Understanding Electric Fields: Superposition, Field Lines, and Conductors - Prof. Yann R. , Study notes of Physics

Download Understanding Electric Fields: Superposition, Field Lines, and Conductors - Prof. Yann R. and more Study notes Physics in PDF only on Docsity! Your questions/comments 1 “Determining how to distinguish which point would have a higher magnitude when it comes to electric fields.” “I did not fully understand the concept of the Superposition of Electric Fields- The Electric Dipole.” “I would like to have a couple examples of mapping out an electric field based on different positions/combinations of negative and positive charges.” “I'm still not sure how to identify which way the vector will go based on the charges.” “I'm having a difficult time understanding the concept of field lines and how to determine the direction to draw them.” Phys 102 – Lecture 3 The Electric field 2 P Calculation: Electric field in H atom – r = 0.53 x 10–10 m + What is the magnitude of the electric field due to the proton at the position of the electron? What is the direction? Direction is the same as for the force that a + charge would feel at that location This is a large electric field! Inside electrical wire: 10–2 N/C Needed to create a spark in air: 106 N/C Phys. 102, Lecture 3, Slide 5 E  2 p e k qF E q r   9 2 2 19 11 10 2 9 10 N m /C 1.6 10 C N 5.1 10 C(0.53 10 m)          3r 2r Electric field from + and – charges Away from + charge, toward – charge – Phys. 102, Lecture 3, Slide 6 r + M ag ni tu de Di re cti on E  2 k q E r  / 4E  / 9E  Superposition principle Total E-field due to several charges = sum of individual E-fields q1 q2 q3 Ex: what is the E-field at point P due to q1, q2, and q3? Order does not matter! P + – + Same approach as for force Phys. 102, Lecture 3, Slide 7 totE  tot E E   1E  2E  3E  1 2 3tot   E E E E     1E  2E  3E  totE  Plane of charge – – – – – – – – – – – – – – – – – – – – – – – – A large plane of charges creates a uniform electric field (constant magnitude, direction) Approach: Superposition principle & symmetry + + + + + + + + + + + + + + + + + + + + + + + ++Q –Q Phys. 102, Lecture 3, Slide 10 planeE  planeE  ACT: two charged planes Consider two large parallel planes with equal and opposite charge +Q and –Q separated by a small distance If the electric field from one plane is Eplane, what is the magnitude of total electric field at position P above the two parallel planes? P A. 0 B. Eplane/2 C. 2Eplane +Q –Q P E field is uniform between plates, 0 everywhere else! Phys. 102, Lecture 3, Slide 11 DEMO 0E  0E  0E  –Q +Q Calculation: Electron microscope We’ll learn an easier way to solve this in Lect. 4 – A uniform E field generated by parallel plates accelerates electrons in an electron microscope. If an electron starts from rest at the top plate what is its final velocity? d = 1 cm E = 106 N/C Electron microscope Kinematics! (Phys. 101) q, m Phys. 102, Lecture 3, Slide 12 0v v at  21 0 0 2x x v t at   2 2 0 2v v a x   2 2 0 2 qEd v v m   19 6 7 31 1.6 10 10 0.01 2 5.9 10 m s 9.11 10 v          eF qE ma CheckPoint 2.1 Charge A is A. positive B. negativeC. unknown Field lines start on positive charge, end on negative. Phys. 102, Lecture 3, Slide 15 • X • Y A B 83% 10% 7% ACT: CheckPoint 2.2 Compare the charges |QA|& |QB| A. |QA|= |QB|/2 B. |QA|= |QB| C. |QA|= 2|QB| # lines proportional to Q Phys. 102, Lecture 3, Slide 16 • X • Y A B 20% 21% 48% ACT: CheckPoint 2.4 The magnitude of the electric field at point X is greater than at point Y A. True B. False Phys. 102, Lecture 3, Slide 17 • X • Y A B Density of lines  E field magnitude 20% 80% Conductors & electric fields – – – – – ++ + + ++ + Imagine placing a conductor inside a uniform external E field Charges are free to move in a conductor Electrons move due to electric force until they feel no more force (F = 0) + – – – True everywhere inside conductor Phys. 102, Lecture 3, Slide 20 extE  0cond q   F E   0cond E  Conductors & electric fields Another way to look at it: –Q moves to the bottom surface leaving excess +Q on top surface Parallel planes of +Q and –Q create own E field, cancel out external E field Imagine placing a conductor inside a uniform external E field + ++ ++ ++ + – – – – ––– – True for electrostatic equilibrium We’ll see exception to this later in semester Phys. 102, Lecture 3, Slide 21 extE  0cond ext charges  E E E    . 0cond E  Summary of today’s lecture • Electric fields Electric field lines • Superposition principle Dipole, line, plane • Dipoles & electric fields • Conductors & electric fields Phys. 102, Lecture 3, Slide 22 tot E E   . 0cond E 