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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. 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

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  1. Unit 9 Electrostatics and Circuits Chapter 15, Chapter 16, Chapter 17, and Chapter 18

  2. 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):

  3. 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

  4. 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

  5. Ch. 15: Electric Charge Contd. • Polarization: the process of TEMPORARILY realigning the charge of an insulator by bringing it near another charged object

  6. 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

  7. 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.

  8. 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.

  9. 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?

  10. Homework • Pg. 524-525 • PROBLEMS: 1, 3, 9a, 10

  11. 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.

  12. 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.

  13. 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).

  14. 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)

  15. 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.

  16. 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?

  17. Homework • Complete the “Electric Field and Potential” Worksheet.

  18. 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)

  19. 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

  20. 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.

  21. Ch. 16: Capacitance Contd. • Energy and Capacitors PEelectric= ½ QV C=Q/V PEelectric = ½ (CV)V= ½ CV2=1/2 (Q2/V)

  22. 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?

  23. 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)

  24. 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.

  25. 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?

  26. 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, )

  27. 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.

  28. 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?

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

  30. 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.

  31. 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

  32. 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 ()

  33. 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?

  34. Homework • Pg. 564 (#22, 24) • Pg. 588 (#10, 11, 31)

  35. Ch. 18: Circuits • Schematic Diagrams

  36. 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?

  37. 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.

  38. 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.

  39. 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==++ • =++

  40. 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.

  41. 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.

  42. Summary of Circuits

  43. 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.

  44. Challenge Problems • Pg. 617-618 (Find Req) • I will show you how to do #5. • You try # 6, 8, 14

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