how to estimate an error for a reading

[david7]

Hello

Can someone help please I have a question I cant seem to work out

If a wattmeter measures a wattage at 14.71w. What do you estimate the error to be for this reading, and there is no manufacturers data.


Thanks in adannce for any help

[factor-fiction]

It depends on the context in which the question has been asked. If the subject is engineering or phsyics, say, there may be a known tolerances.

If it's just a maths question about rounding I'd say the precise value could lie between 14.705 and up to (but not including) 14.715, so the error is plus or minus 0.005w. .

How have you been expected to work these out previously?

[buildersmate]

This used to be the sort of question you got in a Oxbridge Entrance exam - there is no definitive way of working out the answer and you are expected to identify potential root-causes of errors, then have a stab at quantifying the scale of each. Possibilities here could include:
1) Wattmeter tolerance error
2) Operator error (in reading what's on the screen!)
3) Phase error, if the measurement is of AC, and voltage and current are not in phase?

[Teddio]

As Factor 30 points out the context of the question is important. There are two types of error (uncertainty) systematic and random. The wattmeter is measuring power. Is it an analogue or a digital instrument? Is the electrical power constant or varying. Is the meter measuring an rms value? If the value is genuinely to 4 significant figures then the true value will lie between 14.705 and 14.715.