Transcript Guidelines for Calculations

Units with Prefixes and Significant Figures
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.
Units
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
C
Electric Current
Ampere
(Amp)
I or i
A
Voltage
Volt
V or v
V
Power
Watts
P or p
W
Energy
Kilowatt-hours
w
kW-hr
Frequency or
Cycles per second
Hertz
f
Hz
Angular Frequency
radians/s
w
radians/s
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.
Prefixes
Exponent Name Prefix
1018
Exa
E
1015
Peta
P
1012
Tera
T
109
Giga
G
106
Mega
M
103
Kilo
k
102
Hecto
h
101
Deka
da
100
N/A
none
Exponent
10-1
10-2
10-3
10-6
10-9
10-12
10-15
10-18
Name
Deci
Centi
Milli
micro
nano
pico
femto
atto
Prefix
d
c
m
m
n
p
f
a
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.
Expected Format
1.
2.
3.
4.
5.
6.
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.
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.
1. Since it is average power, P  21,372 Watts
2. P  2.1372 x 10 4 W
3. Since 7  5, P  2.14 x 10 4 W
4. P  21.4 x 103 W
5. P  21.4 kW
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.
1. Since it is instantane ous current, i  0.12849 amperes
2. i  1.2849 x 10 1 A
3. Since 4  5, i  1.28 x 10 1 A
4. i  0.128 x 100 A
5. i  0.128 A
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.
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.