percentage uncertainty aqa 2019 paper 1

Hi,

So there's this question that asks for a calculation of the maximum length of an organelle in an image. I used a ruler and got 4mm, then I divided by the magnification (500) to give 0.008mm. They wanted the answer in micrometers so I timesed by 1000 to get 8 micrometers.

They also want me to calculate the uncertainty of my measurement (which is different to percentage uncertainty) and they wanted the uncertainty in units of micrometers.

I know that the uncertainty of a ruler would be 1mm because you make two judgements (when measuring at 0mm and the final length so 0.5 x 2). So that gave me 1, but the answer is 2 micrometers?

Would appreciate some help!

I know you don't need the answer now, but for anyone in the future looking this up:

You work out the percentage uncertainty which is (Uncertainty/Reading)*100 in this case you're right uncertainty = 1mm so (1/4)*100 = 25% then use this percentage to find the end uncertainty. 0.25*8 = 2 micrometers.

Thank you so much for this! I've been trying to figure this out for ages