What Is Ohm's Law?
Ohm's Law is the most fundamental relationship in electrical engineering. It describes how voltage (V), current (I), and resistance (R) interact in any electrical circuit. Named after German physicist Georg Simon Ohm, the law is expressed as:
| Formula | Solve For | Given |
|---|---|---|
| V = I × R | Voltage (V) | Current and Resistance |
| I = V ÷ R | Current (A) | Voltage and Resistance |
| R = V ÷ I | Resistance (Ω) | Voltage and Current |
Understanding the Three Variables
Voltage (V) — 電圧
Voltage is the electrical potential difference between two points. Think of it as the "pressure" pushing electrons through a conductor. It is measured in Volts (V).
Current (I) — 電流
Current is the rate of flow of electric charge through a conductor. It is measured in Amperes (A). Direct current (DC) flows in one direction; alternating current (AC) reverses direction periodically.
Resistance (R) — 抵抗
Resistance opposes the flow of current. It is measured in Ohms (Ω). Every material has some resistance — good conductors like copper have very low resistance, while insulators have very high resistance.
The Ohm's Law Triangle
A handy memory aid is the VIR triangle. Cover the quantity you want to find, and the remaining two show you the operation:
- Cover V → I × R
- Cover I → V ÷ R
- Cover R → V ÷ I
Worked Examples
Example 1: Finding Voltage
A circuit has a current of 2 A flowing through a 10 Ω resistor. What is the voltage?
V = I × R = 2 × 10 = 20 V
Example 2: Finding Current
A 12 V battery is connected to a 4 Ω resistor. How much current flows?
I = V ÷ R = 12 ÷ 4 = 3 A
Example 3: Finding Resistance
A device draws 0.5 A from a 100 V supply. What is its resistance?
R = V ÷ I = 100 ÷ 0.5 = 200 Ω
Limitations of Ohm's Law
Ohm's Law applies to linear (ohmic) components — those where resistance stays constant regardless of voltage or current. It does not apply perfectly to:
- Semiconductors (diodes, transistors)
- Electrolytes and arc discharge
- Non-linear resistive elements at extreme temperatures
Practical Applications
Ohm's Law is used every day by electricians and engineers to:
- Select the correct resistor values in circuits
- Calculate safe current levels in wiring
- Determine voltage drops across cable runs
- Troubleshoot faulty electrical components
Mastering Ohm's Law is the essential first step toward understanding all more advanced electrical calculations.