Electric field

1. Electric field Physics 114 Lecture II

2. Introduction • Instructor Prof. Regina Demina • Office B&L 367 • Phone 275-7357 • Email regina@pas.rochester.edu • Office hour Tue 4:30-5:30 pm Lecture II

3. Workshops • Workshops start this week. • The first homework assignment is due next week • Have questions – use office hours Lecture II

4. Lab sign up • Information about the labs is available at http://www.pas.rochester.edu/~physlabs/ • Questions about the labs will get answered most quickly by email to physlabs@pas.rochester.edu • Labs begin Jan 28th for "A" week students and February 4th for "B" week students. You will receive your "A" or "B" week assignment by email during the week of January 21st. If you do not receive your assignment or have concerns about your "A"/"B" week assignment, please email physlabs@pas.rochester.edu • A pre-lab homework assignment is to be handed in at the beginning of each lab. The manuals, which include the pre-lab homework, are available on the lab web site. Bring a print-out of each lab manual to lab. There will be some changes in grading policy this term. You will receive a handout detailing the new procedures at your first lab. Lecture II

5. Electric field Lecture II

6. Concepts • Primary concepts: • Electric field • Secondary concepts: • Field lines • Shields Lecture II

7. Laws • Superposition principle Lecture II

8. Skills • Calculate electric field of a system of charges Lecture II

9. Coulomb’s law + + 1 2 + - 1 2 Lecture II

10. Test problem #1 • Two point charges are separated by 10meters. If the distance between them is reduced to 5.0meters, the magnitude of the electrostatic force exerted on each charge • A decreases to half its original value. • B increases to twice its original value. • C decreases to one quarter of its original value. • D increases to four times its original value. Lecture II

11. Test problem #2 • If the magnitude of the charge on two identical charged bodies is doubled, the electrostatic force between the bodies will be • A doubled. • B halved. • C quadrupled. • D unchanged. Lecture II

12. Test problem #3 • If the charge of one point charge is doubled and that of the other charge stays the same, the force will be • A doubled. • B halved. • C quadrupled. • D unchanged. Lecture II

13. System of charges Calculate the net electrostatic force on particle 3. Input: • Q1= - 86mC= - 86.10-6 C • Q2= + 50mC= + 50.10-6 C • Q3= + 65mC= + 65.10-6 C • r13=60cm=0.60m • r23=30cm=0.30m • What if the value of Q3 changes? Lecture II

14. F – force between two charges(N) Q – electric charge (C= Coulomb) Electric field • E – electric field created at point 1 by charge 2 • Charge 2 has changed the property of space at point 1 • Charge 1 is experiencing this change + + 1 2 Lecture II

15. Electric field • E – electric field is the force F exerted on a small positive test charge, divided by the magnitude of this charge q. • Electric field is a vector. • Electric field is measured in N/C. Lecture II

16. Electric field • Electric field is like the ski slope • Charge – a skier • The slope is there whether you ski down or not Lecture II

17. Field lines Field lines at each point are parallel to the electric field at this point. Let’s take a sample (+) charge and examine electric field pattern around a point positive charge. • Like signs repel. • Field lines are going radially away from the positive charge + + + + + + + + + Lecture II

18. Field lines Field lines at each point are parallel to the electric field at this point. Let’s take a sample (+) charge and examine electric field pattern around a point negative charge. • Unlike signs attract. • Field lines are going radially toward the negative charge + + + - + + + + + Lecture II

19. Superposition of fields Principle of superposition: Net field created by a system of charges is a vector sum of fields created by individual charges: Positive test charge + + - 1 2 Lecture II

20. System of charges Calculate the net electrostatic force on particle 3. Input: • Q1= - 86mC= - 86.10-6 C • Q2= + 50mC= + 50.10-6 C • Q3= + 65mC= + 65.10-6 C • r13=60cm=0.60m • r23=30cm=0.30m • What if the value of Q3 changes? Lecture II

21. Rules of field lines Field lines • Are parallel to the electric field • Point away from positive charges • Point toward negative charges • Never cross • Can be open ( go to infinity) • Their density represents the intensity of the electric field. Lecture II

22. Constant field -Q +Q • Electric field is constant between two very large parallel plates, • one with a positive charge +Q, • the other one with a negative charge - Q Lecture II

23. Charges in electric fields Positive charges experience force along the direction of the field. Negative charges – against the direction of the field F=qE + - Lecture II

24. Electric field in conductors Conductor – a material with abundant free (to move) charge E=0 in good conductors in the static situation. E is perpendicular to the surface of conductor. Metal hollow boxes are used to shield electric fields. + + + + + - - - - - Lecture II

25. Test problem #4 • Points A, B and C are at various distances from a given point charge.Which statement is most accurate? The electric field strength is • A Greatest at point A. • B Greatest at point B. • C Greatest at point C. • D The same at all three points. Lecture II

26. Test problem #5 • Which diagram best represents the electric field lines around two oppositely charged particles? • A - C • B - D Lecture II

27. Motion in electric field • Remember F=ma ? • This tells about the effect the force will have on an object with mass m • In electric field F=qE • This tells you about the amount of force electric field exerts on the object • Answer the question: what is the origin of forces? E v d E=2.0x104N/C e=1.6x10-19C m=9.1x10-31kg d=1.5cm=0.015m v=? Lecture II

28. a is always || to net F Remember v is not always || to a (projectile, circular motion) Motion in electric fields - - - - vx vy + + + + vx=const vy=at=tF/m=tEq/m Lecture II