Table 1.2.2 - SI derived units

SI derived unit | Symbol | SI base unit | Alternative unit |

newton | N | kg m s-2 | - |

joule | J | kg m2 s-2 | N m |

hertz | Hz | s-1 | - |

watt | W | kg m2 s-3 | J s-1 |

volt | V | kg m2 s-3 A-1 | W A-1 |

ohm | Ω | kg m2 s-3 A-2 | V A-1 |

pascal | Pa | kg m-1 s-2 | N m-2 |

Random errors

A random error, is an error which affects a reading at random.

Sources of random errors include:

* The observer being less than perfect

* The readability of the equipment

* External effects on the observed item

Systematic errors

A systematic error, is an error which occurs at each reading.

Sources of systematic errors include:

* The observer being less than perfect in the same way every time

* An instrument with a zero offset error

* An instrument that is improperly calibrated

ROUNDING OFF

You might think that if a number such as 5.4349 were to be rounded to 3 s.f. it would give 5.44 as the last digit is 9 which is large enough to affect the previous digit which would then become 5. Now that 5 would be large enough to affect the last digit of the number we are keeping which would become 4 (thus 5.44). However, this is not the case, when rounding, we only look at the digit immediately after the one we are rounding to, whether or not that digit would be affected by the one after it is not taken into account. Therefor, the correct result of this question would be 5.43.

How many digits should be kept?

Experimental uncertainties should be rounded to one significant figure. Experimental uncertainties are, by nature, inexact. Uncertainties are almost always quoted to one significant digit (example: ±0.05 s). If the uncertainty starts with a one, some scientists quote the uncertainty to two significant digits (example: ±0.0012 kg).

Wrong: 52.3 cm ± 4.1 cm

Correct: 52 cm ± 4 cm

Always round the experimental measurement or result to the same decimal place as the uncertainty. It would be confusing (and perhaps dishonest) to suggest...