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).