The core relation
| V = I · R | Voltage = Current × Resistance |
|---|---|
| I = V / R | Current = Voltage / Resistance |
| R = V / I | Resistance = Voltage / Current |
Power formulas
| P = V · I | Power (watts) = Volts × Amps |
|---|---|
| P = I² · R | Dissipation in a resistor |
| P = V² / R | From voltage and resistance |
| V = √(P · R) | Voltage from power and resistance |
| I = √(P / R) | Current from power and resistance |
Series / parallel
| R in series | R_total = R1 + R2 + … (current same, voltages add) |
|---|---|
| R in parallel | 1 / R_total = 1/R1 + 1/R2 + … (voltage same, currents add) |
| Two in parallel | R = (R1 · R2) / (R1 + R2) |
Quick reference
| Quantity | Unit | Symbol |
|---|---|---|
| Voltage | Volt (V) | V or E |
| Current | Ampere (A) | I |
| Resistance | Ohm (Ω) | R |
| Power | Watt (W) | P |
| Energy | Joule (J) | W (or E) |
Notes
- Ohm's law holds exactly for linear resistors. Semiconductors, incandescent bulbs, and arcs are nonlinear.
- For AC use V_rms and I_rms. Power factor matters when load is reactive: P = V · I · cos φ.
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