Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, provided temperature remains constant. The formula is V = I × R, where R is resistance in ohms (Ω). It can be rearranged to find current (I = V/R) or resistance (R = V/I). Georg Ohm published this relationship in 1827.

What unit is resistance measured in?

Resistance is measured in ohms, with the symbol Ω (uppercase Greek letter omega), named after Georg Simon Ohm. Common multiples include kilohms (kΩ = 1,000 Ω) and megohms (MΩ = 1,000,000 Ω). One ohm is defined as the resistance that allows one ampere of current to flow when one volt is applied across it.

How do you calculate watts from volts and amps?

Power in watts (W) equals voltage in volts (V) multiplied by current in amps (A): P = V × I. For example, a device drawing 5 A at 240 V uses 240 × 5 = 1,200 W (1.2 kW). You can also calculate power using only resistance: P = I² × R or P = V² / R. These three power formulas are all equivalent via Ohm's Law.

What fuse should I use for a 1200W appliance on 240V?

First calculate the current: I = P/V = 1200/240 = 5 A. UK plug fuses are available in 3 A, 5 A and 13 A ratings. You should select the fuse rated just above the calculated current — so a 5 A fuse for a 1200W appliance at 240V. A 13 A fuse would also protect the wiring but offers less protection to the appliance. Never fit a fuse rated below the calculated current or the fuse will blow under normal operation.

Does Ohm's Law apply to AC circuits?

Ohm's Law applies to purely resistive AC circuits: V = IR holds true where V and I are the RMS (root mean square) values. However, AC circuits often contain capacitors and inductors that introduce reactance (frequency-dependent resistance). In these cases, the generalised form uses impedance Z instead of R: V = IZ, where Z is a complex number combining resistance R and reactance X. For GCSE physics, pure resistors in AC circuits obey Ohm's Law with RMS values.

What is the difference between voltage and current?

Voltage (measured in volts, V) is the electrical potential difference — it is the 'push' or pressure that drives charge through a circuit, analogous to water pressure in a pipe. Current (measured in amperes or amps, A) is the rate of flow of electric charge, analogous to the flow rate of water. Resistance (ohms, Ω) is what opposes the current. A higher voltage with the same resistance produces more current. Ohm's Law (V = IR) describes exactly how these three quantities relate.

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