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Electrical Energy and
Current
Chapter 17
Electrical Potential Energy
 Results from the interaction of two
objects’ charges
 Electrical potential energy can be
associated with a charge in a uniform
field
 A uniform field is a field that has the
same value and direction at all points
 Assume the charge is displaced at a
constant velocity in the same direction
as the electric field
 There is a change in the electrical
potential energy associated with the
charge’s new position in the electric field
 The change in the Peelectric depends on
the charge, q, the strength of the electric
field, E, and the displacement, d
 Peelectric = -qEd
 The negative sign indicates that the electrical
potential energy will increase if the charge is
negative and decrease if the charge is positive
 It is the difference in electrical potential energy
that is physically important
 The SI unit for electrical potential energy is the
joule (J)
 This equation is valid only for a uniform electric
field, such as that between two oppositely
charged parallel plates
 The electric field of a point charge is an
example of a nonuniform electric field
Potential Difference
 Electric potential is defined as the electrical
potential associated with a charged particle in
an electric field divided by the charge of the
particle
 V = Peelectric / q
 Potential difference is a change in electric
potential
 ∆V = Peelectric / q
 Potential difference is a measure of the
difference in the electrical potential energy
between two positions in space divided by the
charge
 The SI unit for potential difference and for
electric potential is the volt (V) and is equivalent
to one joule per coulomb
 Potential Difference in a Uniform Electric Field
∆V = -Ed
 Remember that d is the displacement parallel to
the field and that motion perpendicular to the
field does not change the electrical potential
energy
 Potential difference between a point at infinity
and a point near a point charge
 ∆V = K (q/r)
 Potential difference = Coulomb constant x value
of the point charge / distance to the point
charge
 The superposition principle can be used to
calculate the electric potential for a group of
charges
 Page 599 (1-3)
Capacitance
 A capacitor is a device that is used to
store electrical potential energy
 Capacitance is defined as the ratio of the
net charge on each plate to the potential
difference created by the separated
charges
 C = Q / ∆V

Q is the magnitude of charge on each plate
 The material between a capacitor’s
plates can change its capacitance
 Discharging a capacitor releases its
charge
 A parallel-plate capacitor is often used in
keyboards

Because the area of the plates and the
distance between the plates can be
controlled, the capacitance, and thus the
electric field strength, can also be easily
controlled
Energy and Capacitors
 A charged capacitor stores electrical
potential energy because it requires
work to move charges through a circuit
to the opposite plates of a capacitor
 The work done on these charges is a
measure of the transfer of energy
Electrical Potential Energy
Stored in a Charged Capacitor
 Peelectric = ½ Q ∆V
 Q = charge on the plate, ∆V = final
potential difference
 By substituting capacitance

Pe = ½ C (∆V)2

Pe = Q2 / 2C
Current
 Current is the rate of charge movement

Exists whenever there is a net movement of
electric charge through a medium
 Electric current is the rate at which charges
move through the cross section of the wire.

The direction of current is opposite the movement
of the negative charges (p. 608)
 I=Q/t


Electric current = charge / time
Unit for current is the ampere (Amp or A)
Resistance
 The opposition to the motion of charge
through a conductor is the conductor’s
resistance
 R = ∆V / I


Resistance = potential difference / current
SI unit is the ohm (Ω)
Ohm’s Law
 For many materials including most
metals the resistance is constant over a
wide range of applied potential
differences
 ∆V / I = constant
 Common practice to express Ohm’s law
as ∆V = IR
Ohm’s Law is not a
fundamental law of nature
 It is a behavior that is valid only for certain
materials
 Materials that have a constant resistance over a
wide range of potential difference are said to be
ohmic
 Materials that do not function according to
Ohm’s law are said to be nonohmic

Ex: diode
 Factors that affect resistance

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