Physics Lesson 15.1.1 - Moving Charges. The Meaning of Electric Current

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Welcome to our Physics lesson on Moving Charges. The Meaning of Electric Current, this is the first lesson of our suite of physics lessons covering the topic of Electric Current. Current Density, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Moving Charges. The Meaning of Electric Current

Stationary charges do not do any work. In fact, there are some useful applications of static electricity in daily life such as electrostatic precipitator, paint spray, photocopying machine, etc. as explained in the previous chapter; however, they are not really indispensable for the today's life. Most applications related to electricity in today's world use moving charges (dynamic electricity or electrodynamics) to operate. With moving charges, we usually have in mind electrons, as they are able to move freely between the atoms of conductor. For this reason, electrons are widely recognized as the main carriers of electricity. As we have explained earlier, protons (positive charges) are locked inside the nuclei of conductors and therefore they are not able to move. However, we will see later that in some specific cases, positive charged ions are also carriers of electricity, so we can say that not only negative charges are able to flow but this is an ability of positive charges as well.

Electric charges (typically electrons) always flow from places where there are more electrons to the places in which there are less electrons. This behaviour is similar to the behaviour of water when two containers with different levels of water are connected through a pipe. Water starts flowing from the higher level to the lower level container. This flow stops only after a balance in water level between the two containers is reached.

The same thing occurs to electric charges as well. If there is some misbalance (a potential difference) between the numbers of free charges in the two extremities of a conductor, a charge flow takes place until both extremities have the same density of free charges.

By definition, electric current, is the amount of electric charges flowing through any point of conductor in the unit time.

The symbol of electric current in formulae is I. This is because the term "electric current" is an abbreviation of the longer term "intensity of electric current", but we use the short term "electric current" or simply "current" in conversational form.

Mathematically, we have

The unit of electric current, Ampere (amp), A, is one of the seven fundamental units in the SI system used in science. Rearranging the above formula, we obtain the unit of electric charge, Coulomb in fundamental SI terms. Thus, since

it is easy to see that Coulomb = Ampere ∙ second in the SI system of units.

For example, if a device indicates the value 0.2 A on the display plate, it means there are 0.2 A ∙ 1 s = 0.2 C of charge flowing through the device in every second.

You have reached the end of Physics lesson 15.1.1 Moving Charges. The Meaning of Electric Current. There are 4 lessons in this physics tutorial covering Electric Current. Current Density, you can access all the lessons from this tutorial below.

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