Download Electric Fields and Potential

Electric Fields and Potential
Force Fields

Forces act without contact between objects
 Gravitational field pull mass towards earth
 Electric field surrounds electrically charged
objects
 Explains action at a distance
Electric Field
Proposed by Michael Faraday (1830’s)
 Has both magnitude (strength) and direction
 Magnitude measured by force exerted on
charged particle in the field
 Direction is that of the force on a small
positive “test charge”
 Can be visualized by electric field lines
(lines of force)

Electric Field

Field lines start on positive charges, end on
negative
 Arrows show direction, away from positive,
towards negative
Electric Field: Isolated
Charges
Electric Shielding

Excess charge always located on the surface
of a conductor
 Electric field inside conductor is zero
 Effects of surface charge cancel so no effect
from surface charge inside conductor
 If surface is sphere, excess charge will be
uniformly distributed on surface
Electric Shielding

If shape is irregular, charges are more
concentrated at points & corners
 Electrical components often housed inside
metal boxes to shield from external fields
 Cables often have shielding to reduce stray
signals from surrounding fields
 Cars and airplanes safe from lightning
Electrical Potential Energy

Work must be done to move charged object
into electrical field, like lifting weight in
gravitational field
 Work done increases potential energy of
charged object
 If object is released, potential energy will be
converted to kinetic energy
Electric Potential

If amount of charge is increased, amount of
work must increase to raise potential energy
of object.
 More convenient term is electric potential,
electric potential energy divided by amount
of charge present
 At any point in field, potential is same,
regardless how much charge is present
Electric Potential

Unit of potential is volt (V)
 1 volt = 1 joule/1 coulomb
 Commonly called voltage
 Voltage is independent of amount of charge;
high voltages possible with very little
charge present, therefore little energy
Capacitors

Capacitor is device used to store charge
 Consists of two conducting plates separated
by an insulator
 No electrical contact between plates
 Electrons are pushed onto one plate by
battery which pushes electrons off the other
plate
Capacitors

This stores charge on the plates and energy
in the electric field between the plates
 Capacitor can be discharged, releasing
stored energy
 Commercial capacitors made in many
different forms, very common in circuits