Transcript Electric Potential Energy and Potential Difference

Electric Potential
Difference
Electric Potential Energy (PE)

Potential energy associated with a charged object due to
its position relative to a source of electric force.


Changing the position of the charge in the electric field changes
its PE.
A larger test charge has a greater PE
Electric Potential Difference (∆V)

The work done moving a charged particle
divided by the charge of the particle.
 As
the value of a charge in a field increases,
the value of PE also increases. Electric
potential difference is independent of charge
at a given point.
∆V = Won q / q
 Units = J/C = Volts (V)
Electric Potential Difference (∆V)

The sign of the charge and the direction of the
field determines if ∆V is positive or negative.
 ∆V
is negative if the charge is moved in the same
direction as the net force (negative work done).
 ∆V is positive if the charge is moved opposite the
direction of the net force (positive work done).
 ∆V is zero if the charge is moved perpendicular to
field lines (no work done). These points are called
equipotentials.
Some Points to Remember:



Electric potential difference is also called
potential difference or voltage.
The potential difference between two points can
be measured using a voltmeter.
The zero point for potential can be arbitrarily
assigned.
 Points
that are grounded are usually assigned a
potential of zero.
Electric Potential Difference in a
Uniform Field
E
A
+q
B
d
V = (Vb – Va)
= Won q / q
= (Fd) / q
F=Eq
= (Eq)d /q
V = Ed
 d = displacement parallel to
the field lines
Electric Potential Difference in a
Uniform Field


Charges that move
parallel to the field lines
experience changes in
potential.
Charges that move
perpendicular to the field
lines do not experience
changes in potential.

NOTE: A potential
difference exists between
points in a field even if
there is no charge at those
points.
A charge moves 2.0 m parallel to the direction
of a uniform electric field with a field strength of
1.0 x 103 N/C. What potential difference does
the charge move through?
Given:
E = 1.0 x 103 N/C
d = 2.0 m
Find: ΔV = ?
V = Ed
= (1.0 x 103 N/C)(2.0 m)
= 2.0 x 103 V
Robert A. Millikan’s Oil Drop
Experiment (1909)


Millikan found that
charge always
occurred in multiples
of 1.60 x 10-19 C (the
elementary charge)
He concluded that
charge is quantized
Capacitor

A device that stores electric energy and electric
charge.
 Made
of 2 conducting plates separated by some
distance, each with equal but opposite charge.
 Insulating material is often placed between the plates.
Capacitance (C)

The ability of a capacitor to store energy.
It is the ratio of the amount of charge
stored on each plate to the potential
difference between the plates.
C = q / ∆V
 Units
= farads (F)
1F=1C/V
 Since farads are large, microfarads (F) or
picofarads (pF) are used. (1 pF = 10-12 F)
Some Uses for Capacitors
In a defibrillator, a 10. F capacitor is connected
to a potential difference of 6000. V. What is the
charge stored in the capacitor?
Given:
C = 10. F = 10. x 10 -6 F
∆V = 6000. V
Find: q =?
C = q /∆V
C(∆V) = q
= (10. x 10 -6 F )(6000. V)
= 0.060 C