Electric field Physics 122 Lecture II
Workshops • We are adding new workshop times as we speak. • I’ll send email today to everyone once they are open. Lecture II
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
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
Electric field Lecture II
Concepts • Primary concepts: • Electric field • Secondary concepts: • Field lines • Shields Lecture II
Laws • Superposition principle Lecture II
Skills • Calculate electric field of a system of charges Lecture II
Coulomb’s law + + 1 2 + - 1 2 Lecture II
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
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
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
Electric field • Electric field is like the ski slope • Charge – a skier • The slope is there whether you ski down or not Lecture II
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
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
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
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
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
Charges in electric fields Positive charges experience force along the direction of the field. Negative charges – against the direction of the field. + - Lecture II
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
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
Test problem #5 • Which diagram best represents the electric field lines around two oppositely charged particles? • A - C • B - D Lecture II
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
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
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
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
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
