General Physics II. Electric Charge, Forces & Fields

Transcription

1 General Physics II Electric Charge, Forces & Fields

2 Electric Charge Recall that fundamental particles carry something called electric charge protons have exactly one unit of positive charge x Coulombs [C] electrons have exactly one unit of negative charge x Coulombs [C] Electromagnetic force is one of the basic interactions in nature like charges experience repulsive force opposite charges attracted to each other (like gravity)

3 Charge Balance Neutral atoms are made of equal quantities of positive and negative charges Neutral carbon has 6 protons, 6 electrons, (& neutrons) Electrons can be stripped off of atoms Electrons occupy the vulnerable outskirts of atoms Usually charge flows in such a way as to maintain neutrality Excess positive charge attracts excess negative charge Your body has positive charges and negative charges, balanced within millions or billions

4 Charge Separation Can separate charges by rubbing (contact between dissimilar materials): rubber on carpet atmosphere across ground silk on glass balloon on hair Insulators keep charges where they are (no flow = Static Electricity) Conductors distribute charge equally on surface charge is free to move about why do the charges collect on the surface?

5 Polarization This realignment of charge on the surface of an insulator is known as polarization

6 Charging by Conduction Inside a conductor charges (electrons) are free to move

7 Charging By Induction

8 Electrostatic Force (Coulomb s Law) Two charges, q 1 and q 2, separated by distance r exert a force on each other: F k e q 1 r 2 q 2 q 1 r q 2 k e (Coulomb s Constant) = Nm 2 /C 2, q is in Coulombs, r in meters

9 Coulomb Law Illustrated Like charges repel Unlike charges attract If charges are of same magnitude (and same separation), all the forces will be the same magnitude, with different directions...recall Newton s 3 rd Law!

10 Two balls are suspended from the ceiling with nonconducting wire. One ball is given a charge +3q and the other is given a charge +q; this results in the balls having an equal mass. +3q +q g Which of the following best represents their equilibrium positions? +3q +q +3q +q +3q +q (a) (b) (c)

11 What happens when you consider more than two charges? If q 1 were the only other charge, we would know the force on -q due to q 1. If q 2 were the only other charge, we would know the force on -q due to q 2. What is the force on -q when both q 1 and q 2 are present?? The answer (just as in mechanics): F = F 1 + F 2 The total force on the object is just the vector sum of the individual forces -q F 2 F 1 +q 1 F +q 2

12 1D Example What is the NET force (magnitude & direction) on the each of the four cases shown below? charge in

13 Vector Example What is the net force acting on q 2?

14 Or.Like the net force on q 3 instead

15 Electric Field Can think of electric force as establishing field telling particles which way to move and how fast + Electric field lines tell a positive charge which way to move. Where should I go?! + For example, a positive charge itself has field lines pointing away from it, because this is how a positively-charged test-particle would respond if placed in the vicinity (repulsive force).

16 Electric Field Lines for a Negative Point Charge -

17 Electric Field of a Positive Point Charge The picture shows the electric field vector at several points. E F q o k q r 2 ( Units N C ) ** The same formula is used for the electric field strength of a negative charge **

18 Field Lines From Two Opposite Charges = Dipole + -

19 Field Lines From Two Like Charges There is a zero halfway between the two charges + +

20 A negative charge is placed in a region of electric field as shown in the picture. Which way does it move? a) up c) left e) it doesn't move b) down d) right

21 Rules for Field Lines + Graphical trick for visualizing - E fields Electric field lines leave (+) charges and return to (-) charges Number of lines leaving/entering charge are proportional to the amount of charge Field lines never cross Field lines are always perpendicular to the charged surface

22 Examine the electric field lines produced by the charges in this figure. Which statement is true? (a) q 1 and q 2 have the same sign (b) q 1 and q 2 have the opposite signs and q 1 > q 2 (c) q 1 and q 2 have the opposite signs and q 1 < q 2

23 Problem What is the direction of the net electric field at the center of the square? +q -2q -q +2q

24 End of Electric Charge, Forces & Fields Lecture