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- Basic electrical quantities: current, voltage, power
- Voltage, Current, Resistance, and Ohm's Law
- Current, Voltage and Resistance

*The flow of electricity through an object, such as a wire, is known as the current I. The driving force electrical pressure behind the flow of a current is known as the voltage and is measured in volts V Voltage may also be referred to as the potential difference, or electromotive force. Resistance to alternating current is more properly called impedance but, in this application, resistance and impedance can be considered to be equivalent.*

If you're seeing this message, it means we're having trouble loading external resources on our website. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Donate Login Sign up Search for courses, skills, and videos. Science Physics library Circuits Ohm's law and circuits with resistors. Introduction to circuits and Ohm's law.

An electric circuit is formed when a conductive path is created to allow free electrons to continuously move. This continuous movement of free electrons through the conductors of a circuit is called a current , and it is often referred to in terms of "flow," just like the flow of a liquid through a hollow pipe.

The force motivating electrons to "flow" in a circuit is called voltage. Voltage is a specific measure of potential energy that is always relative between two points.

When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point. Without reference to two particular points, the term "voltage" has no meaning. Free electrons tend to move through conductors with some degree of friction, or opposition to motion. This opposition to motion is more properly called resistance. The amount of current in a circuit depends on the amount of voltage available to motivate the electrons, and also the amount of resistance in the circuit to oppose electron flow.

Just like voltage, resistance is a quantity relative between two points. For this reason, the quantities of voltage and resistance are often stated as being "between" or "across" two points in a circuit. To be able to make meaningful statements about these quantities in circuits, we need to be able to describe their quantities in the same way that we might quantify mass, temperature, volume, length, or any other kind of physical quantity.

For mass we might use the units of "pound" or "gram. Here are the standard units of measurement for electrical current , voltage, and resistance:. The "symbol" given for each quantity is the standard alphabetical letter used to represent that quantity in an algebraic equation.

Standardized letters like these are common in the disciplines of physics and engineering, and are internationally recognized. The "unit abbreviation" for each quantity represents the alphabetical symbol used as a shorthand notation for its particular unit of measurement.

Each unit of measurement is named after a famous experimenter in electricity: The amp after the Frenchman Andre M. The mathematical symbol for each quantity is meaningful as well. The "R" for resistance and the "V" for voltage are both self-explanatory, whereas "I" for current seems a bit weird.

The "I" is thought to have been meant to represent "Intensity" of electron flow , and the other symbol for voltage, "E," stands for "Electromotive force. All of these symbols are expressed using capital letters, except in cases where a quantity especially voltage or current is described in terms of a brief period of time called an "instantaneous" value. For example, the voltage of a battery, which is stable over a long period of time, will be symbolized with a capital letter "E," while the voltage peak of a lightning strike at the very instant it hits a power line would most likely be symbolized with a lower-case letter "e" or lower-case "v" to designate that value as being at a single moment in time.

This same lower-case convention holds true for current as well, the lower-case letter "i" representing current at some instant in time. Most direct- current DC measurements, however, being stable over time, will be symbolized with capital letters. One foundational unit of electrical measurement, often taught in the beginnings of electronics courses but used infrequently afterwards, is the unit of the coulomb , which is a measure of electric charge proportional to the number of electrons in an imbalanced state.

One coulomb of charge is equal to 6,,,,,, electrons. The symbol for electric charge quantity is the capital letter "Q," with the unit of coulombs abbreviated by the capital letter "C. Cast in these terms, current is the rate of electric charge motion through a conductor. As stated before, voltage is the measure of potential energy per unit charge available to motivate electrons from one point to another.

Before we can precisely define what a " volt " is, we must understand how to measure this quantity we call "potential energy. Defined in these scientific terms, 1 volt is equal to 1 joule of electric potential energy per divided by 1 coulomb of charge.

Thus, a 9 volt battery releases 9 joules of energy for every coulomb of electrons moved through a circuit. These units and symbols for electrical quantities will become very important to know as we begin to explore the relationships between them in circuits.

The first, and perhaps most important, relationship between current , voltage, and resistance is called Ohm's Law, discovered by Georg Simon Ohm and published in his paper, The Galvanic Circuit Investigated Mathematically.

Ohm's principal discovery was that the amount of electric current through a metal conductor in a circuit is directly proportional to the voltage impressed across it, for any given temperature.

Ohm expressed his discovery in the form of a simple equation, describing how voltage, current , and resistance interrelate:. In this algebraic expression, voltage E is equal to current I multiplied by resistance R. Using algebra techniques, we can manipulate this equation into two variations, solving for I and for R, respectively:. In the above circuit, there is only one source of voltage the battery, on the left and only one source of resistance to current the lamp, on the right.

This makes it very easy to apply Ohm's Law. If we know the values of any two of the three quantities voltage, current , and resistance in this circuit, we can use Ohm's Law to determine the third. In this first example, we will calculate the amount of current I in a circuit, given values of voltage E and resistance R :. In this second example, we will calculate the amount of resistance R in a circuit, given values of voltage E and current I :.

In the last example, we will calculate the amount of voltage supplied by a battery, given values of current I and resistance R :. Ohm's Law is a very simple and useful tool for analyzing electric circuits. It is used so often in the study of electricity and electronics that it needs to be committed to memory by the serious student.

For those who are not yet comfortable with algebra, there's a trick to remembering how to solve for any one quantity, given the other two. First, arrange the letters E, I, and R in a triangle like this:. If you know E and I, and wish to determine R, just eliminate R from the picture and see what's left:.

Eventually, you'll have to be familiar with algebra to seriously study electricity and electronics, but this tip can make your first calculations a little easier to remember. All About Circuits. Search this site. Print PDF.

An electric circuit is formed when a conductive path is created to allow electric charge to continuously move. Voltage is a specific measure of potential energy that is always relative between two points. When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move charge carriers from one particular point in that circuit to another particular point. Current tends to move through the conductors with some degree of friction, or opposition to motion. This opposition to motion is more properly called resistance. The amount of current in a circuit depends on the amount of voltage and the amount of resistance in the circuit to oppose current flow.

Voltage, Current and Resistance charges, and produce a flow of charge in the wire, an electric current. A voltage source ∆V that produces a current I.

Current Current is a measure of the flow of electric charge through a material. A material that can carry a flow of charge is called a conductor. Current is defined as the amount of charge that flows through a conductor in a certain amount of time. The unit of current is the Ampere A , which is equal to one Coulomb per second Coulomb is the unit of charge , The symbol I is used to represent current though J is often used in engineering sources.

*An electric circuit is formed when a conductive path is created to allow electric charge to continuously move. Voltage is a specific measure of potential energy that is always relative between two points. When we speak of a certain amount of voltage being present in a circuit, we are referring to the measurement of how much potential energy exists to move charge carriers from one particular point in that circuit to another particular point.*

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Resistance simulation. Page 6. 6 of © Boardworks Ltd What is a resistor? A resistor is a component designed to reduce the current. ○ A variable resistor.

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