Ohm's law is the compact relationship V = I × R: voltage V across a component equals current I through it multiplied by resistance R. It is not a complete model of every device—diodes, batteries, lamps, and semiconductors can be nonlinear—but it is an essential first check for resistive circuits and many operating-point estimates. The SI units are volts (V), amperes (A), and ohms (Ω), and keeping those units visible makes mistakes much easier to spot.
The Three Forms of Ohm's Law
Rearrange the same equation to find any missing quantity: I = V/R and R = V/I. For example, a 12 V supply across a 6 Ω resistor produces 2 A, while a 0.5 A load on 24 V has an equivalent resistance of 48 Ω. The Electric Potential Converter can normalize millivolts, volts, and kilovolts before the calculation, and the Electric Resistance Converter handles Ω, kΩ, and MΩ prefixes.
Current, Charge, and Time
Current describes the rate at which charge moves: I = Q/t. One ampere is one coulomb per second, so a steady 2 A current transfers 120 coulombs in one minute. Battery capacity is often specified in ampere-hours rather than coulombs; 1 Ah equals 3,600 C. Use the Charge Converter when translating between coulombs and ampere-hours, and remember that ampere-hours describe stored charge while watt-hours also include voltage.
Power and Energy
Electrical power is P = V × I. Combining this with Ohm's law gives P = I²R and P = V²/R, which are useful for estimating resistor heating. A 12 V, 2 A load consumes 24 W; a 6 Ω resistor carrying 2 A dissipates 24 W and should be selected with an appropriate power rating and safety margin. Power is a rate, whereas energy is power multiplied by time, so a 24 W load running for five hours uses 120 Wh.
Conductance and AC Impedance
Conductance G is the reciprocal of resistance, G = 1/R, and is measured in siemens (S), historically also called mho. A 100 Ω resistor has 0.01 S of conductance. The Electric Conductance Converter is useful when a datasheet uses microsiemens or millisiemens. In alternating-current analysis, impedance extends resistance to include frequency-dependent reactance, so a capacitor or inductor cannot always be reduced to one fixed resistance.
Capacitors and Inductors
Capacitance measures charge stored per volt: C = Q/V, with farads (F) as the SI unit. Inductance measures the tendency of a current-changing conductor to oppose that change and is measured in henries (H). Real components are usually labeled in microfarads, nanofarads, millihenries, or microhenries because one farad or henry is large in many circuits. Convert prefixes before comparing components and check voltage, current, frequency, and tolerance ratings separately.
A Safe Calculation Workflow
Start by drawing the circuit and marking polarity and current direction. Convert every value to a consistent unit system, identify whether the model is DC, steady-state AC, or transient, then apply the simplest valid equation. Finally, estimate heat dissipation and verify that the result is physically plausible: a claimed 10 kA through a small resistor is a warning to recheck prefixes, wiring, and assumptions. Never use a calculator result as permission to probe an energized circuit; isolation and appropriate electrical safety procedures come first.