Electric field Physics 122

Transcription

1 Electric field Physics 122 9/3/13 Lecture II 1

2 Workshops start next week. The first homework assignment is due next week as well! Workshops 9/3/13 Lecture II 2

3 9/3/13 Lecture II 3

4 Concepts Primary concepts: Electric field Secondary concepts: Field lines Shields 9/3/13 Lecture II 4

5 Laws Superposition principle 9/3/13 Lecture II 5

6 Skills Calculate electric field of a system of charges 9/3/13 Lecture II 6

7 Coulomb s law F12 F F 12 F F = k Q Q 1 r 2 2 9/3/13 Lecture II 7

8 F k = k = Q1Q 2 r 2 System of charges Calculate the net electrostatic force on particle Nm / C Input: Q 1 = - 86µC= C Q 2 = 50µC= C Q 3 = 65µC= C r 13 =60cm=0.60m r 23 =30cm=0.30m What if the value of Q 3 changes? 9/3/13 Lecture II 8

9 F = k F E Q Q 1 r Q E Electric field 2 = 2 1 r F force between two charges(n) Q electric charge (C= Coulomb) F = Q E E 9/3/13 Lecture II 9 = k 1 Q 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

10 Electric field E = F q 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. 9/3/13 Lecture II 10

11 Electric field Electric field is like the ski slope Charge a skier The slope is there whether you ski down or not 9/3/13 Lecture II 11

12 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 9/3/13 Lecture II 12

13 - 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 9/3/13 Lecture II 13

14 Superposition of fields E 1 E = E 1 E 2 Positive test charge E Principle of superposition: Net field created by a system of charges is a vector sum of fields created by individual charges: E = E E 1 E /3/13 Lecture II 14

15 F k = k = Q1Q 2 r 2 System of charges Calculate the net electrostatic force on particle Nm / C Input: Q 1 = - 86µC= C Q 2 = 50µC= C Q 3 = 65µC= C r 13 =60cm=0.60m r 23 =30cm=0.30m What if the value of Q 3 changes? 9/3/13 Lecture II 15

16 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. 9/3/13 Lecture II 16

17 Constant field Electric field is constant between two very large parallel plates, one with a positive charge Q, the other one with a negative charge - Q Q -Q 9/3/13 Lecture II 17

18 Charges in electric fields E F - F Positive charges experience force along the direction of the field. Negative charges against the direction of the field F=qE 9/3/13 Lecture II 18

19 Electric field in conductors E external E E exernal Eint = 0 = ernal 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. 9/3/13 Lecture II 19

20 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? 9/3/13 Lecture II 20 E d E=2.0x10 4 N/C e=1.6x10-19 C m=9.1x10-31 kg d=1.5cm=0.015m v=? v

21 Motion in electric fields a is always to net F Remember v is not always to a (projectile, circular motion) v x v y v x =const v y =at=tf/m=teq/m 9/3/13 Lecture II 21