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# PHYSICS Uncertainties watch

1. So basically I know that when you add or subtract two numbers with uncertainties, you add the uncertainties.
But the following didn't make much sense to me:

So on a lab, I got T(1) = 20 +/- 0.5 C T(2) = 18C+/- 0.5C
Temperature change would be
20.1-17.9 = 2C +/- 1C

which would be 2.2C+/- 50%.
would that work? it looks weird to me

Thanks itzRussian. I changed it
2. How are you able to measure to 1dp having an uncertainty of 0.5? You get an uncertainty of +/-0.5 if you measure to the nearest whole unit.
3. (Original post by TheKidd4Ever)
So basically I know that when you add or subtract two numbers with uncertainties, you add the uncertainties.
But the following didn't make much sense to me:

So on a lab, I got T(1) = 20 +/- 0.5 C T(2) = 18C+/- 0.5C
Temperature change would be
20.1-17.9 = 2C +/- 1C

which would be 2.2C+/- 50%.
would that work? it looks weird to me

Thanks itzRussian. I changed it
If you measure a temperature change of 2 degrees with a thermometer that only measures to ± 0.5 degrees, then your maximum possible uncertainty will indeed be ±1 degree. (= ±50%)
1. This is the maximum possible error. It is not the probable error which could be lower.
2. This level of uncertainty would be too large in an experiment.
3. You either need to a) produce a larger change in temperature, or if that is not possible, b) use a more accurate thermometer.
So if that is the maximum temperature change you can produce (you haven't given any details of what you are trying to do so I can't help more on that) then you are using the wrong measuring equipment for the job.

PS
You'll get more replies in the physics forum with questions of this type.

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