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PY212 Electricity and Magnetism

Learn about the fundamental concepts of electrostatics, including electric charge, Coulomb's Law, and the properties of the electric field. Understand why electrostatics is essential in understanding the interactions between charged particles.

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PY212 Electricity and Magnetism

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  1. PY212Electricity and Magnetism I. Electrostatics

  2. I-1 Electric Charge

  3. Main Topics • Why Electrostatics? • Demonstration of Electrostatic Effects. • The Electric Charge and its Properties. • The Coulomb’s Law. • Some Applications of the C. L. • Electric Field and Electric Intensity

  4. Why Electrostatics? • Many important properties of the Nature exist due to electric interactions of charged particles. We shall first deal with fields and charges which are static = do not move. This is for simplicity but they really exist!

  5. Demonstration of Electrostatic Effects • A comb after it has been run through hair attracts little pieces of paper. The force is a long-distance one. It can be sometimes also repulsive. • Bodies can be charged by conduction or induction. • We attribute these forces to the existence of a property we call the electric charge. • Using some materials we can easily discharge charged bodies, they are conductors. By others it is difficult or impossible, these are insulators

  6. Main Properties of the Charge • Since both the attractive and the repulsive forces exist, the charge must be of two types positive and negative. Unlike charges attract and like charges repel themselves. • Charge can be isolated only in multiples of the elementary charge e = 1.602 10-19 C • In all known processes charges appear or disappear only in pairs (+q and -q), so the total charge is conserved • Charge is invariant to the Lorentz transformation

  7. Main Properties of Electrostatic Interactions • Charged particles act by force on each other. Forces: • are Long-distance – mediated by electric field • obey the principle of superposition • Mutual interaction of two static point charges is described by Coulomb’s Law

  8. Coulomb’s Law I • Let us have two point charges Q1 and Q2 at the distance r apart. Then the magnitude of the interaction force is: F = k Q1 Q2 / r2 • k=9 109 Nm2/C2 • Forces act in the line joining the charges • Forces are action and reaction of each other • Positive force is repulsive

  9. Coulomb’s Law II • Let ri be the radius-vector of the charge Qi (i=1, 2). Then the force F21(r2) is: F21(r2) = k Q1Q2(r2-r1)/|r2-r1|3 • k = 1/40 • 0= 8.85 10-12 C2/ Nm2 is the permitivity of vacuum

  10. Comparison with the Force of Gravity • Formally, Coulomb’s Law is Analogous to Newton’s Gravitational Law • But electrostatic force is ~ 1042 (!) times stronger • Such a weak force still dominates the universe because matter is usually neutral • Charging something means to break very slightly the great equilibrium

  11. The Concept of the Field • If a charge is located in some point in the space it sends around an information about its position, polarity and magnitude. The information spreads with the speed of light. It can be “received” by another charge . The interaction of a charge with field produces a force.

  12. Electric Intensity • Electrostatic field could be described by taking some “test” charge and recording the vector of the force F in every point of interest. This description would depend on the test charge. However, we can define electric intensityE=F/q as force per unit charge, which describes the field uniquely.

  13. Electric Field Lines • Electric E(r) field is a “vector field”. In general case it is difficult to visualize it. In simple cases with some symmetry it is possible to use lines which are tangent to E in every point – field lines. A little positive charge would move along a certain field line. • Field lines can’t cross!

  14. Homework 1 • The homework is selected for “problem” sections that are in the end of each chapter. • Due Wednesday ! • 21- 2, 7, 9, 14, 15

  15. Things to read • Giancoli: Chapter 21, Sections 1-8 (ex. 7)

  16. Two electrons 1 m apart They are repelled by electric force but attracted by the force of gravitation. Which force will prevail?

  17. One electron and one proton 0.53 10-10 m apart This corresponds to their distance in hydrogen atom. This could be in principle measured macroscopically! This is the secret why matter holds together.

  18. Let us separate protons and electrons from one gram of H and put each group on each pole of the Earth 1 g is 1 gram-molecule of H, so we have NA=6.02 1023 of both types of particles.

  19. Two 1g iron spheres, 1 m apart are attracted by the force of 10 N. What is their excess charge compared to the total charge? The excess charge: The total charge:

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