# Jan10 - PHYS142 Lecture 2 Electric Fields Chapter 23 All...

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Electric Fields Chapter 23 All sections 23.1 Properties of Electric Charges 23.2 Charging Objects by Induction 23.3 Coulomb’s Law 23.4 The Electric Field 23.5 Electric Field of a Continuous Charge Distribution 23.6 Electric Field Lines 23.7 Motion of a Charged Particle in a Uniform Electric Field PHYS142 Lecture 2 2

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Summary of Coulomb’s Law The magnitude of the electric force is given by Coulomb’s Law, The direction of the force is determined from 2 2 1 r q q k F Charges of the same sign repel one another Charges with opposite signs attract one another. 3
+ Electric Field (23.4) Consider the space surrounding a charge Q 1 . Any charge q placed in that space will experience an electric force. We can calculate the electric force acting on q at any point in space. The collection of all these force vectors is the electric field created by a charge Q 1 . Q 1 1 2 3 q q q F 1 F 2 F 3 4

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Concept of Electric Field • Electric force is a field force . (action through medium) • Electric field is a property of the space induced by charge Q . • Charge q 0 , used in determining the electric field is called a test charge . By convention it is a positive charge. • Test charge is not a real charge. It is a fictitious charge. It has no influence on the electric field of interest. 5
Definition of the Electric Field magnitude of the electric field, E , by a point charge Q at a distance r is defined as the electric force on the test charge at that point divided by the value of the test charge. Units for E : N/C This is the magnitude of the E-field created by the charge Q . 2 r Q k E e q r qQ k q F E e 1 2 Q r In vector form: r r q k E e ˆ 2 q 6

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The direction of E is the direction of the force acting on the positive test charge at that point. + + q Q r F + r E Q If we know the E-field at a point, then if a charge Q ´ is placed at that point it will experience an electric force given by E Q F 7
If Q ´ is a positive charge, the direction of the force acting on it will be in the

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Jan10 - PHYS142 Lecture 2 Electric Fields Chapter 23 All...

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