Ohm's Law

To make a current flow through a resistance there must be a voltage across that resistance. Ohm's Law shows the relationship between the voltage (V), current (I) and resistance (R). It can be written in three ways:
V = I × R or 
I = V
R = V
where:V = voltage in volts (V)
I  = current in amps (A)
R = resistance in ohms (ohm)
or:V = voltage in volts (V)
I  = current in milliamps (mA)
R = resistance in kilohms (kohm)
For most electronic circuits the amp is too large and the ohm is too small, so we often measure current in milliamps (mA) and resistance in kilohms (kohm). 1 mA = 0.001 A and 1 kohm = 1000 ohm.
The Ohm's Law equations work if you use V, A and ohm, or if you use V, mA and kohm. You must not mix these sets of units in the equations so you may need to convert between mA and A or kohm and ohm.

The VIR triangle

V I    R 
Ohm's Law
You can use the VIR triangle to help you remember the three versions of Ohm's Law. 
Write down V, I and R in a triangle like the one in the yellow box on the right.
  • To calculate voltage, V: put your finger over V,
    this leaves you with I R, so the equation is V = I × R
  • To calculate current, I: put your finger over I,
    this leaves you with V over R, so the equation is I = V/R
  • To calculate resistance, R: put your finger over R,
    this leaves you with V over I, so the equation is R = V/I 

Ohm's Law Calculations

Use this method to guide you through calculations:
V I    R 
  1. Write down the Values, converting units if necessary.
  2. Select the Equation you need (use the VIR triangle).
  3. Put the Numbers into the equation and calculate the answer.
It should be Very Easy Now!
  • 3 V is applied across a 6 ohm resistor, what is the current?
    • Values: V = 3 V, I = ?, R = 6 ohm
    • Equation: I = V/R
    • Numbers: Current, I = 3/6 = 0.5 A

  • A lamp connected to a 6 V battery passes a current of 60 mA, what is the lamp's resistance?
    • Values: V = 6 V, I = 60 mA, R = ?
    • Equation: R = V/I
    • Numbers: Resistance, R = 6/60 = 0.1 kohm = 100 ohm
      (using mA for current means the calculation gives the resistance in kohm)

  • A 1.2 kohm resistor passes a current of 0.2 A, what is the voltage across it?
    • Values: V = ?, I = 0.2 A, R = 1.2 kohm = 1200 ohm
      (1.2 kohm is converted to 1200 ohm because A and kohm must not be used together)
    • Equation: V = I × R
    • Numbers: V = 0.2 × 1200 = 240 V