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General Electronics

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Ohm's Law

By Douglas Krantz
If it isn't just a word, and it isn't really something to be memorized in order to pass a test, what do the letters in Ohm's Law really mean?
Picture of an atom, with the electrons in layers around the nucleus.
The electrons are found in layered clouds around the nucleus of atoms. The protons in the atom don't move from atom to atom, but the electrons can be pushed and pulled from atom to atom using electrical force. The force is called Electromotive Force (EMF). EMF is measured in voltage.


The letters in

Ohm's Law show

a relationship


voltage, current,

and resistance,

and at the

heart of that


is the electron.
The Letter E
Electomotive Force (EMF), like pressure pushing on water, or like gravity pulling everything on earth, is what pushes and pulls electrons to cause them to jump from one atom to the next. Electromotive Force is measured in Voltage.

Electromotive Force

"E" stands for

Electromotive Force


It's the driving

muscle that

moves the electrons.
Is Voltage


Measured in Voltage,

EMF pushes and

pulls on the

The Letter I
Electrical Current is the number (quantity) of electrons going past a given point in an electrical circuit, like the number of cubic meters of water that goes over a falls in a second. Electrical Current is measured in Amperage, or Amps for short.

Electrical Current

"I" stands for

"Intensité de Courant"

(French), or Current Intensity.

André-Marie Ampere, who

discovered electrical current,

used this symbol.
Is Electron


Measured in Amps, and

hopping from one atom

to the next, the organized

movement of electrons

going past a point is

electrical current, or "I".
The Letter R
Resistance is the amount of opposition to electrical current. This can be likened to the diameter of a pipe resisting the flow of drainage from the bathtub. The larger the pipe (lower resistance) the faster gravity can pull the water out of the bathtub.


"R" stands for

Resistance. It

restricts, or slows

down, the


of electrons.
 Electron Flow


Measured in Ohms,

resistance opposes

electron flow.

The greater the

resistance, the

less the electrons


Lots of water out of a pipe.
A greater amount of water will flow when the valve is opened wider; more electrons will flow when there is less resistance.

Only a little water out of a pipe
A lessor amount of water will flow when the valve is more closed; less electrons will flow when the resistance is greater.


Assuming the electromotive

force remains the same,

reducing the resistance,

like opening a valve on

a water pipe, will increase

the number of electrons

that flow.

The opposite is also true,

increasing the resistance

will reduce the number of

electrons that flow.
Water flowing out of a pipe
Greater water pressure pushes more water through the same pipe; greater electromotive force will push more electrons past the resistance. Lessor water pressure means less water; lessor electromotive force means fewer electrons.

Current Flow - Quantity of Electrons

The quantity of electrons

flowing, electrical current,

is determined by the

electromotive force -

reduced by the resistance.

Similarly, the quantity of

water flowing is determined

by water pressure,

reduced by the restriction

of the pipe.
EMF (voltage)
Does not affect R (Resistance)
R (Resistance)
Does not affect EMF (voltage)
Electromotive Force (EMF) and Resistance (R) don't affect, or change, each other.
A direct relationship between EMF and Current
The greater the EMF (voltage) the greater the current (amperage); the lessor the EMF the lessor the current.

Voltage and Current

The amount of

electromotive force

(voltage) directly

affects current flow.

Current goes up

and down as

EMF goes up

and down.
As the resistance goes up,the current goes down, and vice-versa.
The greater the resistance (Ohms), the lessor the current (Amps); the lessor the resistance, the greater the current.

Resistance and Current

The amount of

resistance inversely

affects current flow.

As resistance goes

up, current goes

down, and as

resistance goes

down, current

goes up.
The circle of Ohms Law
Voltage and Resistance don't actually affect each other, Voltage (E) and Resistance (R) both will change the current (I).


The relationships

between the

electromotive force,

current and

resistance are all

interrelated, and

described in a single

formula, or law.
The Formula is
Ohm's Law
Volts of Electromotive Force E =
Amps of Current Flow   I x
Ohms of Resistance    R
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Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at