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 
 or 

where:  V = voltage in volts (V) I = current in amps (A) R = resistance in ohms () 
or:  V = voltage in volts (V) I = current in milliamps (mA) R = resistance in kilohms (k) 
The Ohm's Law equations work if you use V, A and , or if you use V, mA and k. You must not mix these sets of units in the equations so you may need to convert between mA and A or k and .
The VIR triangle
V I R 
Ohm's Law triangle 
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 
 Write down the Values, converting units if necessary.
 Select the Equation you need (use the VIR triangle).
 Put the Numbers into the equation and calculate the answer.
 3 V is applied across a 6
resistor, what is the current?
 Values: V = 3 V, I = ?, R = 6
 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
k
= 100
(using mA for current means the calculation gives the resistance in k)
 A 1.2 k
resistor passes a current of 0.2 A, what is the voltage across it?
 Values: V = ?, I = 0.2 A, R = 1.2 k
= 1200
(1.2 k is converted to 1200 because A and k must not be used together)  Equation: V = I × R
 Numbers: V = 0.2 × 1200 = 240 V
 Values: V = ?, I = 0.2 A, R = 1.2 k
= 1200
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