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Announcements

Announcements. Homework for tomorrow… (Ch. 25, CQ 15, Probs. 60 & 67) CQ3: q A =-, q B =0, q c =+, q D =0 CQ11: finger is polarized, net + charge at tip attracts more ex. 4: a) e - ’s transferred from rod to sphere b) 3.1 x 10 10 e - ’ s.

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Announcements

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  1. Announcements • Homework for tomorrow… (Ch. 25, CQ 15, Probs. 60 & 67) CQ3: qA=-, qB=0, qc=+,qD=0 CQ11: finger is polarized, net + charge at tip attracts more ex. 4: a) e-’s transferred from rod to sphere b) 3.1 x 1010 e-’s. ex. 8: a) No, polarization force is a possibility • Office hours… MW 12:30-1:30 pm TR 9-10 am F 10-11 am • Tutorial Learning Center (TLC) hours: M 8-1 pm, 3-6 pm, W 8-1 pm, 2-6 pm TR 8-6 pm, F 8-2 pm

  2. Chapter 25 Electric Forces & Charges (The Electric Field)

  3. Last time… • Coulomb’s Law • So, how does q1 know that q2 is there? q2 r q1

  4. The Electric Field… Postulate a field model that describes how q’s interact: • Source charges alter the space around them by creating an electric field . • A separate charge in the electric field experiences a force exerted by the field.

  5. Electric Field.. SI Units: Notice: • The magnitude of E is the electric field strength.

  6. The Electric Field model… The field is the agent that exerts an electric force on a charged particle. Notice: • Our equation for E assigns a vector to every point in space. • The electric field vector points in the same direction as the force on a positive charge. • Electric field depends only onthe source charge (it’s independent of the test charge ).

  7. The Electric Field of a Point Charge Consider a source charge q and a test charge q’… The FORCE on q’ due to q is… + q + q’

  8. The Electric Field of a Point Charge Consider a source charge q and a test charge q’… The ELECTRIC FIELD due to q is… + q

  9. The Electric Field Diagram… Q: So, what does the electric field diagram look like for a positive charge? +

  10. The Electric Field Diagram… Rules for drawing an electric field diagram.. • Field exists at all points in space. Field diagram is a representative sample. • The arrow indicates the direction and strength of the electric field at the point to which it is attached. +

  11. The Electric Field Diagram… Q: What does the electric field diagram look like for a negative charge? -

  12. The Electric Field Diagram… Q: What does the electric field diagram look like for a negative charge? A: The electric field of a negative charge? -

  13. Quiz Question 1 Point P is located at r = 1 m away from a -1 μC point charge. Which of the following statements is/are true: • The electric field at P points to the right. • The electric field at P is zero since no charge is located there. • Doublingr will halve the electric field. • I only. • II only. • I and II. • II and III. • None of these. -1 μC P

  14. Unit Vector Notation We need to express “away from q” in mathematical notation… Define the unit vector . • Length: 1 • Direction: from origin, radially outward What about the electric field of a negative charge? +

  15. Exercise 25.27 A -12 nC charge is located at (x, y) = (1.0 cm, 0 cm). What are the electric fields at the positions (x, y) = (5.0 cm, 0 cm),(-5.0 cm, 0 cm), and (0 cm, 5.0 cm)? Write the electric field vector in component form.

  16. Problem 25.58 Two 5.0 g point charges on 1.0-m-long threads repel each other after being charged to +100 nC, as shown in FIGURE P25.58. What is the angle θ?You can assume that θ is a small angle.

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