Physics 1202: Lecture 3 Today’s Agenda • Announcements: – Lectures posted on: www.phys.uconn.edu/~rcote/ – HW assignments, solutions etc. • Homework #1: Homework #1: – On Masterphysics today: due next Friday On Masterphysics today: due next Friday – Go to masteringphysics.com and register – Course ID: MPCOTE62465 • Labs: Begin in two weeks • No class Monday: Labor Day
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Physics 1202: Lecture 3Today’s Agenda
• Announcements:– Lectures posted on:
www.phys.uconn.edu/~rcote/– HW assignments, solutions etc.
• Homework #1:Homework #1:– On Masterphysics today: due next FridayOn Masterphysics today: due next Friday– Go to masteringphysics.com and register– Course ID: MPCOTE62465
• Labs: Begin in two weeks
• No class Monday: Labor Day
Today’s Topic :• End of Chapter 15
– Review Electric Field of a point charge
– Electric Field of an electric dipole
– Other geometries
– Conductors and insulators
– Moving charges: Use Newton’s law
Review of E for a point charge Q
• The Old Way: Vector Maps
• Lines leave positive charges and return to negative charges
• Number of lines leaving/entering charge = amount of charge
• Tangent of line = direction of E
• Density of lines = magnitude of E
+O
• A New Way: Electric Field Lines
+ chg - chg
+O O
x
y
a
a
+Q
-Q r
Electric Dipole
EE
Symmetry
Ex = ?? Ey = ??
Calculate for a pt along x-axis: (x,0)
What is the Electric Field generated by this charge
arrangement?
Electric Dipole: Field Lines• Lines leave positive chargeand return to negative charge
• Ex(x,0) = 0
What can we observe about E?
• Ex(0,y) = 0
• Field largest in space betweenthe two charges
• We derived (y=a):
... for r >> a,
Field Lines from 2 Like Charges• Note the field lines from 2
like charges are quite different from the field lines of 2 opposite charges (the electric dipole)
• There is a zero halfway between charges
• r>>a: looks like field of point charge (+2q) at origin.
Lecture 3, ACT 1
• Consider a dipole aligned with the y-axis as shown.
– Which of the following statements about Ex(2a,a) is true?
Insulators vs. Conductors • Insulators – wood, rubber, styrofoam, most ceramics, etc.
• Conductors – copper, gold, exotic ceramics, etc.
• Sometimes just called metals
• Insulators – charges cannot move.
– Will usually be evenly spread throughout object
• Conductors – charges free to move.
– on isolated conductors all charges move to surface.
+++++++
-------
E+++++++
-------
E
Conductors vs. Insulators
++++++
------
+++++++
-------
E
+-
+
+-
-
- +
-+
+
-+ +
--
+
-+
+
-
-
- +
- +
- +
- +
- +
- +
- +
- +
- +
- + - +
- +
- +
- + - +- +
+++++++
-------
E
Ein = 0 Ein < E
Hollow conductors
Conductors & Insulators
• How do the charges move in a conductor??
• Hollow conducting sphere
Charge the inside, all of this charge moves to the outside.
• Consider how charge is carried on macroscopic objects. • We will make the simplifying assumption that there are only two kinds of objects in the world:
• Insulators.. In these materials, once they are charged, the charges ARE NOT FREE TO MOVE. Plastics, glass, and other “bad conductors of electricity” are good examples of insulators.
• Conductors.. In these materials, the charges ARE FREE TO MOVE. Metals are good examples of conductors.
Conductors vs. Insulators
+-
+
+-
-
-
+
-
-
- -
++
- +
+-- -
++++++
-
-- -++++++
- +
- + - +- +
++++++- +
- +
- +
- +
++++++- +
- +
- +
- +
- +
- + - +- +
Charges on a Conductor• Why do the charges always move to the surface of
a conductor ? – E = 0 inside a conductor when in equilibrium (electrostatics) !
» Why?
If E 0, then charges would have forces on them and they would move !
• Therefore, the charge on a conductor must only reside on the surface(s) !
Infinite conductingplane
++++++++++++
++++++++++++
Conducting sphere
++
+
++
+
++
• Homework #1 on Mastering Physics– From Chapter 15