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PHYS1120 Exam II review
Things to remember for PHYS1110: algebra, trig (sin, cos, ..)
vector math, especially vector addition, Fnet = ma problems, free-body diagrams
Exam I Material : E-fields, Gauss's Law
Chapter 23 Voltage = Electric Potential


Definition of voltage: D V є
D Uof q
q
f
= -
v v
т E Чdr
,
D U of q = q D V
i
r
r
dV
 D V = E D x ( if E = constant and E P D x ) , E x = dx
E
hi V
lo V

Voltage is electrical height, E-field is slope

v r
if D r ^ E , D V = 0 , equipotential lines

Voltage due to a point charge Q: V(r) =

Voltage due to many charges: V(r) =
е
i

kQ
r
Vi =
е
i
k Qi
=
ri
т
k dq
r
Work done by external agent to move charge q through a voltage differenceV =
Wext = D U of q = q D V . If charge q falls through a voltage differenceV,
D KE = D PE = q D V

Units of electron volts (eV).
Ch.24 Capacitors
+Q
V = V
E
d
–Q
Q
 capacitance C 
V
10/11/2009 M.Dubson PHYS1120 Notes
©University of Colorado at Boulder
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 C depends only on size and shape: C 
o A
d
area A
d
 Adding capacitors in series and parallel.
 stored energy = U 
 energy per volume = u
1
1
1 Q2
Q V  C V2 
2
2
2 C

U
1

o E 2 .
volume
2
Ch.25, 26 Current and Circuits
 I =
dQ
,
dt
J =
I
= n q vd
A
(where n = #carriers/volume, vd = drift speed)
1
E ,  = resistivity = measure of internal friction, depends on composition
r
I
 Ohm's Law: V  I  R (where R = constant)
lo V
hi V
J = sE =
R
V
 Resistance of a resistor: R =
 power P =
r L
A
area A
L
DW
V2
= I V = I2 R =
Dt
R
 Equivalent Resistance:
R1
in series:
R2
Rtot = R1 + R2
=
R1
R tot =
in parallel:
R2
10/11/2009 M.Dubson PHYS1120 Notes
=
1
1
1
+
R1
R2
©University of Colorado at Boulder
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Things that are always true in circuits:
 elements in series always have the same current:
I
I
I
I
Rbig
R1
R2
V
R3
Ismall
Rsmall
 elements in parallel always have the same voltage:
Ibig
V same
across both R's
 voltage change along an unbroken ideal wire is zero (since
we assume Rwire = 0 ) : Vwire  I R wire  I  0  0
 ideal battery means constant V for any current, current depends on the circuit
 K's Current Law (Junction Law)
total current into junction = total current out of
I2
I1
I1
junction : I1 = I2 + I3
junction
I3
junction
 K's Voltage Law (Loop Law): total voltage change
around any closed loop is zero
 Ammeters and Voltmeters
To prepare for Exam 1:

Review Concept Tests, CAPA problems, and Tutorial HW. (Read question and try to
remember reasoning that gets to the answer)

Prepare your formula sheet.

Take the practice exam.

It is no good to memorize answers. You have to understand and remember how you got
the answers.
10/11/2009 M.Dubson PHYS1120 Notes
©University of Colorado at Boulder