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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?
homework
Homework
  • 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.
practice
Practice

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?

homework1
Homework
  • 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=

    • 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

    • 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

C=Q/V

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

practice1
Practice
  • 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.
practice2
Practice
  • 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?
practice3
Practice
  • 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 ()
practice4
Practice
  • 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?
homework2
Homework
  • 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.
practice5
Practice
  • 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
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