Ohm's Law, Power, and Series/Parallel Circuits

Troubleshooting is applied circuit theory. If you know the two core formulas and how series vs. parallel behaves, you can predict what a meter should read — and spot when it doesn't.

1. The two formulas

• Ohm's law: E = I × R (Voltage = Current × Resistance). Rearranged: I = E/R, R = E/I.
• Power: P = I × E (Watts = Amps × Volts). Also P = I²R = E²/R.

Quick examples:

• 120 V across a 10 Ω load → I = E/R = 120 ÷ 10 = 12 A.
• A 120 V, 1200 W heater → I = P/E = 1200 ÷ 120 = 10 A.
• 3 A through a 20 Ω resistor → P = I²R = 9 × 20 = 180 W.

2. Series circuits

• Current is the same everywhere in the loop.
• Voltages add up (voltage divides across the resistances).
• Total resistance = the sum of the resistances (R_total = R1 + R2 + …).