ELECTRIC FIELD

A number of simple experiments demonstrate the existence of electric forces and charges. For example, after running a comb through your hair on a dry day, you will find that the comb attracts bits of paper. The attractive force is often strong enough to suspend the paper. The same effect occurs when certain materials are rubbed together, such as glass rubbed with silk or rubber with fur. Another simple experiment is to rub an inflated balloon with wool. The balloon then adheres to a wall, often for hours. When materials behave in this way, they are said to be electrified, or to have become electrically charged.

Electric charge, Q is property of bodies of exerting forces on each other by electric fields. The SI unit of charge is the coulomb(C). There are two kinds of electric charges, which were given the names positive and negative by Benjamin Franklin. A negative charge is possessed by electrons and positive charge is possessed by protons. Charges of the same sign repel one another and charges with opposite signs attract one another.

Another important aspect of electricity that arises from experimental observations is that electric charge is always conservedin an isolated system (the law of charge conservation). That is, when one object is rubbed against another, charge is not created in the process. The electrified state is due to a transfer of charge from one object to the other. One object gains some amount of negative charge while the other gains an equal amount of positive charge. For example, when a glass rod is rubbed with silk, the silk obtains a negative charge that is equal in magnitude to the positive charge on the glass rod. Electrons are transferred from the glass to the silk in the rubbing process. This process is consistent with the fact that neutral, uncharged matter contains as many positive charges (protons) as negative charges (electrons).

Thus, electric charge has the following important properties: there are two kinds of charges in nature; charges of opposite sign attract one another and charges of the same sign repel one another; total charge in an isolated system is conserved; charge is quantized.

Charles Coulomb measured the magnitudes of the electric forces between charged objects using the torsion balance, which he invented. Coulomb confirmed that the electric force between two small charged spheres is proportional to the inverse square of their separation distance. The electric force between two stationary charged particles has the following properties: it is inversely proportional to the square of the separation r between the particles and directed along the line joining them; it is proportional to the product of the charges and on the two particles; it is attractive if the charges are of opposite sign and repulsive if the charges have the same sign; it is a conservative force.

Point charge is a particle of zero size that carries an electric charge. Coulomb’s law describes the force acting between two point charges. Coulomb’s law states that the force between two point charges and is directly proportional to the product of the charges and inversely proportional to the square of the distance r between the charges: = , where ke is a constant called the Coulomb constant.Sometimes instead of Coulomb constant , another constant is used. The medium permittivity (dielectric constant) ε is a physical quantity showing by how many times the interaction force F₀ between the charges in vacuum is greater than the force F in the given medium. It is measured in Farad per meter (F/m).

The concept of a field was developed by Michael Faraday in the context of electric forces. An electric field exists in the region of space around a charged object (source charge).When another charged object (test charge) enters this electric field, an electric force acts on it. The electric field vector E is the field produced by some charge or charge distribution separate from the test charge, it is not the field produced by the test charge itself. The existence of an electric field is a property of its source.

The electric field strength (intensity), E is a vector quantity equal to the ratio of the force acting from the side of electric field on the point charge placed in the given point of the field, to the magnitude of this charge. The vector E has the SI units of newtons per coulomb (N/C), equivalent to volt per meter (V/m).

A convenient way of visualizing electric field patterns is to draw curved lines parallel to the electric field vector at any point in space. These lines, called electric field lines and first introduced by Faraday, are related to the electric field in a region of space. The electric field vector E is tangent to the electric field line at each point. The line has a direction, that is the same as that of the electric field vector.

The uniform electric field is a field at all points of which the intensity is the same. The force lines of such a field have the same direction in all points. The uniform field can be created by one large charged metallic plate or by the couple of parallel plates charged by equal opposite charges (from ‘Physics for Scientists and Engineers’).

1. What is electric charge?

2. What is the SI unit of electric charge?

3. What does the law of charge conservation state?

4. What properties does electric charge have?

5. What does Coulomb’s law state?

6. What is the permittivity?

7. Who developed a concept of the electric field?

8. What is the SI unit of electric field strength?

9. What is a force line?

10. What type of a field is called uniform?

Exercise 3. Match the terms in the list with the appropriate definition.

Force line Uniform electric field Electron Coulomb

Point charge Electric field strength Permittivity Proton

Electric field Isolated system Electric charge

Term	Definition
	Property of some particles and of some samples of matter, that leads to a force between them and others.
	Region of space in which charges experience electric forces.
	A negatively charged particle.
	Closed system with no interaction with the outside world.
	Electric charge regarded as concentrated in a mathematical point, without spatial extent.
	The strength of the electric field D influenced in a sample of substance per unit applied outside field E.
	Force on unit charge at that point in a field, measured in N/C.
	SI unit of electric charge.
	An elementary particle having positive charge.
	A field in which the field intensity is constant both in magnitude and direction, at any point in the region of space considered.
	An imaginary line representing a field of force (electric or magnetic field) such that the tangent at any point is the direction of the field vector at that point.

Exercise 4. Complete the table below with appropriate information about some electrical quantities mentioned in the text above. The first line is done.

Quantity	Symbol	Unit name, abbreviation	Formula	Description of formula
Electric charge	Q	coulomb (C)	1C=1A×1s	One coulomb is equal to one Ampere-second.
Permittivity
Electric field strength

Exercise 5. Are the following statements about static charges and electric field True or False?

1. Like charges repel. 2. Like charges attract. 3. Opposite charges repel. 4. Opposite charges attract. 5. A positively charged object has lost electrons. 6. A positively charged object has gained protons. 7. A negatively charged object has lost protons. 8. A negatively charged object has gained electrons. 9. The electric field strength created by object A is dependent upon the separation distance from object A. 10. The electric field strength created by object A is dependent upon the charge on object A. 11. The electric field strength created by object A is dependent upon the charge of the test object used to measure the strength of the field.

True / False True / False True / False True / False True / False True / False True / False True / False   True / False   True / False     True / False

 

Exercise 6. Identify the following objects as positive, negative or neutral.

1. An object possesses more protons than electrons.

2. An object possesses more neutrons than electrons.

3. A formerly neutral object that just lost some electrons.

4. A formerly neutral object that just gained some electrons.

5. An object which attracts a negatively-charged balloon.

6. An object which attracts neutral paper bits and attracts a negatively-charged balloon.

7. An object which attracts neutral attracts paper bits and repels a negatively-charged balloon.

8. An object which attracts a negatively-charged balloon and attracts a positively-charged balloon.

9. An object around which the electric field vector is directed inwards.

10. An object around which the electric field vector is directed outwards.