LECTURE 13 ELECTRIC FIELDS. Instructor: Kazumi Tolich

Transcription

1 LECTURE 13 ELECTRIC FIELDS Instructor: Kazumi Tolich

2 Lecture 13 2 Reading chapter 19.4 to Electric field Electric field lines

3 Electric field 3 If a charge q " experiences an electric force F at a given location, the electric field E at that location is given by E = F q " The electric field of a point charge q points radially away from a positive charge and towards a negative one. The magnitude of electric field due to a point charge is E = k q r * If there are lots of charges, the total electric field at a point can be obtained by vectorially adding the each electric field vector due to each charge.

4 Quiz: 1 4 Charges q and +4q are placed as shown. At which of the five positions indicated by the lettered dots does the electric field due to q point in the opposite direction from the electric field due to +4q? Choose all that apply. q +4q

5 Quiz: 13-1 answer 5 A and E First consider just q; it has negative charge so the electric field points towards it. Now consider just +4q; it has positive charge so the electric field points away from it. q +4q

6 Quiz: 2 6 Charges q and +4q are placed as shown. At which of the five positions indicated by the lettered dots might net electric field be zero? Choose all that apply. q +4q

7 Quiz: 13-2 answer 7 A For the total electric field to be zero, the electric field from each charge must point in opposite directions. The magnitude of electric field due to a point charge is E =. / +4q is four times q, so to have the same magnitude electric field it must be closer to q than +4q q +4q

8 Quiz: 3 8 A point charge of +4 μc is shown. Tests charges with charges +q " and + 3q " are placed at locations A and B respectively. At which of the locations will the strength of the electric field due to the +4 μc be strongest? A A. Location A +q 0 B. Location B C. The electric field strength is equal at both locations. B +3q 0 +4 μc

9 Quiz: 13-3 answer 9 The electric field strength is equal at both locations. Electric field is only dependent on the source charge that creates the field. A E =. / = 0 1 Since the points A and B are located at equal distances from the source charge, the electric field strength at those points are equal. The force exerted on the +3q " charge is three times larger than that on the +q " since they are located at the same distance from the source charge. Since E = F / 8, E 9 = E :. B +3q 0 +q 0 +4 μc

10 Electric field lines 10 Electric field lines indicate the direction of the electric force on a positive test charge would be. Electric field lines start at positive charges or at infinity, and points radially outward from positive charges. Electric field lines end at negative charges or infinity, and points radially inward towards negative charges. The number of lines leaving or entering a charge is proportional to its charge. Field lines never cross. The density of lines is proportional to the electric field strength.

11 Electric dipoles 11 An electric dipole is a system of two equal and opposite charges separated by some distance. The total charge of a dipole is zero.

12 Example: 1 12 In the diagram, q 1 + q 2 = -2.5 µc. Find q 1, q 2, and q 3.

13 Parallel-plate capacitor 13 A parallel-plate capacitor consists of two conducting plates with equal and opposite charges. In an ideal parallel-plate capacitors, the electric field exists only between the plates, and it is constant.

14 Quiz: 4 14 If nonelectric forces are negligible, a positively charged particle released from rest in an electric field A. moves perpendicular to the field with a constant velocity. B. moves parallel to the field with a constant velocity. C. accelerates in the direction of the field. D. accelerates perpendicularly to the field.

15 Quiz: 13-4 answer/demo: 1 15 accelerates in the direction of the field. When a particle with a charge q and mass m is placed in an external electric field, E, it experiences a force, and therefore it accelerates: a = F = /E = = The directions of the electric field and the force on a positive charge are the same. A charged particle does not accelerate due to its own field. Demo: electrostatic deflection Demonstration of electron beam deflection due to an external electric field

16 Example: 2 16 A negatively charged particle moving with speed v enters a region of uniform electric field E. If the charge q = -1 nc, mass m = kg, speed v = 10 5 m/s, the electric field strength E = N/C, and width of the electric field is w = 0.2 m. When the particle emerges from the other side, what is the velocity of the particle? v E w

17 Microwave cooking 17 A microwave oven produces a rapidly oscillating electric field. Water molecules are permanently polarized. The oscillating field exerts a torque on water molecules in the microwave. Water molecules often form groups of molecules, the energy from the rotation can causes these groups to break apart, and the broken apart molecules have increased thermal energy. The water molecules will then form more groups, releasing their electric potential energy as thermal energy, and the process continues.