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Also See: Voltage & Current  Resistance
To make a current flow through a resistance there must be a voltage across that resistance. Ohm's Law shows the relationship between the three quantities: voltage, current and resistance.
Ohm's Law can be written as a word equation:
voltage = current × resistance 
Or using symbols to represent the quantities of voltage (V), current (I) and resistance (R):
V = I × R 
In fact it can be written three ways and you can pick the version that's best for your purpose:

V 
I R 
You can use the VIR triangle to help you remember the three versions of Ohm's Law.
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 (k).
1 mA = 0.001 A
1 k = 1000 .
The Ohm's Law equations work if you use V, A and , or if you use V, mA and k.
It is vital to use the right units for the three quantities in Ohm's Law, otherwise calculations will give the wrong values.
You can use either of these two sets of units:
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) 
You must not mix these sets of units in the equations so you may need to convert between mA and A or k and .
Convert the units when necessary using these rules:
1mA = 0.001A 
1A = 1000mA 
1k = 1000 
1 = 0.001k 
35mA = 0.035A
0.2A = 200mA
3mA = 0.003mA
22k = 22000
470 = 0.47k
3.3k = 3300
Use this method to guide you through calculations:
It should be Very Easy Now! See the examples below:
3V is applied across a 6 resistor, what is the current?
A lamp connected to a 6V battery passes a current of 60mA, what is the lamp's resistance?
A 1.2k resistor passes a current of 0.2A, what is the voltage across it?
9V is applied across a 15k resistor, what is the current?
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