What is Ohm's Law?

Ohm's Law is a fundamental principle of electricity that states the electric current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. It is expressed mathematically as V = IR, where V is voltage (volts), I is current (amperes), and R is resistance (ohms). The law was published by German physicist Georg Simon Ohm in 1827.

Ohm's Law provides the basic framework for analysing and designing electrical circuits. If any two of the three quantities (voltage, current, resistance) are known, the third can be calculated. The law holds true for ohmic conductors — materials where resistance remains constant regardless of voltage — such as most metals at constant temperature.

Materials that do not follow Ohm's Law are called non-ohmic conductors — examples include diodes, transistors, and filament bulbs (whose resistance increases with temperature). Despite this limitation, Ohm's Law remains the single most important equation in basic circuit analysis and is essential for understanding power distribution, electronics, and electrical safety.

Key Features

# Feature Details
1 Formula V = IR (Voltage = Current x Resistance)
2 Rearranged Forms I = V/R (to find current); R = V/I (to find resistance)
3 Unit of Voltage Volt (V) — named after Alessandro Volta; measures potential difference
4 Unit of Current Ampere (A) — named after Andre-Marie Ampere; measures charge flow rate
5 Unit of Resistance Ohm (Ω) — named after Georg Ohm; measures opposition to current flow
6 Ohmic Conductors Follow V = IR linearly (metals at constant temperature, fixed resistors)
7 Non-Ohmic Conductors Resistance varies with voltage/temperature (diodes, filament lamps, electrolytes)
8 Power Relation Electrical power P = VI = I²R = V²/R (derived using Ohm's Law)

Important Concepts

  • Resistance of a wire depends on four factors: length (directly proportional), cross-sectional area (inversely proportional), material (resistivity), and temperature (increases for metals, decreases for semiconductors).
  • Series circuits: resistances add up (R_total = R₁ + R₂ + ...). Parallel circuits: reciprocals add up (1/R_total = 1/R₁ + 1/R₂ + ...). Ohm's Law applies to each component and the entire circuit.
  • Electrical power formulas P = VI, P = I²R, and P = V²/R are all derived by combining Ohm's Law with the power equation. These are used to calculate energy consumption (electricity bills use kilowatt-hours, kWh).
  • Household electrical safety devices — fuses, MCBs (Miniature Circuit Breakers), and earthing systems — all rely on Ohm's Law to detect excess current and protect against short circuits and overloading.
  • Transmission losses in power lines are proportional to I²R — this is why electricity is transmitted at high voltage (and therefore low current) to minimise losses. India uses 400 kV and 765 kV high-voltage transmission lines.
  • Superconductors have exactly zero resistance below their critical temperature, meaning current flows without any energy loss — a field of active research for power grids and maglev trains.
  • Kirchhoff's laws (junction and loop rules) extend Ohm's Law to complex circuits with multiple loops and branches, allowing analysis of any circuit configuration.
  • The V-I graph of an ohmic conductor is a straight line through the origin; its slope equals the resistance. Non-ohmic devices show curved or segmented graphs.

UPSC Exam Corner

Prelims: Key Facts

  • Ohm's Law: V = IR; published by Georg Simon Ohm in 1827
  • Resistance is measured in ohms (Ω); current in amperes (A); voltage in volts (V)
  • Ohmic materials show a linear V-I graph; non-ohmic materials show a curved graph
  • Electrical power P = VI; combining with Ohm's Law gives P = I²R and P = V²/R
  • Resistance of a wire depends on length, cross-sectional area, material, and temperature
  • India's household electricity supply is 230 V AC, 50 Hz
  • In series circuits, total resistance = R₁ + R₂ + ...; in parallel, 1/R_total = 1/R₁ + 1/R₂ + ...
  • Energy consumption is measured in kilowatt-hours (kWh): 1 kWh = 1 unit of electricity
  • Superconductors have zero resistance below a critical temperature — Ohm's Law gives infinite current for any voltage

Mains: Probable Themes

  1. Ohm's Law in the context of India's electrical grid — transmission losses (I²R losses) and efficiency
  2. Understanding resistance and power in household electrical safety (MCBs, fuses, earthing)
  3. Application in renewable energy systems — solar panel circuit design and battery management
  4. Smart meters and energy efficiency — using electrical principles for demand-side management
  5. High-voltage transmission to reduce I²R losses — relevance to India's national grid expansion
  6. Superconductivity research and its potential to revolutionise power transmission and transportation

Sources: Ohm's Law — Wikipedia, Britannica — Ohm's Law, All About Circuits — Ohm's Law