Electrical
Ohm's Law
V = IR — fundamental relationship between voltage, current, resistance
Ohm's Law states that voltage (V) equals current (I) times resistance (R). V = IR. Discovered by Georg Ohm (1827). Foundation of circuit analysis. Voltage drives current through resistance. Higher voltage at constant resistance: more current. Higher resistance at constant voltage: less current. Power: P = VI = I²R = V²/R. Critical for: circuit design, troubleshooting, electronics, electrical engineering. Applies to: linear (ohmic) materials. Many real components nonlinear (diodes, transistors).
- EquationV = IR
- UnitsV (volts), I (amperes), R (ohms)
- PowerP = VI = I²R = V²/R
- DiscoveredGeorg Ohm, 1827
- LinearApplies to ohmic materials
- NonlinearDiodes, transistors don't obey
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Why Ohm's Law matters
- Circuit analysis. Foundation.
- Electronics design. Component sizing.
- Troubleshooting. Diagnosing problems.
- Power calculations. Heat dissipation.
- Education. Introductory physics, EE.
- Safety. Wire sizing, fuses.
- Engineering. All electrical work.
Common misconceptions
- Applies always. Linear materials only.
- One formula. Three forms: V=IR, I=V/R, R=V/I.
- Resistance constant. Varies with temperature.
- Power = voltage. P = VI.
- Just for DC. Applies to AC too (with reactance considerations).
- Easy to apply. Real circuits complex.
Frequently asked questions
What's Ohm's Law?
V = IR. Voltage equals current times resistance. Three forms. (1) V = IR (find voltage). (2) I = V/R (find current). (3) R = V/I (find resistance). Foundation of electrical engineering. Linear relationship for ohmic materials. Power formulations. P = VI, P = I²R, P = V²/R.
How is power calculated?
P = VI. Voltage times current. Combined with Ohm's Law. P = I²R (substituting V = IR). P = V²/R (substituting I = V/R). All three give same answer for ohmic resistors. Different forms useful in different contexts. Heat dissipation: I²R loss in wires.
When does it not apply?
Nonlinear devices. (1) Diodes: exponential V-I curve. (2) Transistors: complex behavior. (3) Light bulbs: hot filament resistance higher. (4) Electrolytes: nonlinear. (5) Semiconductors generally. Ohm's Law: useful approximation for resistors. Nonlinear elements: use device-specific equations.
How do resistors combine?
Series (end-to-end): R_total = R_1 + R_2 + ... Sum. Parallel (side-by-side): 1/R_total = 1/R_1 + 1/R_2 + ... Product over sum for two: R_1 R_2 / (R_1 + R_2). Series: same current. Parallel: same voltage. Important for circuit analysis.
How is it used in design?
Many ways. (1) Sizing resistors: pick resistance for desired current. (2) Voltage dividers: V_out = V_in × R_2 / (R_1 + R_2). (3) Current limiting (LEDs): R = (V_in - V_LED) / I_LED. (4) Calculating power dissipation. (5) Troubleshooting: measure V, I, calculate R. Foundational tool.
What's a voltage divider?
Two resistors in series; output taken between them. V_out = V_in × R_2 / (R_1 + R_2). Used for: signal levels, sensor circuits, biasing. Loaded divider: load affects V_out (pulls it down). Useful for fixed reference voltages, but inefficient for power. Better solutions for many applications: voltage regulators.
What's resistance physically?
Material property opposing current. Free electrons collide with atoms; lose energy as heat. Higher resistance: more collisions; less current. Conductors (copper, gold): low resistance. Insulators (rubber, glass): very high. Semiconductors: variable. Temperature affects: most conductors increase resistance with heat (positive temperature coefficient).