Electrostatics lab - part 2 of 4 — Transcript

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1. 0:00purpose of this step is just hey let's
2. 0:01just make an observation what actually
3. 0:03happened and so it
4. 0:05it attracts uh step six
5. 0:08says do this move the acetate oops i set
6. 0:11it down which is the acetate
7. 0:12here's the acetate it says move the
8. 0:15acetate
9. 0:16strip towards the pit ball very slowly
10. 0:20and carefully
11. 0:21so you could observe the effect of
12. 0:23distance and you might have already seen
13. 0:24that but i'll just do it again here
14. 0:26here when i'm at a far distance i would
15. 0:29say there's very little
16. 0:30attraction kind of a medium distance
17. 0:32there's a medium amount of attraction
18. 0:34and a short did whoa short distance
19. 0:37there's a lot of
20. 0:39attraction and so notice
21. 0:42then that the amount of attraction
22. 0:45depends on how close you are
23. 0:47and in fact they even say here can you
24. 0:49express this as a proportion
25. 0:51so excuse me i'm going to grab a little
26. 0:54water here because it's just not used to
27. 1:00talking so my my voice is a little
28. 1:02hoarse all right first day
29. 1:04okay but the uh proportion i would say
30. 1:08this way i mean how would you describe
31. 1:10it in math class
32. 1:11when when one thing is big so when
33. 1:13distance is big
34. 1:14the force is small on the other hand
35. 1:17when the
36. 1:18distance is small the force is big
37. 1:22and we would call that an inverse
38. 1:24proportion
39. 1:26and so they are inversely proportioned
40. 1:29now if you've gone a little bit farther
41. 1:31in the lecture
42. 1:31just so that i don't confuse anybody um
43. 1:35it's actually inversely proportional to
44. 1:37the distance
45. 1:38squared it's not just the distance it's
46. 1:40the distance squared and that was that
47. 1:44measurement of of of measuring values
48. 1:47that
49. 1:48coulomb did and so we're not measuring
50. 1:51how much the force is so i don't think
51. 1:53we can say it's
52. 1:54inversely proportional to the square we
53. 1:56can't quite get that conclusion but i'm
54. 1:58going to say that because that's what
55. 2:00coulomb did and that's why
56. 2:01you know we named the units of charge
57. 2:04coulombs in
58. 2:05his honor because he of coulomb's law
59. 2:07and he found that proportionality but
60. 2:09it's inversely proportional
61. 2:10is is good enough for what what what we
62. 2:13have here in step six
63. 2:15all right now step seven is kind of
64. 2:17where the the physics starts to to come
65. 2:19in and i've
66. 2:20explained a lot of this but i'll but
67. 2:21i'll keep going here watch this uh
68. 2:23number seven
69. 2:24it says the vinyl plastic
70. 2:27and so let me find the my vinyl here's
71. 2:28my vinyl
72. 2:30has a greater affinity which is a fancy
73. 2:33word for attraction
74. 2:34there's a greater attraction for the
75. 2:36electrons than the wool does
76. 2:38so when the wool and the vinyl are
77. 2:40brought together
78. 2:43what will be the charge on the vinyl and
79. 2:45where did it come
80. 2:47from and although i answered this one
81. 2:50time before let me say it again so when
82. 2:52i'm rubbing this this is back to this
83. 2:53picture right here
84. 2:54when you rub them and you first free up
85. 2:55the electrons and then they snap back
86. 2:57they're saying the electrons are
87. 2:58snapping back they have a greater
88. 3:00affinity
89. 3:01for the vinyl than for the wool and so
90. 3:03as i rub them the electrons then
91. 3:05get uh over to
92. 3:09excuse me get over to
93. 3:13the vinyl and of course what
94. 3:16comes on this vinyl is not just the
95. 3:19original ones that were scraped off
96. 3:21when i was rubbing them but also ones
97. 3:23that were scraped off of the wool
98. 3:25and so that's why we say the wool is
99. 3:26negative because it's got more electrons
100. 3:28than it does positive and where did they
101. 3:30come from they came from
102. 3:32the wool and so that's the answer to to
103. 3:35those two parts
104. 3:36the vinyl is negative and where did
105. 3:38those negatives come from
106. 3:39they came from the wool
107. 3:44all right so let's
108. 3:47keep going look at step eight here step
109. 3:50eight says
110. 3:51touch the pitbull with your fingers and
111. 3:53this will neutralize it
112. 3:55so remember that stuff we just did back
113. 3:57in step one talking about grounding it
114. 3:59and let's just make sure it's
115. 4:03grounded now in case you're wondering
116. 4:06well
117. 4:06why wouldn't it be remember we touched
118. 4:10it back in step four
119. 4:12and that touching we're going to come
120. 4:14back to we could have put some charges
121. 4:15on it and we actually did
122. 4:18uh they haven't asked about that yet but
123. 4:20that's why it's so important here
124. 4:22that we touch it and neutralize in fact
125. 4:24i'll even neutralize myself back over
126. 4:26here at the
127. 4:27at the sink okay but the big thing is
128. 4:30getting that pit ball neutral
129. 4:31all right so i'm going to start again
130. 4:33with a neutral pit ball so it says touch
131. 4:35the pit ball with your fingers to
132. 4:36neutralize it charge the acetate all
133. 4:38right so let me find my acetate
134. 4:41here's my acetate and let me rub
135. 4:44charge the acetate strip by rubbing the
136. 4:47wool
137. 4:48and then touch the pith ball
138. 4:53and so now we're touching it in fact if
139. 4:55you'll notice this is exactly
140. 4:57the same thing we did in step four
141. 4:58except step four we were using the vinyl
142. 5:00to touch it
143. 5:02and so now we are touching it with the
144. 5:05acetate and just like in step four
145. 5:11it takes some effort to see this and
146. 5:13this one's a little bit harder still
147. 5:17so let's see if i can get
148. 5:20the effects and this is
149. 5:24probably the hardest one of the
150. 5:26experiment ah i
151. 5:27can you see it repelling
152. 5:32we want to get it there we go to repel
153. 5:37and so i have now brought
154. 5:40the acetate which as we said earlier it
155. 5:43will be positive when we rub it
156. 5:45near that pit ball that i touched and
157. 5:48they repel
158. 5:50and that's the answer you want to put
159. 5:51here in step 8 it says what happens and
160. 5:53you just want to put they repel
161. 5:55but watch this and i better do this
162. 5:57before the moisture in the air and it's
163. 5:59really humid today
164. 6:00uh takes all the charges off here but
165. 6:03step nine then it says now that you've
166. 6:05touched it bring
167. 6:06the vinyl so i've rubbed the vinyl
168. 6:08remember the vinyl is negative near it
169. 6:10and see what happens and hopefully
170. 6:12you'll see
171. 6:13it attracts so it's attracted to the
172. 6:17vinyl
173. 6:18but it is repelled
174. 6:21from the acetate
175. 6:26let's see if we can get it repelling a
176. 6:27little more don't touch it
177. 6:30and so maybe should have left it alone
178. 6:33working a little bit
179. 6:34better there we go that's a good one
180. 6:36there all right so it repels
181. 6:38from the acetate
182. 6:41but is attracted
183. 6:46ooh to the vinyl that's our step
184. 6:49eight and and nine
185. 6:53and they even say what happened so you
186. 6:55could put rappel here
187. 6:56but uh uh hopefully i'm i'm sorry uh
188. 6:59nine is
189. 7:00they they they attract they repelled in
190. 7:01innate and it says contrast the strength
191. 7:04of what happened in step
192. 7:06three okay because remember in step
193. 7:08three they were
194. 7:09attracted but this one was a lot
195. 7:13stronger in its attraction and so you
196. 7:16could
197. 7:16write that down and finally we're
198. 7:20really getting to the the heart of the
199. 7:22matter uh like i said it's
200. 7:25too bad you know we can't have a
201. 7:28face-to-face setting and you guys get to
202. 7:30do this yourself it's kind of fun to
203. 7:31you know actually rub these and and
204. 7:33watch what what happens
205. 7:35but hopefully the big picture you're
206. 7:36you're still getting and then that's
207. 7:38where this
208. 7:39this physics is because step 10 says
209. 7:41this the wool
210. 7:42has a higher affinity for the electrons
211. 7:45than the acetate so here's the acetate
212. 7:48so when you rub them together here's the
213. 7:50question what is the sign of the charge
214. 7:53on the acetate
215. 7:55what charges moved and how
216. 7:58so again let me just emphasize when you
217. 8:00take two materials and you rub them
218. 8:01together
219. 8:02the rubbing and the bumping is going to
220. 8:04strip electrons off and then since they
221. 8:06are made out of different materials
222. 8:07that means the positives that are left
223. 8:09over one will be a little bit more
224. 8:11attracting those electrons we call that
225. 8:13an
226. 8:13affinity for the electrons and so
227. 8:15they're trying to say here
228. 8:17that when you take acetate and wool
229. 8:20the wool has a higher affinity than the
230. 8:22the acetate
231. 8:23so when you rub them where do the
232. 8:27electrons go after they get bumped off
233. 8:29they go to the
234. 8:31wool and so the first part of this
235. 8:33question says what is the sign of the
236. 8:35charge on the acetate
237. 8:36so i would say the acetate then it's
238. 8:38positive there are more
239. 8:40positives than negatives on here and by
240. 8:43the way did you notice i did not say
241. 8:45that positives were put on here to make
242. 8:46the positives
243. 8:48right did you notice that the reason
244. 8:50this is positive because the electrons
245. 8:52were off did you remember those first
246. 8:55four
247. 8:56basic ideas at the beginning of this
248. 8:57chapter that or this this lab that said
249. 9:00that the things that move the mobile
250. 9:02pieces are the electrons
251. 9:04and so when i rub this and say this is
252. 9:06positive i didn't put positives on here
253. 9:09what i did was take negatives off
254. 9:13which makes them more positive than
255. 9:15negatives on here there's still a lot of
256. 9:16negatives on here don't get me wrong
257. 9:18really percentage-wise i took off very
258. 9:20very few
259. 9:21so i only took off a few hundred billion
260. 9:23you know which sounds like a big number
261. 9:25but
262. 9:25that's a small percentage of the number
263. 9:28of electrons that that are that are on
264. 9:29here
265. 9:30in fact if you want to always kind of
266. 9:32have a fun number there's more electrons
267. 9:34in this piece of acetate
268. 9:36then there are stars in the universe
269. 9:41in fact there's there's more electrons
270. 9:42on this piece of acetate than there are
271. 9:44grains of sand
272. 9:45on all the deserts and all the seas
273. 9:48of the earth in fact there are more
274. 9:51electrons in here
275. 9:52than the sum of all the grains of sand
276. 9:54on the earth
277. 9:55and all of the stars in the universe
278. 9:57there's a lot of electrons so a few
279. 9:58hundred billion billion
280. 9:59is a small number that came off of here
281. 10:02but more on the numbers as we get into
282. 10:04the calculations and that's not what
283. 10:05today's lab is is is all about but i am
284. 10:07trying to answer then
285. 10:09this this first question uh which says
286. 10:12what is the sign so this is positive the
287. 10:14acetate it says what charges
288. 10:16moved and that was key here the
289. 10:18negatives moved i want to keep
290. 10:19emphasizing that
291. 10:20because one of the things that hopefully
292. 10:21you'll get straight in your mind right
293. 10:23away here at the beginning otherwise
294. 10:24things won't make sense
295. 10:26not only for this lab but for most of
296. 10:27the semester if you start thinking about
297. 10:29these positives they're moving the
298. 10:30positives aren't moving the negatives
299. 10:32are and i should say it again at least
300. 10:33in the solid because
301. 10:35in a liquid you know the atoms
302. 10:36themselves could actually move in a gas
303. 10:38the atoms could actually move and those
304. 10:39are where the positives might be but
305. 10:41in a solid the positives do not not move
306. 10:45okay all right well let me continue on
307. 10:48uh number eleven number eleven says from
308. 10:51the behavior of the balls
309. 10:54and the strips in step four and step
310. 10:57eight now since i've been doing a lot of
311. 10:59talking maybe i should review
312. 11:01what i do in step four well in step four
313. 11:03i took
314. 11:05a pith ball that was neutral
315. 11:08and i rubbed a piece of vinyl which now
316. 11:11made it negative
317. 11:12i brought it near and i touched it and i
318. 11:16got something like that
319. 11:17i got them to repel
320. 11:20and in step eight i did something like
321. 11:22this i neutralized the pitbull
322. 11:25i took the acetate i then touched the
323. 11:29acetate
324. 11:30and i might have to do it a couple of
325. 11:31times
326. 11:34and i got it to repel
327. 11:39and so that's our summary or a recap it
328. 11:41says
329. 11:42state here the general behavior of like
330. 11:44charges
331. 11:46here's hopefully what you observed in
332. 11:48that that when i
333. 11:49touched it with say the vinyl which
334. 11:52remember the vinyl has a bunch of
335. 11:53negatives on it
336. 11:54and i touched it what happened is
337. 11:58the sum of the negatives came off of the
338. 12:01the vinyl that's why it's not easy to do
339. 12:02vinyl doesn't like to get rid of
340. 12:03negatives
341. 12:04okay but if i can get it
342. 12:07to transfer over
343. 12:11then there will be negatives on the pit
344. 12:12ball
345. 12:14and of course there's still negatives on
346. 12:16the acetate
347. 12:17and so i would say i have like charges
348. 12:19here i have negatives
349. 12:20and so that's what step four was about
350. 12:22whereas step
351. 12:25eight was this way i started with a
352. 12:27neutral pit ball
353. 12:30and i took the acetate and made it
354. 12:31positive
355. 12:34and of course positive means it lacks
356. 12:35electrons so when i touched it a few
357. 12:37times
358. 12:38some of the electrons from the pit ball
359. 12:39jumped over to here
360. 12:41so now the pith ball is lacking
361. 12:44electrons and so the pitbull is now
362. 12:45positive
363. 12:46as well as this is being positive and
364. 12:48they repelled
365. 12:50and so both step four and step eight
366. 12:53they repelled and they were like charges
367. 12:54in step eight they were both negatives
368. 12:56instead
369. 12:57no step four they were both negatives
370. 12:59and in step eight they were both
371. 13:01positives
372. 13:02and so the lesson here if you didn't
373. 13:05catch it in the lecture
374. 13:06let me repeat it here is that if you
375. 13:07ever have like charges together they
376. 13:09repel
377. 13:09that's what this experiment is trying to
378. 13:11show you if they are both
379. 13:13negative they repel or if they are both
380. 13:15positive they
381. 13:16repel and so that's what they want you
382. 13:20to do can you state a general principle
383. 13:21of like charges yes
384. 13:22like charges repel that's the answer you
385. 13:24should should type in here for
386. 13:26for step 11. all right let's look at
387. 13:28step 12.
388. 13:29and step 12 it says look at the behavior
389. 13:32and what happened in step five and step
390. 13:33nine
391. 13:34now let me just repeat step five here's
392. 13:37what we did in step five
393. 13:38a step five was a continuation of four
394. 13:41because what we did
395. 13:43in four was to make the pit ball
396. 13:46negative by touching it and the two
397. 13:48negatives repelled
398. 13:49so the pit ball is now negative then we
399. 13:51put down the vinyl and in step five we
400. 13:53rub the acetate which we now know is
401. 13:55positive
402. 13:55and so we brought the positive near the
403. 13:57negative and they
404. 13:59attracted and so we made the pitbull
405. 14:03negative by touching it in step four
406. 14:05and then we brought the acetate and made
407. 14:07it positive by rubbing it with wool
408. 14:09and so now we got the two positive then
409. 14:10the i'm sorry
410. 14:12we i said that we we got that negative
411. 14:15by touching
412. 14:16it with the vinyl okay and some
413. 14:17electrons transfer it over and we got
414. 14:19this positive by rubbing it with the
415. 14:21wool and so we have then of course a
416. 14:23negative pit ball
417. 14:25and we have a positive acetate and we
418. 14:27have an attraction between them
419. 14:30we actually saw the same thing in step
420. 14:32nine we did it in reverse order we did
421. 14:34this we neutralized it
422. 14:37in step eight
423. 14:41we then touched it a few times
424. 14:46with the positive
425. 14:50acetate and like i said this is the
426. 14:54hardest one of the experiment
427. 14:58i'm not getting much of a repulsion
428. 15:06there now there i got a little bit but
429. 15:08now that pit ball is positive so then in
430. 15:10step nine we were asked to make
431. 15:12the vinyl which we made the vinyl
432. 15:13negative by rubbing it with the wool
433. 15:15and so we've got a negative and a
434. 15:17positive and they attract
435. 15:19and so hopefully in both of those steps
436. 15:22five and nine
437. 15:23you see that you had opposite charges
438. 15:26and they attracted and so that's the the
439. 15:28lesson he says can you state
440. 15:29a general behavior of unlike charges
441. 15:31yeah unlike charges attract
442. 15:33it doesn't matter if this one is
443. 15:34positive or
444. 15:36negative as long as this one is is
445. 15:38opposite so if i had a positive and
446. 15:39negative they would attract or if i have
447. 15:40a negative and a positive they would
448. 15:43attract and so we saw solve both of
449. 15:45those
450. 15:47now here's a fun one this 13.
451. 15:5013 says maybe the most difficult
452. 15:53phenomenon to explain is actually what
453. 15:55we did at the beginning back in step
454. 15:57three where our pit ball was neutral
455. 16:01do you remember that one let me go back
456. 16:03to step three just as kind of a recap
457. 16:05here
458. 16:05see step three you were asked to bring
459. 16:07the vinyl near it
460. 16:08after you had neutralized it back in
461. 16:10step one so let me neutralize my pit
462. 16:12ball again here so i'm gonna neutralize
463. 16:13my pith ball
464. 16:16and i'm gonna grab my vinyl
465. 16:20and i'm to rub it
466. 16:24now we've been learning that the vinyl
467. 16:26is now negative
468. 16:27but keep in mind the pith ball is not
469. 16:30negative it's not positive it's neutral
470. 16:34and remember it attracted
471. 16:39so the neutral was attracted
472. 16:42to a negative
473. 16:45you even saw that in step two in step
474. 16:48two
475. 16:49we rub the acetate which we now know is
476. 16:51positive
477. 16:52and so here is the positive and a
478. 16:54neutral pit ball and what do we get
479. 16:59we also got attraction
480. 17:02and so your author here in step 13 it
481. 17:04says one of the most
482. 17:06difficult phenomenon to explain is the
483. 17:08interaction between a
484. 17:09charged body and a neutral pit ball can
485. 17:12you explain that
486. 17:14so let me help you with that one well
487. 17:16let me do that
488. 17:18with a picture
489. 17:23and here's where the idea of conductor
490. 17:25is going to be so
491. 17:26crucial and the concept that a neutral
492. 17:30object
493. 17:31has equal number of positives and
494. 17:33negatives it it's not that it has no
495. 17:35charges
496. 17:36it's that it has an equal number of
497. 17:38positives and negatives watch
498. 17:39uh let me take this and call this
499. 17:43my piece of vinyl so if this is the
500. 17:47neutral pit ball
501. 17:48and this is the piece of vinyl let me
502. 17:52just put some negatives here
503. 17:55to indicate that
504. 17:58this vinyl strip is is negative charge
505. 18:01that has more negatives than positive
506. 18:03that's what happened when we rubbed it
507. 18:04with the wool but this is neutral
508. 18:08and your first thought is well it's not
509. 18:10going to interact because we know if
510. 18:11this was negative it would repel and if
511. 18:13it was positive it would attract
512. 18:15but remember step six and remember it's
513. 18:18a conductor watch here's what i mean by
514. 18:20that
515. 18:21the negatives that are
516. 18:25in the position of the ball that is
517. 18:28close to the vinyl strip
518. 18:32would get pushed away
519. 18:35because remember like charges
520. 18:39repel and remember this is a conductor
521. 18:44so i would still say that this pit ball
522. 18:47is
523. 18:47neutral if you were to add up
524. 18:50all the negatives in this pit ball and
525. 18:52all the positives in this pit ball you
526. 18:55would say
527. 18:55they balance but they have separated
528. 18:58themselves
529. 19:00and we call that then polarized
530. 19:04this ball is neutral but it's also
531. 19:08polarized now the reason that's
532. 19:10important
533. 19:11is because if you kind of imagine
534. 19:15the pith ball as being two halves i'll
535. 19:18call this the front half
536. 19:20the front half is being attracted
537. 19:23towards this strip
538. 19:25and the back half is being repelled
539. 19:30if i call this force number one
540. 19:33and force number two again you might
541. 19:36still think well okay then the net force
542. 19:38is zero
543. 19:39no no the net force would only be zero
544. 19:41if those forces were equal
545. 19:42so remember back in step six where we
546. 19:44said the force was related to the
547. 19:46distance it was inversely proportional
548. 19:48that would tell me that force number one
549. 19:51which is made out of positives
550. 19:54and it's closer produces a force
551. 19:57a little bit greater than force number
552. 19:59two
553. 20:02and so there is a net attractive force
554. 20:07in fact that's really how you they want
555. 20:09you to answer number 13 but i'll take
556. 20:11this a step further
557. 20:12because if this was
558. 20:17step 2 and the strip that i'm bringing
559. 20:20near the neutral pit ball was acetate
560. 20:23meaning it was positive
561. 20:26what would happen is this positive would
562. 20:28attract the electrons
563. 20:31to the front part of the ball so they
564. 20:33would come in from the back side
565. 20:35so we would lose electrons over to here
566. 20:37so that's why it's positive and they
567. 20:38would come over
568. 20:39to here and if you split this in half
569. 20:42you would still say this is attracted so
570. 20:43that's f1
571. 20:45this is then repelled so that's f2 but
572. 20:48because these are closer
573. 20:49f one is still greater than f two
574. 20:53and so you get a attraction and so a
575. 20:56neutral
576. 20:56pith ball or a neutral anything
577. 21:00is attracted to anything that's charged
578. 21:02it doesn't matter whether it's positive
579. 21:04or negative
580. 21:05it's attracted and so that's why step
581. 21:07two we saw attraction
582. 21:08and then step three we saw attraction
583. 21:11and of course what you need to answer
584. 21:12and turn in is is this description
585. 21:14why does a negative brought near a
586. 21:16neutral
587. 21:18attract and so hopefully you can you
588. 21:21know kind of say these three steps
589. 21:22step one the ball becomes polarized
590. 21:26step two you get both an attraction and
591. 21:29a
592. 21:29repulsion step three the attraction is a
593. 21:34little bit greater than the repulsion
594. 21:36and therefore we get a net attraction
595. 21:38force a small
596. 21:40net attraction force and that's that's
597. 21:42how they want you to answer 13. so i
598. 21:44hope i didn't take too long getting to
599. 21:45number 13.
600. 21:46but i hopefully then you you get this
601. 21:48bigger picture in fact
602. 21:49a fun experiment to do is to realize
603. 21:53then that neutral objects and
604. 21:56just go around your house especially on
605. 21:58these hot humid days you're probably
606. 21:59running a fan
607. 22:00running a ceiling fan well remember a
608. 22:03lot of these fans are made out of
609. 22:04plastic or made out of vinyl
610. 22:06and they spin those blades spin through
611. 22:08the air so they're being rubbed
612. 22:10and they get charged so the blades on
613. 22:12the fan are actually charged to
614. 22:13themselves
615. 22:14so what happens to the dust as it goes
616. 22:16through
617. 22:18gets attracted to those blades and so it
618. 22:20doesn't take long for a
619. 22:21fan blade to get really dusty
620. 22:25if you have a ceiling fan i got a
621. 22:27challenge for you
622. 22:29step on a a chair a ladder
623. 22:33and draw your finger across the top of
624. 22:36that fan blade
625. 22:38if it hasn't been cleaned in a while
626. 22:40it'll be really
627. 22:42really dusty it attracts a lot of dust
628. 22:45or if you have an entertainment center
629. 22:47or some place you you know you put your
630. 22:49video games or you know your
631. 22:51your xbox if you play video games or
632. 22:53your tv
633. 22:54or your dvr player and it's sitting on a
634. 22:56shelf if it's been a while since you
635. 22:58moved it
636. 22:59all the electronics in there have an
637. 23:00electrical charge and they love dust
638. 23:03and so i bet it is really dusty on the
639. 23:06back of that thing
640. 23:07underneath the thing it just sucks up
641. 23:11dust like crazy uh
642. 23:14i know i have a nice little air filter
643. 23:17that i like
644. 23:18because i have an allergies and health
645. 23:21problems
646. 23:22uh breathing particularly at night but
647. 23:25it's
648. 23:25a really neat it's just a fan but the
649. 23:28design is great so if the air blows
650. 23:30through the fan there's a metal grid on
651. 23:31the outside of the fan
652. 23:33and i'm sorry well there is a metal grid
653. 23:37too that's a little more sophisticated
654. 23:38but there's
655. 23:39just a plastic grid uh it's vinyl and so
656. 23:42as the air blows through
657. 23:43that plastic grid gets rubbed by the air
658. 23:46blowing through and it builds up charge
659. 23:48and so then the dust and the pollen that
660. 23:50comes up sticks to it
661. 23:52and so what comes out is pretty clean
662. 23:54air
663. 23:56and so maybe once every three or four
664. 23:58days i take off that little plastic
665. 24:00cover
666. 24:01and look at it's just coated with dust
667. 24:03and so then i can just
668. 24:04hose it off outside or even just put it
669. 24:06up into the shower and it'll
670. 24:08just kind of wash off all that dust and
671. 24:10i put it back in there and so it's
672. 24:11it's cleaning out all the dust because a
673. 24:14really neat property
674. 24:15here is that neutral objects are
675. 24:18attracted
676. 24:19to anything that's positive and so and
677. 24:22anything that's positive could be
678. 24:23anything that rubbing so
679. 24:24it's very simple just to have this
680. 24:26plastic grid in front of the fan
681. 24:28it gets charged up and then it attracts
682. 24:30all the pollen and dust and
683. 24:31dander in there all right anyways
684. 24:34i should keep going because i don't i
685. 24:36didn't want this to go quite this this
686. 24:37long here but
687. 24:38let's keep going here because the last
688. 24:41one before we kind of switch gears here
689. 24:43is 14
690. 24:44and this is one that is near the end of
691. 24:47the
692. 24:47lecture so unless you've listened to
693. 24:49both lectures let me just kind of
694. 24:51explain
695. 24:51this i'll i'll explain it in a in a
696. 24:54picture here
697. 24:57so let me kind of clean off the on the
698. 24:59board here
699. 25:05but it says here since the charge strips
700. 25:08did not actually
701. 25:09touch the bodies that exerted forces on
702. 25:12them
703. 25:13what physical phenomenon is being
704. 25:15illustrated here
705. 25:19so why don't i just look at step number
706. 25:22four see in step number four
707. 25:26we had taken the vinyl
708. 25:30and once we rubbed it with the wool it
709. 25:31became negative
710. 25:35and we first touched the pitbull
711. 25:39so that made the pit ball negative in
712. 25:41fact you noticed i had to do it maybe
713. 25:42two or three times before i really got
714. 25:44it negative okay
715. 25:46but then we saw that they repelled each
716. 25:48other and so that's why we answered that
717. 25:50like charges two negative charges would
718. 25:52repel each other
719. 25:56but there is something very very
720. 25:59important to comprehend
721. 26:02and let's see if i've got blue on here i
722. 26:05do
723. 26:06because the author of your book at least
724. 26:07the survey book likes to label
725. 26:09electric fields in blue
726. 26:12uh i need to check that out i
727. 26:16try to remember from last year i'm
728. 26:18pretty sure it's blue
729. 26:22or is it orange
730. 26:25well i'm going to draw electro fills in
731. 26:28blue and i'll double check that
732. 26:30but this is what they're trying to say
733. 26:33it's not
734. 26:33just that this is negative and this is
735. 26:36negative and they
736. 26:37they repel that's what coulomb
737. 26:39discovered
738. 26:40but gauss and others went a little bit
739. 26:42further and discovered
740. 26:44that there is something a little bit
741. 26:47more going on let me get rid of the
742. 26:50pitball for a second
743. 26:53they said
744. 27:00that if you didn't even have a second
745. 27:03charge
746. 27:04here the first charge isn't just a
747. 27:07charge it also makes something something
748. 27:09you can't see with your eyes but you can
749. 27:10detect with experiments
750. 27:13it makes an electric field
751. 27:16and then
752. 27:20when a second charge
753. 27:25is put near
754. 27:28the first charge it's pushed away
755. 27:32not just because this is a negative
756. 27:34charge but it's pushed away by the field
757. 27:37itself
758. 27:38the field is the mechanism of how the
759. 27:40force
760. 27:41gets to that object it would be like
761. 27:44saying
762. 27:45you see this pin in physics 121 well we
763. 27:48did this we we we held
764. 27:49something up above the air we let it go
765. 27:51and we said gravity pulled it down we
766. 27:53said the earth pulled it down
767. 27:55but how did the earth pull it down
768. 27:59and in physics 21 we didn't go that far
769. 28:02but what's happening
770. 28:03is the earth itself makes above it
771. 28:06a field and this then is placed in the
772. 28:09field
773. 28:10and it's the field that grabs it and
774. 28:12it's a field that that pulls it down
775. 28:17and so it's not just that the earth
776. 28:19pulls down
777. 28:20on the pen it's that the earth creates a
778. 28:24field and the field pulls down on the
779. 28:26pen there is a mechanism of how that
780. 28:29force gets across here
781. 28:31and that's what we're trying to say here
782. 28:33you may not be able to see these blue
783. 28:35lines
784. 28:36you may not see what we're going to call
785. 28:37the electric field
786. 28:39but that electric field is is there you
787. 28:41can test for it human eyes can't detect
788. 28:44it but we can run little experiments
789. 28:45with it
790. 28:47and so this is a bit more complicated
791. 28:49than just as coulomb first discovered it
792. 28:51and maybe as
793. 28:51you might have learned it in your high
794. 28:53school physics it's not just oh yeah two
795. 28:54negatives repel each other no no no
796. 28:56it's the first negative makes a field
797. 28:59and then the second one is pushed by
798. 29:01that
799. 29:02field and so that's why far more
800. 29:04important is not coulomb's law
801. 29:06but the electric field notice the title
802. 29:08of the first chapter is not coulomb's
803. 29:10law
804. 29:11title the first chapter is electric
805. 29:12fields
806. 29:15far more important far more fundamental
807. 29:17anyways and so that's what they're
808. 29:18trying to get out here and so like i
809. 29:20said
810. 29:20uh that more advanced part of it doesn't
811. 29:23really show up until
812. 29:24late in the first lecture or the or the
813. 29:26second lecture
814. 29:28and it's something that may even be new
815. 29:30to you because if you took
816. 29:31in in your high school physics you you
817. 29:34know you might have just talked about
818. 29:35oh yeah two negatives they repel each
819. 29:37other coulomb's law sure
820. 29:39two positives oh yeah they repel each
821. 29:41other negative positive and you never
822. 29:42got into the
823. 29:43mechanism to what we can't see with our
824. 29:46eyes but it's that that field so that's
825. 29:47why we like to draw a lot of pictures we
826. 29:48can kind of kind of understand that
827. 29:50a little bit better all right well i
828. 29:52better keep going because i
829. 29:53hopefully giving you a lot more
830. 29:54information than you probably would have
831. 29:56discovered like i said
832. 29:58this uh online uh lab
833. 30:01it's got its pros and it's gone and i
834. 30:03think one of the pros here is that i can
835. 30:05elaborate a little bit more
836. 30:09but also hopefully you pick that up in
837. 30:10the reading in the lecture too but you
838. 30:12know if not
839. 30:12that's why we do it multiple times here
840. 30:14and so hopefully you're getting that
841. 30:16all right well we're going to switch
842. 30:16gears i probably should have moved my
843. 30:18electroscope
844. 30:20under the heat lamp uh you'll probably
845. 30:22notice this whole time i keep putting
846. 30:24things under the heat lamp because
847. 30:25really humid today a lot of moisture and
848. 30:28uh
849. 30:29h2o molecules
850. 30:33have their charges and they totally
851. 30:35interrupt our experiments and
852. 30:37kind of kind of hard to do so that's why
853. 30:40i
854. 30:41put the heat lamp out here today so that
855. 30:43we'll have success hopefully
856. 30:45and so so far we've had success a lot of
857. 30:47times these are these are hard in fact
858. 30:49i guess that's one advantage of this
859. 30:50online uh lab also is that
860. 30:52i can make sure you see the right thing
861. 30:54i know that uh a lot of times i
862. 30:56in an online setting i've got to go
863. 30:58around and people are doing getting
864. 30:59weird results and it's because of the
865. 31:00moisture or they're not rubbing hard
866. 31:02enough
867. 31:02there's not doing it right here and so
868. 31:04at least at least this way i can keep
869. 31:06doing until i get it right and i know
870. 31:08what it's supposed to
871. 31:09to illustrate so maybe that's a positive
872. 31:11all right but
873. 31:12we still got what 10 more questions here
874. 31:14to do together so let's go to number 15.
875. 31:16number 15
876. 31:17introduces you to a thing called an
877. 31:18electroscope and number 15 is just
878. 31:21asking you to to label it and so let me
879. 31:23kind of why it's under the heat lamp and
880. 31:25hopefully not too hot that i can still
881. 31:26handle it i can kind of hold it up here
882. 31:29and i don't want to take it too
883. 31:31far apart but i'm hoping you could it's
884. 31:35really delicate
885. 31:36i'm hoping you can see that this metal
886. 31:37piece at the top which i'll call a knob
887. 31:41is connected to two metal pieces
888. 31:44all the way through down here and
889. 31:47there's two this side hopefully looks
890. 31:49like gold if it is a gold foil
891. 31:51and the other side looks like a
892. 31:53stainless steel
893. 31:55if i hold it this way and tilt it maybe
894. 31:57gravity will make the the foil
895. 32:00uh tilt down and so you can see that
896. 32:03one of the pieces of melt of metal and
897. 32:06again metal
898. 32:07conductor this is going to be important
899. 32:09uh can move
900. 32:10and the other one can't it's a little
901. 32:12stiffer because it's a little thicker
902. 32:14it's an eighth of an inch stainless
903. 32:15steel piece gravity wouldn't make it
904. 32:17move
905. 32:17you'd take a lot of force to bend that
906. 32:20eighth of an inch but the foil
907. 32:22really really thin that's why we make it
908. 32:24out of gold
909. 32:26it's really really thin uh we
910. 32:29you can't cut aluminum foil this thin
911. 32:32and so it's really really light and it's
912. 32:33a conductor
913. 32:34so notice that it's a conductor all the
914. 32:37way through that's what's important so
915. 32:38these lower two let's call them leaves
916. 32:40one is moved so let's call it a movable
917. 32:42leaf and the other one is fixed
918. 32:43so let's call it a fixed leaf and the
919. 32:45top is a knob
920. 32:47and so you're asked one two three four
921. 32:49five uh
922. 32:50descriptions here and so label the the
923. 32:53two lower things the fixed leaf and the
924. 32:55movable leaf and the top part the knob
925. 32:57they actually got a round thing we don't
926. 32:59we don't have a round thing on ours but
927. 33:00in the picture they show this big round
928. 33:02thing looks like a knob
929. 33:03so label that as a knob another
930. 33:06important thing and i
931. 33:08will tilt it a little towards you so you
932. 33:10you you can see
933. 33:12but up here is a
934. 33:16rubber stopper what's important is it's
935. 33:20rubber and rubbers and insulator
936. 33:21remember i said a few times what's
937. 33:23what's key here is to understand
938. 33:26the behavior is different for a
939. 33:28conductor versus
940. 33:29an insulator okay i'll draw a few
941. 33:32pictures here but this is important to
942. 33:34know that i have an insulator
943. 33:35here okay and so
944. 33:39we are going to call that just the
945. 33:41insulated stopper
946. 33:43um let's see i think he uses the words
947. 33:47here sketch the electroscope he says
948. 33:51notice there's a stopper so it's called
949. 33:52stop right it's called an insulated
950. 33:53stopper but
951. 33:54but i want you to know it's insulated
952. 33:57now it's also
953. 33:58shielded with this metal thing so let's
954. 34:00call this a
955. 34:01metal can around the outside and that's
956. 34:04what the other thing
957. 34:06they're taught they're talking about is
958. 34:07shield and in our case
959. 34:09we have a shield that not is only metal
960. 34:12around
961. 34:13this side of it but the front and back
962. 34:14are made out of glass
963. 34:16and that way wind or just me standing
964. 34:19here
965. 34:20my breath would be too much
966. 34:23and it would hit that foil and and tear
967. 34:26it off
968. 34:27so and there's no wind currents in here
969. 34:31it's all
970. 34:31locked up you can see we've got tape
971. 34:33around here
972. 34:35all right so that's enclosed and those
973. 34:38are the names of the parts of this
974. 34:40electroscope
975. 34:41so that's what step 15 is so so so write
976. 34:43those down
977. 34:44uh now if you're doing this on a
978. 34:47computer screen
979. 34:48uh do some kind of rough drawing you can
980. 34:52just take
981. 34:53circles and lines and make an
982. 34:54electroscope and label them
983. 34:56if you're doing this by hand just kind
984. 34:57of draw freehand sketch the electroscope
985. 35:00and label those
986. 35:02five parts all right but you need to
987. 35:05know those names before we go further
988. 35:06but importantly you also need to know
989. 35:09what parts were a conductor which parts
990. 35:11can move and which parts were an
991. 35:12insulator
992. 35:13okay because watch this step 16.
993. 35:16it says bring the negatively charged
994. 35:18strip now if you remember the negatively
995. 35:20charged strip was this
996. 35:21vinyl so i'm going to bring the vinyl
997. 35:23and i'm going to rub it all right
998. 35:25bring the negatively charged strip near
999. 35:27the electroscope but
1000. 35:29do not touch the knob what happens and
1001. 35:32what happens when the strip is removed
1002. 35:34all right so they're not really asking
1003. 35:35you to explain what happened yet so i'll
1004. 35:38be careful not to
1005. 35:39go that far yet but let's just watch
1006. 35:41what happens
1007. 35:43and i think i've got the angle
1008. 35:47so you can see it on the video camera
1009. 35:49and so if i bring it near the knob
1010. 35:52oh wow
1011. 35:55i take it away wow and so hopefully what
1012. 35:58you're seeing is when i bring it close
1013. 35:59the movable leaf moves out and when i
1014. 36:02pull it back the movable leaf falls down
1015. 36:07and that's what they want you to observe
1016. 36:09so
1017. 36:10what happens the moveable leaf out
1018. 36:13that's that's what they want you to
1019. 36:14answer
1020. 36:14and then of course what happens when the
1021. 36:16strip is removed the movable leaf
1022. 36:18you can say falls back or it collapses
1023. 36:20back
1024. 36:22okay all right now 17
1025. 36:26is they want you to explain it i think
1026. 36:28look at 17 it says describe the movement
1027. 36:30of electrons when the negatively charged
1028. 36:33charged strip is brought near the
1029. 36:35electroscope and please
1030. 36:37remember that air is not
1031. 36:40a conductor and i think it's best if i
1032. 36:44draw a picture here and so let me kind
1033. 36:46of clean this
1034. 36:47off here and see if this made any sense
1035. 36:51to you
1036. 36:52i have up at top the knob
1037. 36:57now surrounding the knob
1038. 37:00is this rubber