Douglas Krantz - Technical Writer - Describing How It Works

What Does E=IR Really Mean?

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?

By Douglas Krantz

E = I x R is a formula used to figure out the Voltage "E", or the Current "I", or the Resistance "R" of an electrical or electronic circuit.


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 between voltage, current, and resistance, and at the heart of that relationship is the electron.

Electromotive Force

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.

"E" stands for Electromotive Force (EMF). It's the driving muscle that moves the electrons.


Is Voltage

Measured in Voltage, EMF pushes and pulls on the electrons.

Electrical Current

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.

"I" stands for "Intensité de Courant" (French), or Current Intensity. André-Marie Ampere, who discovered electrical current, used this symbol.

 Amperage (Amps)

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 movement of electrons.

Resistance (Ohms)

 Electron Flow

Measured in Ohms, resistance opposes electron flow. The greater the resistance, the less the electrons flow.


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.

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.
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.

The opposite is also true, increasing the resistance will reduce the number of electrons that flow.

Current Flow - Quantity of Electrons

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.

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) do not affect, or change, each other.

Voltage and Current

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.

The amount of electromotive force (voltage) directly affects current flow. Current goes up and down as EMF goes up and down.

Resistance and Current

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.

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
E(Voltage)   =   I(Amps)   x     R(Ohms)
Life Safety
This website uses cookies. See Privacy for details.
Make It Work Series of Books by Douglas Krantz
Want Regular Updates on Articles Like These?

No Charge - Unsubscribe Anytime