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Download Physics 30 – Unit 2 Forces and Fields – Part 2
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Transcript
Physics 30 – Unit 2 Forces and Fields – Part 2 To accompany Pearson Physics Electric Fields • QuickLab 11.1 Shielding Cellular Phones • Watch electric cable inspector video • Ancient Greeks: “violent” and “natural” forces • Effluvium Theory • Field Theory developed to explain forces at a distance: gravity, electrostatic, and magnetic forces • Quantum Theory necessary to understand “how” these forces work Electric Fields • Because electric force direction will vary depending on the type of charge on the object, it is necessary to define electric field direction • It is defined as the direction of force on a positive test charge • As the diagram shows, field direction is away from the (+) charge and towards the (-) charge Electric Fields • The symbol for electric field is E • The symbol for electric field strength is E • Note the difference between this and the symbol for energy which is a scalar! If you get them confused, the consequences can be disastrous Electric Fields Fe E q where q is the charge of the charge in the electric field (the test charge) not the field source • Review Example 11.1, page 548 and try Practice Problem 1, page 548 Electric Fields • Practice Problem 1, page 548 E Fe q 1.00 10 N / C 3 Fe 1.60 10 19 C Fe 1.00 10 3 N / C 1.60 10 19 C 1.60 10 16 N • This question asked for the magnitude of the field therefore the absolute value signs were used Electric Fields • If q1 represents the field source and q2 the test charge, the electric field due to q1 is k q1 q2 Fe 2 k q1 kq r E 2 commonly written as E 2 q2 q2 r r • Review Example 11.2, page 549 • Try Practice Problem 1, page 549 Electric Fields • Practice Problem 1, page 549 E kq r2 N m2 8.99 10 q 2 C 40.0 N / C 2 0.0200 m 9 40.0 N / C 0.0200 m q 1.78 10 12 C 2 Nm 8.99 10 9 C2 2 • Because the question states that the field is directed away from the charge, the charge must be (+) Electric Fields • Review Example 11.3, page 550 • Try Practice Problem 1, page 550 Electric Fields • Practice Problem 1, page 550 x . 2.10 x 10-2 m • What is the field at point X? • Find the field due to A and the field due to B and add them together • They are both in the same direction – to the left Electric Fields 2 N m 6 8.99 10 9 1.50 10 C 2 kq 7 C E 2 3.06 10 N / C left 2 A r 0.0210 m N m2 6 8.99 10 2.00 10 C 2 kq 6 C E 2 6.17 10 N / C left 2 B r 0.0210 m 0.0330 m 9 E at X 3.06 107 N /C 6.17 106 N /C 3.67 107 N /C left Electric Fields • Do Check and Reflect, page 553, questions 1, 2, 4, 5 and • SNAP p. 81, questions 2, 3, 5, 7-9, 13 Electric Field Lines • Drawing electric field lines: Text gives rules and rationale on page 554 • Light particles sprinkled in oil will line up if an electric field is set up within the oil • Read pages 555 – 559 • At the simplest level the field lines show the path of movement of a (+) charge if placed along a field line Electric Fields • Diagrams of electric fields can be drawn using the principles given • Example: + - • Try the following: a single positive charge, 2 negative charges, a (+)ly charge hollow sphere, a (+)ly charged plate with a (-)ly charged plate • Discuss Electric Potential • Electrical potential energy is similar to gravitational potential energy • Gravitational potential energy is easier to understand • We’ll use a comparison between the two to help you understand electrical Electric Potential Gravitational Electrical work done in lifting an object is stored as gravitational potential energy work done in moving a charge with respect to another charge is stored as electrical potential energy W E p F d W E p F d Reference point → surface of earth Reference point → charges in contact ∆Ep between surface of earth and final point ∆Ep between 2 charges in contact and in the final position ∆Ep for macroscopic objects easy to measure and sensible ∆Ep for sub-microscopic objects easy to measure but not sensible for individual particles No analogous concept for gravitational Electric potential (voltage) defined as V E p q 1 Volt = 1 J/C Electric Potential • Electric Potential Difference (commonly called voltage) is the difference in electric potential between any two points V Vfinal Vinitial • For example the potential difference between the (-) and (+) electrodes of an alkaline dry cell is 1.5 V Electric Potential V E p q can be rewritten as E p qV e- • Basis for a non-SI unit used for energy of subatomic particles • If one electron was accelerated across a potential difference of 1 V it would have: Ep qV 1e 1V 1 eV energy • 1 eV = 1.60 x 10-19 J (page 2 of Data Sheets) 1V Electric Potential • Review Example 11.8, page 566 • Do Practice Problem 2, page 566 Electric Potential • Practice Problem 2, page 566 Ep qV 1e 4.00 104 V 4.00 104 eV 4.00 10 4 eV 1.60 10 19 J / eV 6.40 10 15 J Electric Field between Parallel Plates • kq E 2 r is not valid for electric field between parallel plates • Recall that W E p F d for both electric and gravitational fields • Also recall that Fe q E • Put the 2 together and you get a new formula for electric field between parallel plates: Electric Field between Parallel Plates E p q E d Ep q E d V E d V E d Field between parallel plates is constant everywhere between the plates Electric Field between Parallel Plates • Electric field between parallel plates has units V/m • Earlier you used N/C for electric field units • Your book shows on page 568, that these are really the same units • You should be capable of showing this Electric Field between Parallel Plates • Review Example 11.9, page 568 • Do Practice Problem 2, page 568 Electric Field between Parallel Plates • Practice Problem 2, page 568 V E d V 5.00 10 3 m V 3.00 106 V / m 5.00 10 3 m 1.50 10 4 V 3.00 106 V / m Electric Field between Parallel Plates • Do Check and Reflect, p. 569 • Questions 10a, 11, 12 Conservation of Energy and Electric Charges • Review Example 11.10, page 571 • Do Practice Problem 2, page 571 Conservation of Energy and Electric Charges • Practice Problem 2, page 571 smaller charge initially at rest larger charge q = -2.00 μC m = 1.70 x 10-3 kg E p i Ek i E p f Ek f E p i 0 0 1.70 10 5.20 10 m / s 1 2 E p i 2.30 106 J 3 4 2 Conservation of Energy and Electric Charges • Concept Check, page 572 initial motion perpendicular to plates initial motion parallel to plates or Conservation of Energy and Electric Charges • Review Example 11.11, page 572 • Do Practice Problem 1, page 573 Conservation of Energy and Electric Charges • Practice Problem 1, page 573 E p i Ek i E p f Ek f qV 0 0 21 m v 2 3.20 10 19 C 4.00 10 4 V 0 0 21 6.65 10 27 kg v 2 19 4 3.20 10 C 4.00 10 V 2 12 2 2 v2 3.85 10 m / s 6.65 10 27 kg m2 6 v 3.85 10 1.96 10 m/ s 2 s 12 Conservation of Energy and Electric Charges • Review Example 11.12, page 574 F q E F 2.6 10 12 C 1.7 105 V / m 4.4 10 7 N F 4.4 10 7 N 8 2 a 1.5 10 m / s m 3.0 10 15 kg Note: at this acceleration, it would be travelling at half the speed of light in 1 s! Why is this not possible? d v i t 21 at 2 d 0 1.5 10 m / s 6.0 10 s 1 2 d 2.6 10 3 m 8 2 6 2 Conservation of Energy and Electric Charges • Do Check and Reflect, page 575 • Questions 1, 2, 3, 7 • SNAP p. 90 1, 3, 4, 6, 7, 8, 10, 12, 14, 16 Conservation of Energy and Electric Charges