preproom.orgThe online science prep room
You are here: Info Library > Electricity > Electricity Equations
Info Library

Electricity > Electricity Equations

Listed below are some of the most commonly used electricity equations. We can rearrange these simple equations to work out the unknown values by using the triangle method below.

Fig 1: Triangle method (Ohm's Law Triangle)
Fig 1:
Triangle method (Ohm's Law Triangle)


Using the triangle method, we can rearrange the Ohm's Law equation to work out the Voltage (if we know the resistance and current), the Current (if we know the voltage and resistance) or the Resistance (if we know the current and voltage).

If you place your thumb over the unknown letter, for example the R, you are left with the V over I and therefore R=V/I. Using the triangle method will help you to rearrange other three way equations.

  Equation Written Equation
Potential Difference (in volts)
V = I x R
Potential Difference (in volts) =
Current x Resistance
(can be expressed as V=IR This equation
is also known as Ohm’s Law)
Power
P = V x I
Voltage x Current
(can be expressed as P=VI)
Charge
Current x time
Charge = Current x time
(can be expressed as Q=It)
Energy
E = V x Q
Energy = Voltage x Charge
(can be expressed as E=VQ)
Total Cost
-
Number of Units x Cost per Unit
Electromotive force (EMF)
E = V + Ir
Electromotive force (EMF) =
Voltage + Current x internal resistance
Capacitance
C = Q / V
Capacitance = Charge / Voltage