Electric field Physics 114 9172020 Lecture II 1

Electric field Physics 114 9/17/2020 Lecture II 1

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 9/17/2020 Lecture II 2

Workshops • Workshops start this week. • The first homework assignment is due next week • Have questions – use office hours 9/17/2020 Lecture II 3

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 28 th for "A" week students and February 4 th for "B" week students. You will receive your "A" or "B" week assignment by email during the week of January 21 st. 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. 9/17/2020 Lecture II 4

9/17/2020 Lecture II 5

Concepts • Primary concepts: – Electric field • Secondary concepts: – Field lines – Shields 9/17/2020 Lecture II 6

Laws • Superposition principle 9/17/2020 Lecture II 7

Skills • Calculate electric field of a system of charges 9/17/2020 Lecture II 8

Coulomb’s law + 1 + 2 + - 1 2 9/17/2020 Lecture II 9

Test problem #1 • Two point charges are separated by 10 meters. If the distance between them is reduced to 5. 0 meters, the magnitude of the electrostatic force exerted on each charge –A –B –C –D 9/17/2020 decreases to half its original value. increases to twice its original value. decreases to one quarter of its original value. increases to four times its original value. Lecture II 10

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 –B –C –D 9/17/2020 doubled. halved. quadrupled. unchanged. Lecture II 11

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 –B –C –D 9/17/2020 doubled. halved. quadrupled. unchanged. Lecture II 12

System of charges Calculate the net electrostatic force on particle 3. Input: – – – Q 1= - 86 m. C= - 86. 10 -6 C Q 2= + 50 m. C= + 50. 10 -6 C Q 3= + 65 m. C= + 65. 10 -6 C r 13=60 cm=0. 60 m r 23=30 cm=0. 30 m • What if the value of Q 3 changes? 9/17/2020 Lecture II 13

Electric field • F – force between two charges(N) • Q – electric charge (C= Coulomb) + 1 9/17/2020 + 2 • 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 Lecture II 14

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. 9/17/2020 Lecture II 15

Electric field • Electric field is like the ski slope • Charge – a skier • The slope is there whether you ski down or not 9/17/2020 Lecture II 16

Field lines + + + + 9/17/2020 + 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 17

Field lines + + + - + + 9/17/2020 + 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 18

Superposition of fields + Positive test charge + - 1 2 9/17/2020 Principle of superposition: Net field created by a system of charges is a vector sum of fields created by individual charges: Lecture II 19

System of charges Calculate the net electrostatic force on particle 3. Input: – – – Q 1= - 86 m. C= - 86. 10 -6 C Q 2= + 50 m. C= + 50. 10 -6 C Q 3= + 65 m. C= + 65. 10 -6 C r 13=60 cm=0. 60 m r 23=30 cm=0. 30 m • What if the value of Q 3 changes? 9/17/2020 Lecture II 20

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. 9/17/2020 Lecture II 21

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 9/17/2020 Lecture II 22

Charges in electric fields + - 9/17/2020 Positive charges experience force along the direction of the field. Negative charges – against the direction of the field F=q. E Lecture II 23

Electric field in conductors + + + - - - 9/17/2020 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 24

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 –B –C –D 9/17/2020 Greatest at point A. Greatest at point B. Greatest at point C. The same at all three points. Lecture II 25

Test problem #5 • Which diagram best represents the electric field lines around two oppositely charged particles? –A -C –B -D 9/17/2020 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=q. E • This tells you about the amount of force electric field exerts on the object • Answer the question: what is the origin of forces? 9/17/2020 Lecture II E v d E=2. 0 x 104 N/C e=1. 6 x 10 -19 C m=9. 1 x 10 -31 kg d=1. 5 cm=0. 015 m v=? 27

Motion in electric fields • a is always || to net F • Remember v is not always || to a (projectile, circular motion) - - vx + + + vy + vx=const vy=at=t. F/m=t. Eq/m 9/17/2020 Lecture II 28