Ohm's Law
The fundamental relationship between voltage, current, and resistance
Overview
Ohm's Law states that the electrical current flowing through an object is proportional to the applied voltage. For materials with constant resistance (called ohmic behavior), the ratio of voltage to current remains constant regardless of the applied voltage and resulting current.
The assumption of constant resistance applies primarily to metals at constant temperature. For other materials, especially semiconductors, resistance can change significantly with temperature and voltage.
For detailed information about temperature effects on resistance, see our Resistance Temperature Calculator .
Ohm's Law Formulas
Ohm's Law defines one of the most important relationships for calculating electrical circuits. There are three equivalent forms depending on which quantity you want to calculate.
Calculate Resistance (R)
When voltage \(U\) (in volts) and current \(I\) (in amperes) are known:
Resistance Formula
\(\displaystyle 1\,\Omega = \frac{1\,\text{V}}{1\,\text{A}}\)
Calculate Current (I)
When voltage \(U\) (in volts) and resistance \(R\) (in ohms) are known:
Current Formula
\(\displaystyle 1\,\text{A} = \frac{1\,\text{V}}{1\,\Omega}\)
Calculate Voltage (U)
When current \(I\) (in amperes) and resistance \(R\) (in ohms) are known:
Voltage Formula
\(\displaystyle 1\,\text{V} = 1\,\text{A} \cdot 1\,\Omega\)
Worked Example
Calculate Resistance from Voltage and Current
Given:
- Voltage: \(U = 230\,\text{V}\)
- Current: \(I = 4.5\,\text{A}\)
Calculate resistance \(R\):
Result: The resistance of this load is approximately \(51.1\,\Omega\).
Key Points
- Ohm's Law relates voltage \(U\), current \(I\), and resistance \(R\)
- Current is proportional to voltage (when resistance is constant)
- The three equivalent forms allow calculation of any one quantity from the other two
- Ohmic behavior assumes constant resistance independent of voltage and current
- For metals at constant temperature, Ohm's Law is a good approximation
- For semiconductors and non-linear materials, resistance can vary
Quick Calculation
Use the interactive calculator below to quickly compute voltage, current, or resistance using Ohm's Law, plus additional electrical power calculations.
Ohm's Law and Power Calculator →
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