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Unit 9 Electrostatics and Circuits

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Unit 9 Electrostatics and Circuits

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

- Like charges repel; opposites attract
- Electric charge is conserved
- Electrons are transferred from one
atom to another

- Gains electrons= - ion
- Loses electrons= + ion

- Electrons are transferred from one
- Robert Millikan’s Oil Drop Experiment gave (on yellow sheet):

- 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

- 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

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

- 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

- 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?

- Charges are directly related to force.
- If there are more than 2 charges, the resultant force is the vector sum of individual forces.

- 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.

- 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

- 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.

- 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.

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).

- 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)

- V=-Ed

- 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.

- 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)

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)

- C and A are directly proportional
- C is inversely related to the distance between the plates

- 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.

- Energy and Capacitors
PEelectric= ½ QV

C=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?

- 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)

- 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?

- 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 is potential difference (volts, V)
- I is current (amperes, A)
- R is resistance (ohms, )

- 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.

- 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?

- 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.

- 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

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

- 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)

- Schematic Diagrams

- 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?

- 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.

- 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.

- 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==++
- =++

- 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.

- 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.

- Most circuits today have both.

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