# The Electric Field equation, what does the (V-IR) mean?

#### johnwandering

7+ Year Member
The electric field across a capacitor equation is

E=V/d=(V-IR)/L

I was wondering what the line V-IR meant. It clearly represents 2 different voltages, so I assume IR refers to the circuit voltage. I was wondering what voltage the V variable stood for.

Also, does this apply for general electric fields, or just capacitors?

#### howlovely

(V-IR)/L would apply if you had a resistor in series with the capacitor. The V represents the voltage of the EMF, and IR would represent the voltage difference across the resistor. Thus, the voltage across the capacitor would be V-IR, making the second equation equivalent to the first.

E = V/d applies only to electric fields generated by infinitely parallel plates (like capacitors). The formula for electric fields created by a point charge, shell, or sphere is E = kq/r^2.

OP
J

#### johnwandering

7+ Year Member
V represents the voltage of the EMF, and IR
Thanks that's awesome.

But why are we subtracting the Voltage over the resistor from the EMF?

#### howlovely

You subtract the voltage over the resistor from the EMF because that's the voltage difference over the capacitor. Note that this equation only works for a resistor in series with a capacitor (in parallel, they would receive the same voltage).