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Electric Charges. Conductors and Insulators Revision Notes

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14.1Electric Charges. Conductors and Insulators


In these revision notes for Electric Charges. Conductors and Insulators, we cover the following key points:

  • What are electric charges?
  • How do the electric charges behave?
  • What is the unit of electric charge?
  • What is an elementary electric charge?
  • What is static electricity?
  • What are electric conductors and insulators?
  • How do we charge objects electrically?
  • What is the charge distribution in objects?
  • What is lightning? How can we protect ourselves from it?
  • What is grounding process and why is it so useful in daily life?
  • What is an electroscope and how can we charge or discharge it?
  • What are some applications of electrostatics in daily life?

Electric Charges. Conductors and Insulators Revision Notes

Atoms, as building blocks of matter are composed by three main particles:

Protons (p) and neutrons (n), located inside a hard structure called "nucleus", and

Electrons (e), which are smaller in dimensions that protons and neutrons. Electrons revolve around the nucleus just like the planets orbiting the Sun.

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.

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 neutral.

To make a quantitative evaluation of the interacting ability of electric charges, we use a unit called Coulomb (C). 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

Q = 1C ⟹ Ncharges = 6.25 × 1018 e

The value of an elementary charge e in Coulombs is:

1e = 1 C/6.25 × 1018 = 1.6 × 10-19 C

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.

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.

Objects can be charges through three possible methods:

  1. By contact. 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.
  2. By induction. 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. We use grounding as an additional action to produce a permanently charged object by induction.
  3. By friction (rubbing). 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.

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.

To avoid damages caused by lightning on buildings, high metal poles known as lightning poles are installed near them.

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.

A number of useful applications of electrostatics already exist in daily life. Some of them include:

  1. Charging due to flow. To avoid discharges caused due to the flow of inflammable liquids such as fuels, the cars are grounded when they are being refuelled.
  2. Electrostatic precipitator. Electrostatic precipitator is a structure installed inside chimneys to prevent the smoke from turning back to the room because of air currents.
  3. Paint spray systems. Metal objects such as cars, chairs, etc., are painted using electrostatics by charging the metal object and the nozzle of paint bottle with opposite signs. This saves a lot of paint and makes the painted surface more uniform.
  4. Photocopying machine. It works by charging the ink and the metal drum with opposite signs to produce a durable printed document.

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