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|14.1||Electric Charges. Conductors and Insulators|
In this Physics tutorial, you will learn:
Try to do the following activities at home:
If you cannot do these activities, don't worry. We will tell you what happens in each case. Thus, in the first and the second case, the ruler and balloon will attract the small pieces of paper, while in the third case you will get a slight electric shock when touching the metal object.
All these examples are demonstrations of the electric nature of matter, for which we will discuss in this tutorial.
Atoms, as building blocks of matter are composed by three main particles:
A simplified structure of atom is shown in the figure below.
Protons and electrons are able to interact with other particles. They can attract or repel particles near them. Therefore, protons and electrons are known as electric charges as they carry electricity when moving, due to the interaction with other charges. However, electric charges behave differently depending on the type of charge they interact. They obey to the following rule:
Like charges repel each other while unlike charges attract each other.
The above rule is explained more clearly in the table below:
To make distinction between protons and electrons, scientists have classified them into positive and negative. Thus, protons are known as positive charges while electrons as negative ones.
Neutrons do not carry any electric charge, as they do not interact with other particles. Therefore, they are neither positive nor negative but rather, neutral (as their name implies).
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.
"Static" means not moveable or at rest. Therefore, static electricity or electrostatics is the part of electromagnetism that deals with electric charges at rest. Static electricity causes charged object to attract or repel other charged objects.
Static electricity obeys the law of conservation of charge, which represents another version of the law of conservation of energy. It states that:
Electric charges may flow from one place to another; however, the total amount of charge of the system remains the same.
We must first emphasize the fact that only electrons can move independently from one place to another as they rotate in the outer part of atoms. Protons cannot move as they are locked inside the nuclei. This means when discussing about the movement of electric charges, we imply the electrons movement only.
Electric charges can travel from one object into another when the proper conditions are created. These conditions are related to the ability of intermediate media to "host" and "escort" electric charges during flow from the source to the destination. Some materials are more charges-friendly, i.e. they allow free electrons to travel in transit through them. Such materials are called electric conductors. Metals for example, are good conductors of electricity as they allow free electrons to easily flow through them.
On the other hand, some materials provide high resistance to the charges flow. This means electric charges are blocked when they try to move through them. Such materials are known as electric insulators. Vacuum, air, wood, plastics, paper etc., are all examples of electric insulators.
Please note the similarities between heat and electricity conduction. In general, good conductors of heat are also good conductors of electricity. The same is also true for bad conductors.
Objects can be charges through three possible methods, which we will explain in this paragraph.
This method is applied when both materials are conductors and at least one of them is electrically charged. After placing the two objects in contact, the extra charge flows between them until the balance is established. This means by the end of process, both objects will have the same type of extra charge.
For example, if two identical objects are charged as shown in the figure,
the distribution of charge after the contact will be such that each object has
after the contact, as shown in the figure below.
Therefore, the amount of charge that has flowed between objects is 5 C of negative charges flowed from the second to the first object.
Obviously, when objects have different dimensions or when they are made from different materials, the charge distribution is not simply the arithmetic mean as it was in the above example.
Induction is the second method of electrisation. It consists on bringing a charged object near a neutral one. The charged object attracts the unlike charges and repels the like charges of the neutral object. As a result, the neutral object is charged locally by opposite signs as shown in the figure.
However, this method has a serious drawback. When the charged object is moved away, the electric charges in the locally charged object go back to the original positions and the object becomes again neutral in all its parts. In order to produce a permanently charged object, we can connect the part of the object which contains the undesired charges to the ground through a conducting wire. Such an action, balances the electric charges in that part of the object as charges flow between the Earth and the object until the object becomes neutral in that side. Then, we move away the charged object after removing the wire from the grounded object. In this way, the former locally charged object remains permanently charged by the same type of charge (here positive).
In reality, the main purpose of grounding process is not to produce extra charges of the same type in a locally charged object but to make a charged object neutral by allowing extra charges to flow from the object to the ground or vice-versa. This prevents hazards created due to the excessive extra charges accumulated in any faulty electrical appliance or cable.
This method consists on rubbing with each other two insulators made from different materials. For example, when we rub a plastic material (ruler, balloon etc., as mentioned in the introduction paragraph) with another insulator (any stringy one, such as hair, woollen cloth, etc.), the objects become both charged but with opposite signs. This occurs due to the prolonged dynamic contact between objects, which removes some electrons from one object and sends them to the other.
As stated above, despite being both insulators, the objects must have different characteristics: one must be more disposed to expel electrons and the other to host them. For example, the stringy objects release electrons and become positively charged while the plastic objects get electrons, becoming negative.
As explained in the previous paragraph, a permanently charged object contains only one type of extra charge. When an object is electrically charged, the like extra charges it contains push each other as far as possible. As a result, the extra charges go to the outer surface of object, leaving the inside of the object neutral.
When objects are irregularly shaped, electric charges density increases in sharp parts of the object. As a result, a discharge may occur when these sharp parts, producing a large amount of dynamic electricity.
Lightning for example is an example of natural discharge, during which a lot of extra electrons gathered at bottom of the cloud due to friction between water droplets, are discharged to the ground through a high building, tree, pole, etc. Lightning is very dangerous as it produces a very high amount of current, which causes a burning effect. However, after flowing to the ground, electrons are distributed instantly throughout the Earth surface. Since the Earth is very large, these extra charges are not dangerous anymore, as their concentration at a given position is very low.
To avoid damages caused by lightning on buildings, high metal poles known as lightning poles are installed near them. These poles have a wide metal plate at their bottom, which is buried in the ground.
Electroscope is a device used to detect whether an object is electrically charged or not. It consists in a metal knob on the top, a metal body and two thin metal leaves attached to the end of the metal body. Leaves are kept inside an insulated case to prevent the interference of external factors such as air currents.
When an electroscope is neutral (not charged), its leaves always stay in vertical position due to the attraction of gravity. If the knob is electrically charged, the extra charges flow through all metal parts of the electroscope including the leaves. As a result, they open because the extra charge is of the same type everywhere and the leaves are very light to resist to the repulsion caused by the like charges accumulated in them.
Electroscope can be charged in two ways: by contact and by induction. If we touch the knob of an electroscope with a charged object, the leaves open. The extra charges then distribute throughout the metal parts including the leaves, which open as stated earlier. This is the method of charging the electroscope by contact.
On the other hand, when be bring a charged object near a neutral electroscope, the opposite charges are accumulated to the knob due to the electrostatic attraction, sending the like charges as far away as possible to the leaves (electrostatic repulsion). As a result, the leaves open as the electroscope is locally charged by opposite signs. This is the method of charging the electroscope by induction.
The negatively charged object repels the negative charges of electroscope A from the knob to the leaves. Since the leaves close due to this action, it means they have been positive because the negative charges that have flowed to them have balanced the extra positive charges they previously had. Hence, the electroscope A has been positively charged prior to sending the object near its knob.
The negatively charged object sends electrons throughout the electroscope B by means of direct contact. Given that the leaves open more, it means the leaves become even more negative. This means the electroscope B has been negatively charged before the contact with the negative object.
There are not too many applications of electrostatic in daily life, as static charges are not able to do any mechanical work. However, a number of useful applications of electrostatics already exist. Some of them include:
Some inflammable liquids such as fuels can be charged due to friction while flowing through pipes. Since such liquids are inflammable, the flowing process can be very dangerous because if a spark occurs, a fire will start immediately. To prevent this, fuel trucks are always grounded through a conducting wire when they fill containers in a gasoline station. Likewise, airplanes are grounded when being refueled, in order to prevent unexpected explosions and fires.
Electrostatic precipitator is a structure installed inside chimneys to prevent the smoke from turning back to the room because of air currents.
Two negatively charged plates are installed in the lateral sides of chimney and a positively charged grid at its central area. The smoke is caught up in the plates after passing through the grid and charged positively, as opposite charges attract each other. This prevent the smoke from turning back to the room when air currents blow outside.
Metal objects such as cars, chairs, etc., are painted using electrostatics. Thus, the object that needs to be painted is raised from the ground to avoid the grounding effect before starting the process. The nozzle of paint spray bottle is charged positively and the metal object is given a negative charge. In this way, when paint comes out in spray form from the nozzle, it is charged positively and is stuck better to the metal surface. This saves a lot of paint and makes the painted surface more uniform.
It is not the case to explain the whole procedure of how a photocopying machine works, but we can say its operation principle is an example of electrostatics application in technology. In few words, a photocopying machine includes a drum-shaped metal plate coated with selenium, which becomes conductive in light and insulator in darkness. The ink is also charged by opposite sign to the metal drum, and when dark and bright regions formed by the reflection of the original document are in contact with the selenium, they produce charged and neutral regions on the paper, which on the other hand is charged by rolling over the metal drum. In this way, the document not only will have the same look as the original, but this look will last for a long time as the printing process is more qualitative, because the ink sticks better on the paper, similarly to the quality obtained in paint spray systems we discussed earlier.
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