Definition – What does Ohm’s Law mean?
The Ohm’s law is a formula used to calculate the relationship between voltage, current, and resistance in an electrical circuit.
For electronics students, the Ohm’s Law (E = IR) it is as important as Einstein’s equation of Relativity (E = mc²) for physicists.
E = I x R
When specified, it means voltage = current x resistance, or volts = amps x ohms, or V = A x Ω.
Named after the German physicist Georg Ohm (1789-1854), Ohm’s Law addresses the key quantities at work in circuits:
If you know the voltage (E) and current (I) and want to know the resistance (R), X-out the R in the pyramid and calculate the remaining equation (see the first, or leftmost, pyramid above ).
Note: Resistance cannot be measured in an operating circuit, so Ohm’s Law is especially useful when you need to calculate it.
Instead of turning off the circuit to measure resistance, a technician can determine R using the aforementioned variation of Ohm’s Law.
Now, if you know the voltage (E) and resistance (R) and want to know the 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 right-hand pyramid, above).
Basics of electricity
When beginning to explore the world of electricity and electronics, it’s vital to start by understanding the basics of voltage, current, and resistance. These are the three basic elements necessary to manipulate and use electricity.
At first, these concepts can be hard to understand because we can’t «see» them. One cannot see with the naked eye the power flowing through a wire or the voltage of a battery sitting on a table.
Even the lightning in the sky, although visible, is not really the exchange of energy that occurs from the clouds to the ground, but a reaction in the air to the energy that passes through it.
To detect this energy transfer, we must use measurement tools like multimeters, spectrum analyzers, and oscilloscopes to visualize what is happening to the charge on a system.
electric charge
Electricity is the movement of electrons. Electrons create charge, which we can harness to do work.
Your light bulb, your stereo, your phone, etc., are taking advantage of the movement of electrons to do their job. They all operate using the same basic energy source: the movement of electrons.
The three basic principles of this tutorial can be explained using electrons, or more specifically, the charge they create:
Voltage is the difference in charge between two points. Current is the rate at which charge flows. Resistance is the tendency of a material to resist the flow of charge (current).
Therefore, when we talk about these values, we are really describing the movement of the charge, and therefore, the behavior of the electrons.
A circuit is a closed circuit that allows charge to move from one place to another. Components in the circuit allow us to control this charge and use it to do work.
Georg Ohm was a Bavarian scientist who studied electricity. Ohm begins by describing a unit of resistance that is defined by current and voltage. So let’s start with voltage and go from there.
Electric current
Current is the flow of an electrical charge. It is an important quantity in electronic circuits. Current flows through a circuit when a voltage is placed across two points on a conductor.
electron flow
In an electronic circuit, current is the flow of electrons. However, current is usually shown in the direction of positive charges.
This is actually in the opposite direction of the movement of the electrons in the circuit.
How is current measured?
The standard unit of measurement for current is the ampere. It is sometimes abbreviated as A or amps. The symbol used for current is the letter «i».
Current is measured as the flow of electrical charge through time through a given point in an electrical circuit. One ampere is equal to 1 coulomb in 1 second. A coulomb is a standard unit of electrical charge.
current calculation
The current can be calculated using Ohm’s law. It can also be used to calculate the resistance of a circuit if the voltage is also known, or the voltage of a circuit if the resistance is known.
I = V/R
where I = current, V = voltage and R = resistance
Current is also used to calculate power using the following equation:
P = I * V
where P = power, I = current and V = voltage.
AC vs. DC
There are two main types of current used in most electronic circuits today. They are alternating current (AC) and direct current (DC).
Direct current (DC)
Direct current is the constant flow of electrical charge in one direction. The batteries generate direct current to power carry-on items.
Most electronic components use direct current for internal power, often converting alternating current (AC) to direct current (DC) using a transformer.
alternating current (AC)
Alternating current is the current in which the flow of electrical charge constantly changes direction. Today, alternating current is mainly used to transmit power to power lines.
In the United States the frequency at which alternating current is 60 Hertz. Other countries use 50 Hertz as the standard frequency.
Electromagnetism
Current also plays an important role in electromagnetism. Ampere’s law describes how a magnetic field is generated by an electric current. This technology is used in electric motors.
Interesting facts about today
The direction of current flow is often shown with an arrow. In most electronic circuits, current flows to ground.
The current in a circuit is measured using a tool called an ammeter.
The flow of electrical current through a wire can sometimes be thought of as the flow of water through a pipe.
The electrical conductivity of a material is a measure of the material’s ability to allow the flow of electrical current.
examples
Try some sample calculations based on a simple series circuit, including only a voltage source (battery) and resistance (light). 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 = E/R = 12V/6Ω = 2A
Example 2: The voltage (E) and the current (I) are known.
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?
What is the voltage in the circuit?
E = I x R = (5A)(8Ω) = 40V
When Ohm published his formula in 1827, his main finding was that the amount of electrical current flowing through a conductor is directly proportional to the voltage imposed on it.
In other words, it takes one volt of pressure to push one amp of current through one ohm of resistance.
Uses of Ohm’s Law
Ohm’s Law can be used to validate:
Static values of circuit components Current levels Voltage supplies Voltage drop
If, for example, a test instrument detects a higher than normal current measurement, it could mean:
The resistance has decreased. The voltage has increased, causing a high voltage situation. This could indicate a supply or circuit problem.
In direct current (DC) circuits, a lower than normal current measurement could mean:
The resistance of the circuits has increased. Possible Cause: Poor or loose connections, corrosion and/or damaged components. Voltage has dropped.
Loads within a circuit consume electrical current. The loads can be of any type of component: small electrical devices, computers, household appliances or a large motor.
Most of these components (loads) have a nameplate or information sticker. These nameplates provide safety certification and multiple reference numbers.
Technicians refer to component nameplates to learn standard voltage and current values.
During tests, if technicians find that the usual values aren’t registering on their digital multimeters or clamp meters, they can use Ohm’s Law to detect which part of a circuit is failing and from there determine where the problem may be.
Basic Circuit Science
Circuits, like all matter, are made of atoms. Atoms consist of subatomic particles:
Protons (with positive electrical charge)Neutrons (no charge)Electrons (negatively charged)
Atoms are held together by attractive forces between the nucleus of an atom and the electrons in its outer shell.
When influenced by voltage, the atoms in a circuit begin to reform and their components exert an attractive potential known as a potential difference.
The mutually attracted loose electrons move towards the protons, creating a flow of electrons (current). Any material in the circuit that restricts this flow is considered resistance.
electric power capacity
Electrical components are given a «wattage rating» in watts that indicates the maximum rate at which the component converts electrical energy into other forms of energy such as heat, light, or motion. For example, a 1/4W resistor, a 100W bulb, etc.
Electrical devices convert one form of energy into another. So, for example, an electric motor will mask electrical energy into a mechanical force, while an electrical generator converts mechanical force into electrical energy. A light bulb converts electrical energy into light and heat.
Also, we now know that the unit of power is WATT, but some electrical devices like electric motors are rated in the old «horsepower» or CV measure. The relationship between horsepower and watts is given as: 1hp = 746W. So, for example, a two horsepower motor has a power of 1492W, (2 x 746) or 1.5kW.
Ohms Law Pie Chart
To help us understand the relationship between the various values a bit more, we can take all of the Ohm’s Law equations from above to find Voltage, Current, Resistance and of course Power and condense them into a simple Ohms Law graph for your use in AC and DC circuits and calculations as shown.
How Ohm’s Law Works
Ohm’s law describes how current flows through a resistor…