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Basic electrical quantities: current, voltage, power

Build an intuitive understanding of current and voltage, and power. Written by Willy McAllister.
Voltage and current are the cornerstone concepts in electricity. We will create our first mental models for these basic electrical quantities. We will also talk about power, which is what happens when voltage and current act together.

Charge

The concept of electricity arises from an observation of nature. We observe a force between objects, that, like gravity, acts at a distance. The source of this force has been given the name charge. A very noticeable thing about electric force is that it is large, far greater than the force of gravity. Unlike gravity, however, there are two types of electric charge. Opposite types of charge attract, and like types of charge repel. Gravity has only one type: it only attracts, never repels.

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 14). 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

Current is the flow of charge.
Charge flows in a current.
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, equals, start fraction, d, q, divided by, d, t, end fraction
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 Nastart superscript, plus, end superscript and Clstart 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.
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.

Voltage

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

Voltage resembles gravity

For a mass m, a change of height h corresponds to a change in potential energy, delta, U, equals, m, g, delta, h.
For a charged particle q, a voltage V corresponds to a change in potential energy, delta, U, equals, q, V.
Voltage in an electric circuit is analogous to the product of g, dot, delta, h. Where g is the acceleration due to gravity and delta, 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.
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.
We can express the voltage between two points mathematically as the change of energy experienced by a charge:
V, equals, start fraction, delta, U, divided by, q, end fraction
That's an intuitive description of voltage in a nutshell.

Power

Power is defined as the rate energy (start text, U, end text) is transformed or transferred over time. We measure power in units of joules/second, also known as watts.
(1, 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)
start 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:
start 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.

Summary

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.

Want to join the conversation?

  • aqualine seedling style avatar for user Lynch.Steven
    I still don't get Amperes, can someone please help with a simple analogy so I can understand better?
    (2 votes)
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    • male robot johnny style avatar for user Bradley Steinbauer
      Amperes (or amps as a shortened version) is the standard unit of measurement for current. One amp is equal to one coulomb per second (Q/t) flowing within a circuit. A coulomb is 6.25 x 10^18 electrons! As you can imagine, an ampere would be a relatively large unit in most electronics applications. In terms of a water analogy, think of voltage as pressure in a pipe (it is essentially the force that causes the electrons to move) and current as the rate of flow of water molecules in the same pipe.
      (6 votes)
  • primosaur tree style avatar for user Ruchit patel
    what is quantum mechanics ? please explain in brief
    (14 votes)
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  • leaf green style avatar for user Arulx Z
    Can someone elaborate the formula dU/dt
    (12 votes)
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  • leaf green style avatar for user Pragatheeswaran
    The electric power consumed by a device may be calculated by using either of the two expressions P=I^2 R or P=V^2 /R. The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality


    I Know, if you have a constant voltage, increasing the resistance decreases the current flowing in the system by Ohm's law and hence decreases the power consumption

    However, if you have a constant current source, increasing the resistance increases the voltage dropped across the resistor and hence increases the power consumption.


    But more intuitively I wanna know about this, with some example....can anyone please explain me? Thanks is advance
    (2 votes)
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    • purple pi purple style avatar for user APDahlen
      Hello Leo,

      CAUTION - this is an answer from an electrical engineer...

      In my opinion the voltage source is relatively straightforward. Examples include a battery or wall outlet. As you stated as the resistance is decreased more current flows. With more current and a fixed voltage there will be more power.

      The current source is a different animal. I'll give you a few examples to consider:

      1) The current source is a mathematical construct that maintains a constant current. It will take on whatever voltage is necessary to do so including both positive and negative voltages. It has an infinite impedance. To my knowledge there are no perfect constant current sources. To make one you would need a device that could produce an infinite voltage (not possible).

      2) Please search "Thevenin Norton Equivalent." Here you will find that a voltage source with series resistance can be modeled as a current source with parallel resistor. Note that there is no such thing as a perfect constant voltage source. Such a device would demand infinite current (again, not possible in this universe).

      3) Please search "inductor kickback." Here you will find than an inductor acts as a constant current source for a limited amount of time. As the inductor is "turned off" it will act as a constant current source and do whatever is necessary to maintain the same current before and after the transition.

      4) Please search "transistor characteristic curves." Here you will find that a transistor appears to operate as an acceptable constant current source.

      I hope you like these examples. Know that it will take some time for the material to sink in. May I recommend you print this note and check things off as you have master the topics.

      Please leave a comment below if you have any questions.

      Regards,

      APD
      (12 votes)
  • blobby green style avatar for user DC83
    In the analogy for voltage, where you compare it to a ball rolling down a hill, does voltage increase as it rolls down the hill, i.e. does it build momentum with gravity?
    (5 votes)
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  • duskpin ultimate style avatar for user ANI
    Why is I the symbol for current?
    (3 votes)
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  • blobby green style avatar for user Eric George
    If water is a poor conductor of electricity, why do we get electrocuted if we stand in water and electricity is introduced?
    (5 votes)
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    • spunky sam red style avatar for user V_Keyd
      Pure water is poor conductor of electricity but the water in your tap contains many minerals that provide the ions for conduction of electricity. Similarly if you take pure water and dissolve a teaspoon of table salt (NaCl), it would become a good conductor because now it would have sodium and chlorine ions to help conduct electricity.

      Do note that at very high voltages, even pure water starts to conduct electricity because water molecules become ionized and separate into H+ and OH- ions which results in increased conductivity.
      (2 votes)
  • blobby green style avatar for user Jane He
    what's the difference between charges and electron/proton? since in my mind, current is caused by the moving of electron, but the text says that charges cause it.
    (3 votes)
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  • starky ultimate style avatar for user Jack Kovski
    Is there a reason why proton's charge is called positive and electron's charge is called negative?
    (2 votes)
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    • spunky sam orange style avatar for user Willy McAllister
      A long time ago, Ben Franklin (the American statesman) proposed a theory of electricity where he thought electricity was a fluid. This was back when the only thing anyone knew about electricity were those static electricity experiments you can do with a comb through your hair or rubbing a balloon on your sweater. Nobody knew about atoms or electrons or protons. Anyway, Franklin proposed an object could have extra electric fluid or lack fluid, depending on what you rubbed it with, and that's what caused the static attraction, the different levels of electric fluid in two materials. He called the "lacking" material negative, and the extra material "positive". About 150 years later the electron was discovered, and it turned out that the "lacking" materials actually had an excess of electrons. And that's how the electron got its negative sign.
      (6 votes)
  • piceratops seedling style avatar for user Renata Karen Abreu
    I am still confused about voltage, I can't understand why the eletron moves towards a lower energy state?
    (3 votes)
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