Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance. The formula is V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

What is the relationship between voltage, current, resistance and power?

The four fundamental relationships are: V = I × R (Ohm's Law), P = V × I (power), P = I² × R (power from current and resistance), and P = V² / R (power from voltage and resistance). Knowing any two values allows you to calculate the other two.

How do series and parallel resistances differ?

In series, resistances add directly: R_total = R1 + R2 + R3. In parallel, reciprocals add: 1/R_total = 1/R1 + 1/R2 + 1/R3. Series increases total resistance, while parallel decreases it. For two parallel resistors: R_total = (R1 × R2) / (R1 + R2).

What is electrical conductance?

Conductance (G) is the reciprocal of resistance: G = 1/R. It is measured in siemens (S). A higher conductance means the material conducts electricity more easily. It is useful when analyzing parallel circuits, where conductances add directly.

Does Ohm's Law apply to AC circuits?

Ohm's Law applies to AC circuits when using impedance (Z) instead of resistance (R). In AC, V = I × Z, where Z includes resistance, inductive reactance (XL = 2πfL), and capacitive reactance (XC = 1/2πfC). For purely resistive AC loads, V = IR still applies directly.

免費工程工具

歐姆定律計算器

輸入任意兩個已知的電氣參數——電壓、電流、電阻或功率——即可即時計算全部四項，以及電導與每小時能耗。

V = I × R P = V × I 歐姆定律輪圖

結果

電壓

—

電流

—

電阻

—

功率

—

電導 (G = 1/R)

—

每小時能耗

—

歐姆定律

歐姆定律定義了電路中電壓、電流與電阻之間的基本關係：

功率關係

電氣功率可透過任意兩個已知數值的組合來計算：

歐姆定律輪圖

由 V = IR 與 P = VI 衍生的全部十二個公式：

求解	由 V 與 I	由 V 與 R	由 I 與 R	由 P
V	—	—	V = I × R	V = P / I 或 √(P × R)
I	—	I = V / R	—	I = P / V 或 √(P / R)
R	R = V / I	—	—	R = V² / P 或 P / I²
P	P = V × I	P = V² / R	P = I² × R	—

實務範例

範例 — LED 電阻計算

已知： 電源 = 12 V，LED 順向電壓 = 2 V，目標電流 = 20 mA

電阻兩端電壓：12 − 2 = 10 V

R = V / I = 10 / 0.020 = 500 Ω

P = V × I = 10 × 0.020 = 0.2 W （至少使用 ¼W 電阻）

提示： 電導 (G) 是電阻的倒數：G = 1/R，單位為西門子 (S)。在並聯電路分析中非常實用，因為電導可直接相加：G_總 = G₁ + G₂ + G₃。