Review 108 - Chapter 21 Electric Charge Experiments show...

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Chapter 21 Electric Charge Experiments show that they are two distinct type of electric charge: Positive and Negative . When we rub a glass rod with silk cloth both objects acquire electric charge. The sign on the charge on the glass rod is defined as positive When we rub a plastic rod with fur both objects acquire electric charge. The sign on the charge on the plastic rod is defined as negative Atoms consist of electrons and the nucleus . The nucleus itself consists of two types of particles: protons and neutrons. The electrons are negatively charged The protons are positively charged, The neutrons are neutral (zero charge) Neutron (n) : Mass m = 1.675 × 10 -27 kg ; Charge q = 0 Proton (p) : Mass m = 1.673 × 10 -27 kg ; Charge q = +1.602 × 10 -19 C Electron (e) : Mass m = 9.11 × 10 -31 kg ; Charge q = -1.602 × 10 -19 C
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() Electric charge cannot take any arbitrary value but only values that are multiples of the elementary charge . 0 net e p n p e Qe N e N N e N N n e e =− + + = = Charge Quantization Conservation of Charge if QQ = N e N p N n silk glass rod silk glass rod - - - - + + + +
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12 Consider two charges and placed at a distance . The two charges exert a force on each other that has the following characteristics: The force acts along the line connecting the tw qq r Coulomb's law 1. o charges The force is attractive for charges of opposite sign The force is repulsive for charges of the same sign The magnitude of the force, known as Coulomb force is given by the equation: 2. 3. 2 o -12 2 2 The constant is known as =8.85 10 N m /C 1 4 o o F r π ε × = permitivity constant 2 1 4 o F r πε =
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11 2 1 3 The net electric force exerted by a group of charges is equal to the vector sum of the contribution from each charge. FF F =+ Coulomb's law and the Principle of Superposition GG G
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Chapter 22 Electric Fields 12 2 1 4 o qq F r πε = Electric interactions propagate in empty space with a large but finite speed ( c = 3 × 10 8 m/s). In order to take into account correctly the finite speed at which these interactions propagate we have to abandon the “action at a distance” point of view and still be able to explain how does q 1 know about the presence of q 2 . The solution is to introduce the new concept of an electric field vector as follows: Point charge q 1 does not exert a force directly on q 2 . Instead, q 1 creates in its vicinity an electric field It is the electric field vector that exerts a force on q 2 . 1 2 generates electric field exerts a charg force e on E q EF q →→ G GG
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Consider the posively charged rod shown in the figure For every point P in the vicinity of the rod we define the electric field vector as follows: 1. We place a E Definition of the electric field vector G test charge at point P. 2. We measure the electrostatic force exerted on by the charged rod. 3. We define the electric field vector at poit P as positive SI Units: N : o o o q F q E E F q = G G G G /C o F E q = G G
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q r P E G If q > 0 points radially Electric field generated by a point charge ou If q < 0 points radially twards inwa ds r E E G G 2 1 4 o q E r πε =
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O O The net electric electric field
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This note was uploaded on 12/01/2011 for the course PHY 108 taught by Professor Iashvili during the Fall '08 term at SUNY Buffalo.

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Review 108 - Chapter 21 Electric Charge Experiments show...

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