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Lesson_2.2

# Lesson_2.2 - Lesson 2.2 Electric Field 1 Intensity of...

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Lesson 2.2 Electric Field 1. Intensity of Electric Field. Force between two objects whether it is electrical, magnetic or gravitational can be explained using the concept of field. A field is a three dimensional space where a force can be detected. For example, the space around the earth where the gravitational pull of the earth is felt on an object is called the gravitational field of the earth. In the same way there is space around an electric charge where it exerts force on other electric charges. This space is called the electric field of that charge. Electric field is a vector quantity which has a magnitude and a direction . The magnitude of the electric field at a pointy is called the intensity of electric field or the strength of the electric field. It is represented by E . The intensity of electric field E at a point is defined as the force experienced per unit test charge placed at that point. In fig. 1 q o is a test charge placed at Pat a distance r from a charge q. According to Coulomb’s law, the force experienced by q o is: 2 0 r qq k F = The force experienced by unit test placed at P is: q q o r Fig. 1 P 2 0 F kq q r = This is a measure of the intensity of electric filed E at P due to charge q. Therefore, intensity of electric field at a point distance r from a charge q is given by:

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2 kq E r = ….1 E is measured in N.C -1 . Since E is force per unit charge, the force F on a charge q 1 placed at a point where the electric field intensity is E is given by F = E q 1 . ….2 2. Direction of an electric field: The direction of an electric field is the direction in which a positive charge would move . Since a positive charge will move away from a positive charge, the field due to a positive charge is directed away from it. Similarly, the field due to a negative charge is directed towards it. Fig. 2 below illustrates this phenomenon. + - fig. 2 Field due to a positive charge is directed away from it and field due to a negative charge is directed towards it . 3. Electric Field Lines The intensity and direction of an electric field can be better understood by drawing lines of force called field lines . A line of force is the path taken by a positive test charge placed in the field. Since a positive charge always moves away from a positive charge and towards a negative charge, lines of force are always directed away from a positive charge and towards the negative charge. The following are some of the properties of field lines. (i) Field lines always originate at a positive charge and terminate at a negative charge.
(ii) The number of lines originating from a charge or terminating at a charge is proportional to the magnitude of the charge. In other words, the lines will be crowded around a large charge and less crowded around a small charge. (iii)

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Lesson_2.2 - Lesson 2.2 Electric Field 1 Intensity of...

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