WEBVTT
00:00:00.092 --> 00:00:02.763
This is College Physics
Answers with Shaun Dychko.
00:00:03.144 --> 00:00:06.787
We have a galvanometer with
a sensitivity of 25 microamps
00:00:06.923 --> 00:00:09.686
so full scale deflection occurs when
that current goes through it,
00:00:10.030 --> 00:00:12.492
and it has an internal
resistance of 40 ohms.
00:00:13.132 --> 00:00:18.184
So the question is what resistance
should be put in series with it
00:00:18.535 --> 00:00:23.310
to allow it to be used as a voltmeter with
a full scale deflection of 0.5 millivolts?
00:00:24.584 --> 00:00:26.000
Well if it's a voltmeter
00:00:26.000 --> 00:00:28.178
we're going to have the voltage
between this point and this point
00:00:28.560 --> 00:00:32.664
and that's going to equal the full
scale current flowing through it
00:00:32.892 --> 00:00:36.246
multiplied by this *R* that
we're trying to find,
00:00:36.689 --> 00:00:41.483
plus the current multiplied by this internal
resistance of the galvanometer.
00:00:43.095 --> 00:00:46.344
We can put this term by itself on one side
00:00:46.344 --> 00:00:53.230
by subtracting *I max* times
little *r* from both sides.
00:00:53.476 --> 00:00:57.507
We get *I max* times the
extra resistance capital *R*,
00:00:57.507 --> 00:01:00.646
equals *V* minus *I max* times
internal resistance *R*.
00:01:01.489 --> 00:01:03.421
Then divide both sides by the current.
00:01:06.904 --> 00:01:11.655
So *R* is the voltage at which full
scale deflection is meant to occur,
00:01:13.064 --> 00:01:17.729
minus the sensitivity times the internal
resistance divided by the sensitivity.
00:01:18.430 --> 00:01:20.646
So we have 0.5 times ten
to the minus three volts,
00:01:20.892 --> 00:01:23.852
minus 25 microamps times 40 ohms,
00:01:23.852 --> 00:01:27.150
divided by 25 microamps which
gives negative 20 ohms.
00:01:27.858 --> 00:01:31.606
Now that answer is not realistic because
resistance can't be negative.
00:01:32.264 --> 00:01:38.332
So the voltage must in fact
be more than 0.5 millivolts.
00:01:38.436 --> 00:01:45.120
This galvanometer cannot be used to have
a full scale deflection at this voltage.