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# Question 2cd: 2015 AP Physics 1 free response

## Video transcript

let's now tackle the rest of this problem so they say a light bulb is non-ohmic if its resistance changes as a function of current you're set up from Part A which we have right over here is to be used or modified to determine whether the light bulb is non ohmic how if at all does the setup need to be modified well we want to be able to change the current that's going through the light bulb and then see how the resistance changes so what we could do we can always measure with the setup right over here it's very easy to measure the resistance so we can measure resistance by we know that V is equal to we know that voltage is equal to current times resistance or that resistance is equal to voltage divided by current so if we know the current going through a light bulb and we know the voltage across it we can always figure out the resistance of the light bulb so our setup right here is actually very good for figuring out the resistance of the actual light bulb and so we need to change the current and see how the resistance changes and there's a couple of ways that we could change the current we could change some other resistance here but lucky for us we have a variable power source so we can change the voltage we could change the voltage across the entire circuit and by changing the voltage across the entire circuit well that's going to change the current that is going this is a simple circuit it's just going in series the current is just doing like that and so if we change the current through the entire circuit we would be changing the current through the light bulb and then we can just measure V and I and figure out whether R changes as well so how if all does the setup need to be modified I could say it doesn't need to be modified doesn't need modification modification alright and then the next section what additional data if any would need to be collected well I would want to I would want to measure let me do this part down let me do it part C over here so Part C so part one of Part C was no modification no modification Nomad I can explain that I can change resistance or I can change I can change current by varying power source by varying power source power source I can measure I can measure resistance of bulb by voltage across bulb across bulb divided by divided by current entering or exiting bulb and I know the first part of this test we were saying hey do we have the same number of electrons per second entering the bulb or exiting the bulb but from what we know of currents we know that the current is going to be the same on either side that you have a voltage drop but the current is the same and so entering or exiting you're going to get the same value and even if you didn't know what I just told you the first part of the experiment would have told you that that your courage is the same entering and exiting the bulb and so this right over here is going to be equal to the resistance of the bulb so I can change I can change the current by varying the power source and then as I can see what value I get for R by measuring these things as I change that overall current for my for my platform for my setup let's see B what additional data if any would need to be collected so part two part two here I need to collect collect voltage across across bulb as as current changes or maybe as I change my variable power source as very power source power source and I would also have to collect we've already this way I'll collect that voltage alright like that voltage across bulb is a very power source and I could say current current entering exiting bulb as I vary power source power source so I'd have to measure these things and then I'll be able to figure out if my R changes alright let's tackle Part D now how would you analyze the data to determine whether the bulb is non ohmic include a discussion of how the uncertainties in the voltmeters and ammeters would affect your argument for concluding whether the resistance is non ohmic well I explained it a little bit well up here but I'll write it again D so let me read that one more time how would you analyze the data so what I would do is so power source I would make one voltage for I would very power source source to first voltage voltage and measure measure what is described above and measure above use to calculate use to calculate the resistance resistance of bulb and then vary power source to second voltage voltage and once again measure above measure above see if resistance changes see if resistance changes so we could say if V across bulb across bulb over let me actually just write it this way all the V's are crossed above this v1 over the current one so this is a voltage across the bulb over the current across the bulb if this is equal to after I change it I the voltage across the bulb divided by the current across the bulb if this over here if these are equal then I am dealing with the resistance has not changed and so we could say well non ohmic non ohmic is when the resistance does change if resistance so we could say non non ohmic so I could or I could say not non ohmic non ohmic otherwise if v1 over I 1 does not equal let me write that our VI v1 v1 over i 1 does not equal v2 over I 2 then we are non ohmic and we have to be careful because the sensitivities of our of our voltmeters and ammeters might give us an aberrant result for example we might be non ohmic but because of how we're rounding in the measurement it actually might give us a change especially because our ammeter is more sensitive here and on the other hand we might be a situation where we are non ohmic but it doesn't you don't register the change because it hasn't been large enough to be registered by the sensitivity of your instruments so need to be careful careful about about instrument sensitivity instrument sensitivity sensitivity the rounding rounding to nearest nearest 0.1 volt or 0.01 amp could result in in difference in V over I when there isn't any there isn't any or vice versa it might not result in a difference but there actually might be a change but it's not big enough to be measured or vice versa so you always have to be careful when you're doing any experiment in any signs about how sensitive your actual instruments are and what the rounding might do for these calculations
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