ohm – (symbol: Ω) – electrical unit and Ohm’s Law

Ohm is the unit of electrical resistance in the International System of Units (SI). It is indicated by the symbol “Ω” and is named after the German physicist Georg Simon Ohm, who first formulated Ohm’s Law.

Resistance Ohm Value:

Ohm’s Law:

Ohm’s Law states that the current passing through a conductor between two points is directly proportional to the voltage across the two points, assuming the temperature remains constant.

Mathematically, Ohm’s Law is expressed as:

Mathematically, Ohm's Law


  • V is the voltage across the conductor in volts (V),
  • I is the current passing through the conductor in amperes (A),
  • R is the resistance of the conductor in ohms (Ω).

Ohm’s Law Triangle Formula:

The Ohm’s Law Magic Triangle is a simple way to remember the relationships between voltage (v)
current (I) and resistance (R) in a circuit.

1# Example: you have a circuit, and you’re asked to find the current (I), given the values of voltage and resista. Instead of memorizing complicated formulas, you can use the Ohm’s Law Magic Triangle.

Now, if you’re given the values of V and R and you need to find I, all you have to do is cover I with your finger. The remaining two corners will reveal that you need to divide V by R to get the current value.

Let’s say you have a circuit with a voltage (V) of 24 volts and a resistance (R) of 8 ohms. You want to calculate the current (I).

Here are Given Value:

V = 24 v

R = 8 Ω

2 # example Calculate Resistance:

Let’s say you have a circuit with a voltage (V) of 15 volts and a current (I) of 3 amperes. You want to calculate the resistance (R) using the formula:

V = 15 v

I = 3 A

3 # example calculate Voltage:

Let’s consider a circuit with a current (I) of 2 amperes and a resistance (R) of 6 ohms. You want to calculate the voltage (V) using the formula:

Here are the steps:

I = 2A

R = 6 ohm

  • R is the resistance in ohms (Ω)
  • V is the voltage in volts (V)
  • I is the current in amperes (A)

Measuring Resistance:

Resistance can be measured using an instrument called an ohmmeter. Here are the general steps:

Turn Off the Circuit:

Make sure the circuit or device you are measuring is turned off. This prevents any interference from other electrical components.

Disconnect Power:

Disconnect the power source from the circuit to ensure safety during measurement.

Select the Range:

If your ohmmeter has multiple ranges, select the appropriate range. Choose a range higher than the expected resistance to get accurate measurements.

Connect the Leads:

Connect the ohmmeter leads across the component or conductor whose resistance you want to measure. Ensure a good connection.

Read the Value:

The ohmmeter will display the resistance value directly. If the reading is on an analog scale, note the position of the needle, and read the resistance value from the scale.

Resistance Color Code:

Resistance Color chart

or resistors, there is a color code system to represent their resistance. The colors on a resistor correspond to numerical values. You can use a resistor color code chart to determine the resistance value based on the color bands.

Here is a table of standard resistance values in ohms along with tolerance values are also included:

Color1st Digit2nd Digit3rd DigitMultiplierTolerance
Golden0.1+5%( J)
Red ET2221002%(G)
Purple 77710M+0.1%(B)
Gray 888
White E999
6-band resistor color code table

Note: The tolerance values indicate the maximum permissible variation in resistance from the specified value.