The Electron Volt

� The electron volt (eV) is defined as the energy that an electron gains when accelerated through a potential difference of 1 Vdifference of 1 V� Electrons in normal atoms have energies of 10’s of eV

� Excited electrons have energies of 1000’s of eV

� High energy gamma rays have energies of millions of eV

� 1 eV = 1.6 x 10-19 J

Chapter 16:Lecture II

We have talked about Electric Potentials.

V = U/q = ∑ irq

k||

-q1

q2

q

Scalar (not a vector)

Adds like numbers.

∑i ir || q3

-q4

−++−=

4

4

3

3

2

2

1

1

r

q

r

q

r

q

r

qkV

F U

∆∆−=

x

UFx

E V

qFE /rr

= qUV /=

∆x

Equipotential Surfaces

� An equipotential surface is a surface on which all points are at the same potentialNo work is required to move a charge � No work is required to move a charge at a constant speed on an equipotential surface

� The electric field at every point on an equipotential surface is perpendicular to the surface

Equipotentials and Electric

Fields Lines – Positive Charge

� The equipotentials for a point charge are a family of spheres centered on spheres centered on the point charge

� The field lines are perpendicular to the electric potential at all points

Equipotentials and Electric Fields Lines – Dipole

� Equipotential lines are shown in blue

� Electric field lines are shown in goldare shown in gold

� The field lines are perpendicular to the equipotential lines at all points

Application – Electrostatic Precipitator

� It is used to remove particulate matter from combustion gases

� Reduces air pollution� Reduces air pollution

� Can eliminate approximately 90% by mass of the ash and dust from smoke

� Recovers metal oxides from the stack

Application – Electrostatic Air Cleaner

� Used in homes to reduce the discomfort of allergy sufferers

� It uses many of the same � It uses many of the same principles as the electrostatic precipitator

Application – Xerographic Copiers

� The process of xerography is used for making photocopies

� Uses photoconductive materials� Uses photoconductive materials

� A photoconductive material is a poor conductor of electricity in the dark but becomes a good electric conductor when exposed to light

The Xerographic Process

Application – Laser Printer

� The steps for producing a document on a laser printer is similar to the steps in the xerographic process� Steps a, c, and d are the same� Steps a, c, and d are the same

� The major difference is the way the image forms on the selenium-coated drum

� A rotating mirror inside the printer causes the beam of the laser to sweep across the selenium-coated drum

� The electrical signals form the desired letter in positive charges on the selenium-coated drum

� Toner is applied and the process continues as in the xerographic process

Capacitance

� A capacitor is a device used in a variety of electric circuits

� The capacitance, C, of a capacitor � The capacitance, C, of a capacitor is defined as the ratio of the magnitude of the charge on either conductor (plate) to the magnitude of the potential difference between the conductors (plates)

Capacitance, cont

�

� Units: Farad (F)

� ∆V is the potential difference across a circuit element or device

VQ

C∆

≡

� Units: Farad (F)

� 1 F = 1 C / V

� A Farad is very large

� Often will see µF or pF

or device

� V represents the actual potential due to a given charge at a given location

Parallel-Plate Capacitor

� The capacitance of a device depends on the geometric arrangement of the conductorsarrangement of the conductors

� For a parallel-plate capacitor whose plates are separated by air:

dA

C oε=

Parallel-Plate Capacitor, Example

� The capacitor consists of two parallel plates

� Each have area A

� They are separated by a distance ddistance d

� The plates carry equal and opposite charges

� When connected to the battery, charge is pulled off one plate and transferred to the other plate

� The transfer stops when ∆Vcap = ∆Vbattery

Electric Field in a Parallel-Plate Capacitor

� The electric field between the plates is uniformNear the center� Near the center

� Nonuniform near the edges

� The field may be taken as constant throughout the region between the plates