Current, Voltage and Resistance

Current flows in a circuit as a result of the the electrical force or voltage applied.
The amount of current flow is limited by the amount of resistance in the circuit:

The amount of current flow around a circuit is dependent on both voltage and resistance.

Georg Simon Ohm

The relationship between current, voltage and resistance was discovered by Georg Simon Ohm, a German physicist, in 1826.

Ohm's Law
The current flow in a circuit is directly proportional to the source voltage applied and inversely proportional to the resistance of the circuit.

Formulas

Ohm's Law:

       E
I = ---         or
       R


 
 
E=IR

Symbol in formula Signifies Units of measurement Accepted Abbreviation
E EMF voltage V
I current amperes A or amps
R resistance Ohms per unit length Ω
Series Circuits
When resistors are connected in a circuit so that the same current flows through each resistor, the resistors are said to be connected in series. It is possible to determine the resistance of a single resistor that has the same resistance as a number of resistors in series. This single equivalent resistance is just the sum of the individual resistances. For three resistors in series, the equivalent resistance R is given by:

Rtotal = R1 + R2 + R3


A series connection of two resistors is called a voltage divider
Parallel Circuits
When resistors are connected so that the same voltage is applied across each resistor, the resistors are said to be connected in parallel. The single equivalent resistance R of three parallel resistors can be determined from:

1/Rtotal=1/R1+1/R2+1/R3







A connection of two resistors in parallel is called a current divider

Calculations

How can you calculate voltage (V=?) if I and R are known?
  1. Start with the formula:
    
     V
I = ---    		
     R
  2. Solve for V by multiplying both sides by R 
    
I*R = V
How can you calculate resistance (R=?) if I and V are known?
  1. Start with the formula:
    
     V
I = ---    		
     R
  2. Solve for V by multiplying both sides by R 
    
I*R = V
  3. Divide both sides by I 
    
     V
R = ---
     I