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Units vs. Quantities in electromagnetism

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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

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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

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