Ohm's law states that voltage equals current multiplied by resistance (V=IR), and it can be used to calculate any two of the three variables if the third is known. The letter I represents current because it is named after André-Marie Ampère, who undertook experiments on current. V typically represents voltage but may also be represented by E, which stands for electromotive force. Examples are provided for using Ohm's law to calculate voltage, current, and resistance.

1. Ohms law

2. To Find Voltage
So if we want to find the voltage, we write V = and then cover up the V in the
triangle, that leaves us with I and R. So we write I x R. Which means Voltage =
Current multiplied by Resistance. You can write a little multiplication symbol in
the triangle between the two letters if it helps you.
Find Voltage
With Ohm’s Law
We know what you’re thinking. Why is current represented with the letter I and
not a C for current or even an A for the unit for Amperes. Well the unit of
current is the Ampere or Amp which is named after André Ampère, a french
physicist. A couple of hundred years ago he undertook lots of experiments many
involved varying the amount of electrical current, so he called this intensité du
courant or intensity of current. So when they published his work, they took the
letter I and it become standard until this day

3. You might also come across formulas where the letter E is used instead of V.
The E stands for EMF or electromotive force but don’t worry about that, just
stick with using V and substitute V for E if you see it used in an Ohms law
question.
Sometimes “E” is
used instead of “V” for Ohm’s Law
By covering V we get voltage = Current multiplied by Resistance.

4. To Find Current
If we want to find Current then we write down I = and then cover up the letter
I. That gives us V and R and as V is above the R like a fraction, we can write V
÷ R. Therefore Current is equal to Voltage divided by Resistance.
Find Current With
Ohm’s Law

5. To Find Resistance
If we want to find Resistance then we write R = and cover up the R, that leaves
us with V and I so we write V ÷ I which gives us resistance = Voltage divided by
current.
Find Resistance With Ohm’s
Law
Let’s look at some examples for how to use these formulas. First, lets see how
we find voltage and how it relates to the other parts

6. Finding Voltage Example
Lets say we have a simple electrical circuit with a battery and a resistor. We
don’t know what voltage the battery is though. The resistor is 3Ω and when we
connect a multimeter into the circuit we see that we get a reading of 2 Amps.
Voltage Ohm’s Triangle
We want to find the voltage so using ohms triangle we cover up V and that
gives us V = I x R. We know the current is 2 Amps so we write that in and we
know the Resistance is 3 Ohms so we write that in too. Therefore 2A multiplied
by 3Ω gives us 6 Volts. The battery is therefore 6V.

7. Finding Current Example
Lets say we now have a 3Ω lamp connected to a 6V power supply. To find the
current we write down I = and cover up the I in the triangle. That gives us V ÷
R so Current equals Voltage divided by resistance. We know the voltage is 6V
and the resistance is 3 ohm so the current is therefore 2A and that’s what we
see with the multimeter.
Current Ohm’s
Triangle
By the way if you don’t have a multimeter then we highly recommend you get
one, it’s essential for trouble shooting and also building your electrical
knowledge. There are some links down below for which one to get and from
where.
So we saw what happens when we use a resistance of 3 ohms in the circuit. But
If we double the resistance to 6 ohms by placing another 3 ohm lamp into the
circuit, the current halves to 1A.
If we double the resistance again to 12 Ohms, the current will half again to
0.5A. We can visually see this because the lamps will become less bright as the
current reduces from the increase in resistance.

8. Finding Resistance Example
Say we have a resistive lamp connected to a 12V power supply. We don’t know
how much resistance it’s adding to the circuit but we measure the current as
0.5 A.
Resistance Ohm’s Law
To find resistance we write down R = and cover up the R on the triangle. We’re
left with V and I so Resistance = Voltage divided by current. We know the
voltage is 12V and the current is 0.5A so 12 divided by 0.5 gives us 24 Ohms of
resistance.