Ohm's Law

Ohm's law defines a linear relationship between the voltage and the current in an electrical circuit.

The DC current flow through a resistor is set by the resistor's voltage drop and the resistor's resistance.

With water flow analogy we can imagine the electric current as water current through pipe, the resistor as a thin pipe that limits the water flow, the voltage as height difference of the water that enables the water flow.

Ohm's Law Formula / Equation

When we know the voltage and resistance, we can calculate the current.

Ohm's law definition

The resistor's current I in amps (A) is equal to the resistor's voltage VR=V in volts (V) divided by the resistance R in ohms (Ω):

 

 

V is the voltage drop of the resistor, measured in Volts (V). Sometimes E is used instead of V . E denotes electromotive force.

I is the electrical current flowing through the resistor, measured in  Amperes (A)

R is the resistance of the resistor, measured in Ohms (Ω)

 

Voltage calculation

When we know the current and resistance, we can calculate the voltage.

The voltage V in volts (V) is equal to the to the current I in amps (A) times the resistance R in ohms (Ω):

 

V=I\times R

 

Resistance calculation

When we know the voltage and the current, we can calculate the resistance.

The resistance R in ohms (Ω) is equal to the voltage V in volts (V) divided by the current I in amps (A):

 

R=\frac{V}{I}

 

Since the current is set by the values of the voltage and resistance, the Ohm's law formula can show that:

  • If we increase the voltage, the current will increase.
  • If we increase the resistance, the current will reduce.

Example #1

Find the current of an electrical circuit that has resistance of 50 Ohms and voltage supply of 5 Volts.

Solution:

V = 5V

R = 50Ω

I = V / R = 5V / 50Ω = 0.1A = 100mA

Example #2

Find the resistance of an electrical circuit that has voltage supply of 10 Volts and current of 5mA.

Solution:

V = 10V

I = 5mA = 0.005A

R = V / I = 10V / 0.005A = 2000Ω = 2kΩ

Ohm's Law for AC Circuit

The load's current I in amps (A) is equal to the load's voltage VZ=V in volts (V) divided by the impedance Z in ohms (Ω):

 

V is the voltage drop on the load, measured in Volts (V)

I is the electrical current, measured in Amps (A)

Z is the impedance of the load, measured in Ohms (Ω)

Example #3

Find the current of an AC circuit, that has voltage supply of 110V∟70° and load of 0.5kΩ∟20°.

Solution:

V = 110V∟70°

Z = 0.5kΩ∟20° = 500Ω∟20°

I = V / Z = 110V∟70° / 500Ω∟20° = (110V / 500Ω) ∟ (70°-20°) = 0.22A ∟50°

Ohm's Law Calculator (short form)

Ohm's law calculator: calculates the relation between Voltage, Current and Resistance.

Enter 2 values to get the third value and press the Calculate button:

             
  Enter Resistance: R = ohms (Ω)  
  Enter Current: I = amps (A)  
  Enter Voltage: V = volts (V)  
             
   
             

 

Ohm's law calculator II ►

 

See also

Write how to improve this page

  URL
  Name
  E-mail