Ohm's Law

Voltage, current and resistance

Ohm's law states that

"the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them".

Ohm's law can be expressed as 

I = U / R                     (1)


I = current (ampere, A)

U = electrical potential (volts, V)

R = resistance (ohms, Ω)

Example - Ohm's law

A 12 volt battery supplies power to a resistance of 18 ohms. The current in the elctrical circuit can be calculated as

I = (12 volts) / (18 ohm)

    = 0.67 ampere

ohm's law

Equivalent Expressions

Ohm's law (1) can also be expressed as

U = R I                         (2)


R = U / I                       (3)

ohms law - voltage, resistance and current diagram

Example - Electric Circuit Resistance

A current of 1 ampere is flowing through a 230 V electric circuit. From the diagram above this indicates resistance 

R ≈ 220 Ω

This can alternatively be calculated with Ohm's law

R = (230 V) / (1 A)

   = 230 Ω

Example - Ohm's Law and Multiples and Submultiples

Currents, voltages and resistances in electric circuits may often be very small or very large - so multiples and submultiples are often used. 

The voltage required applied to a 3.3 kΩ resistor to generate a current of 20 mA can be calculated as

U = (3.3 kΩ) (1000 Ω/kΩ) (20 mA) (10-3 A/mA)

   = 66 V


Electric power can be expressed as 

P = U I

  = R I2 

  = U2 / R                     (4)


P = electrical power (watts, W)

Example - Power Consumed

The power consumed in the 12V electrical circuit above can be calculated as

P = (12 volts)2 / (18 ohm)

   =  8 W

Example - Power and Electrical Resistance

A 100 W electric light bulb is connected to a 230 V supply. The current flowing can be calculated by reorganizing (4) to

I = P / U

  = (100 W) / (230 V)

  = 0.43 ampere

The resistance can be calculated by reorganizing (4) to

R = U2 / P

   = (230 V)2 / (100 W) 

   = 529 Ω

Related Topics

  • Electrical - Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors

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