Uncertanty in Meaurements- PHYSICS

Lab.No- 1    IB DP Topic No- 1.1 & 1.2  IB Criteria- DCP & CE                                 Time Taken:-  1.0 Hours                                      Date:- 27/7/2012

Uncertainties in Physics Internal Assessment

Using Meter Scale

The treatment of errors and uncertainties is directly relevant in the internal assessment of:

·         data collection and processing, aspects 1, 2 and 3 (recording raw data, processing raw data, and presenting processed data)

·         conclusion and evaluation, aspects 1 and 2 (concluding, and evaluating procedure(s))—a reasonable interpretation, with justification, may include the appreciation of errors and uncertainties, and evaluation of procedures may, if relevant, include the appreciation of errors and uncertainties.

The core physics syllabus covers errors and uncertainties in the following section of the Physics guide (2007):

·         Measurement and uncertainties (topic 1.2).

Both standard and higher level students are to be assessed by the same syllabus content and the same assessment criteria.

Expectations at standard level and higher level

All physics students are expected to deal with uncertainties throughout their investigations. Students can make statements about the minimum uncertainty in raw data based on the least significant figure in a measurement. They can calculate the uncertainty using the range of data in a repeated measurement, and they can make statements about the manufacturer's claim of accuracy. Students can estimate uncertainties in compound measurements, and can make educated guesses about uncertainties in the method of measurement. If uncertainties are small enough to be ignored, the student should note this fact.Students may express uncertainties as absolute, fractional, or percentages. They should be able to propagate uncertainties through a calculation—addition and subtraction, multiplication and division, as well as squaring and trigonometric functions.

All students are expected to construct, where relevant, uncertainty bars on graphs. In many cases, only one of the two axes will require such uncertainty bars. In other cases, uncertainties for both quantities may be too small to construct uncertainty bars. A brief comment by the student on why the uncertainty bars are not included is then expected. If there is a large amount of data, the student need only draw uncertainty bars for the smallest value datum point, the largest value datum point, and several data points between these extremes. Uncertainty bars can be expressed as absolute values or percentages.Arbitrary or made-up uncertainty bars will not earn the student credit. Students should be able to use the uncertainty bars to discuss, qualitatively, whether or not the plot is linear, and whether or not the two plotted quantities are in direct proportion. In respect of the latter, they should also be able to recognize if a systematic error is present.

Using the uncertainty bars in a graph, students should be able to find the minimum and maximum slopes, and then use these to express the overall uncertainty range in an experiment.Qualitative and quantitative comments about errors and uncertainties may be relevant in the data collection and processing criterion, aspect 1. Qualitative comments might include parallax problems in reading a scale, reaction time in starting and stopping a timer, random fluctuation in the read-out, or difficulties in knowing just when a moving ball passes a given point. Students should do their best to quantify these observations. For example, one student measured a voltage from an unstable power supply, and wrote the following qualitative and quantitative comments:

The voltage varied slightly over time; it went up and down by several hundredths of a volt. Therefore, the values recorded have an uncertainty greater than the least significant digit of each measurement. The uncertainty was estimated to be more like ±0.04 V.