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Ohm's Law
Relates the voltage across a conductor to the current flowing through it and its resistance, the most fundamental relationship in circuit analysis.
Interpretation
Ohm's law: V = I·R, where V is voltage, I is current, R is resistance. It relates the three quantities in a linear resistor. Example: 12 V across 4 Ω → I = 3 A.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| V | Voltage across the conductor | V |
| I | Current flowing through the conductor | A |
| R | Resistance of the conductor | ohm |
What it means
Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant. The constant of proportionality is the resistance R. This law is fundamental in electrical engineering and physics, used to analyse simple circuits, design power supplies, and calculate power dissipation (P = VI = I²R). It applies to ohmic materials (metals) but not to semiconductors, diodes, or capacitors under AC. The law is used in circuit analysis (Kirchhoff’s laws), in sizing wires, and in troubleshooting electrical systems. Understanding Ohm’s law is essential for anyone working with electricity.
Worked example
Ohm's Law – Two Examples
Real‑World| Parameter | Value |
|---|---|
| I | 2 A |
| R | 10 Ω |
| Parameter | Value |
|---|---|
| I | 1 A |
| R | 220 Ω |
Common mistakes
- Ohm’s law: V = I·R applies only to ohmic materials (constant resistance). Semiconductors and diodes are non‑ohmic.
- Voltage V: Potential difference across the resistor – not the total voltage of the circuit.
- Current I: Through the resistor – direction matters for sign.
- Resistance R: In ohms (Ω) – depends on material, temperature.
- AC vs. DC: For AC, use RMS values; for instantaneous, use v(t)=i(t)R.
Applications
Ohm's law, V = I·R, relates voltage, current, and resistance in an electrical circuit. It is the most fundamental law in electrical engineering, used in the design of every electronic device, power system, and circuit. Engineers use it to calculate current flow, to size resistors, and to design power supplies. In telecommunications, it ensures signal integrity. In power distribution, it determines voltage drops and losses. The law also underpins the design of sensors, heating elements, and electrical safety devices like fuses and circuit breakers. By applying Ohm's law, professionals can design efficient, safe, and reliable electrical and electronic systems across all industries.
- Design of electrical circuits and PCBs
- Sizing of resistors and other components
- Power distribution and grid analysis
- Sensor design and signal conditioning
- Safety device selection (fuses, breakers)
Frequently Asked Questions
Ohm's law states that the voltage (V) across a resistor is directly proportional to the current (I) through it, with the resistance (R) as the proportionality constant: V = I·R. It applies to ohmic materials at constant temperature.
- Voltage V – volts (V).
- Current I – amperes (A).
- Resistance R – ohms (Ω).
Applying it to non‑ohmic devices (e.g., diodes, transistors, light bulbs) where resistance changes with voltage or current. Ohm's law only holds for linear resistors.
For most conductors, resistance increases with temperature (positive temperature coefficient). For semiconductors, resistance decreases with temperature (negative coefficient).
Resistance (R) is a property of a particular object: R = ρ·L/A, where ρ is resistivity (material property), L is length, and A is cross‑sectional area. Resistivity is intrinsic; resistance depends on geometry.
- Series: same current flows through each resistor; total R = R₁ + R₂ + …; V = I·R_total.
- Parallel: same voltage across each resistor; total 1/R = 1/R₁ + 1/R₂ + …; I = V/R_total.
The power is P = V·I = I²·R = V²/R. This is the rate at which electrical energy is converted to heat.
It is the fundamental relationship used to calculate voltage drops, current flows, and to select appropriate resistor values for biasing and limiting current in electronic circuits.