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Calculate Voltage (V), Current (I), Resistance (R), and Power (P). Simply enter two known values to solve for the rest.
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Fill in any 2 boxes on the left to solve.
Understanding the relationship between V, I, R, and P is fundamental to all electronics.
The "pressure" that pushes electricity. Like water pressure in a hose.
The "flow rate" of electricity. Like the amount of water flowing through the hose.
The "friction" opposing flow. Like narrowing the hose, restricting the water.
The "work done" by electricity. The total force and flow combining to perform a task.
Ohm's Law is the foundational relationship between voltage, current, and resistance in electrical circuits. It tells you how electrical pressure (voltage) and opposition (resistance) determine flow (current). This simple relationship powers practical decisions in electronics design, troubleshooting, and safety.
Whether you are building a hobby circuit, sizing resistors for LEDs, analyzing battery-powered devices, or validating power loads, Ohm's Law gives you the baseline math needed to avoid underperformance and component failure.
V = I x R
I = V / R
R = V / I
P = V x I = I^2 x R = V^2 / R
V (Voltage): electrical potential difference in volts.
I (Current): charge flow in amperes.
R (Resistance): opposition to current in ohms.
P (Power): energy transfer rate in watts.
Tip: convert mA to A and kΩ to ohms before calculation to avoid unit errors.
A 9V battery powers an LED at 20mA with ~2V forward drop. Resistor voltage is 7V, so R = 7 / 0.02 = 350 ohms. Nearest standard value: 360 ohms for safer current limiting.
A 5V load with equivalent resistance 10 ohms draws I = 5 / 10 = 0.5A. Power is P = 5 x 0.5 = 2.5W, useful for cable and adapter checks.
A 220 ohm resistor with 12V across it dissipates P = V^2 / R = 144 / 220 = 0.65W. A 0.25W resistor would fail; choose at least 1W with safety margin.
Compare common circuit scenarios and expected outcomes for current and power.
| Scenario | Inputs | Calculated Current | Calculated Power | Practical Note |
|---|---|---|---|---|
| Low-voltage sensor | 3.3V, 330 ohms | 0.01A | 0.033W | Efficient, low heat |
| USB accessory | 5V, 10 ohms | 0.50A | 2.5W | Check adapter limits |
| Automotive load | 12V, 6 ohms | 2.00A | 24W | Moderate heat dissipation |
| High current branch | 24V, 2 ohms | 12.0A | 288W | Needs heavy wiring and cooling |
Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points. The standard formula is V = I × R, where V is voltage, I is current, and R is resistance.
Voltage (V) is measured in Volts (V). Current (I) is measured in Amperes or Amps (A). Resistance (R) is measured in Ohms (Ω). Power (P) is measured in Watts (W).
While Ohm's Law strictly deals with V, I, and R, Joule's law helps us find Power (P). Power is the rate at which electrical energy is transferred by an electric circuit. The fundamental formula for power is P = V × I. By substituting Ohm's Law into this equation, you can also get P = I²R or P = V² / R.
The relationships between Voltage, Current, Resistance, and Power form a complete mathematical system. If you know any two of these values, the formulas allow you to perfectly calculate the other two. Entering only one value doesn't provide enough information, and entering three might create contradictions if they don't align mathematically.
Using the fundamental Ohm's Law formula, you multiply the Current in Amps by the Resistance in Ohms (V = I × R) to find Voltage in Volts.
Yes, but for Alternating Current (AC), Resistance is replaced by Impedance (Z), which accounts for both standard resistance and the effects of capacitance and inductance.
Since Current and Resistance are inversely proportional (I = V/R), if voltage stays the same, increasing the resistance will proportionally decrease the current flowing through the circuit.
You can combine Joule's Law and Ohm's Law. First calculate Current (I = V/R), or use the direct formula: Power = Voltage squared divided by Resistance (P = V² / R).
It is the basic foundation of circuit design. It ensures components receive the correct voltage and current so they function properly without burning out.
It applies strictly to "ohmic" materials (like standard wires and resistors) where resistance stays constant. It does not apply to non-ohmic components like diodes, where resistance changes based on voltage.
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