Electric field

1. Electric field Physics 122 Lecture II

2. Workshops • We are adding new workshop times as we speak. • I’ll send email today to everyone once they are open. Lecture II

3. Website • Both workshop modules and homework problems are from Giancoli • Different, but deal with the same concepts • Workshop problems are a bit easier, helpers for your homework problems • Questions and problems are assigned • http://www.pas.rochester.edu/~regina/PHY122/SCHF05.html Lecture II

4. Lab sign up • Labs start this week! • Remember to do a little homework from lab manual before the first class • Sign up at http://www.pas.rochester.edu/~physlabs/ • In case of problems send email to physlabs@pas.rochester.edu 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. 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

11. 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

12. 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

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

14. 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

15. 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

16. 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

17. 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

18. 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

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

20. 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

21. 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

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

23. 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

24. 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

25. 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

26. 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

27. 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