00:00:00.030 a youtuber sent me an email and was 00:00:06.38000:00:06.390 wondering about capacitors and their 00:00:12.23000:00:12.240 ability to block DC but more puzzling to 00:00:18.68000:00:18.690 him was the ability of capacitors to 00:00:23.56000:00:23.570 pass AC and he was wondering how can 00:00:29.54000:00:29.550 this be 00:00:31.51000:00:31.520 well this is my representation of a 00:00:34.79000:00:34.800 capacitor that is not charged and if we 00:00:41.86900:00:41.879 were to add a battery to it immediately 00:00:46.18000:00:46.190 electrons would start the flow and 00:00:49.84000:00:49.850 notice at each side of the capacitor 00:00:58.31000:00:58.320 which way they're flowing and this will 00:01:03.68000:01:03.690 continue until the capacitor is charged 00:01:10.46000:01:10.470 up to in this case the level of the 00:01:14.53900:01:14.549 battery and then the current will stop 00:01:18.73000:01:18.740 let's take a look at this a couple of 00:01:21.59000:01:21.600 times 00:01:27.85000:01:27.860 at this time the capacitor is charged 00:01:32.71000:01:32.720 and you could consider it now that it is 00:01:37.75000:01:37.760 blocking DC but all that means is the 00:01:41.89000:01:41.900 capacitor is charged up to the DC value 00:01:45.49000:01:45.500 of where it is in the circuit now if I 00:01:51.67000:01:51.680 would have reversed the battery of 00:01:55.05000:01:55.060 course we would end up with this a fully 00:01:59.68000:01:59.690 charged capacitor with the opposite 00:02:05.11000:02:05.120 polarity now let's take a look at AC and 00:02:11.07000:02:11.080 of course it's a good idea to understand 00:02:14.29000:02:14.300 what alternating current is it means 00:02:17.89000:02:17.900 that the current will move in one 00:02:21.40000:02:21.410 direction and then reverse direction and 00:02:26.69900:02:26.709 then reverse back again here we have 00:02:30.79000:02:30.800 reached the most positive part of the 00:02:37.15000:02:37.160 alternating current and now it's going 00:02:39.72900:02:39.739 to start going back down and go past 00:02:43.27000:02:43.280 zero and then continue on until it 00:02:48.94000:02:48.950 reaches maximum negative voltage and 00:02:54.60000:02:54.610 then it's going to go back in the other 00:02:57.52000:02:57.530 direction towards zero and when this is 00:03:01.81000:03:01.820 complete this is one cycle let's watch 00:03:07.33000:03:07.340 this one cycle a few times 00:03:18.50900:03:18.519 now of course this is only one cycle it 00:03:22.89900:03:22.909 actually continues going up and down and 00:03:28.19900:03:28.209 here I'm showing three cycles but it 00:03:32.50000:03:32.510 just continues this changing from 00:03:36.28000:03:36.290 positive to negative the definition of 00:03:40.34900:03:40.359 alternating current now here is a 00:03:45.09900:03:45.109 capacitor hooked up to an AC source and 00:03:49.94900:03:49.959 when it's positive on the Left it'll 00:03:55.50900:03:55.519 charge the capacitor like this and when 00:03:59.50000:03:59.510 it's positive on the right it'll charge 00:04:01.92900:04:01.939 the capacitor this way but notice as 00:04:06.72000:04:06.730 this alternating current goes back and 00:04:09.87900:04:09.889 forth the electrons shown by the arrows 00:04:16.53900:04:16.549 keeps flipping back and forth also 00:04:20.34900:04:20.359 notice that there are no electrons that 00:04:23.83000:04:23.840 travel between the plates which was also 00:04:29.02000:04:29.030 true with the DC battery no electrons 00:04:37.29000:04:37.300 travel between the two plates it is the 00:04:42.31000:04:42.320 charging and discharging of the plates 00:04:44.95000:04:44.960 that is causing the current flow and 00:04:49.27000:04:49.280 because this is alternating current it 00:04:53.21900:04:53.229 is a continuous process now all that we 00:04:59.68000:04:59.690 can really see in the circuit is a 00:05:04.18000:05:04.190 capacitor on an AC source and when we're 00:05:08.92000:05:08.930 looking at it like this it does appear 00:05:12.27000:05:12.280 like the AC current is flowing through 00:05:16.99000:05:17.000 the capacitor that is the effect but 00:05:24.01000:05:24.020 however what is really happening is this 00:05:28.11000:05:28.120 the capacitors 00:05:30.82000:05:30.830 plates are being charged and discharged 00:05:35.08000:05:35.090 alternately by the alternating current 00:05:39.10000:05:39.110 I hope this demonstration clears up some 00:05:46.30000:05:46.310 questions about capacitors being able to 00:05:50.46900:05:50.479 block DC and capacitors being able to 00:05:57.39000:05:57.400 pass AC current thanks for watching 00:06:11.12900:06:11.139 you
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