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Unit 9 Electrostatics and Circuits. Chapter 15, Chapter 16, Chapter 17, and Chapter 18. Ch. 15: Electric Charge. Like charges repel; opposites attract Electric charge is conserved Electrons are transferred from one atom to another Gains electrons= - ion Loses electrons= + ion

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Unit 9 electrostatics and circuits

Unit 9 Electrostatics and Circuits

Chapter 15, Chapter 16, Chapter 17, and Chapter 18

Ch 15 electric charge
Ch. 15: Electric Charge

  • Like charges repel; opposites attract

  • Electric charge is conserved

    • Electrons are transferred from one

      atom to another

      • Gains electrons= - ion

      • Loses electrons= + ion

  • Robert Millikan’s Oil Drop Experiment gave (on yellow sheet):

Ch 15 electric charge contd
Ch. 15: Electric Charge Contd.

  • Transfer of Charges

    • Conductors: Electric charge moves freely (metals)

    • Insulators: Charges do not move freely (glass, rubber, plastic)

  • Charges by CONTACT

    • Rub insulators together to transfer charges

    • Can do with a metal if held with an insulator (so the charge doesn’t move through the human to Earth)

    • If connected to Earth, it is “grounded” because Earth can accept an unlimited amount of electrons

Ch 15 electric charge contd1
Ch. 15: Electric Charge Contd.

  • Charges by Induction: process of charging a conductor by bringing it near another charged object and grounding the conductor

    • Charged object brought near

    • Conductor’s charged realigned because opposites attract

    • Grounded so the electrons leave to Earth

    • Only positive charges are left on the conductor

Ch 15 electric charge contd2
Ch. 15: Electric Charge Contd.

  • Polarization: the process of TEMPORARILY realigning the charge of an insulator by bringing it near another charged object

Ch 15 coulomb s law
Ch. 15: Coulomb’s Law

  • Charged particles near each other cause an acceleration toward or away from each other because they exert a force on each other.

  • Electric force acts along the line that connects the charges centers.

  • Felectric=kcq1q2/r2

    • kc (Coulomb’s constant)=9.0 x 109 Nm2/C2

    • q is the charge

    • r is the distance

Ch 15 coulomb s law contd
Ch. 15: Coulomb’s Law Contd.

  • Felectric=kcq1q2/r2

  • Relationships:

    • Charges are directly related to force.

      • What happens to the electric force if q1 doubles?

      • What happens to the electric force if q1 and q2double?

    • Force is inversely related to the square of the distance.

      • What happens to the electric force if the charges are closer?

      • If the distance is doubled, what happens to the force?

  • If there are more than 2 charges, the resultant force is the vector sum of individual forces.

Practice pg 504
Practice (pg.504)

  • Consider the following diagram, where q1=6.00 x 10-9 C, q2=-2.00 x 10-9 C, and q3=5.00 x 10-9.

    • Find the components of force exerted by q2 on q3.

    • Find the components of the force exerted by q1 on q3.

    • Find the resultant force on q3, including direction.

Practice pg 503
Practice (pg. 503)

  • Three charges lie along the x-axis. The positive charge q1=15 C is at x=2.0 m, and the positive charge q2=6.0 C is at the origin. Where must a negative charge q3 be placed so that the resultant electric force on it is zero?


  • Pg. 524-525

    • PROBLEMS: 1, 3, 9a, 10

Ch 15 electric field
Ch. 15: Electric Field

  • A region where an electric force on a charge can be detected.

    E==( )(

  • The direction of E (electric field, N/C) is the direction of the electric force that would be exerted on a small positive charge.

    • If +q, E is directed away from q.

    • If –q, E is directed towards q.

Ch 15 electric field contd
Ch. 15: Electric Field Contd.

  • Electric Field lines are used to represent strength and direction of the field.

    • E is stronger when lines are closer together, so the strength of the field increases near the charge.

    • Rules:

      • Lines for positive charge go outward and inward for negative charge.

      • # of lines is proportional to the magnitude of the charge.

      • No two field lines from the same field can cross each other.

Ch 15 electric field contd1
Ch. 15: Electric Field Contd.

Electrostatic Equilibrium (no NET motion of charge)

  • The electric field is zero everywhere inside the conductor.

  • Any excess charge on an isolated conductor resides on the outer surface.

  • The electric field just outside a charged conductor is perpendicular to the conductor’s surface.

  • On an irregularly shaped conductor, the charge tends to accumulate where the radius of the curvature of the surface is smallest (at the sharp points).

Ch 16 electric potential
Ch. 16: Electric Potential

  • Electric Potential Energy (PEelectric) results from the interaction between charges.

    • ME=KE+GPE+PEspring+PEelectric(ME is conserved)

    • PEelectric=-qEd

  • Electric Potential (V) is the amount of work needed to move a particular charge.

    • V=-Ed

      • V is electric potential (Volts, V)

      • E is the electric field (N/C)

      • d is the distance from a reference point (m)

Ch 16 electric potential contd
Ch. 16: Electric Potential Contd.

  • Usually we are interested in the potential DIFFERENCE

    • The difference of potential energies between two positions.

    • V=-Ed=-(=-( )(

  • Batteries Voltage

    • The + terminal has the higher potential.

    • The electrons produced from a chemical reaction (REDOX) collect along the negative terminal.

    • The charges move from the positive to the negative terminal.


A charge moves 2.0 cm in the direction of a uniform electric field of 215 N/C. As the charge moves, the potential energy decreases by 6.9 x 10-19 J. Find the charge on the moving particle. What is the potential difference?


  • Complete the “Electric Field and Potential” Worksheet.

Ch 16 capacitance
Ch. 16: Capacitance

  • Capacitor: a device used to store PEelectric

  • Capacitance: the ability of a conductor to store energy in the form of electrically separated charge


    • C is the capacitance (farad, F)

    • Q is the charge on a plate (C)

    • V is potential difference (V)

Ch 16 capacitance contd
Ch. 16: Capacitance Contd.


  • C is the capacitance (farad, F)

  •  permittivity of free space

  • A area of a capacitor plate(m2)

  • D is the distance between the plates (m)

  • Capacitance depends on the size and shape of the capacitor.

    • C and A are directly proportional

    • C is inversely related to the distance between the plates

  • Ch 16 capacitance contd1
    Ch. 16: Capacitance Contd.

    • Material between the plates can change the capacitance.

      • Insulating material (dielectric) can increase the capacitance by increasing the charge.

    • Discharging a capacitor releases the charge.

    Ch 16 capacitance contd2
    Ch. 16: Capacitance Contd.

    • Energy and Capacitors

      PEelectric= ½ QV


      PEelectric = ½ (CV)V= ½ CV2=1/2 (Q2/V)


    • A capacitor, connected to a 12 V battery, holds 36 C of charge on each plate. What is the capacitance? How much electric potential energy is stored?

    Ch 17 current
    Ch. 17: Current

    • Current: the rate at which charges move through the cross section of a wire

    • I=

      • I is current (Amperes, A)

      • Q is charge (Coulombs, C)

      • T is time (seconds, s)

    Practice pg 569
    Practice (pg. 569)

    • The amount of charge that passes through the filament of a certain lightbulb in 2.00 s is 1.67 C. Find the current in the bulb.


    • The current in a light bulb is 0.235 A. How long does it take for a total charge of 3.67 C to pass through the filament of the bulb?

    Ch 17 resistance
    Ch. 17: Resistance

    • Resistance: the opposition presented to electric current by a material or device

      • Insulators have a high resistance.

      • Conductors have a low resistance.

      • The amount of resistance varies by material.

  • V=IR

    • V is potential difference (volts, V)

    • I is current (amperes, A)

    • R is resistance (ohms, )

  • Ch 17 resistance contd
    Ch. 17: Resistance Contd.

    • Resistance depends on length, area, material, and temperature.

      • Longer wire=higher R

      • Wider wire=lower R

      • Higher temperature=higher R

        • The atoms vibrating make it difficult for an electron to flow through.

    • If resistance increases, current decreases.

      • Inversely proportional

      • Idea used to control currents.

    • Salt water and perspiration lower your resistance.

      • Ions allow electricity to flow easier.

    Practice pg 575
    Practice (pg. 575)

    • All electric devices are required to have identifying plates that specify their electrical characteristics. The plate on a certain steam iron states that the iron carries a current of 6.40 A when connected to a source of 120. V. What is the resistance of the steam iron?


    • The resistance of a steam iron is 19.0 . What is the current when connected to a 120. V source?

    Ch 17 electric power
    Ch. 17: Electric Power

    • Sources of Current

      • Charge move from HIGH PEelectric to LOW PEelectric

      • The potential difference maintains the current.

        • Batteries keep V by converting chemical energy to PEelectric until the chemicals are depleted.

        • Generators convert ME to PEelectric.

    Ch 17 electric power contd
    Ch. 17: Electric Power Contd.

    • Types of Current:

      • Direct Current (DC): charges move in 1 direction

        • Electrons move from low to high.

      • Alternating Current (AC): terminals of V are always changing signs

        • Charge carriers vibrate back and forth

    Ch 17 electric power contd1
    Ch. 17: Electric Power Contd.

    • Electric Power

    • P=IV=I(IR)=I2R=

      • P is power (Watts, W)

      • I is current (A)

      • V is potential difference (V)

      • R is resistance ()


    • An electric space heater is connected across a 120. V outlet. The heater dissipates 1320 W of power in the form of electromagnetic radiation and heat. What is the resistance of the heater?


    • Pg. 564 (#22, 24)

    • Pg. 588 (#10, 11, 31)

    Ch 18 circuits
    Ch. 18: Circuits

    • Schematic Diagrams

    Ch 18 circuits contd
    Ch. 18: Circuits Contd.

    • Short Circuit: when there is little resistance

      • Wire can’t withstand the increase in current

      • Wires overheat

      • Wires may melt or cause a fire.

    • Which of the following will have NO current?

    Ch 18 series circuits
    Ch. 18: Series Circuits

    • Series: 2 or more components of a circuit with a single path for current

      • Because charge is conserved, the current to each resistor is conserved.

      • V=IR=I(R1+R2+R3……)=IReq

        • Reqis the equivalent resistant (sum of individual resistances)

      • If one part is removed (a bulb goes out), then the circuit becomes open.

    Practice pg 595
    Practice (pg. 595)

    • Four resistors are arranged in a series circuit with 2.0 ohms, 4.0 ohms, 5.0 ohms, and 7.0 ohms. Find the equivalent resistance of the circuit. Find the current in the circuit if a 6.0 V battery is used.

    Ch 18 parallel circuits
    Ch. 18: Parallel Circuits

    • Parallel: 2 or more components that provide separate conducting paths for current

      • The same V applies to each resistor.

      • The sum of the currents equals the total current.

      • This type does NOT require all parts to conduct

        • I==++

        • =++

    Practice pg 597
    Practice (pg. 597)

    • Three resistors are connected in parallel (3.0 ohms, 6.0 ohms, and 9.0 ohms) to a 18 V battery.

      • Find the current in each resistor.

      • Calculate the power delivered to each resistor and the total power.

      • Find the equivalent resistance of the circuit.


    • Four resistors are arranged in a parallel circuit with 2.0 ohms, 4.0 ohms, 5.0 ohms, and 7.0 ohms. Find the equivalent resistance of the circuit. Find the current in the circuit if a 6.0 V battery is used.

    Ch 18 complex dc circuits
    Ch. 18: Complex DC Circuits

    • Resistors combined both in parallel and in series are considered COMPLEX.

      • Most circuits today have both.

        • Fuse or circuit breakers are in series to numerous outlets.

        • Outlets are parallel to each other, so appliances operate independently.

      • Safety Features:

        • Circuit breaker opens if the current is too high.

        • Fuse metal strip will melt if the current is too high.

    Challenge problems
    Challenge Problems

    • Pg. 617-618 (Find Req)

      • I will show you how to do #5.

      • You try # 6, 8, 14