# Introduction ## Specific Bibliography (1) BARTKOWIAK, Robert A., Circuitos Elétricos, 2a. Edição, São Paulo: Makron Books, 1999 ## Summary [[#Chapter 1 - Units and Conversion Factor]] [[#Chapter 2 - International System Units (SI) Prefixes]] Chapter 3 - The Electrical Charge: Concept ## Chapter 1 - Units and Conversion Factor ### 1.1 - Units and the International Units System Physics has three main quantities: length, mass, and time. The main system used to measure those quantities is the MKS, Meter-Kilogram-Second, also known as the Metric System. In 1960, the 11th General Conference on Weights and Measures (CGPM) proposed the International Units System (SI), which has the MKS as a subsystem for mechanics and the MKSA for mechanics, electricity, and magnetism. The unit Ampere (A) for electrical current is included. > [!info] Table 1 - MKSA System | | Length | Mass | Time | Electricity | | ------------ | :----: | :------: | :----: | :---------: | | System | M | K | S | A | | Unit | Meter | Kilogram | Second | Ampere | | Abbreviation | m | kg | s | A | ### 1.2 - Conversion Factors Frequently, it is required to convert some units to others of another measurement system. This is done using conversion factors, the most common of which are shown in Table 2. > [!info] Table 2 - Conversion Factors | Unit 1 | Unit 2 | |:---------------------------:|:-----------------------:| | 1 inch (") | 2.54 centimetres (cm) | | 1 metre (m) | 39.37 inches (") | | 1 pound (lb) | 453.6 grams (g) | | 1 kilogram (kg) | 2.2046 pounds (lb) | | 1 newton (N) | 0.224 pound-force (lbf) | | 1 foot-pound force (ft.lbf) | 1.3549 joules (J) | | 1 horsepower (hp) | 746 watts (W) | Note that: $\frac{1\,inch}{2.54\,centimetres}=1$ Hence, to transform centimetres into inches, multiply the quantity as exemplified below: $508\,\cancel{centimetres} \cdot \frac{1\, inch}{2.54\,\cancel{centimetres}} = 200\,inches$ To transformer inches to centimetres, invert the unit division, as shown below: $200\,\cancel{inches} \cdot \frac{2.54\, centimetres}{1\,\cancel{inch}} = 508\,centimetres$ The same method applies to the other units' conversion. >[!Important] Every engineer must know the units behind the equations and calculations. Calculation mistakes around units, like considering kW instead of kVA, can be onerous to equipment and cabling sizing and protection design. ## Chapter 2 - International System Units (SI) Prefixes