Chapter_22

# Chapter_22 - Chapter 22: Electric Field Electric (22-1) In...

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1 In this chapter we will introduce the concept of an electric field . If charges are stationary the Coulomb’s law describes adequately the forces among the charges. If the charges are not stationary we must use an alternative approach by introducing the electric field (symbol ). We will cover the following topics: Electric field generated by a point charge . Using the principle of superposition, we will determine the electric field created by a collection of point charges, and by continuous charge distributions. Once we know the electric field at a point P we will be able to calculate the electric force on any charge placed at P . Introduce electric dipole . Determine the net force and the net torque, exerted on an electric dipole by an uniform electric field. (22-1) Chapter 22: Electric Field Electric Field E

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2 The Electric Field The Electric Field In previous chapter we discussed Coulomb’s law that gives the force on a charge q 2 when it is placed near a charge q 1 : A question arises: how does particle 2 “know” of the presence of particle 1 ? How can there be “action at a distance”? The answer to this question is that particle 1 sets up an electric field in the space surrounding itself. The particle 2 “knows” of the presence of particle 1 because it is affected by the electric field that particle 1 has created. E F The idea of electric field was introduced by Michael Faraday in 19 th century. F = 1 4 pe 0 | q 1 || q 2 | r 2

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4 The Electric Field (cont’d) The Electric Field (cont’d) The temperature at every point in a room has a definite value. One can measure the temperature at any given point by putting a thermometer there. We call the resulting distribution of temperatures in space a temperature field . In much the same way, one can imagine a pressure field in the atmosphere. It consists of the distribution of air pressure values, one for each point in the atmosphere. The above two examples are of scalar fields because temperature and air pressure are scalar quantities.
5 The Electric Field (cont’d) The Electric Field (cont’d) The electric field is a vector field ; it consists of a distribution of vectors, one for each point in the region around a charged object, such as a charged rod. We can determine the electric field at some point P near the charged object using the following procedure: we place a positive charge q 0 at point P . This charge is called a test charge . we then measure the electrostatic force on the test charge. finally, we define the electric field at point P due to the charged object as F E 0 q F E = The SI unit for the electric field is (N/C).

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6 Electric Field Lines Electric Field Lines Electric field lines are useful for visualization of electric field. The relation between electric field lines and
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## This note was uploaded on 10/04/2010 for the course PHY 340410 taught by Professor Shi,z during the Fall '10 term at SUNY Buffalo.

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Chapter_22 - Chapter 22: Electric Field Electric (22-1) In...

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