Basic electrical quantities: current, voltage, power (article) | Khan Academy (2023)

Conductors and insulators

Conductors are made of atoms whose outer, or valence, electrons have relatively weak bonds to their nuclei, as shown in this fanciful image of a copper atom. When a bunch of metal atoms are together, they gladly share their outer electrons with each other, creating a "swarm" of electrons not associated with a particular nucleus. A very small electric force can make the electron swarm move. Copper, gold, silver, and aluminum are good conductors. So is saltwater.

There are also poor conductors. Tungsten—a metal used for light bulb filaments—and carbon—in diamond form—are relatively poor conductors because their electrons are less prone to move.

Insulators are materials whose outer electrons are tightly bound to their nuclei. Modest electric forces are not able to pull these electrons free. When an electric force is applied, the electron clouds around the atom stretch and deform in response to the force, but the electrons do not depart. Glass, plastic, stone, and air are insulators. Even for insulators, though, electric force can always be turned up high enough to rip electrons away—this is called breakdown. That's what is happening to air molecules when you see a spark.

Semiconductor materials fall between insulators and conductors. They usually act like insulators, but we can make them act like conductors under certain circumstances. The most well-known semiconductor material is Silicon (atomic number 14141414). Our ability to finely control the insulating and conducting properties of silicon allows us to create modern marvels like computers and mobile phones. The atomic-level details of how semiconductor devices work are governed by the theories of quantum mechanics.


Current is the flow of charge.

Charge flows in a current.

[Why did you say that twice?]

(Video) Basic Electrical Quantities (Full Lecture)

Current is reported as the number of charges per unit time passing through a boundary. Visualize placing a boundary all the way through a wire. Station yourself near the boundary and count the number of charges passing by. Report how much charge passed through the boundary in one second. We assign a positive sign to current corresponding to the direction a positive charge would be moving.

Since current is the amount of charge passing through a boundary in a fixed amount of time, it can be expressed mathematically using the following equation:

i=dqdti = \dfrac{dq}{dt}i=dtdqi, equals, start fraction, d, q, divided by, d, t, end fraction

[ What does the d mean?]

That's current in a nutshell.

A few remarks on current

What carries current in metal? Since electrons are free to move about in metals, moving electrons are what makes up the current in metals. The positive nuclei in metal atoms are fixed in place and do not contribute to current. Even though electrons have a negative charge and do almost all the work in most electric circuits, we still define a positive current as the direction a positive charge would move. This is a very old historical convention.

Can current be carried by positive charges? Yes. There are lots of examples. Current is carried by both positive and negative charges in saltwater: If we put ordinary table salt in water, it becomes a good conductor. Table salt is sodium chloride, NaCl. The salt dissolves in water, into free-floating Na+^++start superscript, plus, end superscript and Cl^-start superscript, minus, end superscript ions. Both ions respond to electric force and move through the saltwater solution, in opposite directions. In this case, the current is composed of moving atoms, both positive and negative ions, not just loose electrons. Inside our bodies, electrical currents are moving ions, both positive and negative. The same definition of current works: count the number of charges passing by in a fixed amount of time.

(Video) Introduction to circuits and Ohm's law | Circuits | Physics | Khan Academy

What causes current? Charged objects move in response to electric and magnetic forces. These forces come from electric and magnetic fields, which in turn come from the position and motion of other charges.

What is the speed of current? We don't talk very often about the speed of current. Answering the question, "How fast is the current flowing?" requires understanding of a complex physical phenomenon and is not often relevant. Current usually isn't about meters per second, it's about charge per second. More often, we answer the question "How much current is flowing?" all the time.

How do we talk about current? When discussing current, terms like through and in make a lot of sense. Current flows through a resistor; current flows in a wire. If you hear, "the current across ...", it should sound odd.


To get our initial toehold on the concept of voltage, let's look at an analogy:

Voltage resembles gravity

For a mass mmmm, a change of height hhhh corresponds to a change in potential energy, ΔU=mgΔh\Delta U = mg\Delta hΔU=mgΔhdelta, U, equals, m, g, delta, h.

For a charged particle qqqq, a voltage VVVV corresponds to a change in potential energy, ΔU=qV\Delta U = qVΔU=qVdelta, U, equals, q, V.

Voltage in an electric circuit is analogous to the product of gΔhg\cdot \Delta hgΔhg, dot, delta, h. Where gggg is the acceleration due to gravity and Δh\Delta hΔhdelta, h is the change of height.

A ball at the top of the hill rolls down. When it is halfway down, it has given up half of its potential energy.

An electron at the top of a voltage "hill" travels "downhill" through wires and elements of a circuit. It gives up its potential energy, doing work along the way. When the electron is halfway down the hill, it has given up, or "dropped", half of its potential energy.

(Video) Voltage | Introduction to electrical engineering | Electrical engineering | Khan Academy

For both the ball and the electron, the trip down the hill happens spontaneously. The ball and electron move towards a lower energy state all by themselves. On the trip down, there can be things in the way of the ball, like trees or bears to bounce off. For electrons, we can guide electrons using wires and make them flow through electronic components —circuit design— and do interesting things along the way.

[Why use an analogy?]

[Limits of this analogy]

[I'm still puzzled by voltage]

We can express the voltage between two points mathematically as the change of energy experienced by a charge:

V=ΔUqV = \dfrac{\Delta U}{q}V=qΔUV, equals, start fraction, delta, U, divided by, q, end fraction

(Video) Basic electrical quantities: current, voltage, power

That's an intuitive description of voltage in a nutshell.


Power is defined as the rate energy (U\text UUstart text, U, end text) is transformed or transferred over time. We measure power in units of joules/second, also known as watts.

(1watt=1joule/second1 \,\text{watt} = 1\,\text{joule}/\text{second}1watt=1joule/second1, start text, w, a, t, t, end text, equals, 1, start text, j, o, u, l, e, end text, slash, start text, s, e, c, o, n, d, end text)

power=dUdt\text{power} = \dfrac{\text dU}{\text dt}power=dtdUstart text, p, o, w, e, r, end text, equals, start fraction, start text, d, end text, U, divided by, start text, d, end text, t, end fraction

An electric circuit is capable of transferring power.Current is the rate of flow of charge, and voltage measures the energy transferred per unit of charge. We can insert these definitions into the equation for power:

power=dUdt=dUdqdqdt=vi\text{power} = \dfrac{\text dU}{\text dt} = \dfrac{\text dU}{\text dq} \cdot \dfrac{\text dq}{\text dt} = v \,i power=dtdU=dqdUdtdq=vistart text, p, o, w, e, r, end text, equals, start fraction, start text, d, end text, U, divided by, start text, d, end text, t, end fraction, equals, start fraction, start text, d, end text, U, divided by, start text, d, end text, q, end fraction, dot, start fraction, start text, d, end text, q, divided by, start text, d, end text, t, end fraction, equals, v, i

Electrical power is the product of voltage times current. in units of watts.


These mental models for current and voltage will get us started on all sorts of interesting electric circuits.

If you want to reach beyond this intuitive description of voltage you can read this more formal mathematical description of electric potential and voltage.

(Video) Basic Electrical Quantities current,voltage, resistance ,power| Explain practical with water bottle


What are the 4 basic electrical quantities? ›

Volts, amps, ohms, and watts are therefore the four fundamental units of electricity.

What are the 3 basic electrical quantities? ›

Ohm's law is the most important, basic law of electricity. It defines the relationship between the three fundamental electrical quantities: current, voltage, and resistance.

What is current, voltage and power? ›

How much the electricity wants to move from one point to another. Measured in volts. Current - the current flow from one point to another, literally based on how many electrons are moving per second. Measured in amps. Power - work that is being done per second.

What are the basic concepts of electricity? ›

The definition of electricity is the flow of charge. Usually our charges will be carried by free-flowing electrons. Negatively-charged electrons are loosely held to atoms of conductive materials. With a little push we can free electrons from atoms and get them to flow in a generally uniform direction.

What are the 3 major components of electrical power system? ›

The electrical power system consists of three major components: generation, a high voltage transmission grid, and a distribution system. The high voltage transmission system links the generators to substations, which supply power to the user through the distribution system.

What are the 5 parts of a electrical system? ›

The Basics of Electrical Components
  • Resistors.
  • Capacitors.
  • Light Emitting Diode (LED)
  • Transistors.
  • Inductors.
  • Integrated Circuit (IC)
  • Circuit Breaker.
  • Fuse.

Can current flow without voltage? ›

Current cannot flow without Voltage. Voltage is the cause and current is its effect. Voltage can exist without current. 1 ampere =1 coulomb/second.

What are the 3 measurable quantities of a circuit? ›

Capacitance, inductance, and resistance are electrical quantities that characterize an ideal simple electronic circuit, component, and materials used to make components, and determine a relation between ac voltage and current (see Eq. (2), (3), and (4)).

What is the basic electrical quantity? ›

The basic electrical quantities are electrical current and voltage, electrical charge, resistance, capacitance, inductance and electric power.

What is difference between voltage and power? ›

In terms of electromagnetism, it is the current flow of one ampere with a voltage of 1 volt. A volt is a derived unit for electric potential, electromotive force, and electric potential difference.
AC and DC.
Current TypeFormulaTerms
DCPW= VV × IAPW is Power in watt
VV is voltage in volts
IA is current in ampere
4 more rows

Is voltage a power or energy? ›

Voltage measures the energy that a charge will get if it moves between two points in space. The unit for voltage is the volt (V) and 1 Volt = 1 J/C.

What is difference between current and power? ›

Electric current is the actual rate of flow of electrons past a given point, in a unit of time. ... Power, measured in Watts, is the amount of work electricity can do in a circuit. Watts are calculated by the Voltage times Amps in a circuit.

What are the 10 rules of electricity? ›

Let's look at ten electrical safety tips every homeowner should know.
  • Always Cut the Power. ...
  • Have the Appropriate Fire Extinguisher on Hand. ...
  • Use More Than One Outlet. ...
  • Feel Your Outlets. ...
  • Child-Proof Your Outlets. ...
  • Investigate Flickering Lights. ...
  • Install Arc-Fault Circuit-Interrupters. ...
  • Don't Use Extension Cords Long-Term.
Jul 11, 2019

Do amps push volts? ›

AMPS is amount of electricity. VOLTS is the Push, not the amount.

What are the 7 types of electricity? ›

Electricity takes different forms: coal, water, solar, wind, nuclear, hydro and solar.

What are the four 4 components of power supply? ›

A power supply unit consists of four core components.
Let's take a look at each of them before delving into power supply component availability.
  • The Transformer. ...
  • The Rectifier. ...
  • The Filter. ...
  • The Regulator Circuits.

What are the six 6 main components of the power system? ›

A modern electric power system has mainly six main components: 1) power plants which generate electric power, 2) transformers which raise or lower the voltages as needed, 3) transmission lines to carry power, 4) substations at which the voltage is stepped down for carrying power over the distribution lines, 5) ...

What are the 4 important elements of electrical circuit? ›

All circuits contain four elements: a source, a load a transmission system and a control. The source provides the electromotive force. This establishes the difference in potential which makes current flow possible. The source can be any device which supplies electrical energy.

What are the 7 components of the electrical panel? ›

Electrical Control Panel Electrical Components
  • Main circuit breaker. This is like the disconnect of the main electrical panel leading into a home or office. ...
  • Surge arresters. ...
  • Transformers. ...
  • Terminal blocks. ...
  • Programmable Logic Controller (PLC). ...
  • Relays and contactors. ...
  • Network switches. ...
  • Human Machine Interface (HMI).
Sep 16, 2022

What is Ohm's law rule? ›

Ohm's law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant.

How many electrical quantities are there? ›

The electrical quantities are charge, current, voltage, power and energy.

What are the important electrical quantities? ›

In electrical and electronic circuits, there are five major electrical quantities that used to analyze circuits. These quantities are electric charge, electric current, voltage, electric power and electrical energy.

What are basic electrical measurements? ›

The Volt V, Ampere A, and Ohm are the standard units of electrical measurement for voltage, current, and resistance, respectively. Other commonly used electrical units are derived from SI base units.

What are the 4 sources of electrical energy? ›

According to the U.S. Energy Information Administration, most of the nation's electricity was generated by natural gas, nuclear energy, and coal in 2020. Electricity is also produced from renewable sources such as wind, hydropower, solar power, biomass, wind, and geothermal.


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