# Ohm's law calculator

> Ohm's law calculator: enter any two of voltage, current, resistance and power to get the other two, shown in the units you pick (mA, kΩ…).

Интерактивная версия: https://www.calcopenly.com/ru/science/ohms-law-calculator
Тема: Научные калькуляторы

Ohm's law says the current through a resistor equals the voltage across it divided by its resistance, I = V/R. Combined with the power law P = VI, any two of voltage, current, resistance and power fix the other two: P = I²R and P = V²/R follow by substitution. Solved values are shown in the unit you choose, so a milliamp current does not appear as 0.002 A.

The default, 12 V across 6 Ω, gives 2 A and 24 W. The same arithmetic sizes fuses and cables (a 2,300 W kettle on 230 V draws 10 A) and checks resistor ratings (a 100 Ω, ¼ W resistor carries at most 50 mA).

The law holds for ohmic conductors, whose resistance stays constant as the current changes. Diodes, LEDs and filament lamps are not ohmic, so a single resistance value describes them only at one operating point.

## Входные данные

- **I know** (варианты: Voltage and resistance, Voltage and current, Current and resistance, Voltage and power, Current and power, Resistance and power)
- **Voltage**
- **Voltage unit** (варианты: mV, V, kV)
- **Сила тока**
- **Current unit** (варианты: µA, mA, A, kA)
- **Resistance**: Accepts 4.7k or 2.2M
- **Resistance unit** (варианты: mΩ, Ω, kΩ, MΩ)
- **Мощность**
- **Show voltage in** (варианты: mV, V, kV)
- **Show current in** (варианты: µA, mA, A, kA)
- **Show resistance in** (варианты: mΩ, Ω, kΩ, MΩ)
- **Show power in** (варианты: mW, W, kW)

## Результаты

- Сила тока (A) — основной результат
- Voltage (V)
- Resistance (Ω)
- Мощность (W)
- Conductance (S)

## Формула

$$
V = IR,\qquad P = VI = I^2R = \frac{V^2}{R}
$$

## Примеры с решением

### 12 V across 6 Ω

- I know: Voltage and resistance
- Voltage: 12
- Voltage unit: V
- Resistance: 6
- Resistance unit: Ω
- Show current in: A
- Show power in: W
- **Сила тока: 2 A**
- **Мощность: 24 W**
- Источник проверки: Python 3.8 decimal: I = 12/6, P = 12²/6

### Kettle: 230 V drawing 10 A

- I know: Voltage and current
- Voltage: 230
- Voltage unit: V
- Сила тока: 10
- Current unit: A
- Show resistance in: Ω
- Show power in: W
- **Resistance: 23 Ω**
- **Мощность: 2,300 W**
- Источник проверки: Python 3.8 decimal: R = 230/10, P = 230 × 10

### Quarter-watt 100 Ω resistor at full rating

- I know: Resistance and power
- Resistance: 100
- Resistance unit: Ω
- Мощность: 0.25 W
- Show voltage in: V
- Show current in: A
- **Сила тока: 0.05 A**
- **Voltage: 5 V**
- Источник проверки: Python 3.8 decimal: I = √(0.25/100), V = √(0.25 × 100)

### 5 V, 500 mW

- I know: Voltage and power
- Voltage: 5
- Voltage unit: V
- Мощность: 500 mW
- Show current in: A
- Show resistance in: Ω
- **Сила тока: 0.1 A**
- **Resistance: 50 Ω**
- Источник проверки: Python 3.8 decimal: I = 0.5/5, R = 25/0.5

### 20 mA dissipating 40 mW

- I know: Current and power
- Сила тока: 20
- Current unit: mA
- Мощность: 40 mW
- Show voltage in: V
- Show resistance in: Ω
- **Voltage: 2 V**
- **Resistance: 100 Ω**
- Источник проверки: Python 3.8 decimal: V = 0.04/0.02, R = 0.04/0.02²

### 2 mA through 4.7k (typed with the k suffix)

- I know: Current and resistance
- Сила тока: 2
- Current unit: mA
- Resistance: 4.7k
- Resistance unit: Ω
- Show voltage in: V
- Show power in: W
- **Voltage: 9.4 V**
- **Мощность: 0.0188 W**
- Источник проверки: Python 3.8 decimal: V = 0.002 × 4700, P = 0.002² × 4700

## Вопросы

### How do you calculate current from voltage and resistance?

Divide the voltage by the resistance: I = V/R. With 12 V across 6 Ω the current is 2 A; with 5 V across 1 kΩ it is 0.005 A, or 5 mA. Convert kilohms to ohms (×1,000) before dividing, or pick kΩ and mA in the unit selectors and let the calculator convert.

### How do you convert watts to amps?

Divide the power by the voltage: I = P/V. A 2,300 W kettle on a 230 V supply draws 10 A, and a 60 W load on 12 V draws 5 A. For AC this holds for resistive loads such as heaters and kettles; motors and power supplies with a power factor below 1 draw more current than P/V.

### What is the formula for electrical power?

P = VI, which becomes P = I²R or P = V²/R once Ohm's law is substituted. A 100 Ω resistor with 5 V across it dissipates 5²/100 = 0.25 W, exactly the limit of a ¼ W part, so a 0.5 W resistor is the safer choice there.

### Does Ohm's law apply to LEDs and light bulbs?

Not with a single fixed resistance. An LED conducts almost nothing below its forward voltage (about 2 V for red, about 3 V for blue and white) and then current rises steeply, so it needs a series resistor. A filament lamp's resistance rises as the tungsten heats, so the V²/P value, 882 Ω for a 60 W, 230 V bulb, applies only when it is lit.

### What is conductance?

Conductance is the reciprocal of resistance, G = 1/R, measured in siemens (S). A 6 Ω resistor has a conductance of 1/6 ≈ 0.167 S. Conductances in parallel add directly, which is why it is convenient for parallel networks: 1/R_total = G₁ + G₂ + …

### Насколько точен «Ohm's law calculator»?

Точность зависит от введённых данных и допущений метода. Десятичная арифметика использует 50 значащих цифр, но оценки, численные методы и исходные данные могут быть менее точными; округление на экране не устраняет эти ограничения. Решённые примеры, проверенные по независимым источникам: 6. Например, «12 V across 6 Ω» проверяется по источнику Python 3.8 decimal: I = 12/6, P = 12²/6.

### Откуда взята методика?

OpenStax University Physics Volume 2, §9.4 Ohm's law and §9.5 Electrical energy and power; HyperPhysics — Ohm's law, power.

## Источники

- [OpenStax University Physics Volume 2, §9.4 Ohm's law and §9.5 Electrical energy and power](https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law)
- [HyperPhysics — Ohm's law, power](http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html)