# lecture1 - ELECTROMAGNETISM 1 Electromagnetism a branch of...

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1 ELECTROMAGNETISM 1. Electromagnetism - a branch of physics that deals with the interactions between objects, related to their charge and spin (attributes of the object), and all the phenomena associated with these two attributes. 2. (31.7) Maxwell's equations All electric and magnetic phenomena are based on four equations (axioms of electromagnetism). For a free space the four equations are: r r E A = d Q ε 0 or div r E = ρ ε 0 (Gauss' law) r r B A = d 0 or r B = (Gauss' law) r r E s B = - d d dt Φ or rot t r r E B = - (Faraday's law) r r B s E = + d I d dt μ ε 0 0 Φ or rot t r r r B J E = + μ ε μ 0 0 0 (Ampere-Maxwell law)

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2 3. The attributes of elementary particles Each elementary particle has a set of attributes that affect the interaction of the particle with the rest of the universe. Mass, electric charge and spin determine gravitational and electromagnetic interactions. Mass and electric charge are scalar quantities and spin is a vector quantity. 4. (23.1) The charge of some elementary particles electron: -e = -1.6 10 -19 C proton: e = 1.6 10 C neutron: 0 C 5. (23.1) The charge of a body (system of particles) By adding the charge of all the particles constituting the system of particles (body) we obtain the total charge of the system (body). Bodies are neutral (they have zero charge) when the positive charge is balanced by the negative charge. When we say that a body is charged we mean that it has a slight charge imbalance. nucleus: Ze; where Z is the number of protons in the nucleus.
3 6. (23.1) The law of charge conservation The electric charge of an isolated system is always conserved. (The charge of the universe is constant.) 7. (23.2) Conductivity in substances. a) If the "charged" particles in a system cannot move we say that the system is an insulating medium (insulator). b) If the "charged" particles can move almost freely, we say that the medium is a conductor. In metals, electrons move freely; in electrolytes, the ions move freely. c) In semiconductors (Si, Ge, GaAs, InSb, CdTe ), the charged particles move with some significant restrictions. d) In superconductors the moving "charged" particles are not slowed down by other particles in the system. 8. Force field (Webster's Dictionary) - a region in which a position dependent force is exerted on particles and bodies placed in this region. Force field vector and potential are commonly used to describe the force field quantitatively.

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4 9. (23.4) Electric field vector a) Definition: The electric field vector at a certain position r r is a vector ( 29 r E r r such that the electric force exerted on "a charge" q (a particle with charge q) placed at this position would be r r F E el = q E E F F - + In SI the unit of an electric field vector is N/C. b) Theorem: The net electric field caused by a group of charged objects equals the (vector) sum of the electric fields produced by each object separately: r r E E i i net = Conclusion: For a continuous body = body d E E r r
5 Example: The electric field, at a certain location, produced by a charged particle (“point charge”) depends on the

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lecture1 - ELECTROMAGNETISM 1 Electromagnetism a branch of...

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