Measurements in Science is the process of obtaining the magnitude of a physical quantity and expressing it relative to a certain basic, arbitrarily chosen, internationally accepted reference standard called UNIT.

We need only a limited number of units to express all the physical quantities because these quantities are dependent on each other. We assign a set for fundamental or base quantities only, the units for these are called the Fundamental/Base units.

All remaining quantities can be derived from the base quantities. The units for these derived quantities are called derived units.

**A system of units = Fundamental Units + Derived Units**

There are different systems of units e.g. CGS(centimeter, gram and second), MKS(meter, gram and second) etc. However, the most commonly used standard is the SI system.

# SI System

#### 7 Base quantities

Base Quantity |
Name |
Symbol |

Length | metre | m |

Mass | kilogram | kg |

Time | second | s |

Electric Current | ampere | A |

Thermodynamics Temperature | Kelvin | K |

Amount of substance | mole | mol |

Luminous Intensity | candela | cd |

#### 2 additional quantities

Base Quantity |
Name |
Symbol |

Plane Angle | radian | rad |

Solid Angle | steradian | sr |

## Errors

Degree of uncertainty associated with a measurement.

**Result = measured value ± absolute error**

**Accuracy** is a measure of how close the measured value is to the true value of a quantity.

**Precision** tells us to what resolution or limit the quantity is measured.