Archive

SI unit prefixes

No Comments

Even with scientific notation, very large numbers and very small numbers can be awkward to work with. That is why scientists have developed a set of standard prefixes that are attached to a unit to indicate multiples of that unit. Each of the prefixes represents a particular power of ten. There are currently 20 prefixes in use:

 

Prefixes greater than 1

Factor

Name

symbol

1024

yotta

Y

1021

zetta

Z

1018

exa

E

1015

peta

P

1012

tera

T

109

giga

G

106

mega

M

103

kilo

k

102

hecto

h

101

deka

da

 

Prefixes less than 1

Factor

Name

symbol

10-1

deci

d

10-2

centi

c

10-3

milli

m

10-6

micro

μ

10-9

nano

n

10-12

pico

p

10-15

femto

f

10-18

atto

a

10-21

zepto

z

10-24

yocto

y

 

Do you have to memorize all of these? No. If you are a high school chemistry or physics student, you should know the prefixes in blue above. If you are an AP chemistry or AP physics student, you should also learn the prefixes in red. If you plan to be a scientist or an engineer, it wouldn’t hurt you to learn the rest, but the extreme prefixes are not so common.

Why do the prefixes skip three powers of 10? This allows us to use numbers that are always between 1 and 999. This is easiest to explain with an example. Let’s say we are measuring very small electrical currents. The following table shows how we would state the values without scientific notation, with scientific notation, and with prefixes. You can see how convenient this prefix method is:

Current Scientific notation Prefix notation
0.50 A 5.0 x 10-1 A 500 mA
0.050 A 5.0 x 10-2 A 50 mA
0.0050 A 5.0 x 10-3 A 5 mA
0.00050 A 5.0 x 10-4 A 500 μA
0.000050 A 5.0 x 10-5 A 50 μA
0.0000050 A 5.0 x 10-6 A 5 μA
0.00000050 A 5.0 x 10-7 A 500 nA
0.000000050 A 5.0 x 10-8 A 50 nA
0.0000000050 A 5.0 x 10-9 A 5 nA
0.00000000050 A 5.0 x 10-10 A 500 pA

Units vs. Quantities in electromagnetism

No Comments

 

Keeping the abbreviations straight in electromagnetism is particularly hard because sometimes the abbreviation we use for a QUANTITY is the same letter we use to define a UNIT of a different quantity. For example, a capital C is used for capacitance in the equation C = Q/V. But in the statement Q = 0.050 C, the C stands for ‘coulomb’, the unit of charge. It is up to you to understand from the context whether the symbol refers to a quantity or a unit.

Also, the symbol for a QUANTITY often comes from a German abbreviation, so it’s not the letter you expect. For example, the symbol for inductance is L, not I, and the symbol for current is I, not C. Again, it’s up to you to learn these well enough that you can figure them out on your own. Here’s a table that shows the various symbols:

Quantity

Symbol   for quantity

Unit

Symbol   for unit

force

F

newton

N

charge

Q

coulomb

C

electric field

E

N/C or V/m

current

I

ampere

A

potential difference

V

volt

V

resistance

R

ohm

Ω

power

P

watt

W

capacitance

C

farad

F

magnetic flux

Φ

weber

Wb

inductance

L

henry

H

magnetic field

B

tesla

T

Mastering units in electromagnetism

No Comments

 

Almost all of the units we use in electromagnetism are named after dead physicists, so they can be difficult to remember and distinguish. Consider making a flash card with a list of all the quantities and units as a quick reference guide until you have them memorized.

Quantity

SI   Unit

Symbol

Equivalent   units

force

newton

N

kg·m/s2

charge

coulomb

C

A·s
current ampere

A

potential difference

volt

V

resistance

ohm

Ω

V/A
power watt

W

V·A

capacitance farad

F

A·s/V

magnetic flux

weber

Wb

V·s

inductance

henry

H

V·s/A

magnetic field tesla

T

Wb/m2

Blue Taste Theme created by Jabox