Ohm's Law
Ohm's Law is one of the most important principles in electronics. It describes the relationship between voltage, current and resistance and forms the basis for analysing almost every electrical circuit.
What is Ohm's Law?
Ohm's Law states that the current flowing through a conductor is directly proportional to the applied voltage and inversely proportional to the resistance, provided the temperature remains constant.
This simple relationship allows engineers and technicians to calculate voltage, current or resistance whenever the other two quantities are known.
The Three Electrical Quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | V | Volt (V) |
| Current | I | Ampere (A) |
| Resistance | R | Ohm (Ω) |
Ohm's Law Formulas
By rearranging Ohm's Law, three useful equations are obtained.
| To Calculate | Formula |
|---|---|
| Current | I = V ÷ R |
| Voltage | V = I × R |
| Resistance | R = V ÷ I |
Example 1 – Finding Current
A resistor of 100 Ω is connected to a 12 V battery.
Current:
I = 12 ÷ 100 = 0.12 A
Therefore the resistor carries 120 mA.
Example 2 – Finding Voltage
A current of 2 A flows through a 10 Ω resistor.
Voltage:
V = 2 × 10 = 20 V
Example 3 – Finding Resistance
A circuit operates from 24 V and draws 3 A.
Resistance:
R = 24 ÷ 3 = 8 Ω
The Ohm's Law Triangle
Many students use the Ohm's Law triangle to remember the formulas.
|
V
I R |
Cover the quantity you want to calculate:
- Cover V → V = I × R
- Cover I → I = V ÷ R
- Cover R → R = V ÷ I
Practical Applications
Ohm's Law is used in almost every area of electronics.
- Selecting LED current-limiting resistors.
- Designing power supplies.
- Amplifier bias calculations.
- Voltage divider circuits.
- Battery charging systems.
- Electrical fault finding.
- Speaker crossover design.
- Current shunt calculations.
Limitations of Ohm's Law
Ohm's Law applies only to materials and components whose resistance remains constant.
Devices such as diodes, LEDs, transistors and thermistors do not obey Ohm's Law over their entire operating range because their resistance changes with voltage, current or temperature.
Common Mistakes
- Using the wrong units.
- Confusing milliamperes with amperes.
- Ignoring resistor tolerance.
- Applying Ohm's Law to non-linear devices without considering their characteristics.
Key Points
- Ohm's Law relates voltage, current and resistance.
- Increasing voltage increases current if resistance remains constant.
- Increasing resistance decreases current if voltage remains constant.
- Voltage is measured in volts (V).
- Current is measured in amperes (A).
- Resistance is measured in ohms (Ω).
- Ohm's Law is fundamental to electrical and electronic engineering.