Ohm's Law
Calculator and formulas for U, I, and R according to Ohm's Law
Ohm's Law is one of the most fundamental laws in electrical engineering. It describes the linear relationship between electrical voltage U, electrical current I, and electrical resistance R.
This function automatically calculates the voltage U, current I, or resistance R according to Ohm's Law as soon as two of the three values are known.
Application: Select the value to be calculated, enter the two known values and click the 'Calculate' button. The result is displayed immediately.
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The three basic formulas of Ohm's Law
1. Calculating resistance
When voltage and current are known, resistance can be calculated:
\[ R = \frac{U}{I} \]
Example: With a voltage of 12V and a current of 2A, the resistance is: R = 12V ÷ 2A = 6Ω
2. Calculating current
When voltage and resistance are known, current can be calculated:
\[ I = \frac{U}{R} \]
Example: With a voltage of 9V and a resistance of 3Ω, the current is: I = 9V ÷ 3Ω = 3A
3. Calculating voltage
When current and resistance are known, voltage can be calculated:
\[ U = I \times R \]
Example: With a current of 0.5A and a resistance of 100Ω, the voltage is: U = 0.5A × 100Ω = 50V
Applications of Ohm's Law
Calculating simple circuits
Ohm's Law is the most important law for calculating electrical circuits, since they generally consist of metallic conductors to which it applies.
In its original form \(\displaystyle R=\frac{U}{I} \), Ohm's Law is used to determine:
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the resistance from current and voltage measurements:
\[ R=\frac{U}{I} \]
when U is in volts and I in amperes:
\[ 1\; \Omega=\frac{1\;V}{1\;A} \]
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the current in the form solved for I:
\[ I=\frac{U}{R} \]
when U is in volts and R in ohms:
\[ 1\;A=\frac{1\;V}{1\;\Omega} \]
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the voltage in the form solved for U:
\[ U=I \cdot R \]
when I is in amperes and R in ohms:
\[ 1\;V=1\;A \cdot 1\;\Omega \]
Practical application examples
Example 1: Calculate LED series resistor
An LED requires 2V at 20mA. The supply voltage is 5V.
Find: Series resistor
Solution: R = (5V - 2V) ÷ 0.02A = 150Ω
Example 2: Current consumption of an incandescent lamp
A 60W incandescent lamp is operated at 230V.
Find: Current consumption
Solution: First R = U²/P = (230V)² ÷ 60W = 883Ω, then I = 230V ÷ 883Ω = 0.26A
Limitations of Ohm's Law
Ohm's Law only applies to ohmic resistances (mainly metallic conductors) at constant temperature. It does not apply to:
- Semiconductors (diodes, transistors)
- Incandescent lamps (resistance changes with temperature)
- Capacitors and inductors with AC current
- Gas discharge lamps
Relationship to electrical power
Ohm's Law is closely related to electrical power calculations. The basic power formula is:
\[ P = U \times I \]
Combined with Ohm's Law, this gives us additional power formulas:
- \[ P = I^2 \times R \] (when current and resistance are known)
- \[ P = \frac{U^2}{R} \] (when voltage and resistance are known)
Historical background
Ohm's Law was formulated by the German physicist Georg Simon Ohm in 1826. It was initially met with skepticism but is now recognized as one of the fundamental laws of electrical engineering.
The law established the concept of electrical resistance and laid the foundation for quantitative analysis of electrical circuits.