Download Electrical Energy and Capacitance

Electrical Energy and
Capacitance
• Potential difference and electrical potential
• Work and potential energy:
• Potential energy is a scalar quantity with
charge to the negative of the work done by
the conservative force
• ΔPE=Pef-Pei =- Wf
• Coulomb force is conservative
• If imagine a small + charge placed in a
uniform electric field E. As the charge
moves from A to B, the work done on the
charge by the electric field:
• W=FxΔx =q Ex (xf-xi)
• Work –energy theorem
• W=q Ex Δx =ΔKE
• But the work done by a conservative force
can be reinterpreted as the negative of the
charge in a potential energy associated
with that force
• ΔPE of a system consisting on an object of
charge q through a displacement Δx in a
constant electric field E is given by:
• ΔPE =-WAB= -q Ex Δx
• SI unit J (Joule)
• Δ KE + ΔPE el = ΔKE +(0-ΙqΙ E d =0
• ΔKE = ΙqΙ E d
• Similarly , KE equal in magnitude to the
loss of gravitational potential energy:
• ΔKE +ΔPEg =ΔKE +(0 –mgd) =0
• ΔKE=mgd
• Electric Potential
• F = qE
• The electric potential difference between
points A and B is the charge in electric
potential energy as a charge q moves from
A to B, divided by the charge q:
ΔV =VA-VB = ΔPE/q
• SI unit J/C or V (Joule/Coulomb or Volt)
• Electric potential is a scalar quantity
• Electric potential and potential energy due
to point charges
• The electric field of a point charge extends
throughout space, so its electrical potential
also
• Electric potential created by a point
charge: V=ke q/r
• The electric potential of two or more
charges is obtained by applying the
superposition principle: the total electric
potential at some point P due to several
point charges is the algebraic sum of the V
due to the individual charges
• Potentials and charged conductors
• The electric potential at all points on a
charged conductor
• W= -ΔPE =-q( VB-VA)
• No net work is required to move a charge
between two points that are at the same
electric potential
• All points on the surface of a charged
conductor in electrostatic equilibrium are
at the same potential
• The electric potential is a constant
everywhere on the surface of a charged
conductor
• The electric potential is constant
everywhere inside a conductor and equal
to the same value at the surface
• The electron volt is defined as KE that an
electron gains when accelerated through a
potential difference of 1V
• 1eV =1.6x 10-19 C V =1.6x10-19 J
• Equipotential surface is a surface on which all
points are at the same potential
• The electric field at every point of an
equipotential surface is perpendicular to the
surface.
• Capacitance
• A capacitor- is a device used in variety of
electric circuits
• The capacitance C of a capacitor is the
ratio of the magnitude of the charge on
either conductor (plate) to the manitude