Skip to main content

Other Fluke companies:

Fluke Calibration Fluke Biomedical Fluke Networks Fluke Process Instruments
  • Accelix
  • Amprobe
  • Beha-Amprobe
  • Comark
  • Landauer
  • Pacific Laser Systems
  • Pomona
  • RaySafe
  • Schad
  • Pruftechnik

Select your country site

  • Africa and Middle East
    Africa
    ( English | Français )
    Middle East
    ( English )
  • Americas
    Argentina
    ( Español )
    Costa Rica
    ( Español )
    Mexico
    ( Español )
    Bolivia
    ( Español )
    Dominican Republic
    ( Español )
    Peru
    ( Español )
    Brazil
    ( Português )
    Ecuador
    ( Español )
    United States
    ( English | Español )
    Canada
    ( English | Français )
    El Salvador
    ( Español )
    Uruguay
    ( Español )
    Chile
    ( Español )
    Guatemala
    ( Español )
    Venezuela
    ( Español )
    Colombia
    ( Español )
  • Asia Pacific
    Australia
    ( English )
    Japan
    ( 日本 )
    South Korea
    ( 한국어 )
    China
    ( 中国 )
    Malaysia
    ( English )
    Taiwan
    ( Chinese Traditional )
    India
    ( English )
    Philippines
    ( English )
    Thailand
    ( English | ภาษาไทย )
    Indonesia
    ( English | Bahasa )
    Singapore
    ( English )
    Vietnam
    ( English | Tiếng Việt Vietnam )
  • Europe
    Austria
    ( Deutsch )
    Ireland
    ( English )
    Rest of Europe
    ( English )
    Belgium
    ( Français | Dutch )
    Italy
    ( Italiano )
    Spain
    ( Español )
    Czechia
    ( Czech )
    Netherlands
    ( Dutch )
    Sweden
    ( Swedish )
    Denmark
    ( Dansk )
    Norway
    ( Norsk )
    Switzerland
    ( Deutsch | Français )
    Finland
    ( Finish )
    Poland
    ( Polski )
    Turkey
    ( Türkçe )
    France
    ( Français )
    Portugal
    ( Português )
    United Kingdom
    ( English )
    Germany
    ( Deutsch )
  • Russia
    Russia
    ( Russian )

Select your country site

  • Africa and Middle East
    • Africa English
    • Africa Français
    • Middle East English
  • Americas
    • Argentina Español
    • Colombia Español
    • Bolivia Español
    • Brazil Português
    • Canada English
    • Canada Français
    • Chile Español
    • Costa Rica Español
    • Dominican Republic Español
    • Ecuador Español
    • El Salvador Español
    • Guatemala Español
    • Mexico Español
    • Peru Español
    • United States English
    • United States Español
    • Uruguay Español
    • Venezuela Español
  • Asia Pacific
    • Australia English
    • China 中国
    • India English
    • Indonesia English
    • Indonesia Bahasa
    • Japan 日本
    • Malaysia English
    • Philippines English
    • Singapore English
    • South Korea 한국어
    • Taiwan Chinese Traditional
    • Thailand English
    • Thailand ภาษาไทย
    • Vietnam English
    • Vietnam Tiếng Việt Vietnam
  • Europe
    • Austria Deutsch
    • Belgium Français
    • Belgium Dutch
    • Belgium Français
    • Belgium Dutch
    • Czechia Czech
    • Denmark Dansk
    • Finland Finish
    • France Français
    • Germany Deutsch
    • Ireland English
    • Italy Italiano
    • Netherlands Dutch
    • Norway Norsk
    • Poland Polski
    • Portugal Português
    • Rest of Europe English
    • Spain Español
    • Sweden Swedish
    • Switzerland Deutsch
    • Switzerland Français
    • Turkey Türkçe
    • United Kingdom English
  • Russia
    • Russia Russian
Fluke
  • Products
      • Accessories
        • See all
      • Building infrastructure
        • HVAC tools
        • Indoor air quality testing
        • Industrial Ethernet tools
        • Laser distance meters
        • Laser levels
        • Network cable testers
      • Calibration tools
        • Fluke Calibration
        • mA loop calibrators
        • Multifunction calibrators
        • Pressure calibrators
        • Temperature calibrators
      • Electrical tools
        • Basic electrical testers
        • Battery analyzers
        • Clamp meters
        • Digital multimeters
        • Earth ground
        • Installation testers
        • Insulation testers
        • Insulated tools
        • Portable appliance testers
        • Portable oscilloscopes
        • Power quality
      • Fluke software
        • Fluke Calibration software
        • Fluke Networks software
        • Fluke SmartView
      • Industrial imaging
        • See all
      • Intrinsically safe
        • See all
      • Mechanical maintenance
        • Laser alignment
        • Vibration analysis
      • Temperature measurement
        • Contact thermometers
        • IR thermometers
      • Thermal imaging
        • IR windows
        • Thermal cameras
        • Visual IR thermometers
      • Awards
      • New Fluke products
      • Test tools catalog
  • Learn
      • Best practices
        • Custody transfer
        • Electrical inspection
        • Energy efficiency
        • Equipment and production line testing
        • Fluid systems
        • Measurement basics
        • Motors and drives
        • Pressure calibration
        • Preventive maintenance
        • Safety
        • Temperature calibration
        • Test tools basics
      • Blog
      • Fluke education partnership program
      • Leak detector
      • Library
      • Online courses
      • Tools and calculators
        • Air leakage calculator
        • FOV calculator
      • Tradeshows and seminars
      • Webinars
  • Support
      • About us
        • Careers
        • Companies
        • Contact us
        • Corporate profile
        • Directions
        • Integrity
      • Calibration Services
      • Manuals
      • Press releases
      • Product registration
      • Repair
      • Return materials authorization
      • Safety notices
      • Service center locations
      • Software downloads
      • Warranties
  • Where to buy
      • Where to buy
      • Promotions and contests
Privacy policy
Talk to Sales

What is Ohm’s Law?

Ohm's Law is a formula used to calculate the relationship between voltage, current and resistance in an electrical circuit.

To students of electronics, Ohm's Law (E = IR) is as fundamentally important as Einstein's Relativity equation (E = mc²) is to physicists.

E = I x R

When spelled out, it means voltage = current x resistance, or volts = amps x ohms, or V = A x Ω.

Named for German physicist Georg Ohm (1789-1854), Ohm's Law addresses the key quantities at work in circuits:

QuantityOhm's Law
symbol
Unit of measure
(abbreviation)
Role in circuitsIn case you're wondering:
VoltageEVolt (V)Pressure that triggers electron flowE = electromotive force (old-school term)
CurrentIAmpere, amp (A)Rate of electron flowI = intensity
ResistanceROhm (Ω)Flow inhibitorΩ = Greek letter omega

If two of these values are known, technicians can reconfigure Ohm's Law to calculate the third. Just modify the pyramid as follows:

How to reconfigure Ohm’s Law formula.

If you know voltage (E) and current (I) and want to know resistance (R), X-out the R in the pyramid and calculate the remaining equation (see the first, or far left, pyramid above).

Note: Resistance cannot be measured in an operating circuit, so Ohm's Law is especially useful when it needs to be calculated. Rather than shutting off the circuit to measure resistance, a technician can determine R using the above variation of Ohm's Law.

Now, if you know voltage (E) and resistance (R) and want to know current (I), X-out the I and calculate the remaining two symbols (see the middle pyramid above).

And if you know current (I) and resistance (R) and want to know voltage (E), multiply the bottom halves of the pyramid (see the third, or far right, pyramid above).

Try a few sample calculations based on a simple series circuit, which includes just one source of voltage (battery) and resistance (light). Two values are known in each example. Use Ohm's Law to calculate the third.

Example 1: Voltage (E) and resistance (R) are known.

Known voltage and resistance shown in a circuit

What is the current in the circuit?

I = E/R = 12V/6Ω = 2A

Example 2: Voltage (E) and current (I) are known.

Known voltage and current shown in a circuit

What is the resistance created by the lamp?

R = E/I = 24V/6A = 4Ω

Example 3: Current (I) and resistance (R) are known. What is the voltage?

Known current and resistance shown in a circuit

What is the voltage in the circuit?

E = I x R = (5A)(8Ω) = 40 V

When Ohm published his formula in 1827, his key finding was that the amount of electric current flowing through a conductor is directly proportional to the voltage imposed on it. In other words, one volt of pressure is required to push one amp of current through one ohm of resistance.

What to validate using Ohm’s Law

Ohm’s Law can be used to validate the static values of circuit components, current levels, voltage supplies, and voltage drops. If, for example, a test instrument detects a higher than normal current measurement, it could mean that resistance has decreased or that voltage has increased, causing a high-voltage situation. This could indicate a supply or circuit issue.

In direct current (dc) circuits, a lower than normal current measurement could mean that the voltage has decreased, or circuit resistance has increased. Possible causes for increased resistance are poor or loose connections, corrosion and/or damaged components.

Loads within a circuit draw on electrical current. Loads can be any sort of component: small electrical devices, computers, household appliances or a large motor. Most of these components (loads) have a nameplate or informational sticker attached. These nameplates provide safety certification and multiple reference numbers.

Technicians refer to nameplates on components to learn standard voltage and current values. During testing, if technicians find that customary values do not register on their digital multimeters or clamp meters, they can use Ohm's Law to detect what part of a circuit is faltering and from that determine where a problem may lie.

The basic science of circuits

Circuits, like all matter, are made of atoms. Atoms consist of subatomic particles:

  • Protons (with a positive electrical charge)
  • Neutrons (no charge)
  • Electrons (negatively charged)

Atoms remain bound together by forces of attraction between an atom's nucleus and electrons in its outer shell. When influenced by voltage, atoms in a circuit begin to reform and their components exert a potential of attraction known as a potential difference. Mutually attracted loose electrons move toward protons, creating a flow of electrons (current). Any material in the circuit that restricts this flow is considered resistance.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.

Related articles

  • Troubleshoot failed motors with insulation resistance testing
  • Electrical testing safety – Preparing for absence of voltage testing

Recommended products

Fluke 3000 FC Series Wireless Multimeter
Fluke 3000 FC Series Wireless Multimeter
Fluke 87V Industrial Multimeter
Fluke 87V Industrial Multimeter
Fluke 179 True-RMS Digital Multimeter
Fluke 179 True-RMS Digital Multimeter
Fluke 233 Remote Display Digital Multimeter
Fluke 233 Remote Display Digital Multimeter
©2019 Fluke Corporation. Specifications subject to change without notice. 6004178

Products

  • Where to buy
  • Manuals
  • Product registration

Company

  • Careers
  • Companies
  • Integrity
  • Supplier handbook

Customer service

  • Contact us
  • Directions
  • Safety notices
Facebook YouTube Twitter LinkedIn Instagram
Fluke. Keeping your world up and running.®
  • Privacy policy
  • Fluke terms of use
  • Fluke terms and conditions of sale
  • Fluke terms and conditions of service
  • Feedback
  • Site map
©1995 - 2019 Fluke Corporation
    • Products
      • Accessories
        • See all
      • Building infrastructure
        • HVAC tools
        • Indoor air quality testing
        • Industrial Ethernet tools
        • Laser distance meters
        • Laser levels
        • Network cable testers
      • Calibration tools
        • Fluke Calibration
        • mA loop calibrators
        • Multifunction calibrators
        • Pressure calibrators
        • Temperature calibrators
      • Electrical tools
        • Basic electrical testers
        • Battery analyzers
        • Clamp meters
        • Digital multimeters
        • Earth ground
        • Installation testers
        • Insulation testers
        • Insulated tools
        • Portable appliance testers
        • Portable oscilloscopes
        • Power quality
      • Fluke software
        • Fluke Calibration software
        • Fluke Networks software
        • Fluke SmartView
      • Industrial imaging
        • See all
      • Intrinsically safe
        • See all
      • Mechanical maintenance
        • Laser alignment
        • Vibration analysis
      • Temperature measurement
        • Contact thermometers
        • IR thermometers
      • Thermal imaging
        • IR windows
        • Thermal cameras
        • Visual IR thermometers
      • Awards
      • New Fluke products
      • Test tools catalog
    • Learn
      • Best practices
        • Custody transfer
        • Electrical inspection
        • Energy efficiency
        • Equipment and production line testing
        • Fluid systems
        • Measurement basics
        • Motors and drives
        • Pressure calibration
        • Preventive maintenance
        • Safety
        • Temperature calibration
        • Test tools basics
      • Blog
      • Fluke education partnership program
      • Leak detector
      • Library
      • Online courses
      • Tools and calculators
        • Air leakage calculator
        • FOV calculator
      • Tradeshows and seminars
      • Webinars
    • Support
      • About us
        • Careers
        • Companies
        • Contact us
        • Corporate profile
        • Directions
        • Integrity
      • Calibration Services
      • Manuals
      • Press releases
      • Product registration
      • Repair
      • Return materials authorization
      • Safety notices
      • Service center locations
      • Software downloads
      • Warranties
    • Where to buy
      • Promotions and contests
    • Other Fluke companies
      • Fluke Calibration
      • Fluke Biomedical
      • Fluke Networks
      • Fluke Process Instruments
      • See more Fluke brands
        • Accelix
        • Amprobe
        • Beha-Amprobe
        • Comark
        • Emaint CMMS
        • Landauer
        • Pacific Laser Systems
        • Pomona
        • Pruftechnik
        • RaySafe
        • Schad