Notes on Chapter 22 - Chapter 22: Electric Fields 22 - 1...

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Chapter 22: Electric Fields 22 - 1 What is Physics? The concept of the electric field allows us to give a description of how a charged particle can exert a force on another particle even though the two particle are not in contact – action at a distance. The chapter is about calculating the electric field for various charge configurations.
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The concept of electric fields allows us to answer the question: How does particle 1 ‘know’ of the presence of particle 2? Recall: 1 q 2 q 1 q 2 q + + + 21 F 21 F 22 - 2 The Electric Field ˆ r ˆ r
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Suppose I have a fluid in a certain region and I define: (1) a function T(x, y, z, t) that gives the temperature in the fluid at the coordinates x, y , and z and at time t . (2) a function v (x, y, z, t) that gives the velocity of the fluid at the coordinates x, y, z , and at time t . The function T(x, y, z, t), ( called the temperature field ) is an example of a scalar field while v(x, y, z, t), (called the velocity field ) is an example of a vector field These functions give the temperature and velocity at the coordinate and time specified by the dependent function You can imagine other fields like pressure fields , etc The electric field , E (x,y,z) is an example of vector fields Examples of fields
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We use the following diagram to get a mathematical expression for the electric field. We consider the mutual repulsion between two positively charged bodies A and B . Mathematical expression for the electric field The electric field at P is defined mathematically as the electric force per unit charge : 0 F E q =
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is the force experienced by the charge q 0 at P . The body placed at P to determine the electric field at P is called a test charge . By convention a test charge has positive sign . The SI unit of electric field is N/C (Later on we will find that the unit can also be V/m ) . If the test charge is to determine the electric field due to the body A alone, then it is important that test charge does not disturb the electric field of A . For this reason a test charge is assumed to have negligible charge : F
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We thus extend the definition of electric field as a limiting process: The magnitude of the force on the test charge q 0 at P is: Hence, by definition, the magnitude of the electric field at P is: 2 0 || 1 4 q E r πε = 0 0 0 lim q F E q = 0 2 0 | || | 1 4 qq F r =
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Value ( N/C ) Field at 3 x 10 21 Surface of U nucleus 5 x 10 11 Within H atom at Bohr radius 3 x 10 6 Electric breakdown in air 10 5 Near charged drum of photocopier 10 3 Near a charged comb 10 2 In lower atmosphere 10 -2 Inside a copper wire Some electric field values
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Electric Field Lines By definition an electric field line is an imaginary line or curve drawn through a region of space so that its tangent at any point is in the direction of the electric field vector at that point http://www.mta.ca/faculty/science/physics/suren/FieldLines/FieldLines.html 22 - 3
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Notes on Chapter 22 - Chapter 22: Electric Fields 22 - 1...

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