1
00:00:00,967 --> 00:00:02,637
- [Voiceover] Let's say
you have a circuit here

2
00:00:02,637 --> 00:00:05,075
and you had a battery with a voltage v

3
00:00:05,075 --> 00:00:07,280
and there were resistors one,

4
00:00:07,280 --> 00:00:08,650
resistor two,

5
00:00:08,650 --> 00:00:10,369
and resistor three up here,

6
00:00:10,369 --> 00:00:12,204
and there was current
flowing through here.

7
00:00:12,204 --> 00:00:14,525
What if you wanted to
experimentally measure

8
00:00:14,525 --> 00:00:16,986
the voltage across some of these elements?

9
00:00:16,986 --> 00:00:18,566
You'd have to use a voltmeter.

10
00:00:18,566 --> 00:00:19,820
Voltmeter looks like this.

11
00:00:19,820 --> 00:00:21,770
So a circle with a v in it

12
00:00:21,770 --> 00:00:24,045
is the symbol we use for a voltmeter.

13
00:00:24,045 --> 00:00:25,648
How do you use it?

14
00:00:25,648 --> 00:00:26,530
You take that voltmeter,
you bring it over to here.

15
00:00:27,737 --> 00:00:29,827
I can't plug it in the circuit like that.

16
00:00:29,827 --> 00:00:32,079
What I do is I take the
leads of the voltmeter

17
00:00:32,079 --> 00:00:33,310
and I just connect them

18
00:00:33,310 --> 00:00:35,678
to either side of the circuit element

19
00:00:35,678 --> 00:00:37,559
that I want to determine
the voltage across.

20
00:00:37,559 --> 00:00:40,067
So if I do this and I connect
those leads right here,

21
00:00:40,067 --> 00:00:43,479
this voltmeter will tell me
the voltage across R three.

22
00:00:43,479 --> 00:00:46,084
Or take the voltmeter,

23
00:00:46,084 --> 00:00:47,404
put it over here,

24
00:00:47,404 --> 00:00:49,192
and if I connect the leads

25
00:00:49,192 --> 00:00:52,419
across R one in parallel,

26
00:00:52,419 --> 00:00:53,859
notice I'm hooking up the voltmeter

27
00:00:53,859 --> 00:00:55,113
in parallel.

28
00:00:55,113 --> 00:00:56,947
Voltmeters you always hook up in parallel.

29
00:00:56,947 --> 00:00:59,362
This now will tell me
the voltage across R one

30
00:00:59,362 --> 00:01:00,887
and if I wanted to make sure

31
00:01:00,887 --> 00:01:02,897
my battery was functioning correctly,

32
00:01:02,897 --> 00:01:05,349
I could take my voltmeter and I can

33
00:01:05,349 --> 00:01:07,766
hook up the leads across
the positive and negative

34
00:01:07,766 --> 00:01:08,858
terminals of the battery

35
00:01:08,858 --> 00:01:10,414
and see if the voltage across the battery

36
00:01:10,414 --> 00:01:11,900
is what I think it is.

37
00:01:11,900 --> 00:01:13,270
That's how you use a voltmeter:

38
00:01:13,270 --> 00:01:15,081
always hooked up in parallel.

39
00:01:15,081 --> 00:01:16,915
But if I wanted to measure the current,

40
00:01:16,915 --> 00:01:19,329
I don't use a voltmeter, I use an ammeter.

41
00:01:19,329 --> 00:01:22,278
And for an ammeter you
do not hook up an ammeter

42
00:01:22,278 --> 00:01:25,357
in parallel with the element
you're trying to measure.

43
00:01:25,357 --> 00:01:27,025
You will probably blow out the ammeter.

44
00:01:27,025 --> 00:01:28,478
I've done it a few times.

45
00:01:28,478 --> 00:01:29,407
It's embarrassing.

46
00:01:29,407 --> 00:01:32,263
Don't hook up the ammeter in parallel,

47
00:01:32,263 --> 00:01:33,494
tell you why in a minute.

48
00:01:33,494 --> 00:01:35,304
But what you have to do
is hook it up in series.

49
00:01:35,304 --> 00:01:36,540
So if I wanted to know the current

50
00:01:36,540 --> 00:01:38,162
going through R three,

51
00:01:38,162 --> 00:01:40,822
I could just stick the
ammeter right in here.

52
00:01:40,822 --> 00:01:44,621
One lead would plug into
one side of the ammeter,

53
00:01:44,621 --> 00:01:46,032
the other lead would
plug into the other side.

54
00:01:46,032 --> 00:01:46,961
This current would have to

55
00:01:46,961 --> 00:01:48,328
flow straight through the ammeter

56
00:01:48,328 --> 00:01:49,492
and this is telling me how much current

57
00:01:49,492 --> 00:01:50,652
goes through R three.

58
00:01:50,652 --> 00:01:52,394
It doesn't matter what side I put it on,

59
00:01:52,394 --> 00:01:53,740
the current going into R three

60
00:01:53,740 --> 00:01:55,088
will equal the current going out.

61
00:01:55,088 --> 00:01:56,388
So you can put it over here too,

62
00:01:56,388 --> 00:01:58,617
but it's gotta be hooked up in series.

63
00:01:58,617 --> 00:01:59,801
So you have to disconnect,

64
00:01:59,801 --> 00:02:00,892
it's kind of a pain

65
00:02:00,892 --> 00:02:02,731
to hook up an ammeter sometimes.

66
00:02:02,731 --> 00:02:04,793
You have to disconnect something here,

67
00:02:04,793 --> 00:02:07,353
then connect that connection
to the one side of the ammeter,

68
00:02:07,353 --> 00:02:09,217
connect to the other side of the ammeter.

69
00:02:09,217 --> 00:02:11,086
For a voltmeter, you
didn't have to do that.

70
00:02:11,086 --> 00:02:13,565
For a voltmeter, just kept it out here

71
00:02:13,565 --> 00:02:15,236
and just touch those leads

72
00:02:15,236 --> 00:02:16,684
wherever you needed to touch them.

73
00:02:16,684 --> 00:02:17,907
But for an ammeter,

74
00:02:17,907 --> 00:02:20,461
you have to break the circuit
to let this ammeter in.

75
00:02:20,461 --> 00:02:22,063
But I can move it wherever I want.

76
00:02:22,063 --> 00:02:23,156
I could put it down here,

77
00:02:23,156 --> 00:02:25,104
that tells me the current in this strip.

78
00:02:25,104 --> 00:02:28,402
Again, ammeters always hooked up in series

79
00:02:28,402 --> 00:02:30,282
with the element that
you're trying to measure.

80
00:02:30,282 --> 00:02:33,115
So this ammeter position
will let me measure

81
00:02:33,115 --> 00:02:35,321
the current that's flowing
through the battery.

82
00:02:35,321 --> 00:02:38,011
But why is the voltmeter
always hooked up in parallel

83
00:02:38,011 --> 00:02:40,429
and the ammeter always
hooked up in series?

84
00:02:40,429 --> 00:02:42,890
We want the ammeter to
be hooked up in series

85
00:02:42,890 --> 00:02:44,423
because we want to measure the current

86
00:02:44,423 --> 00:02:46,605
through a line in the circuit.

87
00:02:46,605 --> 00:02:47,581
We want to measure the current

88
00:02:47,581 --> 00:02:49,344
flowing through this resistor.

89
00:02:49,344 --> 00:02:50,460
So if we want to measure the current

90
00:02:50,460 --> 00:02:52,224
flowing through something,

91
00:02:52,224 --> 00:02:53,896
we need to make sure
that the current flows

92
00:02:53,896 --> 00:02:55,266
through our ammeter

93
00:02:55,266 --> 00:02:56,706
and that's how we get our reading.

94
00:02:56,706 --> 00:02:57,611
Because of this,

95
00:02:57,611 --> 00:03:01,585
people design ammeters with
very little resistance.

96
00:03:01,585 --> 00:03:04,917
An ammeter has very little resistance.

97
00:03:04,917 --> 00:03:05,854
And the reason is,

98
00:03:05,854 --> 00:03:08,362
if you took this ammeter
and it had a big resistance

99
00:03:08,362 --> 00:03:09,838
and you stuck it in here,

100
00:03:09,838 --> 00:03:11,659
you'd be changing how much current flowed

101
00:03:11,659 --> 00:03:13,354
through this part of the circuit.

102
00:03:13,354 --> 00:03:14,220
We don't want to do that.

103
00:03:14,220 --> 00:03:15,304
Whenever we measure something,

104
00:03:15,304 --> 00:03:16,581
we don't want to disturb it.

105
00:03:16,581 --> 00:03:18,276
So when I stick my ammeter in here,

106
00:03:18,276 --> 00:03:19,970
I don't want to disturb how much current

107
00:03:19,970 --> 00:03:20,684
was going through here.

108
00:03:20,684 --> 00:03:22,595
I wanted to know how much current flows

109
00:03:22,595 --> 00:03:24,267
without my ammeter being in there.

110
00:03:24,267 --> 00:03:25,706
So when I put my ammeter in there,

111
00:03:25,706 --> 00:03:28,470
it better have very little
affect on this circuit.

112
00:03:28,470 --> 00:03:30,289
That's why we make this ammeter

113
00:03:30,289 --> 00:03:32,370
have a very small resistance.

114
00:03:32,370 --> 00:03:33,671
And that's also why you can't

115
00:03:33,671 --> 00:03:35,551
hook this ammeter up in parallel,

116
00:03:35,551 --> 00:03:37,471
cause if you did, look
at what would happen.

117
00:03:37,471 --> 00:03:38,756
This is why it's bad.

118
00:03:38,756 --> 00:03:39,893
If I took this ammeter

119
00:03:39,893 --> 00:03:41,495
and I hooked it up right here,

120
00:03:41,495 --> 00:03:43,817
and I hooked the other side up right here,

121
00:03:43,817 --> 00:03:45,187
look what the current's gonna do.

122
00:03:45,187 --> 00:03:46,766
I've got current flowing through here,

123
00:03:46,766 --> 00:03:48,780
current comes this way, goes this way,

124
00:03:48,796 --> 00:03:50,110
reaches this fork in the road

125
00:03:50,110 --> 00:03:51,014
and it's got a choice.

126
00:03:51,014 --> 00:03:52,316
It can go to the left

127
00:03:52,316 --> 00:03:53,639
or flow up through here

128
00:03:53,639 --> 00:03:54,939
and go through R three

129
00:03:54,939 --> 00:03:56,658
or flow through my ammeter,

130
00:03:56,658 --> 00:03:59,374
but my ammeter has very little resistance.

131
00:03:59,374 --> 00:04:02,207
I mean small, maybe on
the order of a milliohm.

132
00:04:02,207 --> 00:04:05,108
So all of this current
that's flowing through here,

133
00:04:05,108 --> 00:04:08,128
all this current's gonna choose
to go through my ammeter.

134
00:04:08,128 --> 00:04:11,376
It's gonna just skip all
those resistors, forget that.

135
00:04:11,379 --> 00:04:12,910
it just goes through the ammeter.

136
00:04:12,910 --> 00:04:14,676
If you've got a normal-sized voltage,

137
00:04:14,676 --> 00:04:16,880
maybe nine volts, three volts,

138
00:04:16,880 --> 00:04:18,947
hooked up to a milliohm,

139
00:04:18,947 --> 00:04:20,527
you're gonna burn out your ammeter.

140
00:04:20,527 --> 00:04:22,059
There's usually a fuse in here

141
00:04:22,059 --> 00:04:23,940
because they know people
are gonna hook it up wrong.

142
00:04:23,940 --> 00:04:25,007
I've done that,

143
00:04:25,007 --> 00:04:26,308
and you burn out a fuse,

144
00:04:26,308 --> 00:04:28,299
you gotta go replace the
fuse and it's a pain.

145
00:04:28,305 --> 00:04:30,850
So don't hook up your ammeter in parallel.

146
00:04:30,850 --> 00:04:32,136
What about voltmeters?

147
00:04:32,136 --> 00:04:33,621
Why do we hook those up in parallel?

148
00:04:33,621 --> 00:04:36,269
Well, a voltmeter is hooked
up in parallel because

149
00:04:36,269 --> 00:04:37,732
we want to know the voltage

150
00:04:37,732 --> 00:04:39,683
across a circuit element,

151
00:04:39,683 --> 00:04:41,005
so on either side.

152
00:04:41,005 --> 00:04:43,859
Voltage, remember, is
defined to be the difference

153
00:04:43,859 --> 00:04:46,602
between electric potential
at two points in space.

154
00:04:46,602 --> 00:04:49,363
It makes no sense to
ask what's the voltage

155
00:04:49,363 --> 00:04:51,757
through a certain point in a circuit.

156
00:04:51,757 --> 00:04:53,127
You can ask what current flows

157
00:04:53,127 --> 00:04:54,565
through that point in the circuit.

158
00:04:54,565 --> 00:04:56,375
But asking what the voltage is

159
00:04:56,375 --> 00:04:59,511
at a particular point in
a circuit makes no sense.

160
00:04:59,511 --> 00:05:00,742
The only thing that would make sense

161
00:05:00,742 --> 00:05:02,398
is asking what's the voltage

162
00:05:02,398 --> 00:05:05,031
across two points in a circuit.

163
00:05:05,031 --> 00:05:07,176
So I can ask what's the voltage between

164
00:05:07,176 --> 00:05:08,916
this point and that point,

165
00:05:08,916 --> 00:05:09,984
that makes sense,

166
00:05:09,984 --> 00:05:12,397
or I can ask what's the voltage between

167
00:05:12,397 --> 00:05:15,736
this point and that
point, that makes sense.

168
00:05:15,788 --> 00:05:18,320
But asking what's the voltage at a point

169
00:05:18,320 --> 00:05:19,480
or through a point,

170
00:05:19,480 --> 00:05:20,525
makes no sense.

171
00:05:20,525 --> 00:05:21,500
That's what current is.

172
00:05:21,500 --> 00:05:22,870
Current flows through a point,

173
00:05:22,870 --> 00:05:25,424
voltage is across two points.

174
00:05:25,424 --> 00:05:27,862
The difference in electric
potential between two points.

175
00:05:27,862 --> 00:05:30,138
That's why we hook up
voltmeters in parallel

176
00:05:30,138 --> 00:05:33,064
and because we hook up
voltmeters in parallel,

177
00:05:33,064 --> 00:05:36,012
voltmeters have to have a huge resistance.

178
00:05:36,012 --> 00:05:39,426
Sometimes on the order of
hundreds of thousands of ohms

179
00:05:39,426 --> 00:05:40,958
or even millions of ohms.

180
00:05:40,958 --> 00:05:43,442
So this can be big, big number of ohms.

181
00:05:43,442 --> 00:05:44,418
And the reason is,

182
00:05:44,418 --> 00:05:45,207
think about it,

183
00:05:45,207 --> 00:05:47,413
again our key idea is that
we don't want to disturb

184
00:05:47,413 --> 00:05:48,875
the thing we're measuring.

185
00:05:48,875 --> 00:05:51,104
I'm measuring the voltage
across this resistor.

186
00:05:51,104 --> 00:05:52,959
If I were to hook up a voltmeter

187
00:05:52,959 --> 00:05:54,615
with very little resistance,

188
00:05:54,615 --> 00:05:55,888
I just told you what would happen.

189
00:05:55,888 --> 00:05:58,510
This current that's
flowing out of the battery,

190
00:05:58,510 --> 00:06:00,323
would all try to go
through this voltmeter.

191
00:06:00,323 --> 00:06:03,151
Not only would it try to
mess up the voltmeter,

192
00:06:03,151 --> 00:06:04,869
but that's current that's not flowing

193
00:06:04,869 --> 00:06:06,216
through R three anymore,

194
00:06:06,216 --> 00:06:07,701
and so I wouldn't get a correct reading

195
00:06:07,701 --> 00:06:09,350
for the voltage through R three.

196
00:06:09,350 --> 00:06:12,413
So we want to make sure our
voltmeter has a big resistance

197
00:06:12,413 --> 00:06:16,789
so that yes, technically
a very, very small amount

198
00:06:16,789 --> 00:06:18,567
of current, maybe a milliamp,

199
00:06:18,567 --> 00:06:20,216
will flow through this voltmeter,

200
00:06:20,216 --> 00:06:21,749
because it's gotta take a reading.

201
00:06:21,749 --> 00:06:23,931
But, we want as small amount as possible,

202
00:06:23,931 --> 00:06:26,694
because we want to keep this
current flowing through R three

203
00:06:26,694 --> 00:06:29,505
the same as it was before
we were measuring it,

204
00:06:29,505 --> 00:06:32,160
because I know v equals IR.

205
00:06:32,160 --> 00:06:34,079
And if I can measure
this voltage across here,

206
00:06:34,079 --> 00:06:35,727
I want to make sure
the current's the same,

207
00:06:35,727 --> 00:06:37,654
or I won't be getting
an accurate measurement

208
00:06:37,654 --> 00:06:39,164
for the voltage.

209
00:06:39,164 --> 00:06:40,347
You could ask what would happen

210
00:06:40,347 --> 00:06:43,808
if we did hook the voltmeter
in series instead of parallel.

211
00:06:43,808 --> 00:06:45,340
Voltmeters have a huge resistance,

212
00:06:45,340 --> 00:06:46,826
so if I stuck that here,

213
00:06:46,826 --> 00:06:48,597
the voltmeter has a huge resistance,

214
00:06:48,597 --> 00:06:49,844
you wouldn't break it,

215
00:06:49,844 --> 00:06:50,820
it's just that,

216
00:06:50,820 --> 00:06:52,352
think about what the current's gonna do.

217
00:06:52,352 --> 00:06:54,068
Current comes out of this battery,

218
00:06:54,068 --> 00:06:55,083
it's got a choice,

219
00:06:55,083 --> 00:06:58,083
it can go up here through
R three and the voltmeter

220
00:06:58,083 --> 00:07:00,139
or through R one and R two.

221
00:07:00,139 --> 00:07:02,244
I said the voltmeter has
hundreds of thousands,

222
00:07:02,244 --> 00:07:03,589
even millions of ohms,

223
00:07:03,589 --> 00:07:06,074
so this current's just
all gonna go this way.

224
00:07:06,074 --> 00:07:07,212
Forget that.

225
00:07:07,212 --> 00:07:08,674
It's gonna skip this entirely.

226
00:07:08,674 --> 00:07:12,367
If you hook up a voltmeter in
series instead of in parallel,

227
00:07:12,367 --> 00:07:15,246
you just kill off any
current through this portion

228
00:07:15,246 --> 00:07:17,528
of the circuit that the
voltmeter was hooked up in.

229
00:07:17,528 --> 00:07:18,845
You probably won't break it,

230
00:07:18,845 --> 00:07:21,189
so it's not as delicate as the ammeter,

231
00:07:21,189 --> 00:07:23,536
but you still mess up your measurement

232
00:07:23,536 --> 00:07:25,672
because it wasn't designed
to be used that way.

233
00:07:25,672 --> 00:07:28,226
So remember voltmeters
are hooked up in paralled

234
00:07:28,226 --> 00:07:29,525
to the circuit element that you

235
00:07:29,525 --> 00:07:31,430
want to determine the voltage across.

236
00:07:31,430 --> 00:07:33,728
But ammeters are connected in series

237
00:07:33,728 --> 00:07:35,935
to the circuit element
that you want to measure.

238
00:07:35,935 --> 00:07:38,160
And if you're sitting there thinking,

239
00:07:38,160 --> 00:07:41,577
"Pfft, I'm never gonna hook
up my ammeter in parallel.

240
00:07:41,577 --> 00:07:43,316
"How dumb do you think I am?"

241
00:07:43,316 --> 00:07:46,480
Well, gotta be careful,
cause most multimeters

242
00:07:46,480 --> 00:07:49,216
are both voltmeters and ammeters,

243
00:07:49,216 --> 00:07:51,143
depending on where you set the dial.

244
00:07:51,143 --> 00:07:52,351
So if you're sitting there all day

245
00:07:52,351 --> 00:07:55,508
measuring current with
your ammeter setting.

246
00:07:55,508 --> 00:07:57,230
Everything's going well.

247
00:07:57,230 --> 00:07:58,480
And then you go to measure a voltage,

248
00:07:58,480 --> 00:08:00,965
but you forget to switch the dial

249
00:08:00,965 --> 00:08:03,654
to volt instead of amps,

250
00:08:03,654 --> 00:08:05,721
you'll be hooking up an ammeter

251
00:08:05,721 --> 00:08:07,981
in parallel erroneously.

252
00:08:07,981 --> 00:08:09,792
That's what happened to me.

253
00:08:09,792 --> 00:08:10,878
Don't let it happen to you.

254
00:08:10,878 --> 00:08:12,667
Check the dial on your multimeter.

255
00:08:12,667 --> 00:08:14,107
Make sure it's on the function

256
00:08:14,107 --> 00:08:15,128
that you want it to be

257
00:08:15,128 --> 00:00:00,000
so you don't burn out a fuse.