# Ohm's Law

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) *

* where *

* I = electrical current (ampere, A) *

* U = electrical potential difference, voltage (volts, V) *

* R = electrical 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

### Equivalent Expressions of Ohm's Law

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

* U = R I (2) *

* or *

* R = U / I (3) *

Download and print the Ohm's Law 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 Resistance Nomogram

The default values in the nomogram above indicates * 230 volts *, resistance * 24 ohm * and * current 10 amps *.

### Power dissipated Resistors

Electric power in resistor circuit can be expressed as

* P = U I *

* = R I ^{2}*

* = U ^{2}/ R (4) *

* where *

* P = electrical power (watts, W) *

#### Example - Power Consumed

The power consumed or dissipated in the 18 ohm resistor 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 = U ^{2}/ P *

* = (230 V) ^{2}/ (100 W) *

* = 529 Ω *

#### Electric Power Nomogram

This nomogram can be used to estimate power vs. voltage and ampere.

The default values in the nomogram above indicates * 240 volts *, resistance * 10 amps * and power * 2.4 kW * for DC or single phase AC - and * 4 kW * for three phase AC.

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