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P132_ch23

# P132_ch23 - Chapter 23 Electric Fields Previously(Ch 22 we...

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R. Kass P132 Sp04 Ch23 1 Chapter 23: Electric Fields Previously (Ch. 22) we have seen that two electric charges will either attract or repel each other. How is it that one electric charge even knows about the existence of the other charge?? Based on an idea of Michael Faraday’s we say that one of the charges sets up an electric field throughout space and the other charge reacts to this field. The electric field (E) due to a charged object at a location in space is defined to be the force (F) per unit charge (q 0 ) as felt by a test charge placed at that location in space. 0 ) , , ( ) , , ( q z y x F z y x E = Important Points about the above definition of the electric field: 1) The electric field is a vector (just like force). It has magnitude (units=N/C) and direction. 2) The test charge, q 0 , in the definition is assumed to be positive. It is also assumed to be so small that it does not change the distribution of charge that is causing the electric field. 3) Since the test charge, q 0 , is positive , by DEFINITION the electric field is in the same in the same direction as the force. positive charge: electric field points away from the charge. negative charge: electric field points towards the charge. Direction of E for a negative charge. HRW Fig 23-2

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R. Kass P132 Sp04 Ch23 2 Electric Field of a point Charge Recall Coulomb’s Law for a point charge and our test charge, q 0 . The magnitude of force (F) on q 0 due to q is: 2 0 r qq k F = 2 2 9 0 / 10 99 . 8 4 1 C Nm k × = πε Therefore by definition the magnitude of the electric field (E) due to q is: 2 0 r q k q F E = = What about the direction of E? There are two cases, positive (+) and negative (-) q. + q 0 r E - q 0 r E example : what is the magnitude of E 1 m away from a charge of 1C? (+) C N m C C Nm r q k E / 10 99 . 8 ) 1 ( ) 1 )( / 10 99 . 8 ( 9 2 2 2 9 2 × = × = = (+) This is a very large electric field! ~10 3 times large than a small spark in air
R. Kass P132 Sp04 Ch23 3 Visualizing the Electric Field One way to visualize the electric field is the use the concept of electric field lines . 1) The direction of the electric field is given by the direction of a straight field line or the tangent to a curved field line. 2) The magnitude of the electric field is related to the number of field lines per unit area in a plane perpendicular to the lines. In other words, the electric field is large when the field lines are close together and weaker when the lines are further apart.

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P132_ch23 - Chapter 23 Electric Fields Previously(Ch 22 we...

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