23-electric fields

23-electric fields - Physics 4B Lecture Notes Chapter 23 -...

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Physics 4B Lecture Notes 23-1 Chapter 23 - Electric Fields Problem Set #2 - due: Ch 23 - 1, 2, 5, 8, 14, 19, 26, 30, 31, 35, 38, 39, 50, 58, 61, 62 The motion of an electric charge is determined by the forces that act on it. The forces it feels that are exerted by all the other charges can be described in terms of the electric field due to the other charges. Lecture Outline 1. The Definition of Electric Field 2. Electric Field Lines 3. The Electric Field Due to Point Charges 4. The Electric Field Due to Continuous Charge Distributions 5. The Force on Charges in Electric Fields 1. The Definition of Electric Field The idea of a field is required in order to explain the "action at a distance." Recall the idea of gravitational field. M m F g In this view, Earth creates a force on the mass m. This is “insane.” Earth isn’t even touching the mass. So we introduce the idea of a gravitational field. m F g g Now we take the view that the field due to Earth, g, is exerting the gravitational force on the mass. Since the force is r F g = m r g , the gravitational field is defined as, r g r F g m . Let’s take the same approach with the electric force. r 1 q q 2 e F Instead of thinking of q 1 exerting the force on q 2 , we think of q 1 creating a field and the field exerting the force on q 2 . E q 2 e F
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Physics 4B Lecture Notes 23-2 Mathematically, we can write r F 21 = q 2 r E 1 r E 1 r F 21 q 2 which is the definition of the electric field. r E r F q The Definition of Electric Field Note that the field is the force felt by each Coulomb of charge. 2. Electric Field Lines We need a more descriptive image of the field. The most useful idea is “Electric Field Lines.” Field Lines or Lines of Force are used to visualize the field. The rules for drawing them are: 1. The tangent to the field line points in the direction of the force on a positive test charge. 2. The density of lines is proportional to the strength of the field (bigger charges make more lines). Example 1: Sketch the field due to a positive point charge. + + strong field weak field test charge Example 2: Sketch the field due to a dipole. A dipole consists of two equal and opposite charges. Notice that all the field lines that leave the positive charge end on the negative charge. Beyond the Mechanical Universe (vol. 29 ch 14)
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Physics 4B Lecture Notes 23-3 3. The Electric Field Due to Point Charges We have now reduced the problem of figuring out what a charge will do, to a problem of finding the field that it feels. The next step is to develop techniques to calculate the fields created by charges. Starting with the definition of the electric field
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23-electric fields - Physics 4B Lecture Notes Chapter 23 -...

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