The Core Relationship
Voltage is electrical potential difference, current is the rate of charge flow, and resistance is opposition to that flow. For an ideal linear resistor:
V = I × R
The same equation can be rearranged depending on the unknown:
I = V ÷ R R = V ÷ I
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | V | volt (V) |
| Current | I | ampere (A) |
| Resistance | R | ohm (Ω) |
Adding Electrical Power
Electrical power is the rate at which energy is transferred. The basic relationship is:
P = V × I
Substituting Ohm's law gives two additional forms:
P = I² × R P = V² ÷ R
These forms are useful because power can be found from any two of voltage, current, and resistance. For a resistor, this power normally becomes heat.
Worked Example
A 6 Ω resistor is connected across 12 V. Current is:
I = 12 V ÷ 6 Ω = 2 A
Power is:
P = 12 V × 2 A = 24 W
A resistor rated at exactly 24 W would generally be a poor choice. Its datasheet, ambient temperature, mounting, and derating curve should be used to select a higher practical rating.
Common Unit Mistakes
- 1 milliampere is 0.001 A, so 20 mA is 0.020 A.
- 1 kilohm is 1,000 Ω, so 4.7 kΩ is 4,700 Ω.
- 1 megohm is 1,000,000 Ω.
- Power equations require consistent base units unless a converted formula is used.
Where Ohm's Law Needs Care
Ohm's law is exact for an ideal linear resistance, but many components are nonlinear. LEDs, diodes, transistors, motors, lamps, batteries, and switching power supplies do not maintain one constant resistance across all conditions.
Even a resistor changes value with temperature and tolerance. In AC circuits, impedance can include capacitance and inductance, so voltage and current may not be in phase.
Use the component's actual operating model and datasheet when behavior is not purely resistive.