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ELECTROSTATICS 1. Something called "electric charge" exists on matter. We detect it's presence by attraction or repulsion to other "charge". 2. Two kinds of charge: 1. Positive - which we attribute to a deficit of electrons 2. Negative - which we attribute to an excess of electrons 3. "Electrons" are carriers of negative electric charge 4. Like charges repel; unlike charges attract 5. Charge is conserved in a closed system. The number of electrons always remains the same 6. Conductors permit electrons to flow; Insulators inhibit the flow of electrons ELECTROSTATICS What makes a good conductor? SHELL Sub shell Max # of electrons K s 2 L s 2 p 6 s 2 p 6 d 10 M Nickel Atom ELECTROSTATICS Copper atom 29 Protons 29 Electrons 29 Neutrons ELECTROSTATICS Let's introduce some definitions before we continue: to quantify "electric charge" we label the amount of charge on a body as: q q = quantity of electric charge We can have -q (negative charge) or +q (positive charge) ELECTROSTATICS We further define a basic unit of charge (just as we defined the basic unit of mass as a kilogram) as: the "Coulomb" One Coulomb = 1.0 C = 6.242 x 1018 electrons This means that a SINGLE electron carries a very small charge. Can you figure out how much charge (in "Coulombs") are on a single electron? -____________ C on 1 eThis number is a constant and a very important value. It also represents the charge on the PROTON ( but + ) ELECTROSTATICS Typical current in a lightning bolt is 40,000 Amperes (that’s about 40,000 Coulombs per second) with a voltage of up to 100,000,000 volts. ELECTROSTATICS THE ELECTRIC FIELD Between two charged bodies there is a force, F, of attraction or repulsion: ++++++ + + + +++++ + - A We don't understand why; we can only say this is what happens. We can think of a charged body as changing the nature of the space surrounding it. ELECTROSTATICS The field gets weaker as we move away from the charge…it follows the “inverse square law” just as Gravitation did and just as our recent study of Sound Intensity showed. Why? + +++ + + + A ELECTROSTATICS Direction of the Electric Field Outward (away) from a positive charge + These are called “field arrows” Inward (towards) a negative charge _ ELECTROSTATICS We draw the number of arrows proportional to the charge…more charge, more arrows. Say the charges are in “mCoulombs” (that’s micro-coulombs, or 10-6 Coulombs) 6 12 ? ELECTROSTATICS We draw the number of arrows proportional to the charge…more charge, more arrows. Say the charges are in “mCoulombs” (that’s micro-coulombs, or 10-6 Coulombs) +6 +12 -8 -10 ELECTROSTATICS When charges get near each other, these fields interact For unlike charges, the arrows go from the positive charge to the negative charge: +6 -4 ELECTROSTATICS For like particles the arrows are repelled: -4 -4 The field arrows never cross in either case ELECTROSTATICS Coulomb’s Law quantifies the attractive and repulsive behavior we observe in electrostatics Coulomb used a torsion balance (similar to the one Cavendish used to determine the gravitational constant) k|q1q2| F= r2 + - degrees of twist determines force ELECTROSTATICS F= k|q1q2| r2 Force = Newtons q1 = charge on particle one (in Coulombs) q2 = charge on particle two (in Coulombs) r = distance between particles (in meters) k = 9 x 109 (N·m2)/C2 (Coulomb's constant) ELECTROSTATICS SUPERPOSITION OF ELECTRIC CHARGE (super - pose: to place on top of; to add) The electrostatic force is additive q1 - q2 1.0 m + The force on this charge = force from q1 + force from q3 q3 1.8 m + The force on this charge = force from q1 + force from q2 ELECTROSTATICS q1 q2 q3 -1.3 x 10-6 C +2.3 x 10-6 C +1.8 x 10-6 C - 1.0 m + The force on this charge = force from q1 + force from q3 F = k|q1q2| / (r12)2 + k|q2q3|/ (r23)2 1.8 m + The force on this charge = force from q1 + force from q2 F = k|q1q3| / (r13)2 + k|q2q3|/ (r23)2 The subscripts on "r" refer to the particle distance; that is, r12 represents the distance between particles 1 and 2. ELECTROSTATICS Just as we defined a gravitational field, we define an "electric field" in a similar manner: GmM F 2 r GmM mg 2 r GM g 2 r This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg) ELECTROSTATICS Just as we defined a gravitational field, we define an "electric field" in a similar manner: GmM F 2 r GmM mg 2 r GM g 2 r F k qQ r 2 This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg) ELECTROSTATICS Just as we defined a gravitational field, we define an "electric field" in a similar manner: GmM F 2 r GmM mg 2 r GM g 2 r F k qQ q (?) 2 r k qQ r 2 This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg) ELECTROSTATICS Just as we defined a gravitational field, we define an "electric field" in a similar manner: GmM F 2 r GmM mg 2 r GM g 2 r F k qQ r q( E ) 2 k qQ r 2 This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg) ELECTROSTATICS Just as we defined a gravitational field, we define an "electric field" in a similar manner: GmM F 2 r GmM mg 2 r GM g 2 r F k qQ r q( E ) E 2 k qQ r 2 kQ r 2 This is the gravitational field (Earth = 9.8 m/s2 or 9.8 N/kg) ELECTROSTATICS The general equation for an ELECTRIC FIELD is: E kQ r 2 Newtons N Coulomb C (compare this to the equation for the gravitational field) ELECTROSTATICS Notice that for gravity, F = mg We see that in electrostatics, F = qE GmM F 2 r GmM mg r2 GM g 2 r F k qQ r q( E ) E 2 k qQ kQ r 2 r 2 ELECTROSTATICS Here’s our equations to date. At first we will only deal with POSITIVE charges: Electric Field Force F k qQ r 2 F = |q|E E kQ r 2