1. Units with Prefixes and Significant Figures

2. Objectives of this lecture Introduce the electrical quantities or variables that are used in the field of Electrical and Computer Engineering. Present the prefixes to the units associated with the variables that will be used frequently in this course. Review significant figures. Explain how you should present the results of your calculations in this course.

3. Units Quantity or Variable Name Basic Measure SymbolUnit TimeSecondts TemperatureKelvin (Celsius) TK ( o C) ResistanceOhmR  CapacitanceFaradCF InductanceHenryLH ChargeCoulombQ or qC Electric CurrentAmpere (Amp) I or iA VoltageVoltV or vV PowerWattsP or pW Energy Kilowatt-hours wkW-hr Frequency or Cycles per second HertzfHz Angular Frequencyradians/s 

4. Uppercase or Lowercase Symbols Using an uppercase symbol means that either the value of the variable is a constant or the value is the average of a time-varying signal. Using a lowercase symbol means that the value of the variable is what the value was at the instant that you measured or calculated the value.

5. Prefixes ExponentNamePrefixExponentNamePrefix 10 18 ExaE10 -1 Decid 10 15 PetaP10 -2 Centic 10 12 TeraT10 -3 Millim 10 9 GigaG10 -6 micro  10 6 MegaM10 -9 nanon 10 3 Kilok10 -12 picop 10 2 Hectoh10 -15 femtof 10 1 Dekada10 -18 attoa 10 0 N/Anone

6. Significant Figures Give a reader an indication of how confident you are in the accuracy of the answer that you are presenting. Electrical and electronic components are commonly binned into what is known as nominal values. The range of values within each bin is known as the component tolerance. For example, a 2 k  resistor with a 5% tolerance could actually have a resistance value between 1.90 – 2.10 k 

7. Expected Format 1. Determine whether the variable name should be written upper or lower case. 2. Use the proper unit for the variable. 3. Write your answer in scientific notation where all numbers include a factor of ten raised to an exponent – even if the exponent is zero. 4. Round the value of the digit in the one-hundredths place up if the digit in the one-thousandths place is equal to or greater than 5. 5. Rewrite the number such that the exponent is equal to closest multiple of 3. 6. Replace the factor of ten raised to an exponent with the appropriate prefix.

8. Example 1 Suppose you calculated that the average power generated by a solar cell array is 21,372 Watts. If you wrote this value as your final answer to a question, you will lose some points. Follow the steps outlined in the previous slide.

9. Example 2 Suppose you measure the instantaneous current flowing from the battery in your Tablet PC to power the screen and the rest of the electronics and determine that it is -0.12849 amperes.

10. Exceptions We use scientific notation when writing the values for Coulombs and occasionally for energy, frequency, and angular frequency. Other variables where the answers are written using scientific notation include resistivity and conductivity.

11. Summary Examples of how your answers should be written using appropriate symbols, significant figures, and units with prefixes. I will indicate when your answers should be written in scientific notation for the cases where this format is used.