Chapter1_Notes_v0

Electric charge can neither be created nor destroyed

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Unformatted text preview: negative charges, the total is q = np e − nn e = (np − nn )e (C) (8) 2. Principle of linear superposition: the total vector electric field at a point in space due to a system of point charges is equal to the vector sum of the electric fields at that point due to the individual charges. • As noted, eq. 7 is valid for free space; what happens if an electron is introduced inside electrically neutral material? • This situation is illustrated in Fig. 5. • Each atom has an electrically positive nucleus and an electron “cloud” surrounding it. Notes based on Fundamentals of Applied Electromagnetics (Ulaby et al) for ECE331, PSU. + + - -+ -+ + -+ -+ -+ -+ -+ -+ - - +- - + + ++- +- + -+ -+ +- + - -+ -+ -+ + - + - - -+ + - -+ -+ - -+ -+ + + q + - - + +- - + +- - -+ +- +- +- + - +- +- - -+ +- +- - +- + + +- 11 +- - - +- + + - +- + -+ Electromagnetics I: Introduction: Waves and Phasors + + Figure 1-6 Figure 5: Polarization of atoms in a dielectric material. • Each atom is electrically neutral. • After introducing positive charge ⇒ different forces cause the atoms to become distorted. • Now we can talk about one end (pole) of atom becoming more positive while the other becomes more negative ⇒ this effect is called polarization and the atom is now a electric dipole . Notes based on Fundamentals of Applied Electromagnetics (Ulaby et al) for ECE331, PSU. Electromagnetics I: Introduction: Waves and Phasors 12 Observe that: • The amount of polarization depends on the distance between the atom and the point charge, • The orientation of the dipole is such that the dipole axis is directed toward the point charge, as illustrated in fig. 5. • The electric dipoles tend to counteract the electric field from the point charge • The electric field inside the material is different than in free space Notes based on Fundamentals of Applied Electromagnetics (Ulaby et al) for ECE331, PSU. Electromagnetics I: Introduction: Waves and Phasors 13 The form of eq. 7 describing electric field is still valid, but we need to change the electrical permittivity so that ˆ E=R r q (V/m) where 4π R2 = r0 (F/m) (9) is dimensionless quantity called relative permittivity or dielectric constant of the material. What are some typical values? See Appendix B. Later on we will use additional quantity electric flux density D = E (C/m2 ). Electric field E and electric flux density D constitute one of the two fundamental pairs of quantities in electromagnetics. Notes based on Fundamentals of Applied Electromagnetics (Ulaby et al) for ECE331, PSU. Electromagnetics I: Introduction: Waves and Phasors 14 • Magnetic fields • Experimental observation of magnetism goes way back in history (Greeks 800 B.C.). • Observation of the magnetic force direction led to realization that magnetic field lines enter magnets at two points: north and south poles, independent of magnet’s shape. • Magnetic field lines for a bar magnet are given in fig. 6. In addition:...
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This note was uploaded on 09/25/2013 for the course ECE 331 taught by Professor Martinsiderious during the Fall '12 term at Portland State.

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