. 34 00:03:22,730 --> 00:03:28,880 But here you can see also that he analyzed this in terms of another quantity, which is e the electric 35 00:03:28,880 --> 00:03:29,330 field. 36 00:03:29,840 --> 00:03:33,710 So the electric field times, that charge gives rise to the force. 37 00:03:34,760 --> 00:03:40,700 And these lines of forces are actually really helpful because they allow to understand what would happen 38 00:03:40,850 --> 00:03:44,000 if you would bring an additional charge into the system. 39 00:03:45,110 --> 00:03:51,260 For example, here you can see in the background I have in this bright color here, the electric field 40 00:03:51,260 --> 00:03:54,740 lines for a simple radial symmetric charge. 41 00:03:54,920 --> 00:03:58,580 So these will be radial arrows pointing away from the charge. 42 00:03:59,120 --> 00:04:04,100 And if you have reversed a sign of the charged and you will also reverse the sign of the electric field. 43 00:04:04,950 --> 00:04:11,960 However, in total, this system, consisting out of two oppositely charged charges, will give rise 44 00:04:11,960 --> 00:04:14,240 to such an electric field profile. 45 00:04:14,540 --> 00:04:21,260 So I basically took these two arrows here, for example, and you are in a way, interpolated the electric 46 00:04:21,260 --> 00:04:23,530 field between them. 47 00:04:23,540 --> 00:04:28,460 So you when you and, you know, the electric field in the vicinity of this charge and in this charge, 48 00:04:28,790 --> 00:04:32,660 you can guess what's the total electric field will look like. 49 00:04:33,770 --> 00:04:39,290 So this shape of these lines really allows us to understand what would happen if we would put another 50 00:04:39,290 --> 00:04:40,820 charge, for example, here. 51 00:04:42,350 --> 00:04:50,210 And also this electric field was quite useful to explain earlier experiments or even devices. 52 00:04:50,210 --> 00:04:57,770 For example, in the 1748, there was already the very first capacitor that's in fact relies on storing 53 00:04:57,770 --> 00:05:00,280 charge and generating electric fields. 54 00:05:01,610 --> 00:05:07,430 So there was already quite a lot known about charges, electric fields and also the coolant law. 55 00:05:07,910 --> 00:05:13,610 And this all in the end was condensed in the Maxwell's equations. 56 00:05:13,940 --> 00:05:18,590 And similarly, there was already quite a lot early knowledge about magnetism. 57 00:05:19,130 --> 00:05:21,770 And this is what we want to discuss in the next lecture.