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.