QUALITY OF MEASUREMENT PART 3

SYSTEMATIC ERRORS This is an error INHERENT in the experiment or due to human error. Examples: Failing to account for background radiation in a radioactivity experiment, failing to zero a meter.

Sometimes you might not even know you have a systematic error! They have to be identified and put right, or your “precise” measurements will cluster round a wrong value! It’s important to estimate the size of your systematic error and subtract it from the results. At the very least, you must write about it. Examples: a shrunken ruler, a clock running fast or slow. A good ploy with thermometers finding a temperature difference: exchange them and repeat the reading.

Systematic errors Example 1 Suppose you are measuring with a ruler: If the ruler is wrongly calibrated, or if it expands, then all the readings will be too low (or all too high):

Systematic errors Example 2 If you have a parallax error: with your eye always too high then you will get a systematic error All your readings will be too high.

Systematic errors A particular type of systematic error is called a zero error. Here are some examples . . .

Zero errors Example 3 A spring balance: Over a period of time, the spring may weaken, and so the pointer does not point to zero: What effect does this have on all the readings?

Zero errors Example 4 Look at this top-pan balance: It has a zero error. There is nothing on it, but it is not reading zero. What effect do you think this will have on all the readings?

Zero errors Example 5 Look at this ammeter: If you used it like this, what effect would it have on your results?

Zero errors Example 6 Look at this voltmeter: What is the first thing to do? Use a screwdriver here to adjust the pointer.

Zero errors Example 7 Look at this ammeter: What can you say? Is it a zero error? Or is it parallax?

Zero error, Parallax error Example 8 Look at this ammeter: It has a mirror behind the pointer, near the scale. What is it for? How can you use it to stop parallax error? When the image of the pointer in the mirror is hidden by the pointer itself, then you are looking at 90o, with no parallax.

The speed of light measurement. 2 famous systematic errors: The Hubble Space telescope – the most expensive zero error in history. Cost: hundreds of millions of dollars, all due to an end cap! See AS book p229 The speed of light measurement.

Light travels slower in air than a vacuum. 1926: Light speed measured between 2 mountain tops 35 km apart. Speed taken as 299 796 +/- 4 kms-1 Light travels slower in air than a vacuum. Weather conditions affect speed, too. Uncertainty corrected to +/- 67 kms-1 1932: Use a 1 km long evacuated tube buried in the ground to find speed in a vacuum. Speed taken as 299 774 +/- 4 kms-1 Whoops! Radar confirms speed in vacuum to be 299 792 458 +/- 1 ms-1. What’s the error in 1932? Still a mystery……could be expansion of the buried tube.