What is Ohm's law?
The Ohm's law is a formula used to calculate the relationship between electric voltage, electric current and resistance in a circuit.
For people learning about electronics, Ohm's law, U = I x R, is as important as Einstein's theory of relativity (E = mc²) is for physicists.
U = I x R
Written out, this means voltage = current x resistance, or volt = amp x ohm or V = A x Ω.
Ohm's law, named after the German physicist Georg Ohm (1789-1854), relates to the most important quantities in circuits:
| Greatness | Symbol for the way of Ohm | Unit of measurement (abbreviation) | Role in circuits | In case you were wondering: |
|---|---|---|---|---|
| Tension | U | Volt (V) | Pressure activating the flow of electrons | U = urgere (Latin for ‘to propel’) |
| Current | I | Ampere, amp (A) | Speed of electron flow | I = intensity |
| Resistance | R | Ohm (Ω) | Inhibits flow | Ω = Greek letter omega |
If two of these values are known, engineers can use Ohm's law to calculate the third. The pyramid can be changed as follows:
If the voltage (E) and current (I) are known and you want to calculate the resistance (R), cross out the R in the pyramid and calculate the remaining equation (see the first pyramid, on the far left, above).
Remark: resistance cannot be measured when a circuit is in operation. Ohm's law is then especially useful if it needs to be calculated. It is not necessary to switch off the circuit to measure the resistance, because using the above variation on Ohm's law, a technician can calculate R.
If the voltage (E) and resistance (R) are known and you want the stream (I), cross out the I in the pyramid and calculate the remaining equation (see middle pyramid above).
If the current (I) and resistance (R) are known and you want the tension (E) calculation, multiply the two values at the bottom of the pyramid by each other (see the third pyramid, far right, above).
Try some example calculations for a simple serial circuit with one voltage source (battery) and resistor (bulb). In each example, two values are known. Use Ohm's law to calculate the third.
Example 1: Voltage (E) and resistance (R) are known.
What is the current in the circuit?
I = U/R = 12 V/6 Ω = 2 A
Example 2: Voltage (E) and current (I) are known.
What is the resistance caused by the bulb?
R = E/I = 24 V/6 A = 4 Ω
Example 3: Current (I) and resistance (R) are known. What is the voltage?
What is the voltage in the circuit?
E = I x R = (5 A)(8 Ω) = 40 V
When Ohm published his formula in 1827, his main conclusion was that the amount of electric current flowing through a conductor was directly proportional is with the voltage to which it is subjected. In other words, it takes one volt of pressure to push one ampere of current through one ohm of resistance.
What can be validated with Ohm's law
Ohm's law can be used to validate the static values of circuit components, current levels, voltage inputs and voltage drops. For example, if a measuring instrument observes a higher current reading than normal, it may mean that resistance has decreased or voltage has increased, causing a high-voltage situation. This may indicate a problem in the power supply or with the circuit.
In direct current (DC) circuits, a lower than normal current measurement value may mean that the voltage has decreased or the circuit resistance has increased. Possible causes for increased resistance are poor or loose connections, corrosion and/or damaged components.
Loads in a circuit take electrical current. Loads can be all kinds of components: small electrical appliances, computers, household appliances or a large motor. Most of these components (loads) have a rating plate or information sticker. This shows the safety certification and various reference numbers.
Technicians consult component nameplates to find out what the standard voltage and current values are. If, when measuring, a technician finds out that the digital multimeter or current clamp registers different values from the usual ones, he/she can use Ohm's law to determine which part of the circuit is not working properly and what could be causing it.
Basic knowledge about circuits.
Like everything else, circuits are made up of atoms. Atoms, in turn, are made up of subatomic particles:
- Protons (with a positive electrical charge)
- Neutrons (no charge)
- Electrons (with a negative charge)
Atoms are held together by the forces of attraction between the nucleus of the atom and the electrons in the outer layer. When atoms in a circuit are exposed to voltage, they re-form and their parts exert an attraction potential known as a potential difference. Loose electrons with mutual attraction move towards protons, creating an electron flow (current). Any material in the circuit that interferes with this current is considered resistance.
Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.
