Ohm's Law

Calculator and formulas for calculating voltage, current and resistance

Ohm's Law Calculator

U-R-I Calculator

Select what should be calculated and enter the two known values. The result will be automatically calculated according to Ohm's law.

What should be calculated?
Results
Resistance R:
Current I:
Voltage U:

Ohm's Law

U
I × R

Ohm's Triangle: U = I × R

Basic Formulas
U = I × R (Voltage)
I = U ÷ R (Current)
R = U ÷ I (Resistance)
Units
  • U: Voltage in Volts (V)
  • I: Current in Amperes (A)
  • R: Resistance in Ohms (Ω)
Memory Aid
"UIR"
Voltage equals current times resistance

The Three Basic Formulas of Ohm's Law

1. Calculate resistance

When voltage and current are known:

\[R = \frac{U}{I}\]

Example: 12V ÷ 2A = 6Ω

2. Calculate current

When voltage and resistance are known:

\[I = \frac{U}{R}\]

Example: 9V ÷ 3Ω = 3A

3. Calculate voltage

When current and resistance are known:

\[U = I \times R\]

Example: 0.5A × 100Ω = 50V

Practical Application Examples

Example 1: LED Series Resistor

Given: LED needs 2V at 20mA, supply voltage 5V

Find: Series resistor

Voltage drop: 5V - 2V = 3V
\[R = \frac{3V}{0.02A} = 150Ω\]
Result: 150Ω series resistor
Example 2: Incandescent Bulb

Given: 60W bulb at 230V

Find: Current consumption and resistance

From P = U²/R: \[R = \frac{(230V)^2}{60W} = 883Ω\]
Then: \[I = \frac{230V}{883Ω} = 0.26A\]
Result: 260mA, 883Ω
Example 3: Voltage Divider

Given: 12V should be divided to 5V, load current 100mA

Find: Resistor values

Total resistance: \[R_{total} = \frac{12V}{0.1A} = 120Ω\]
R₂ (for 5V): \[R_2 = \frac{5V}{0.1A} = 50Ω\]
R₁: R₁ = 120Ω - 50Ω = 70Ω
Result: R₁ = 70Ω, R₂ = 50Ω

Application Range and Limitations

Ohm's Law applies to

  • Metallic conductors: Copper, aluminum, silver at constant temperature
  • Carbon film resistors: Standard resistors in electronics
  • Wire-wound resistors: Precision resistors for measurement technology
  • Heating elements: At constant temperature
Historical Background

Ohm's law was formulated in 1826 by Georg Simon Ohm. It describes the linear relationship between voltage, current and resistance in metallic conductors and forms the basis for calculating electrical circuits.

Mathematical Relationships
\[1\;Ω = \frac{1\;V}{1\;A}\]
\[1\;A = \frac{1\;V}{1\;Ω}\]
\[1\;V = 1\;A \times 1\;Ω\]
Limitations of the Law

Ohm's law does NOT apply to:

  • Semiconductors (diodes, transistors)
  • Incandescent bulbs (temperature dependent)
  • Capacitors with AC
  • Inductors with AC
  • Gas discharge lamps
  • Batteries (have internal resistance)
Practical Tips
  • Always pay attention to units (V, A, Ω)
  • Consider temperature for precision measurements
  • Check load capacity of components
  • Maintain safety clearances

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Circuits with resistors

Ohms Law  •  Total resistance of a resistor in parallel  •  Parallel- total resistance of 2 resistors  •  Series resistance for a voltmeter  •  Parallel resistance for an ampere meter  •  Voltage divider  •  Loaded voltage divider  •  Pi Attenuator  •  T Attenuator  •