In this post we will discuss on the concepts of electric charge. We will also cover types of charges (positive charge and negative charge). Interaction between like charges and opposite charges will be discussed. Discussion on Law of conservation of electric charge and triboelectric charging will also follow.
Atom is an object with a small, dense nucleus with a positive charge surrounded by a cloud of negatively charged electrons. It is electrically neutral. Electrons have negative charge, protons have positive charge, and neutrons have zero charge. An atom has no electrical charge because the number of protons and electrons are equal in it.
Many centuries ago it was noticed that if a piece of amber is rubbed with animal fur, it attracts small, lightweight objects.
Later, it was discovered that other materials can behave in the same way as well. For example, if glass is rubbed with a piece of silk, it too can attract small objects. Atoms are electrically neutral. Therefore it was well understood by the scientists that it is not atom which is behind this, but something else.
This is something else that is created by this rubbing and which is exhibiting this attraction ability. This phenomenon is found by them to be the effect of something named later as electric charge which is generated because of the rubbing.
If we see this ‘post rubbing attraction ability’ of glass and amber those seem to be the same for both the materials. It turns out, however, that these two materials have different types of charge. Let’s go to the concepts of Like CHARGE and Opposite CHARGE.
Like Charge versus Opposite Charge
If a piece of charged amber is brought close to another piece of charged amber then we see repulsion between them i.e. they tend to move away from each other. On the other hand, if a piece of charged glass is brought near the charged amber rod, the amber moves toward the glass, indicating an attractive force.
Thus it was concluded that like electric charges repel and different electric charges attract one another. We refer to different charges as being the “opposite” of one another, and can say that opposite charges attract.
We know today that only these the two types of charge (as found on amber and glass) exist, and we still use two purely arbitrary names, positive electric charge and negative electric charge, which were proposed by Benjamin Franklin in 1747.
Calling the different charges this way is actually quite useful for mathematical calculation. For example, an object that contains an equal amount of positive and negative charge has zero net charge. Objects with zero net charge are said to be electrically neutral.
Law of Conservation of Electric Charge
We now know, however, that rubbing the fur across the amber simply results in a transfer of charge from the fur to the amber. However the total amount of charge remains unchanged.
Before charging, the fur and the amber are both neutral. During the rubbing process some electrons are transferred from the fur to the amber, giving the amber a net negative charge, and the fur is left with a net positive charge.
Note that during this process no charge is ever created or destroyed.
This, in fact, is an example of one of the fundamental conservation laws of physics– Conservation of Electric Charge.
This law says that the total electric charge of the universe is constant. No physical process can result in an increase or decrease in the total amount of electric charge in the universe.
When charge is transferred from one object to another, it is generally due to the movement of electrons.
Positive Charge and Negative Charge
The outer electrons of these atoms are often weakly bound and fairly easily separated. In the next paragraph we can see quick answers to the following questions.
What is Electric charge? What is positive charge and what is negative charge?
As a piece of fur rubs across amber, for example, some of the electrons that were originally part of the fur are separated from their atoms and deposited onto the amber. The atom that loses an electron is now a positive ion or positive electric charge.
And the atom that receives an extra electron becomes a negative ion or negative electric charge.
A material with net positive ions is taken as positively charged material. Similarly a negatively charged material will have net positive ions. And both these types constitute the concept of electric charge.
Now the question is which material will lose electron/s and which one will gain.
As we have seen in an example that if silk is rubbed against glass, the glass acquires a positive charge. It follows that electrons have moved from the glass to the silk, giving the silk a negative charge.
But is it always true that silk will be at the receiving end always to have a negative charge finally for all such interactions with other materials as well?
NO! If silk is rubbed against amber, however, the silk becomes positively charged, as electrons in this case pass from the silk to the amber.
In general, when two materials are rubbed together, then who will lose and who will gain electrons and to what extent(how many electrons get transferred) depend on how strongly they hold onto their electrons. In other words the magnitude and sign of the charge that each material acquires after such rubbing depend on how strongly it holds onto its electrons.
These results can be understood by referring to the relative charging due to rubbing—also known as triboelectric charging—for a variety of materials.
The more plus signs associated with a material, the more tendency to be positively charged readily by giving up electrons. Similarly, the more minus signs for a material, the more readily it acquires electrons.
In general, when two materials in triboelectric charging table are rubbed together, the one higher in the list becomes positively charged. And the one lower in the list becomes negatively charged.
The greater the separation on the list, the greater the magnitude of the charge.(more the number of electrons transferred)