Ohm's Law Cheat Sheet
Ohm's law and power formulas — V, I, R, P in every combination.
Reference
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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