Ohm's Law | Electronics Club

Ohm's Law shows the relationship between voltage, current and resistance

To make a current flow through a resistance there must be a voltage across that resistance. Ohm's Law shows the relationship between the three quantities: voltage, current and resistance.

Ohm's Law can be written as a word equation:

voltage = current × resistance

Or using symbols to represent the quantities of voltage (V), current (I) and resistance (R):

V = I × R

In fact it can be written three ways and you can pick the version that's best for your purpose:

V = I × R

I =	V
	R

R =	V
	I

The VIR triangle - a way to remember Ohm's Law

I R

You can use the VIR triangle to help you remember the three versions of Ohm's Law.

To calculate voltage, V: put your finger over V, this leaves I R, so the equation is V = I × R
To calculate current, I: put your finger over I, this leaves V over R, so the equation is I = ^V/_R
To calculate resistance, R: put your finger over R, this leaves V over I, so the equation is R = ^V/_I

Use the right units

For most electronic circuits the amp is too large and the ohm is too small, so we often measure current in milliamps (mA) and resistance in kilohms (kΩ).

1 mA = 0.001 A
1 kΩ = 1000 Ω.

The Ohm's Law equations work if you use V, A and Ω, or if you use V, mA and kΩ.

It is vital to use the right units for the three quantities in Ohm's Law, otherwise calculations will give the wrong values.

You can use either of these two sets of units:

V = voltage in volts (V)
I = current in amps (A)
R = resistance in ohms (Ω)

V = voltage in volts (V)
I = current in milliamps (mA)
R = resistance in kilohms (kΩ)

You must not mix these sets of units in the equations so you may need to convert between mA and A or kΩ and Ω.

Converting units

Convert the units when necessary using these rules:

For current:

1mA = 0.001A

1A = 1000mA

For resistance:

1kΩ = 1000Ω

1Ω = 0.001kΩ

Example unit conversions

35mA = 0.035A

0.2A = 200mA

3mA = 0.003mA

22kΩ = 22000Ω

470Ω = 0.47kΩ

3.3kΩ = 3300Ω

Ohm's Law Calculations

Example 1:

3V is applied across a 6Ω resistor, what is the current?

Values: V = 3V, I = ?, R = 6Ω
Equation: I = ^V/_R
Numbers: Current, I = ³/₆ = 0.5A

Example 2:

A lamp connected to a 6V battery passes a current of 60mA, what is the lamp's resistance?

Values: V = 6V, I = 60mA, R = ?
Equation: R = ^V/_I
Numbers: Resistance, R = ⁶/₆₀ = 0.1kΩ = 100Ω
(using mA for current means the calculation gives resistance in kΩ)

Example 3:

A 1.2kΩ resistor passes a current of 0.2A, what is the voltage across it?

Values: V = ?, I = 0.2A, R = 1.2kΩ = 1200Ω
(1.2kΩ is converted to 1200Ω because A and kΩ must not be used together)
Equation: V = I × R
Numbers: V = 0.2 × 1200 = 240V

Example 4:

9V is applied across a 15kΩ resistor, what is the current?

Values: V = 9V, I = ?, R = 15kΩ
Equation: I = ^V/_R
Numbers: Current, I = ⁹/₁₅ = 0.6mA
(using kΩ for resistance means the calculation gives current in mA)