Numbers from measurements are different compared to the numbers of arithmetic we are use to.

· Measurements represent an action by someone with some measuring instrument.

· No measuring instrument is completely accurate.

· Measurements have built-in uncertainty or error

Error affects:

Accuracy (how close to the true value) and

Precision (the closeness of repeat measurements to each other).

Whenever a measurement is made, an estimate is part of the number.

The uncertainty (estimate) in the measurement needs to be communicated.

Uncertainty:

The level of uncertainty of a measurement should always be expressed in the numbers recorded.

The use of significant figures in a measurement expresses the level of uncertainty.

The number of digits that are known for sure, plus the estimated digit – the last digit in the number – expresses the level of uncertainty.

Measurement:

A measurement always consists of two parts:

• a number

• a unit

Measurements always have error or uncertainty

Measurement practice: Animation of measurement using rulers and graduated cylinders (answer/instant grading)

http://chemsite.lsrhs.net/measurement/images/measurement%20tutorial.swf

Handout

Graduated Cylinder:

• Measurement-4
• Measurement-5

Density: a ratio of weight over volume

Scientific Notation

• Rules: SciNotationRules
• Practice: http://janus.astro.umd.edu/astro/scinote/

Measurement

• Practice: Measurement animation – rulers and grad cylinders

http://chemsite.lsrhs.net/measurement/images/measurement%20tutorial.swf

Significant Figures:

• Practice: Sig. Fig. animation http://chemsite.lsrhs.net/measurement/countingsigdigflash.html

For a video lesson on significant figures and scientific notation, go to:

http://education-portal.com/academy/lesson/significant-figures-and-scientific-notation.html#lesson

For a video lesson on unit conversion (dimensional analysis), go to:

http://education-portal.com/academy/lesson/unit-conversion-and-dimensional-analysis.html#lesson

.

Standarization:

It has long been the goal of scientists to relate all of the unit definitions to fundamental constants of nature, making them stable and universal, and giving them closer links to each other and the quantities they measure.  Key units still to be redefined are the kilogram (mass) and the ampere (electric current).

Presently the kilogram is defined by a physical lump of platinum-iridium and the ampere is defined via the force produced between two wires.

The goal is to define the kilogram in terms the Planck constant h and the ampere in terms of the electron charge e.