Question 1

What is Ohm's Law?

Accepted Answer

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. The formula is V = I x R, where V is voltage in volts, I is current in amps, and R is resistance in ohms. It was formulated by Georg Ohm in 1827 and is the foundation of electrical circuit analysis. The three forms are: V = IR, I = V/R, and R = V/I.

Question 2

How do I calculate power from Ohm's Law?

Accepted Answer

Power is derived from Ohm's Law using P = V x I. You can substitute V = IR to get P = I^2 x R, or I = V/R to get P = V^2 / R. All three forms give the same result. For example, a 12V circuit drawing 2A consumes 24W of power. Equivalently, if the resistance is 6 ohms: P = I^2 x R = 4 x 6 = 24W, or P = V^2 / R = 144 / 6 = 24W.

Question 3

Does Ohm's Law work for AC circuits?

Accepted Answer

Ohm's Law works for DC and purely resistive AC circuits. For AC circuits with inductors or capacitors, you must use impedance (Z) instead of resistance: V = I x Z. Impedance combines resistance and reactance and is frequency-dependent. For practical residential AC calculations with resistive loads (heaters, incandescent bulbs), the standard Ohm's Law applies directly using RMS voltage and current values.

Question 4

What does a zero or infinite resistance mean?

Accepted Answer

Zero resistance is a short circuit - infinite current would flow for any non-zero voltage, limited only by the source capacity and wire resistance. This trips breakers or blows fuses instantly. Infinite resistance is an open circuit - no current flows regardless of voltage. Real-world circuits have small parasitic resistance (milliohms) and leakage currents (microamps), so these are theoretical limits, not literal values.

Question 5

How is resistance measured in circuits?

Accepted Answer

Resistance is measured in ohms using a multimeter set to the resistance function. Always disconnect power before measuring - the meter applies a small test voltage and measures current to calculate resistance. Typical values: household wire is 0.001-0.01 ohm/ft, a 100W incandescent bulb is about 144 ohms when hot, a 1 kOhm resistor is 1000 ohms. Resistance also changes with temperature, especially in metals (positive coefficient) and semiconductors (negative).

Question 6

What's the difference between EMF and voltage?

Accepted Answer

EMF (electromotive force) is the energy per charge provided by a source, like a battery or generator, independent of the circuit. Voltage is the potential difference between two points in a circuit, which equals EMF minus the internal voltage drop of the source. For a 12V battery with 0.5 ohm internal resistance delivering 2A, the terminal voltage is 12 - (2 x 0.5) = 11V. Under no load, terminal voltage equals EMF.

Question 7

Can Ohm's Law help me choose a resistor value?

Accepted Answer

Yes. To limit current in an LED circuit, for example: if you have a 9V source and want 20mA through an LED with 2V forward voltage, you need to drop 7V across the resistor. R = V/I = 7 / 0.020 = 350 ohms, so use a standard 360 ohm resistor. Also check power: P = I^2 x R = 0.0004 x 360 = 0.14W, so a 1/4W resistor is fine. Always pick a resistor rated at least 2x the calculated power.

To Find	Given V, I	Given V, R	Given I, R	Given P + one
Voltage	-	-	I x R	P/I or √(P*R)
Current	-	V/R	-	P/V or √(P/R)
Resistance	V/I	-	-	V²/P or P/I²
Power	V x I	V²/R	I² x R	-

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