How to Use Ohm's Law — Voltage, Current, Resistance & Power

The Four Pillars of Circuit Analysis

Every DC circuit revolves around four key quantities:

• Voltage (V) – Electrical “push” or potential difference, measured in volts (V)
• Current (I) – Flow of electric charge, measured in amperes (A)
• Resistance (R) – Opposition to current flow, measured in ohms (Ω)
• Power (P) – Rate of energy consumption or dissipation, measured in watts (W)

Ohm’s Law: The Foundational Equation

At its heart, Ohm’s Law states:

Voltage = Current × Resistance
V = I × R

This can be rearranged to solve for any unknown:

• I = V / R (Find current)
• R = V / I (Find resistance)

💡 Memory aid: Use the Ohm’s Law triangle—cover the variable you want to find. The remaining symbols show the operation (e.g., cover V → I × R).

Power Calculations: Beyond Ohm’s Law

While Ohm’s Law relates V, I, and R, power requires its own set of formulas—though they integrate seamlessly:

1. P = V × I (Universal for all loads)
2. P = I² × R (Best when you know current and resistance)
3. P = V² / R (Best when you know voltage and resistance)

All three are equivalent for resistive loads (e.g., heaters, incandescent bulbs, resistors).

Combining Resistors: Series and Parallel

Real circuits rarely use a single resistor. Understanding how resistors combine is critical.

Series Circuits

• Current is the same through all components.
• Total resistance: R_total = R₁ + R₂ + R₃ + …
• Voltage divides proportionally to resistance.

Parallel Circuits

• Voltage is the same across all branches.
• Total resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + …
• For two resistors, use: R_total = (R₁ × R₂) / (R₁ + R₂)
• Current splits inversely with resistance.

AC vs. DC: When Ohm’s Law Applies

• DC (Direct Current): Ohm’s Law applies directly to resistive circuits.
• AC (Alternating Current): Use RMS values (e.g., UK mains = 230V RMS) for approximate calculations in resistive loads.
• Reactive loads (motors, capacitors, transformers) require impedance (Z) and power factor—beyond basic Ohm’s Law.

Pro Tips & Common Mistakes

• Derate components: Always choose resistors with at least 2× the calculated power rating (e.g., 1W for 0.5W dissipation).
• Verify with a multimeter: Calculations assume ideal conditions; real circuits have tolerances.
• Beware of voltage ratings: A resistor may handle the power but fail if voltage exceeds its rating.
• Use fuses: Protect prototypes with current-limiting supplies or fuses.
• Temperature matters: Resistance changes with heat—critical in high-power applications.

Practical Applications

• Sizing current-limiting resistors for LEDs
• Designing voltage dividers for sensors
• Calculating power supply requirements
• Troubleshooting open/short circuits
• Estimating battery life (Battery Life = Capacity (Ah) / Current (A))