Chapter 19 Electric Forces and Electric Fields - Historical Overview Properties of Electric Charge Insulators and Conductors Coulomb’s Law - Electric Fields and Field Lines - Motion of Charged Particles in a Uniform Electric Field - Electric Flux - Gauss’ Law and Applications mcdermott.chem.columbia.edu/bioph ys/intro_ima...

-Electrostatics The study of stationary (not moving) Electric Charges. -Electrodynamics The study of moving charges. Where does electric charge come from and why must matter be (almost) electrically neutral? - There are two types of charge and we call them negative and positive - Opposite charges attract, like charges repel! Why? we don’t know. The origin of charge in our daily wanderings is from two elementary particles: the electron and up and down quarks (which combine to form the +1 proton and neutral neutron) q1 q2

Electric Charges: the origin of electricity and magnetism + - Schematic of a Hydrogen atom Nucleus (~ m) with one proton (and zero, 1 or more neutrons) Electron ( < m) Every atom is (1)Composed of a positively charged nucleus of protons (q =+1) and neutrons (q =0) (2)Negatively charged Electrons (-1) “orbit” the nucleus at fixed distances. (3)M proton ~ 1800 M electron (4)Atoms usually have equal numbers of negative and positive charges (Ordinary) matter is composed of lots (10 23 ) of atoms and is almost perfectly neutral! r ~ m

Conservation of Charge Charge can be neither created nor destroyed only transferred from one object to another. Atoms can lose an electron but some other atom must acquire that charge! + A positively charged hydrogen ion = a proton! (and maybe a neutron or 2) Where is my electron! ? -A charge imbalance occurs when electrons are transferred from 1 object to another. -Electrons furthest away from the nucleus are loosely bound and can be “set free” if they are given some energy. -Real materials (like hair, plastic, metals) hold onto their electrons with different strengths.

Conservation of Charge (cont’d) -Preference for holding onto their electrons: Silk > Glass > Rubber > Plastic > Hair -Ease with which they give up their electrons: Hair > Plastic > Rubber > Glass > Silk A Comb transfers electrons from your hair to the comb! Comb becomes negatively charged Hair becomes positively charged! There is an actual difference of integer numbers of electrons, like 123,766,218,196 electrons.

Charging- the transfer of electrons by physical contact or induction Initially uncharged Gains a net charge by touching a charged object (Hollow metal sphere) Initially uncharged Touch the sphere! Net positive charge remains + Direct contact Induction

Charge Polarization The re-arrangement of charges in a material in response to an external Net charge. In conductors, the electrons can move In insulators, the atoms or molecules themselves become polarized! -+ - Negative charge Outside the molecule Rearrangement of charge within The molecule (net charge =0) Some molecules have an intrinsic uneven distribution of charge (like water!) and we say these are “polar” molecules.

Conductors and Insulators Conductor: electrons in outer atomic shells are “free to wander.” Insulator: electrons in outer atomic shells are not “free to wander” All materials can be classified by the ease with which they conduct electric charge Good conductor Insulator Ease with which electrons are Moved about Wood, plastic, rubber, glass Semi-conductors (germanium, silicon) Copper, Iron

Coulomb’s Law charge 1  charge 2 distance 2 Force ~ charge 1 charge 2 d k q 1  q 2 d2d2 Force = Another inverse square law! This one can be attractive or repulsive. This time, however, the proportionality constant k is k = 9,000,000,000 Nm 2 /C!!!

k = 9  10 9 N m 2 /C 2 k q 1  q 2 d2d2 Force = Coulomb’s Law (cont’d) What is a “Coulomb” of charge? 1 electron has a charge of 1.6  C. So 1 C = 6.25 billion billion (6.25  ) electrons worth of charge! -For direct current this is about the amount of charge passing through a 100W bulb in 1s (~100 joules of energy). Electrical force between two stationary charges: Even for highly charged objects the charge imbalance is only 1 part in a trillion. q1q1 q2q2 d

Electric Field: The force per unit charge at some location in space. E = F e /(+q) The direction is defined by the direction of the force a positive “test” unit charge would feel at that point! +q E - Therefore, for a given source of charge, an electric Field is defined everywhere in space. A source of Net charge (-) k q 1  q 2 d2d2 F e =

Electric Field: The force per unit charge at some location in space. ~duffy/PY106/2e.GIF son.nasa.gov/tass/images/electric_fields2.jpg

Electric Field: The force per unit charge at some location in space. Energy is stored in the electric field and energy can be transported by the electric field! (over very long distances). Electric Shielding: Because electric forces can be attractive or repulsive we can shield objects from any external electric field! For any conductor all of its net charge (If any) resides on the surface. The charge is distributed so that the electric field inside the conductor is zero Everywhere! No electric fields allowed in here! Metal wire cage (Faraday cage) -a conducting Surface.