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You are here: Home / Basic Electronics / How to Use Ohms Law

How to Use Ohms Law

July 1, 2012 By Øyvind Nydal Dahl 54 Comments

I usually don’t use a lot of math when doing electronics, but Ohms law is extremely useful!

The law was found by Georg Ohm and is based on how voltage, current and resistance are related:

Ohms law cartoon

Look at the drawing above and see if it makes sense to you that:

  • If you increase the voltage in a circuit while the resistance is the same, you get more current.
  • If you increase the resistance in a circuit while the voltage stays the same, you get less current.

Ohm’s law is a way of describing the relationship between the voltage, resistance and current using math:

V = RI

  • V is the symbol for voltage.
  • I is the symbol for current.
  • R is the symbol for resistance.

I use it VERY often. It is THE formula in electronics.

You can switch it around and get R = V/I or I = V/R. As long as you have two of the variables, you can calculate the last.

Ohm’s law triangle

You can use this triangle to remember Ohm’s law:

How to use it:
Use your hand to cover the letter you want to find. If the remaining letters are over each other, it means divide the top one with the bottom one. If they are next to each other, it means multiply one with the other.

Example: Voltage

Let’s find the formula for voltage:

Place your hand over the V in the triangle, then look at the R and the I. I and R are next to each other, so you need to multiply. That means you get:

V = I * R

Example: Resistance

Let’s find the formula for resistance:

Place your hand over the R. Then you’ll see that the V is over the I. That means you have to divide V by I:

R = V / I

Example: Current

Let’s find the formula for current:

Place your hand over the I. Then you’ll see the V over the R, which means divide V by R:

I = V / R

How to remember Ohms law

A simple way of remembering things is to make a stupid association with it so that you remember it because it’s so stupid.

So to help you remember Ohm’s law let me introduce the VRIIIIIIII! rule.

Pretend that your driving a your car really fast, then suddenly you hit the brakes really hard. What sound do you hear?

“VRIIIIIIIIIIII!”

And this way you can remember V=RI ;)

A practical example

The best way to teach how to use it is by example.

Below is a very simple circuit with a battery and a resistor. The battery is a 12 volt battery, and the resistance of the resistor is 600 Ohm. How much current flows through the circuit?

Demo of ohms law on circuit with battery and resistor

To find the amount of current, you can use the triangle above to the formula for current: I = V/R. Now you can calculate the current by using the voltage and the resistance:

I = 12 V/600 Ohm
I = 0.02 A = 20 mA (milli Ampere)

So the current in the circuit is 20 mA.

If you don’t like calculating things yourself, check out this calculator for Ohm’s law.

Another example

Let us try another example.

Below we have a circuit with a resistor and a battery again. But this time we don’t know the voltage of the battery. Instead we imagine that we have measured the current in the circuit and found it to be 3 mA (milli Ampere).

Demo of ohms law on circuit with battery and resistor

The resistance of the resistor is 600 Ohm. What is the voltage of the battery?

By remembering the “VRIIII!” rule, you get:

V = RI
V = 600 Ohm * 3 mA
V = 1.8 V

So the voltage of the battery must be 1.8 V.

Return from Ohms Law to Electronic Schematics

Filed Under: Basic Electronics

Reader Interactions

Comments


  1. Dipo says

    February 27, 2015

    Why didn’t you convert the 3mA to Ampere before evaluating in the second examlpe?

    Reply

    • Dipo says

      February 27, 2015

      Kindly help remove my comments. Gosh! I suck at math.

      Reply

    • admin says

      February 28, 2015

      I’m glad you figured it out =)

      Oyvind

      Reply

    • vamsi says

      December 1, 2016

      because if we covert 3mA to Amphere we have do a long process

      Reply

  2. jake says

    June 11, 2015

    Hey im new to the whole electronics thing was wondering if anyone could mentor me?

    Reply

  3. jake says

    June 11, 2015

    If so email me, [email protected]

    Reply

    • Basharat ali says

      March 8, 2019

      i am interested in electronics

      Reply

  4. Abdul Latif says

    September 27, 2015

    Hi sir I like electronics kindly help me how to make a circuit on pcb is there any software for it?
    Thanks.

    Reply

    • admin says

      October 5, 2015

      Yes,

      check out https://www.build-electronic-circuits.com/pcb-design/

      Cheers!
      Oyvind

      Reply

  5. SnowBerry says

    September 29, 2015

    Hi. I am not so new to electronics but I didn’t have enough recourses to play around with the various components. Keep being awesome and make more interesting articles about electronics:D

    Reply

    • admin says

      October 5, 2015

      Thanks!

      Reply

  6. SnowBerry says

    September 29, 2015

    Hello Øyvind! I am new to all of these concepts and I really need some help. I have considered joining a robotics club. Please keep on making awesome article about electronics!:D

    Reply

    • admin says

      October 5, 2015

      Join the robotics club! It sounds awesome =)

      Oyvind

      Reply

  7. Seair says

    October 12, 2015

    Hi. Hoping you can help.
    I’m using three(3) 1.5V bulbs(not leds) running off of one(1) 1.5V AAA battery.
    When I start with a brand new battery, the bulbs are nice and bright. However, after about 2 minutes, I can notice the bulbs all starting to get dimmer. They’re just about completely out after 30 minutes.
    Would using resistors keep the bulbs burning at a more equal brightness from start to ending of the battery?
    I believe they are wired in parallel. Would it matter if in series or parallel?
    Thanks,
    Seair

    Reply

    • Mike says

      July 22, 2017

      The rate of discharge you are describing sounds more like a 3W 1.5V LED bulb. AAA batteries are rated 1.5V, with about 1200mAh. So a single 3W 1.5V bulb is going to draw a 2A current. Technically that would give you a 36 minute run time, but since 2A is such a high current for these batteries, the actual run time would be significantly less. Now add 2 more of these bulbs in the equation, for a total of three, and you would see them dimming almost immediately, a lot like how you described. As far as the circuit type, normally if three bulbs we’re to be wired in series they would be 1/3rd the brightness they normally are. However in this case, they probably wouldn’t work at all, because now your at 4.5 volts between the three of them. I would double check your bulb type. If they actually are regular incadseccent, then they are probably around 20-30ma bulb, which should run for at least an hour before noticeably dimming. If your wiring job is good, that leaves the batteries as the only other possible problem. For the most part you get what you pay for, when buying these type batteries.

      Reply

  8. Nick says

    January 25, 2016

    Stupid question, in the second example
    we had a current, I, or .03 and a resistance of 600. Multiplied they give us 18. How do you know to make it 1.8 volts?

    Reply

    • admin says

      February 8, 2016

      Hi Nick,

      3 mA is 0.003 A (so you were lacking a zero ;)

      Best,
      Oyvind

      Reply

      • Bunky says

        June 24, 2016

        Hi,

        I am nerd in electronics, but I guess my maths is good.

        If 1000mA = 1A
        then 100mA = 0.1A
        10mA = 0.01A
        implicates
        30mA = 0.03A

        so howcome your statement, it is 30 mA is 0.003 A (so you were lacking a zero ????

        Am so confused…

        Reply

        • admin says

          June 25, 2016

          @Bunky;

          Oooops! Sorry, that was a mistake by me. I’ve edited my comment now.

          3 mA is 0.003A.
          30 mA = 0.03A

          Reply

    • Jean Paul MUTOMBO NSAPU says

      April 14, 2020

      600 x 3 = 1800
      0r, 3 mA doit être convertit en Ampère donc, on aura 0,003 A
      d’où le calcul est: 600 x 0,003 = 1.8v

      ou 600 x3 = 1800
      1800 divisé par 100 = 1.8v

      Reply

  9. louis says

    April 28, 2016

    Great little website ….really happy i found it

    Reply

  10. Bunky says

    June 24, 2016

    V = RI where V = 1 volt, R = 1 Ohms, I = 1 Ampere
    ========================================

    Given example, R = 600 Ohms, I = 3 mili Ampere

    1000mA = 1A, then 3mA = 0.003A

    On putting the values

    V = 600 (R) x 0.003 (I)

    V = 1.8

    :)

    Reply

    • admin says

      June 25, 2016

      Exactly =)

      Reply

  11. ismaili seleman jafo says

    October 2, 2016

    its good and educated

    Reply

  12. vamsi says

    December 1, 2016

    because if we convert it it will be a long process

    Reply

  13. kiprono says

    December 17, 2016

    I am really new for electronics but i have a little ideas over this. my questions is under programming how can understand it? Am a first year pursuing computer science. my email is [email protected]

    regards
    Kenneth

    Reply

  14. Chris says

    February 24, 2017

    Nice site.

    In your example, 12v batt and 600ohm resister, you determine current is .02 (20ma)

    What if batt was a 3.7v battery (with say 3000mah)

    Reply

    • admin says

      March 1, 2017

      Hey,

      The 3000mah just tells you how much energy your battery can store. It doesn’t affect this calculation. So just switch 12V with 3.7V:

      3.7V / 600 ohm = 0.006A = 6mA

      Best,
      Oyvind

      Reply

  15. Hudson Mesritz says

    March 28, 2017

    Can u please just write me a summary or something on this because i still don’t fully understand all of this. Actually can anyone describe ohms law better for me, thanks

    Reply

    • admin says

      March 30, 2017

      Hey, let me know what you don’t understand and I’ll try to explain.

      Best,
      Oyvind

      Reply

    • Oscar says

      May 28, 2020

      Ohms law states that the current (charges)moving through a conductor(wire) is directly proportional to the applied potential difference(volts) and inversely proportional to the resistance in the wire. This means as the voltage increases, the current increases hence the resistance is kept constant.

      Reply

      • Floyd Stegall says

        December 28, 2020

        I once had it explained to me in terms of water and pipes. Voltage is water pressure, current is how much water is actually flowing (it is possible to have very high pressure, but little actual water flow – think of a pressure washer for a car. You can also have a LOT of water flowing at low pressure – think of a large storm drain pipe during a heavy rain), and resistance is a restriction on the water flow – think of a valve or a narrowing of a water pipe.

        Reply

  16. Amit says

    November 12, 2017

    Give me an example of electrical component which obeys ohm’s law

    Reply

    • admin says

      November 19, 2017

      It’s not about components. It’s about how current, voltage and resistance relate to each other.

      Reply

  17. gayathri says

    November 17, 2017

    Hii! Ohms law explanation is very nice…
    But I have a doubt that in Ohms law there is a direct relation between V & I (V is directly proportional to I).
    P=VI
    In this relation, its look like V is inverse to I
    So, can you explain??

    Reply

  18. irshad says

    November 18, 2017

    you not convert mA into A
    and resistor denoted by v
    and in drawing v denoted by u
    why?
    I think second example is wrong

    Reply

  19. Akash says

    January 29, 2018

    Very Good Information & You Made it easy to learn….

    Reply

  20. MUNEER C says

    March 19, 2018

    Thanks.
    It very useful than my physics class.
    Now I can study electric after a long time of my school days.
    Thanks , you are my teacher. ..

    Reply

  21. Louie says

    April 13, 2018

    Thank you, that explanation was simple yet very complete…before this I had no clue about ohms law, now I understand

    Reply

  22. Lucas says

    August 20, 2018

    What if resistance is 0? (If you dont have a resistor, for example). Does the voltage will be 0 too? Why?

    Reply

    • admin says

      August 20, 2018

      I guess you are referring to the V=R*I form?

      This gives you the voltage drop across the resistance you put into the formula. If the resistance is zero (like a wire), there will be no voltage drop across the wire, so yes, V becomes 0 in this case.

      Best,
      Oyvind

      Reply

  23. mavn divn says

    January 19, 2019

    AM NEW AT LEARNING ELECTRONIC. Am from scratch

    is it possible to to find out about resistance,

    if u have V (volts) and I (current)
    how to find amount of RESISTANCE

    Reply

    • admin says

      January 21, 2019

      It’s voltage divided by the current:

      R=V/I

      Best,
      Oyvind

      Reply

  24. karen renfro says

    March 13, 2019

    I did not know anything before reading the material. I can see how voltage works and how you can use math is a asset.

    Reply

  25. eric says

    June 25, 2019

    Its been cool and by time ill be perfect.

    Reply

  26. Sahil Muhammed says

    July 19, 2019

    Mr. Øyvind, you are truly creative and amazing in teaching electronics. My journey towards electronics has become so easy and interesting because of you! Thank you!

    Reply

  27. Deepak kumar says

    February 10, 2020

    Give me an example of Kirchoff law

    Reply

  28. Jonathan B says

    June 23, 2020

    I’m still a little confused about voltage. Does it influence the speed at which electrons flow?

    Reply

    • admin says

      June 29, 2020

      More voltage means more current if the resistance stays the same. More current means more electrons flowing.

      Reply

  29. Denis says

    August 8, 2020

    In Italy we use to remember this law with a sentence: “Vittorio Re d’Italia” (V=RI) which translated in English means: “Vittorio king of Italy”
    Vittorio was in fact king of Italy in the past ahah.
    I know this comment wasn’t necessary, but it’s fun

    Reply

    • admin says

      August 10, 2020

      Thanks for sharing!

      Reply

  30. Electric Jankari says

    August 16, 2020

    thanks!
    its very easy to understand.

    Reply

  31. ben says

    September 22, 2020

    Hi there, thanks for providing some examples for people to practice Ohm’s law. The first example is incorrect. milliamps is 10^-3 and centiamps is 10^-2. So technically 0.02 is 2cA right?

    Reply

    • admin says

      September 22, 2020

      Hi Ben,

      You’re right that 0.02A is 2cA – although “centi” is usually not used.

      But 2 cA is the same as 20 mA.

      Reply

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