How do I understand Ohm's law

Ohm's law

Ohm's law states that the current I in a conductor and the voltage U between the ends of the conductor are directly proportional. The formula URI is a mathematical representation of this law.

With the help of Ohm's law, the three basic quantities of a circuit can be calculated if at least two of them are known. The three basic quantities are voltage, current and resistance.

The physicist Georg Simon Ohm has established and proven the relationship between voltage, current and resistance. Ohm's law was named after him.

Understanding electrical engineering and electronics is hardly possible without understanding Ohm's law.

Important note for electronics beginners

Normal resistances are linear. Semiconductors are not.

It is important to understand that Ohm's law is only for ohmic resistances applies. This includes normal resistors and components that have a linear relationship between voltage and current. The resistance of a diode or a lamp cannot be calculated using Ohm's law, since current and voltage at these components do not have a linear relationship. Normal resistors always have the same value within their limit values, regardless of voltage and current. Only this resistance value can be calculated using Ohm's law.

Proof of Ohm's law

If a resistor R is applied to a voltage U and a closed circuit is formed, a certain current I flows through the resistor R. The following measurements show the linear relationship that is shown in the subsequent current-voltage characteristic.
The determined values ​​are used for a measurement on a resistor. In the case of a coherent circuit with several different components, further principles must be taken into account.

Note: Current and voltage measuring devices each have an internal resistance that influences the measurement result. In order to rule out measurement errors, it makes sense to measure current and voltage separately from one another.

Measurement 1

In a measuring circuit, the voltage is increased from 5 V to 10 V and 15 V while the resistance (100 Ω) remains the same. How does the current behave?

Resistance R100 Ω100 Ω100 Ω
Voltage U5 V10 V15 V
Current I.50 mA100 mA150 mA

Finding: If the resistance R remains the same and the voltage U increases evenly, the current I increases with the voltage U. The current increases proportionally to the voltage. This means that a voltage that is twice as high leads to a current that is twice as high. Or the other way around, halving the voltage also halves the current.

Measurement 2

In a measuring circuit, the resistance is increased from 50 ohms to 100 ohms and 150 ohms while the voltage (5 volts) remains the same. How does the current behave?

Resistance R50 Ω100 Ω150 Ω
Voltage U5 V5 V5 V
Current I.100 mA50 mA33.3 mA

Finding: If the voltage U remains the same and if the resistance R increases evenly, the current I decreases by 1 / R. This means that a resistance that is twice as high leads to a halving of the current. Or the other way around, halving a resistor leads to a current that is twice as large.

Current-voltage characteristic / resistance characteristic

If you enter the voltages and currents of an associated resistor in a diagram and connect the points with each other, a straight line is formed. The steeper the straight line, the smaller the resistance.

Formulas of Ohm's law

Ohm's law knows three formulas for calculating current, voltage and resistance. The prerequisite is that two of the basic variables are known.

If the voltage U is applied to a resistor R, a current I flows through the resistor R.
If the current I is to flow through a resistor R, the voltage U must be calculated.
If a current I flows through a resistor R, a voltage U is applied to it.

Practical tip: the magic triangle

The magic triangle can be used as an aid to determine the various formulas of Ohm's law.
The value to be calculated is crossed out. The result is calculated with the other two values.
So that you can remember the order of the values, memorize the word URI. Where U is above R and I.

Georg Simon Ohm

Georg Simon Ohm was born on March 16, 1789 in Erlangen born under poor conditions. Although his father was a master locksmith, he dealt with mathematics, physics and philosophy and imparted his knowledge to his two sons Georg Simon and Martin.
Georg Simon Ohm attended grammar school in Erlangen and then went to university. Because he couldn't pay for his studies, he dropped out after a year and worked as a math teacher in Switzerland for a few years. Then he came back to Erlangen and worked there as a private lecturer in mathematics.
Later as a senior teacher for mathematics and physics, he began to be interested in physical research. In the years that followed, many scientists worked on electromagnetic phenomena. Ohm too was concerned with the phenomena of the galvanic chain. In 1826 his book appeared on the quantitative relationship between different quantities of a galvanic chain, which we know as Ohm's law. In principle, he discovered the connection between the current I and the voltage U. For the first time, thanks to Ohm, it was possible to treat the hardly researched theory of electricity mathematically. Although at the beginning of its discovery there was no recognition and the importance of Ohm's law was not recognized, years later it received many awards and honors from abroad within a very short time.
In 1852 his dream came true and he became professor of physics at the University of Munich.
Georg Simon Ohm died in 1854.

Basic quantities of Ohm's law

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