Electrostatics: Charges, Electric Fields, and Coulomb's Law - Prof. Kamil Ucer, Study notes of Physics

Download Electrostatics: Charges, Electric Fields, and Coulomb's Law - Prof. Kamil Ucer and more Study notes Physics in PDF only on Docsity! Electrical Charges Charge is carried by electrons and protons. Can be positive or negative. Like charges repel, opposite charges attract. Total charge in a system is conserved. Charges come in discrete quantities. Charges are measured in Coulombs (C). Usually denoted by q. What Carries Charge? Smallest unit of charge is: e = 1.6 x 10-19 C 1 electron : -1 e 1 proton : +1 e 1 neutron : 0 e Most matter is naturally neutral Charging can be accomplished by rubbing We can consider Earth to be an infinite source of charge - grounding Nitrogen atom Three pith balls are suspended from thin threads. Various objects are then rubbed against other objects (nylon against silk, glass against polyester, etc.) and each of the pith balls is charged by touching them with one of these objects. It is found that pith balls 1 and 2 repel each other and that pith balls 2 and 3 repel each other. From this we can conclude that 1. 1 and 3 carry charges of opposite sign. 2. 1 and 3 carry charges of equal sign. 3. all three carry the charges of the same sign. 4. one of the objects carries no charge. 5. we need to do more experiments to determine the sign of the charges. Concept Question Electrostatic Force The electrostatic force between charges follows an inverse-square law (like gravity). The force can be attractive or repulsive (unlike gravity). As with other forces it is a vector quantity. The superposition principle applies. Coulomb’s Law 2 21 r qq kF ee = 2212 0 229 0 N.mC1085.8 CN.m1099.8 4 1 −×= ×== ε πεe k q1 q2r rF ˆ2 21 12 r qqke= v Electric Field of a Point Charge rFE e ˆ2 0 r qk q e == v v q q0r̂ E v r̂ r The Electric Dipole 2/322 )( 2 ay qakE e + = 3 2 y qakE e= y>>a What is the electric force on a test charge, q0 , at P? Electric Field Lines A way to visualize field patterns over space. The e-field is tangent to the field lines at each point and along the direction of the field arrow. The density of the lines is proportional to the magnitude of the e-field. Field lines start from positive charges an end at negative ones. Field lines can not cross. Uniformly Charged Ring rEd ˆ2r dqke= v θcosdEdEx = 0=⊥dE Q ax xkE dq ax xkdq ax xkE dq ax xk r x r dqkdEdE e x ee x e ex 2/322 2/3222/322 2/3222 )( )()( )( )(cos + = + = + = + === ∫∫ θ Motion in a Uniform E-Field E-Field E v Force EF vv q= Acceleration m q m EFa vv v == constant== ix vv t m eEtav yy −== 2 22 2 1)( 2 1 2 1)( ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = == xv x m eEty t m eEatty tvtx x=)( Example 23.10 vi = 3 x 106 m/s E = 200 N/C l = 0.1 m a = ? T = ? y = ?