Ohm's Law
Ohm's Law (V = I × R) relates voltage, current, and resistance in DC circuits. Power formulas P = VI, P = I²R, and P = V²/R extend the analysis.
Why This Physics Calculation Matters
Why: Ohm's Law is foundational for circuit design, component selection, and power dissipation analysis. Essential for LEDs, resistors, and power supplies.
How: Given any two of V, I, R, or P, solve for the third. Use consistent units: V in volts, I in amperes, R in ohms, P in watts.
- ●Power wheel has 12 formulas relating V, I, R, P
- ●Series: R_total = R₁ + R₂; parallel: 1/R_total = 1/R₁ + 1/R₂
- ●Copper resistivity 1.68×10⁻⁸ Ω·m
- ●Human body: 1kΩ (wet) to 100kΩ (dry skin)
Sample Examples
Click an example to load values, or enter your own below.
🔦 LED Circuit
5V supply, 20mA current → Calculate resistance
⚡ Resistor Power
12V, 100Ω → Calculate power dissipation
🔋 Battery Current
9V battery, 470Ω load → Calculate current
⚙️ Motor Voltage
2A motor, 6Ω resistance → Calculate voltage
🔥 Heater Power
120V heater, 10A current → Calculate power
📊 Voltage Divider
12V input, 1kΩ + 2kΩ → Calculate current
Calculate
Inputs
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
Georg Ohm published his law in 1827; Copley Medal 1841
— Royal Society
Human body resistance: 1kΩ wet to 100kΩ dry
— Electrical Safety
Superconductors have zero resistance below critical temperature
— Physics
Copper resistivity 1.68×10⁻⁸ Ω·m for wiring
— Materials Science
📋 Key Takeaways
- • V=IR is fundamental: Ohm's Law (V = I × R) is the foundation of DC circuit analysis — voltage drives current through resistance
- • Power wheel has 12 formulas: The power wheel shows P=IV, P=I²R, P=V²/R and their rearrangements — 12 total formulas connecting V, I, R, and P
- • Series vs parallel: In series circuits, resistance adds (R_total = R₁ + R₂); in parallel, conductance adds (1/R_total = 1/R₁ + 1/R₂)
- • Temperature affects resistance: Most conductors increase resistance with temperature (positive temperature coefficient) — copper increases ~0.4% per °C
- • Power dissipation matters: Always check that calculated power doesn't exceed component ratings — resistors have wattage limits
💡 Did You Know?
🔬 How It Works
Ohm's Law: V = I × R
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with the constant of proportionality being the resistance. This fundamental relationship governs DC circuit behavior.
The Power Wheel
Power (P) can be calculated using any combination of voltage, current, and resistance:
🎯 Expert Tips
Always check power ratings — ensure calculated power doesn't exceed component wattage ratings to prevent overheating and failure
Use consistent units — convert mA to A (÷1000), kΩ to Ω (×1000), mW to W (÷1000) before calculating to avoid errors
Consider temperature effects — resistance increases with temperature for most conductors, affecting calculations in hot environments
Account for internal resistance — real voltage sources (batteries) have internal resistance that causes voltage drop under load
📊 Circuit Configurations Comparison
| Configuration | Total Resistance | Current | Voltage | Power |
|---|---|---|---|---|
| Series | R_total = R₁ + R₂ + R₃ | Same through all | Divides proportionally | P_total = P₁ + P₂ + P₃ |
| Parallel | 1/R_total = 1/R₁ + 1/R₂ | Divides inversely | Same across all | P_total = P₁ + P₂ + P₃ |
| Mixed | Combine series/parallel | Depends on path | Depends on path | Sum of all branches |
❓ Frequently Asked Questions
Does Ohm's Law apply to AC circuits?
For purely resistive AC circuits, yes — using RMS values. For circuits with capacitors or inductors, use impedance (Z) instead of resistance: V = I × Z, where Z includes reactive components.
Why do power lines use high voltage?
Power loss in wires is P = I²R. For the same power delivery, higher voltage means lower current (since P = VI). Lower current dramatically reduces losses — that's why transmission lines use hundreds of thousands of volts.
What's the difference between EMF and voltage?
EMF (electromotive force) is the energy per unit charge provided by a source. Terminal voltage is the actual voltage available at the terminals, which is EMF minus the voltage drop across internal resistance. At no load, terminal voltage equals EMF.
How do I measure very high or very low resistance?
High resistance (MΩ-TΩ): Use a megohmmeter or insulation tester, which applies high voltage. Low resistance (mΩ): Use 4-wire (Kelvin) measurement to eliminate lead resistance from the reading.
Can current flow without voltage?
In normal conductors, no — you need a voltage difference to drive current (V = IR). However, in superconductors (R = 0), current can persist indefinitely without any voltage once established.
What happens if I exceed a resistor's power rating?
The resistor will overheat, potentially causing failure, fire, or circuit damage. Always use resistors rated for at least 2× the calculated power dissipation for safety margin.
How does temperature affect resistance?
Most conductors increase resistance with temperature (positive temperature coefficient). Copper increases ~0.4% per °C. Some materials like nichrome have very low temperature coefficients for stable resistance.
What is the power wheel?
The power wheel (or Ohm's Law wheel) shows all 12 formulas relating voltage, current, resistance, and power. Each quadrant represents one variable, showing how to calculate it from any two of the other three.
📊 Ohm's Law by the Numbers
📚 Official Data Sources
⚠️ Disclaimer
This calculator is for educational and design purposes. Always verify calculations and use appropriate safety margins. For critical applications, consult a licensed electrical engineer. Component ratings and safety standards must be followed.