What exactly is Ohm's Law?
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, with resistance as the proportionality constant: V = I × R. Formulated by Georg Ohm in 1827, it's foundational to all electrical engineering. The law applies to "ohmic" materials (most metals at constant temperature) but breaks down for semiconductors, plasmas, and other non-linear devices.
Does Ohm's Law work for AC circuits?
It works for resistive AC loads (heaters, incandescent bulbs) — use RMS voltage and current. For reactive components (capacitors, inductors, motors), you must use impedance (Z) instead of resistance: V = I × Z, where Z includes resistance and reactance. Power in AC also requires considering power factor: P = V × I × cos(φ). For purely resistive AC loads, Ohm's Law applies directly with RMS values.
Why is current "I" instead of "C"?
"I" stands for "intensité" — French for intensity — used by André-Marie Ampère (the namesake of the unit) in early 19th-century writings. "C" was reserved for capacitance and the speed of light. The convention stuck globally. It can be confusing for newcomers but is the universal symbol in physics and engineering.
What's the difference between V=IR and V×I=P?
V=IR is Ohm's Law — it relates voltage, current, and resistance in a circuit. V×I=P is the Power Law — it tells you how much energy is being used or dissipated. They're different relationships but both involve the same variables (V and I), which is why they're often used together. Combined, they let you derive variants like P=I²R (power from current and resistance) or P=V²/R (power from voltage and resistance).
How do I calculate resistor power rating?
Calculate the actual power: P = I² × R or P = V²/R, depending on what you know. Then select a resistor rated at least 2× this value for safety margin. Common ratings: 1/8W (small signal), 1/4W (most common), 1/2W, 1W, 5W, 10W (high power). For example, a 470Ω resistor with 9V across it dissipates 9²/470 = 0.172W — a 1/4W resistor works fine (with 30% headroom).
Can voltage and current ever be negative?
Yes — in AC circuits voltage and current alternate between positive and negative many times per second (50-60 Hz mains). In DC circuits, the sign indicates direction relative to a reference. Power is the product, so when V and I have the same sign, power is positive (consumed by the load). When they have opposite signs (like in batteries during charging or generators), power flows the opposite way.
What's "voltage drop" and why does it matter?
Voltage drop is the reduction in voltage along a wire due to its resistance. For a wire with resistance R carrying current I, drop = I × R. For most household wiring it's small (1-3%), but for long DC runs (solar panels, RVs, marine) it becomes significant. Too much drop causes underperformance, voltage starvation of devices, and wire heating. Use thicker wire (lower resistance per foot) or higher voltage to reduce drop.
How accurate is this calculator?
Mathematically perfect — these are exact algebraic relationships. In real circuits, accuracy depends on: component tolerances (resistors typically ±5% or ±1%), temperature (resistance changes with heat), wire resistance (often ignored in calculations), and measurement precision. For practical work, account for these by sizing components conservatively (2× safety margin on power, 25% on current).
What's the difference between Ohms, Watts, Volts, and Amps?
They measure different things: Volts measure electrical pressure (voltage). Amps measure electron flow rate (current). Ohms measure opposition to flow (resistance). Watts measure energy per second (power). Think of plumbing: pressure ≠ flow ≠ pipe size ≠ horsepower of the pump. They're related (Ohm's Law and Power Law connect them) but each represents a distinct physical quantity.
What's the safe voltage to work with?
Up to 50V DC or 30V AC is generally considered safe to touch under normal dry conditions. Above this, sufficient current can flow through your body to be dangerous. Mains voltage (120V/240V) can be lethal — about 100mA across the heart is fatal, and just 10-20mA can cause involuntary muscle contraction. Always assume any circuit is live until proven dead with a meter, and use proper PPE.
Why do I need to know about both voltage and current?
Voltage alone doesn't tell you danger or power — current does. Static electricity can hit you with 10,000+ volts but the current is microscopic, so it just stings. Conversely, a 12V car battery has low voltage but can deliver 600A in a short circuit, which can weld metal. To understand any electrical situation, you need both numbers (and their product, power).
Is Ohm's Law really always true?
For "ohmic" devices (most resistors, wires, etc., at constant temperature), yes. But many real devices are non-linear: LEDs have a sharp threshold voltage; diodes only conduct one direction; transistors are voltage-controlled current sources; light bulbs have resistance that increases dramatically when hot. For these, you need device-specific models (I-V curves, datasheets). Ohm's Law remains the foundation but isn't the whole story.
