# Ch. 23 Electrostatics. Coulombs Law: F=(k Q1 Q2/r^2) ˆ r Electric ```†
Ch. 23 Electrostatics.
Coulombs Law: F=(k Q1 Q2/r^2) rˆ !
(k = 1 will of course be given on the exam)
4pe0
†
Electric charge is conserved.
Charge of electron is negative, -e
(e will of course be given on the exam)
Electric Field: E=F/q (force per unit charge)! [Units: N/C]
points away from +, towards - charges
E-fields superpose ( =&gt; know how to add vectors, find components)
Terminology: Conductor, insulator, dielectric, dipole...
Ch. 24 Gauss' law
Interpret field line diagrams: (Higher density of field lines
means stronger E field, counting lines to find charges)
Definition and computation of flux, F= &Uacute; E⋅dA
Gauss' Law: &Uacute; E⋅dA = Q(enclosed)/
e0
†
(dA points outward)
†
When/how
does symmetry help?
When/how does superposition help?
E field is zero inside conductors in steady state.!
All excess electric charge sits on surfaces of conductors
At surface of conductor, E is normal, and |E|=s /e0
(Do you know WHY all of the above are true?)
†
Ch 25 Electric potential
Work is F*d*cos(q) [Unit is J = N*m]
V = U/q , Voltage = Potential energy/charge, [V = J/C]
B
DVAB =VB -VA =- &Uacute; E⋅dL
A
Isolated systems conserve energy, PEi+KEi=PEf+KEf.
(Where KE = 1/2 m v^2)
If speed is constant:
Wext = DU = q D V.
Point charge: V= k Q/r.
Use superposition if more than one charge, adds up like a
number. (V is neg. if Q is neg. )
You choose where you want to call PE=0, or V=0.
(Here, it was at infinity)
Constant E means DVAB = -E⋅DL
1 eV = 1.6E-19 J! =unit of energy.
† be given on the exam)
(This will of course
(Equipotential graphs not on this exam)
Point P in empty space is equidistant from 3 charges as shown.
(Note: the 4 &quot;answer&quot; arrows are all 45
degrees from the axes)
What is the direction of the electric
field at point P?
PURPLE:
Some completely
other angle/ not sure.
+y
+Q
+x
GREEN
+2Q
r
YELLOW
r
P
BLUE
r
+Q
PINK
B) If a negative test charge -q is now placed at P, which direction is
the force on that charge?
A point charge Q is at is the origin.
Point P is located at the point (x=3, y=4)
What is the x component of Electric field at point P?
BLUE:
(kQ/25)
PINK:
(kQ/9)
YELLOW: (kQ/25)*(3)
GREEN:
(kQ/25)*(3/5)
PURPLE: None of these
What is the voltage at point P?
BLUE:
kQ/3
PINK:
kQ/5
YELLOW: (kQ/5)*(3/5)
GREEN:
kQ/25
PURPLE: None of these
The diagram shows a positive point charge, Q=1 mC, near a metal
bar.
The electric field in the
vicinity of the point charge
and the bar are shown by the
field lines in the figure.
A) From the figure, what can
you say about the net charge
on the bar?
BLUE:
YELLOW:
PINK:
GREEN:
PURPLE:
Q(bar)= +1 mC
Q(bar) = -1 mC
Q(bar) = 0.25 mC
Q(bar) = -.25 mC
None of these
+
Qbar
Q
Three closed surfaces enclose a point charge. Which surface has the
smallest outward flux through it?
PINK: Small cube
green
yellow
YELLOW: smaller sphere
GREEN: larger sphere
pink
+