Introduction to Ohm’s law:
Voltage, current and resistance are interdependent. If the voltage increases while the resistance remains the same, the current will also increase. If the resistance becomes higher while the voltage remains the same, the current decreases. Using a formula from “Ohm’s law”, one of these three factors can be calculated if the other two are known.
The three symbols can be placed in a triangle. On this page, calculations using Ohm’s law are explained step by step with several examples.
The following table shows the letters, quantities, units and symbols of voltage, current and resistance. It is important to keep this table or learn it by heart.
On the page: “basis elektronica” the fundamentals are explained.
Once we know the current (I) and resistance (R) in a circuit, we can calculate the voltage (U). We take the triangle and cover the letter of the value we want to calculate. In this case, the U. What remains is: I * R. By multiplying the current by the resistance, we obtain the voltage.
The same applies to the current: when covering the I, U / R remains. By dividing the voltage by the resistance value, we obtain the current.
Finally, we can also calculate the resistance. Covering the R gives: U / I. Dividing the voltage by the current gives the resistance value.
The following paragraphs contain three examples in which we perform calculations with the Ohm’s law triangle.
Calculating voltage:
In this diagram the multimeter is connected in series with the lamp. For this purpose the switch has been opened and the measurement is taken across the switch. The multimeter is set to the mA range. See the page “basis elektronica” for more information on measuring current.
We measure a current of 500 mA and the resistance value of the lamp is known: it is 28 ohms. Using Ohm’s law, we calculate the voltage of the lamp. We cover the U and see that we must multiply I by R to obtain the value U.
We assume there are no losses (so no faults). In that case, the battery voltage is 14 volts and the voltage across the lamp is 14 volts. The lamp uses the 14 volts to light up.
Calculating current:
The switch is closed and the lamp is on. With the voltmeter, the voltage is measured in parallel across the lamp. The voltage is 13 volts. The resistance value is again known and is 20 ohms.
The current of 650 milliamps flows through the entire circuit and is not consumed.
Calculating resistance:
The resistance value of a lamp that is on is different from when it is off. While current is flowing through it and it has warmed up, we can use the measured voltage and current to determine the actual resistance value. Again we take the Ohm’s law triangle and this time we cover the R.
The resistance is 30 ohms.
