PHYA22 Chapter 29.doc

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12 Mar 2012
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Chapter 29: The Electric Potential
Electric Potential Energy
Kinetic Energy (K): the sum of the kinetic energies of all the particles in the
system.
K = ½ mv2
Potential Energy (U): the interaction energy of the system.
U = -Winteraction forces
W = Frcosθ, where r is a linear displacement and θ is between the force and
r.
Conservative Force: the work done as a particle moves from position i (start) to
position f (end) and is independent of the path followed.
Ugrav = -Wgrav = mgyf – mgyi
Uelec = -Welec = qEsf – qEsi
Electric Potential Energy in a Uniform Electric Field: Uelec = U0 + qEs, where s is
measured from the negative plate and U0 is the potential energy at the
negative plate (s = 0).
In a parallel-plate capacitor, the potential energy of a positive charge decreases
in the direction of E but the charge gains kinetic energy as it moves toward the
negative plate.
In a parallel-plate capacitor, the potential energy of a negative charge
decreases in the direction opposite to E and the charge gains kinetic energy as
it moves away from the negative plate.
If a charged particle is projected outward in a uniform field, it gradually slows
(kinetic to potential) until reaching the turning point where Uelec = Emech.
If the motion of a particle is always perpendicular to the electric force, the work
done on the charged particle is zero.
The Potential Energy of Point Charges
Potential Energy of the Two-Point-Charge System: Uelec = Kq1q2/x = 1/(4πε0) x
(q1q2/r), where r is the distance between their centers.
Two like charges shot toward each other will gradually slow down because of
the repulsive force between them, until the distance between them is rmin, the
distance of closest approach.
Two oppositely charged particles shot apart from each other with equal but
opposite momenta will slow down, losing kinetic energy until reaching
maximum separation rmax.
The work is independent of the path; thus, the electric force is a conservative
force.
Escape Velocity: the initial velocity that allows a particle to reach rf = with vf
= 0.
The potential energy of two charged particles approaches zero as r .
Conservation of Energy: Kf + Uf = Ki + Ui
If more than two charges are present, the potential energy is the sum of the
potential energies due to all pairs of charges: Uelec = Σ(Kqiqj/rij), where rij is the
distance between qi and qj.
The Potential Energy of a Dipole
Udipole = -pEcosφ = -pE
Stable Equilibrium: potential energy is minimum at φ = 0° where the dipole is
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