1. Guidelines for Calculations
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. Quantity or Variable Name
Basic Measure
Symbol
Unit
Time
Second t s
Temperature
Kelvin (Celsius) T
K (oC)
Resistance
Ohm R W
Capacitance
Farad C F
Inductance
Henry L H
Charge
Coulomb
Q or q
Electric Current
Ampere (Amp)
I or i A
Voltage
Volt
V or v V
Power
Watts
P or p
Energy
Kilowatt-hours w
kW-hr
Frequency or
Cycles per second
Hertz f Hz
Angular Frequency
radians/s
Units

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 Exponent Name Prefix 1018 Exa E 10-1 Deci d 1015 Peta P 10-2
Centi c
1012
Tera T
10-3
Milli m
109
Giga G
10-6
micro
106
Mega M
10-9
nano n
103
Kilo k
10-12
pico p
102
Hecto h
10-15
femto f
101
Deka da
10-18
atto a
100
N/A
none

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 kW resistor with a 5% tolerance could actually have a resistance value between 1.90 – 2.10 kW.

7. Expected Format
Determine whether the variable name should be written upper or lower case.
Use the proper unit for the variable.
Write your answer in scientific notation where all numbers include a factor of ten raised to an exponent – even if the exponent is zero.
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.
Rewrite the number such that the exponent is equal to closest multiple of 3.
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 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.