Electrostatics and Electricity

ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – + + – + – + – + – + – – negative charge: excess (gain) of electrons positive charge: deficiency (loss) of electrons neutral: electrons equal protons (no net charge )

ELECTRIC CHARGE law of conservation of charge: total charge stays constant (for every + charge produced, there is a – charge produced) – – – + + – –

ELECTRIC CHARGE law of conservation of charge: total charge stays constant (for every + charge produced, there is a – charge produced) – – + + – – –

ELECTRIC CHARGE law of electrostati cs: like charges repel, unlike charges attract

ELECTRIC CHARGE Charge transfer conductor: readily transfers charge (free electrons) insulator: doesn’t transfer charge (electrons in bonds)

ELECTRIC CHARGE Charging by Conduction direct contact same sign permanent charge divides evenly between objects

ELECTRIC CHARGE Charging by Induction no contact opposite sign temporary unless grounded

Electric Charge Charge by Friction The heat generated by rubbing two objects together energizes electrons causing them to transfer.

ELECTRIC CHARGE Conductor that has induced charge by neighboring positive wall. Free electrons move towards the wall. Insulator that has induced charge by neighboring positive wall. Molecules are polarized.

ELECTRIC CHARGE Why does the water bend towards the cup?

ELECTRIC FORCE electric force is a fundamental force of nature: holds atoms together, holds molecules together, causes friction & most forces (except gravity) Amount of charge, q or Q: measured in coulombs, C 1.00 C = 6.25×10 18 electrons charge of one proton or electron, e = ±1.60×10 – 19 C

ELECTRIC FORCE Coulomb’s Law: force between charges depends on amounts of charge and distance between them inverse square law like the force of gravity F e = kq 1 q 2 /r 2 F e : electric forceq: charge r: distance between chargesk: 8.99×10 9 Nm 2 /C 2 +F e : repulsion, –F e : attraction

ELECTRIC CHARGE Grounding: discharging by connecting to a large charge sink (such as earth) Charge Distribution: only on the surface; spreads evenly on spherical conductor; stays put on insulator; concentrates at points Spark Discharge: when charge is large enough, air ionizes and conducts the charge away (lightning)

ELECTRIC FORCE Electric field: region around a charge where it exerts electric force on other charges field lines: show direction & amount of force (by how close the lines are) on a + test charge

ELECTRIC FORCE electric fields exert force on charged objects electric field strength, E: force exerted on a charge by an electric field E = F/q unit: N/C (Newtons/Coulomb), or V/m (Volts/meter)

ELECTRIC FORCE constant electric fields are used to accelerate charged particles field is constant between parallel plates force F = qE change in kinetic energy K-K 0 = Fd d: distance traveled in electric field, K = ½mv 2

CIRCUIT BOARD INTRO

ELECTRIC CIRCUITS Basic Circuit: conductor loop for transferring energy load: energy user (bulb, resistor, heater, motor) source: energy provider (battery, generator)

ELECTRIC CIRCUITS Current, I : rate of “flow” of electric charge. unit: ampere, A I = Q/t 1 A = 1 C/s Charge, Q, is measured in Coulombs. Think of current as the number of electrons that pass by a point each second!

ELECTRIC CIRCUITS Voltage, V: work done per charge between two points, unit: volt, V The voltage is the “push” on the current! Examples: Batteries, Electrical Outlets, Capacitors.

ELECTRIC CIRCUITS Resistance, R: opposition to charge flow, unit: ohm,  resistance limits the flow of current resistance turns electric energy into heat (& light) resistor: fixed resistance, symbol:

ELECTRIC CIRCUITS Ohm’s law: current is proportional to voltage and inversely proportional to resistance: V = IR V: voltage, V I: current, A R: resistance,  Example: How much current is there if the voltage is 6V and the Resistance is 3  ?

ANALYZING CIRCUITS Resistances in Series: I T = I 1 = I 2 = I 3 V T = V 1 +V 2 +V 3 R T = R 1 +R 2 +R 3 adding resistors in series increases R T, decreases I T removing one resistor stops current in the whole circuit

ANALYZING CIRCUITS EXAMPLE CIRCUIT 1 - assume 12 V battery R T =____ V T =____ I T =____ P T =____ R 1 = 8  V 1 =____ I 1 =____ P 1 =____ R 2 = 8  V 2 =____ I 2 =____ P 2 =____

ANALYZING CIRCUITS EXAMPLE CIRCUIT 2 - assume 4 V per cell R T =____ V T =____ I T =____ P T =____ R 1 = 8  V 1 =____ I 1 =____ P 1 =____ R 2 = 16  V 2 =____ I 2 =____ P 2 =____

ANALYZING CIRCUITS Resistances in Parallel: I T = I 1 + I 2 + I 3 V T = V 1 = V 2 = V 3 1/R T = 1/R 1 +1/R 2 +1/R 3 adding resistors in parallel decreases R T, increases I removing one resistor stops current only in that branch

ANALYZING CIRCUITS EXAMPLE CIRCUIT 3 - assume 12 V R T =____ V T =____ I T =____ R 1 = 8  V 1 =____ I 1 =____ R 2 = 8  V 2 =____ I 2 =____

ANALYZING CIRCUITS EXAMPLE CIRCUIT 4 - assume 12 V R T =____ V T =____ I T =____ R 1 = 1  V 1 =____ I 1 =____ R 2 = 2  V 2 =____ I 2 =____

UNIT 7 FORMULAS F e = kq 1 q 2 /r 2 k = 8.99×10 9 Nm 2 /C 2 e = ± 1.60×10 –19 C F = qE K-K 0 = Fd I = Q/t V = W/Q R =  L/A V = I R P = V I = I 2 R E = Pt R T = R 1 +R 2 +R 3 1/R T = 1/R 1 +1/R 2 +1/R kWh = 3.60×10 6 J