Electrical Energy and
Capacitance
Physics - Chapter 18
Ch. 18-1 (Pages 666-669)
I. Electrical Energy & Electric Force


Electrical potential energy - PE associated
with an object due to position relative to
an electric force.
Results from interactions of 2 objects’
charges
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ΔPE electric  change in electric potential energy
(final minus initial)
Factors determining PE electric
1. Charge
2. Strength of electric field
3. Position in the direction of the field

PE
= -q E d
d = displacement from ref. point
q = charge
E = electric field strength
electric

PE
electric=
Kc (q1q2)
r
r = distance between charges in meters
Kc=8.99 x 109 N(m2/C2)
(pairs of charges)
Reference point assumed to be infinity
18-2 Potential difference
(p.670-675)
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
Electric potential - electric potential energy of a
charged particle in an electric field (divided by its
charge)
Electric potential (V) = electric potential energy
charge of the particle
V = PE electric
q
V is measured in volts 1 volt = 1 J/C
 Potential difference- change in electrical
potential energy (V)
*only changes in electric potential are
significant and important

ΔV = PE electric
q
 ΔV Measured in
volts (V) = 1 J/coulomb

ΔV measures the change in energy per unit
of charge
 A potential difference within a battery causes
charge to move
 ex.  12 Volt car battery
-positive terminal has electric potential of 12 V
-negative terminal has electric potential of 0 V
 positive terminal has a higher electric
potential then the negative terminal


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As 1 coulomb of charge moves from
negative to the positive terminal the
battery does work on the charge to
increase the PE electric of the charge to
12 J of electric potential energy
Each coulomb of charge that leaves the
battery would have 12 J of energy
This energy is given up to the
parts of the device which is run
from the battery
18-3 Capacitance

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
Capacitor - a device used in electrical
circuits to store charge
A storehouse of charge
Stores positive and negative charges
separately

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
Parallel-plate-capacitor-two metal plates
separated by a small distance
Connected to the 2 terminals of a battery
(once charged, battery removed)
Charge is removed from one plate and
accumulates on another


Capacitance- the ability of a conductor to
store energy in the form of electrically
separated charges
 Ratio of net charge on
each plate to
potential
difference

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SI unit is the farad (F) = coulomb/volt
Capacitance depends on the size, shape,
and materials of the capacitor
Larger plates = more storage of energy
Materials between plates can change
capacitance


Charged capacitors release energy rapidly
 Ex. – camera flash, computer keyboard