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Electric Forces and Charges: Understanding Electric Fields and Key Concepts

In this overview, we explore the principles of electric forces and charges as outlined in Chapter 25. Key topics covered include Coulomb's Law, electric field models, and diagrams representing the electric fields produced by point charges. We also present problem sets to analyze electric field strengths, unit vector notation, and evaluation of force exerted on charges. Additionally, information about tutoring and office hours is provided for further assistance in mastering these concepts.

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Electric Forces and Charges: Understanding Electric Fields and Key Concepts

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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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