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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…).

更新于 已验证的示例：6

Accepts 4.7k or 2.2M
试一试
电流
A
电流: 2 A
有效数字：6；取最近值，等距时远离零
Voltage
12V
Resistance
6Ω
功率
24W
Conductance
0.166667S

12 V across 6 Ω drives 2 A and turns 24 W into heat.

Ohm's-law triangle (solved values highlighted)

V12 VI2 AR6 ΩP = V × I24 WCover the one you want: V = I × R, I = V ÷ R, R = V ÷ I
计算方法 S

  1. Given, in base units

    V=12 V,R=6 ΩV = 12\ \mathrm{V},\quad R = 6\ \mathrm{\Omega}

  2. Find I

    I=VR=126=2 AI = \frac{V}{R} = \frac{12}{6} = 2\ \mathrm{A}

  3. Find P

    P=V2R=(12)26=24 WP = \frac{V^2}{R} = \frac{(12)^2}{6} = 24\ \mathrm{W}

  4. Check

    IR=12 V,VI=24 WIR = 12\ \mathrm{V},\quad VI = 24\ \mathrm{W}

    Ohm's law holds for ohmic conductors (resistance independent of current); diodes, lamps and LEDs are not ohmic.

关于Ohm's law calculator

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.

计算示例

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
电流
2 A
功率
24 W

核验来源：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
电流
10
Current unit
A
Show resistance in
Ω
Show power in
W
Resistance
23 Ω
功率
2,300 W

核验来源：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
功率
0.25 W
Show voltage in
V
Show current in
A
电流
0.05 A
Voltage
5 V

核验来源：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
功率
500 mW
Show current in
A
Show resistance in
Ω
电流
0.1 A
Resistance
50 Ω

核验来源：Python 3.8 decimal: I = 0.5/5, R = 25/0.5

常见问题

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 + …

“Ohm's law calculator”有多准确？

准确性取决于输入值和方法的假设。十进制运算使用50位有效数字，但估算、数值方法和源数据的精度可能较低；显示时的舍入并不能消除这些限制。 已按独立来源核验的计算示例：6。 例如，“12 V across 6 Ω”根据Python 3.8 decimal: I = 12/6, P = 12²/6进行核验。

这种方法出自哪里？

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

关于此计算器

V=IR,P=VI=I2R=V2RV = IR,\qquad P = VI = I^2R = \frac{V^2}{R}

来源

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

已对照来源验证

此计算器包含 6 个已解示例，答案来自独立来源。这些示例会在测试套件中运行，你也可以在此运行验证。

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