* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download + E - Purdue Physics
Survey
Document related concepts
Electron mobility wikipedia , lookup
History of quantum field theory wikipedia , lookup
Magnetic monopole wikipedia , lookup
Anti-gravity wikipedia , lookup
Introduction to gauge theory wikipedia , lookup
Fundamental interaction wikipedia , lookup
Speed of gravity wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Maxwell's equations wikipedia , lookup
Electromagnetism wikipedia , lookup
Field (physics) wikipedia , lookup
Lorentz force wikipedia , lookup
Circular dichroism wikipedia , lookup
Transcript
Question 1 (Chap. 14) A penny carrying a small amount of positive charge Qp exerts an electric force F on a nickel carrying a large amount of positive charge Qn that is a distance d away (Qn > Qp ). Which one of the following is not true? A. The electric force exerted on the penny by the nickel is also equal to F. B. The number of electrons in the penny is less than the number of protons in the penny. C. F=KQpQn/d2 , if d is small compared to the size of the coins. D. F=KQpQn/d2 , if d is large compared to the size of the coins. Question 2 (Chap. 14) Which one of the following is not true? The electric force exerted by an electron on an electron: A. decreases by a factor of 25 if the distance is increased by a factor of 5. B. has the same magnitude as the electric force exerted by a proton on a proton at the same distance. C. has the same direction as the electric force exerted by a proton on a proton at the same distance. D. is much weaker than the gravitational force between them. The Superposition Principle The E of a Uniformly Charged Sphere Can calculate using principle of superposition: Esphere 1 Q rˆ 2 40 r Esphere 0 for r>R (outside) for r<R (inside) The Superposition Principle The electric field of a dipole: Electric dipole: Two equally but oppositely charged point-like objects s -q +q Example of electric dipole: HCl molecule What is the E field of the dipole? Calculating Electric Field Choice of the origin y s -q +q x z Choice of origin: use symmetry 1. E along the x-axis E1, x E , x E , x E1, x E1, x 1 q 4 0 r s 2 2 1 q 4 0 r s 2 2 1 qr 2 qrs qs 2 / 4 qr 2 qrs qs 2 / 4 4 0 1 r s 2 r s 2 2 2qrs 4 0 r s 2 2 r s 2 2 2 Approximation: Far from the Dipole E1, x 1 2 srq 2 2 40 s s r r 2 2 2 2 s s if r>>s, then r r r 2 2 2 E1, x 1 2 sq 40 r 3 E1 r = 1,0,0 1 2sq ,0,0 3 40 r While the electric field of a point charge is proportional to 1/r2, the electric field created by several charges may have a different distance dependence. 2. E along the y-axis E 1 q rˆ 2 40 r s s r 0, y ,0 ,0,0 , y ,0 2 2 r r s s r 0, y ,0 ,0,0 , y ,0 2 2 s y2 2 2 E E 1 q 40 s y 2 s , y ,0 2 2 2 1 q 40 s y2 2 s y2 2 2 s , y ,0 2 2 s y2 2 2 s y2 2 2 2. E along the y-axis E 1 q 40 s y 2 2 s , y ,0 2 2 s y2 2 2 1 E2 E E 40 s y2 2 2 E 1 q 40 s y 2 2 qs q 33 2 2 2 s y 2 s , y ,0 2 2 s y 2 2 2 1s,0,0 1 qs ,0,0 if r>>s, then E2 3 4 0 r at <0,r,0> 3. E along the z-axis E1 E2 1 qs ,0,0 3 40 r 1 2sq ,0,0 3 40 r at <0,r,0> or <0,0,r> Due to the symmetry E along the z-axis must be the same as E along the y-axis! at <r,0,0> Other Locations Example Problem y E=? A dipole is located at the origin, and is composed of particles with charges e and –e, separated by a distance 210-10 m along the xaxis. Calculate the magnitude of the E field at <0,210-8,0> m. sq Since r>>s: 40 r 3 2 10 19 Nm 2 10 m 1.6 10 C 9 E1, x 9 10 3 2 8 C 2 10 m N E1, x 7.2 104 C E1, x 200Å 1 x 2Å Using exact solution: 4 N E1, x 7.1999973 10 C Interaction of a Point Charge and a Dipole Edipole F F F QEdipole Q 1 2qs ,0,0 3 40 d • Direction makes sense? - negative end of dipole is closer, so its net contribution is larger • What is the force exerted on the dipole by the point charge? - Newton’s third law: equal but opposite sign Dipole Moment x: y, z: E1 E2 1 2qs p ,0 ,0,0 ,0 33 40 rr r>>s 1 qs p ,,00,,00 33 40 r The electric field of a dipole is proportional to the Dipole moment: p = qs p qs, direction from –q to +q Dipole moment is a vector pointing from negative to positive charge Dipole in a Uniform Field F qE Forces on +q and –q have the same magnitude but opposite direction Fnet qE qE 0 It would experience a torque about its center of mass. What is the equilibrium position? Electric dipole can be used to measure the direction of electric field. Neutral Matter in the Electric Field A wooden dowel rod is balanced on a sharp needle and placed between a pair of parallel plates connected to an electrostatic generator. When the plates are charged, the dowel rod: A) Could not care less B) Will orient perpendicular to the direction of the E – field C) Will orient parallel to the direction of the E – field D) Will jump out of the area with E-field Polarization of Atoms E + - + Force due to E created by positive charge shifts electron cloud and nucleus in opposite directions: electric dipole. An atom is said to be polarized when its electron cloud has been shifted by the influence of an external charge so that the electron cloud is not centered on the nucleus. Induced Dipole An applied electric field creates induced dipoles! E • it is not a permanent dipole • an induced dipole is created when a neutral object is polarized by an applied electric field Choice of System Multiparticle systems: Split into objects to include into system and objects to be considered as external. To use field concept instead of Coulomb’s law we split the Universe into two parts: • the charges that are the sources of the field • the charge that is affected by that field A Fundamental Rationale • Convenience: know E at some location – know the electric force on any charge. • Can describe the electric properties of matter in terms of electric field – independent of how this field was produced. Example: if E>3106 N/C air becomes conductor • Retardation Nothing can move faster than light c c = 300,000 km/s = 30 cm/ns Coulomb’s law is not completely correct – it does not contain time t nor speed of light c. F 1 q1q2 rˆ 2 40 r E 1 q rˆ 2 40 r v<<c !!!