Phy 103: Fundamentals of Physics

Chapter 22: Electrostatics

Lecture Notes

Electric charge The fundamental electrical property to which the mutual

attractions or repulsions between electrons or protons (in the atom) is attributed.

Units of charge are called coulombs (C) Two types (called polarity)

Positive (e.g. protons) Negative (e.g. electrons)

Like charges repel Negative-negative Positive-positive

Opposite charges attract Positive-negative

The attraction-repulsion between charges is called electric force

Electric Charge Tidbits Electric charge is “quantized”

Electric charge has a smallest value that occurs in nature a single “packet” of charge

The smallest quantity of electric charge is called a quantum of charge

an electron has a quantum of negative electric charge (-e) a proton has a quantum of positive electric charge (+e)

1 quantum of electric charge, e, has a value of 1.6 x 10-19C Conservation of Electric Charge

Electric charge is neither created nor destroyed The total electric charge before an interaction equals the

total electric charge after the interaction

Electricity Electricity is the movement (or flow) of electric charge Electricity is due to the presence of electric forces acting

on charges (causing them to move) Electricity can be the movement of:

electrons (in metals) ions (in fluids such as water or air)

The rate of flow of electric charge is called electric current

The SI units for electric current are amperes (A) 1 A is the flow of 1 coulomb of charge per second How many electrons is this (per second)?

Newton’s Laws (quick review) What are Forces?

The pushes or pulls on things that cause changes in motion SI Units of force are called newtons (or N) Types of forces:

Contact (involves direct touching/contact) Non-contact (occurs without direct contact, across space)

Newton’s 1st Law: when no net force acts on something, its state of motion will remain unchanged

Newton’s 2st Law: a net force acts on something, its state of motion will change: Fnet = ma

Newton’s 3rd Law: forces always act in action-reaction pairs between 2 objects

Coulomb’s Law & Electric Force Describes the magnitude of electric force between 2 “point”

charges Magnitude of electric force depends on

Magnitude of the charges involved (q1 & q2) Distance between the charges, 1/d2 (actually the distance squared!)

To calculate electric force:

The direction of the electric force depends on the polarity of the charges Opposite charges (attractive) Like charges (repel)

221

d

qqk F

Charles Augustin de Coulomb(1736-1806)

Engineer by education Won prize for his work on the

subject of friction Won prize for using calculus of

variations to solve engineering problems

Published 7 important papers on electricity & magnetism (between 1785-1791), including:– The law of attraction and repulsion– The electric point charges– Magnetic poles– The distribution of electricity on the surface of charged bodies

The Electric Field The ability of a charge to influence other charges in its

vicinity its electric field The SI units for electric field are N/C The electric field is a vector property

E field lines originate at + charges & terminate at - charges The direction of an electric field vector (at a point in

space) is the direction of electric force that would be exerted by on a positive charge at that location

-+

The Electric Field (cont.)

The force (FE) exerted by an electric field on a charge (qo) placed in the field (E) is

FE = qoE The direction of the electric force is:

The same direction as the field (for positive charge) The opposite direction as the field (for negative charge)

Formal definition of electric field: the electric force per unit charge that acts on a test charge at a

point in space or

E = FE/qo

Conductors & Insulators Conductance is a physical property of matter Conductance describes the ability of a substance to conduct

electricity (allow charge to flow through it) Electrically speaking, matter can be classified as

Conductors: allow electrons/charge to flow through them metals, dissolved salts in water

Insulators: do not readily conduct electric charge non-metals, most ceramics, plastics, pure water

Semiconductors: under certain conditions conduct eleetric charge but not under other conditions

metalloids, some ceramics Superconductors: allow electric charge to flow without any

material resistance superconduction occurs at very low temperatures (<100 K) certain ceramics and metal oxides

Charging Charging is the process of

generating excess electric charge localizing regions of charge within a substance

There are 2 types of Charging: Charging by friction and contact

Electrons can be transferred from one material to another by simply touching.

Charging by induction Redistribution of electric charges in and on objects

caused by the electrical influence of a charged object close by but not in contact.

Work & Energy (review) Work is a measure of mechanical output (movement) due to some effort (a

force)Work = (Force Applied)*(Distance Traveled)

Energy is the capacity to do work Energy comes in many forms:

Kinetic energy (energy of motion) Heat (energy that flows from hot to cold) Potential energy (energy due to position)

Gravitational Electrical Nuclear Chemical

The SI Units of work & energy are Joules (J) Other units include calories & kilowatt-hours (kW .hr)

Electric Potential A charge placed in an electric field has an associated

electric potential energy (EPE) EPE is essentially the work required to place the charge at

that location EPE is the energy a charged object possesses by

virtue of its location in an electric field The electric potential energy reflects the amount of

work the electric field can perform on the charge if it is free to move it

Electric potential is a charged object’s EPE divided by its charge or

Electric potential = EPE/chargeor

V = EPE/q Units of electric potential are J/C (or volts)

Alessandro Volta (1745-1827)

Italian physicist & inventor First person to isolate methane Fascinated with electricity at an

early age Pioneered the field of

electrochemistry Constructed the first battery to produce

electricity (called a voltaic pile)

Capacitors an electrical device that stores electric charge and electrical

potential energy in its simplest form a pair of parallel conducting plates

separated by a small distance

Electric charge of opposite sign accumulates on face of each plate, establishing an electric field between the plates

The electric field between the plates has a constant value (magnitude & direction)

+

+

+

+

-

-

-

-E

+q -q