Ohm's Law
Given any two of voltage U, current I, resistance R, solve the third and output power P=UI (DC).
Inputs
Formula
U = I·R; I = U/R; R = U/I; P = U·I
Fundamentals
Ohm's law is the most basic circuit relation, linking voltage, current and resistance linearly.
$U=IR$: voltage equals current times resistance. Any two give the third; power $P=UI=I^2R=U^2/R$.
History
Ohm published his law in 1827; at first disputed, it became the basis of circuit analysis.
Engineering applications
Used in circuit design, voltage-current-resistance calculation and fault diagnosis.
Glossary
| Voltage $U$ | Potential difference (V). |
| Current $I$ | Charge per unit time (A). |
| Resistance $R$ | Opposition to current (Ω). |
How to use
- Fill in Voltage U, Current I, Resistance R in the Inputs section (watch the unit on each field).
- Click Calculate; the tool evaluates the formula shown above.
- Read Voltage U, Current I, Resistance R, Power P in the results area.
Formula notesOhm's law U = I·R; hence I = U/R, R = U/I. Power P = U·I = I²·R = U²/R.
Formula · Worked Example · Knowledge
Formula
At constant temperature, voltage and current of a linear resistor are proportional — Ohm's law:
$$U=I\cdot R\quad\Rightarrow\quad I=\frac{U}{R},\quad R=\frac{U}{I}$$
Power can be written as:
$$P=U\cdot I=I^{2}R=\frac{U^{2}}{R}$$
Conductance $G=1/R$ in siemens (S).
$$U=I\cdot R\quad\Rightarrow\quad I=\frac{U}{R},\quad R=\frac{U}{I}$$
Power can be written as:
$$P=U\cdot I=I^{2}R=\frac{U^{2}}{R}$$
Conductance $G=1/R$ in siemens (S).
Worked Example
Given $I=2.5\ \text{A}$, $R=4\ \Omega$.
Step 1 — voltage: $U=I\times R=2.5\times4=10\ \text{V}$.
Step 2 — power: $P=U\times I=10\times2.5=25\ \text{W}$ (or $P=I^{2}R=2.5^{2}\times4=25\ \text{W}$).
Step 1 — voltage: $U=I\times R=2.5\times4=10\ \text{V}$.
Step 2 — power: $P=U\times I=10\times2.5=25\ \text{W}$ (or $P=I^{2}R=2.5^{2}\times4=25\ \text{W}$).
Key Points
- The $I$–$V$ curve is a line through the origin with slope $G$; heating raises resistivity (non-linear).
- Keep units consistent: V, A, Ω; common prefixes mA, μA, kΩ, MΩ.
- Normalize magnitudes first (mA→A, kΩ→Ω) before calculating.
Parameters
Inputs
| Parameter | Symbol | Unit | Default |
|---|---|---|---|
| Voltage U (V) | U | V | 12 |
| Current I (A) | I | A | 0 |
| Resistance R (Ω) | R | Ω | 6 |
Outputs
| Result | Symbol | Unit |
|---|---|---|
| Voltage U | U | V |
| Current I | I | A |
| Resistance R | R | Ω |
| Power P | P | W |
Applications
- Common engineering use cases
FAQ
What formula does this tool use?
This tool computes per ISO / AGMA / ASME standard formulas: U = I·R; I = U/R; R = U/I; P = U·I
How accurate are the results?
Results match input precision, based on SI units and common engineering approximations; for critical duty re-check with a safety factor.
Where is it used?
Common engineering use cases