Physics 1041
Notes
March 24th
Electrostatic Energy and Capacitors:
• a capacitor: something that stores charge and energy. For example: biological
membranes, consumer electronics, defibrillators
• capacitors take advantage of energy storage capability by storing charges in a
certain location
If 2 charges are present, and we want to consider the electric potential at position 2:
kq 1
V = to solve for voltage
r
kq1 2
W = E =V ×q = 2 r to solve for the work/energy to put the second charge there
If 3 charges are present, and we want to consider the electric potential at position 3:
kq kq
V = 1 + 2 to solve for voltage (the sum of the voltage of the 2 other charges)
r r
kq q kq q
W = E =V ×q = 2 1 3 + 2 3 to solve for the work/energy to put the third charge there
r r
To solve for the entire electric field:
kq1q 2 kq 1 3 kq2q 3
E = r + r + r
• for 2 parallel plates, if charges are added to one plate, equal and opposite charges
will be added to the other
• electric field will be uniform between the two plates, and 0V/m outside of the
plates
• capacitance: ratio of charge stored to potential difference
• when C= capacitance, measured in Farads (F)
• when Q= charge, measured in Coulombs (C)
• when V= voltage between 2 points, measured in volts (V)
• then C = Q
V
Parallel Plate Capacitors:
• when C= parallel plate capacitance (F)
• when ε = the permitivity of free space constant, equal to 8.85 x 10 F/m
O
• when A= the area of a single plate (m )
• when d= the distance between the plates (m) • when k= the dielectric constant, assumed to be equal to 1 when parallel plates are
in a vacuum or there is only air between them
• then C = kεOA
d
• the electric field within a parallel plate capacitor is constant and uniform
• under that situation, the potential difference between the two plates is given by:
ΔV = Ed where E is the electric field strength, and d is the distance between 2
plates
• electric field can be measured by N/m or V/m
Defibrillator:
• sends a charge throughout the body so that energy stored becomes energy active
to reboot heart and reverse fibrillation
• fibrillation is a state in which contraction of ventricles is out of usual rhythm
• it is designed to be used before a flat line heart rate
March 26th
Optics
• assumes that all light acts as a ray, traveling in a straight line
• optical properties by wave fields
• related to microscopes, corrective lenses, human eye
Reflection
• law of reflection: the angle of incident and the angle of reflection should be equal
if light is reflecting in the same medium
• angles are measured from light ray to the normal line, which bisects the light ray
Refraction
• bending of light when changing materials
• when θ1 is the angle of incident
• when θ2 is the angle of refraction
• when n-values are the indices of refraction, which vary with different material
(n=c/v, when c= speed of light in vacuum and v= speed of light in given material)
• then n sinθ = n sinθ , also known as Snell's Law
1 1 2 2
• if moving from a substance of low index of refraction (n) to high index of
refraction, light is

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