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Welcome to our Physics lesson on Unit of Electric Charge. Elementary Charge, this is the second lesson of our suite of physics lessons covering the topic of Electric Charges. Conductors and Insulators, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
To make a quantitative evaluation of the interacting ability of electric charges, we use a unit called Coulomb (C). The standard unit of electric charges involves a very large number of charges. We need 6.25 × 1018 protons or electrons to produce an electric charge of +1C or - 1C. In formulae, we denote the electric charge by Q.
Since protons and electrons are the smallest particles that contain an electric charge, we call them elementary charges, e. Thus, we can write
From here, we can find the value in Coulombs of an elementary charge e:
Do not confuse the symbol of elementary electric charge e with that of electron e-. As you see, electron contains a minus as superscript, to show that it is a negative charge.
In most cases, we are more interested in the extra charge of objects than in the total amount of electric charge they contain. This is similar to the situations discussed in the previous tutorial about entropy, in which we were more interested in the change in entropy than in the actual entropy of thermodynamic systems. For example, an object may contain 1 000 000 000 protons and 1 000 000 001 electrons; however we don't have to consider such large values but only the extra charge, i.e. the difference between the positive and negative charges, as the rest of charges balance each other. In this case, the extra charge is Q = -1e as only one electron is left over.
An object has an electric charge of + 6.4 micro-Coulombs. How many extra elementary charges does it have and what type of charge are they?
Given that 1 micro-Coulomb = 10-6 C, we have Q = + 6.4 × 10-6 C.
The number N of extra charges therefore is
Since the extra charges have a positive sign, they are protons. Therefore, the object contains 4 × 1013 extra protons.
You have reached the end of Physics lesson 14.1.2 Unit of Electric Charge. Elementary Charge. There are 8 lessons in this physics tutorial covering Electric Charges. Conductors and Insulators, you can access all the lessons from this tutorial below.
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