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Electricity

Electricity. Unit 4: Electricity Chapter 13: Electrical Systems. 13.1 Series Circuits 13.2 Parallel Circuits 13.3 Electrical Power, AC and DC Electricity. Key Question: How can devices be connected in circuits?. 13.1 Investigation: Series Circuits. Objectives:

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Electricity

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  1. Electricity

  2. Unit 4: ElectricityChapter 13: Electrical Systems • 13.1 Series Circuits • 13.2 Parallel Circuits • 13.3 Electrical Power, AC and DC Electricity

  3. Key Question: How can devices be connected in circuits? 13.1 Investigation: Series Circuits Objectives: • Build and analyze series circuits. • Apply an understanding of Ohm’s law to explain their observations. • Describe the effects of short circuits.

  4. Electrical Systems • In in the late 1800s, a major disagreement over the use of AC and DC electricity erupted between two famous inventors. • Thomas Edison favored the direct current (DC) method of moving electrical energy from electrical generation stations to homes and buildings. • George Westinghouse argued that the alternating current (AC) method worked better. • Which system do we use today?

  5. Series Circuits • In a series circuit, current can only take one path, so the current is the same at all points in the circuit.

  6. Series circuits • Inexpensive strings of holiday lights are wired with the bulbs in series. • If you remove one of the bulbs from its socket, the whole string of mini bulbs will go out.

  7. Current and resistance in series circuits • If you know the resistance of each device, you can find the total resistance of the circuit by adding up the resistance of each device.

  8. Current and resistance in series circuits • Adding resistances is like adding pinches to a water hose. • Each pinch adds some resistance. • Everything has some resistance, even thin wire.

  9. Calculating current A series circuit contains a 12-V battery and three bulbs with resistances of 1 W, 2 W, and 3 W. What is the current in amps? • Looking for: …current (amps) • Given: …voltage (12V); resistances = 1Ω, 2 Ω, 3 Ω. • Relationships:Use: Rtot = R1+R2+R3and Ohm’s Law I = V ÷ R • Solution:Rtot= 6Ω and I = 12 V ÷ 6Ω = 2 amps

  10. Energy and series circuits • The devices in a circuit convert electrical energy into other forms of energy. • Remember that the rate of energy transfer is called power, and is measured in watts (W).

  11. Voltage drop • As devices in series use power, the power carried by the current is reduced. • As a result, the voltage is lower after each device that uses power. • This is known as the voltage drop.

  12. Voltage drop and Ohm’s law • The law of conservation of energy also applies to a circuit. • In this circuit, each bulb has a resistance of 1 ohm, so each has a voltage drop of 1 volt when 1 amp flows through the circuit.

  13. Kirchhoff’s Voltage Law • Kirchhoff’s voltage law states that the total of all the voltage drops must add up to the battery’s voltage.

  14. Calculating voltage drops A circuit contains a 9-volt battery, a 1-ohm bulb, and a 2-ohm bulb. Calculate the circuit’s total resistance and current, then find each bulb’s voltage drop. • Looking for:…total resistance; and voltage drop for each bulb • Given:…voltage = 9V; resistances = 1Ω, 2 Ω. • Relationships:Rtot = R1+R2+R3 and Ohm’s Law I = V ÷ R • Solution:- part 1 Rtot= 3Ω I = 9 V ÷ 3Ω = 3 amps

  15. Calculating voltage drop • Solution:- part 2 • Use resistance to find current: I = 9 V ÷ 3Ω = 3 amps • Solution:- part 3 • Rearrange Ohm’s law to solve for voltage. • Use current to find each voltage drop: V = I x R V1 = (3 A) x (1 Ω) = 3 volts V2 = (3 A) x (2 Ω) = 6 volts , so Rtot = (3 + 6 ) = 9 V

  16. Unit 4: ElectricityChapter 13: Electrical Systems • 13.1 Series Circuits • 13.2 Parallel Circuits • 13.3 Electrical Power, AC and DC Electricity

  17. Key Question: How do parallel circuits work? 13.2 Investigation: Parallel Circuits Objectives: • Build parallel circuits. • Compare and contrast series and parallel circuits. • Discuss applications of parallel circuits.

  18. Parallel Circuits • In parallel circuits the current can take more than one path.

  19. Kirchhoff’s Current Law • All of the current entering a branch point must exit again. • This is known as Kirchhoff’s current law.

  20. Voltage and parallel circuits • If the voltage is the same along a wire, then the same voltage appears across each branch of a parallel circuit.

  21. Voltage and parallel circuits • Parallel circuits have two advantages over series circuits: • Each device in the circuit has a voltage drop equal to the full battery voltage. • Each device in the circuit may be turned off independently without stopping the current in the other devices in the circuit.

  22. Resistance in parallel circuits • Adding resistance in parallel provides another path for current, and more current flows. • When more current flows at the same voltage, the total resistance of the circuit must decrease.

  23. Calculating resistance in parallel circuits A circuit contains a 2-ohm resistor and a 4-ohm resistor in parallel. Calculate the total resistance of the circuit. • Looking for:…the resistance • Given:…the type of circuit (parallel) and branch resistances (2  and 4 ) • Relationships:Use: the rule for parallel resistances. • Solution:

  24. Current and parallel circuits • Each branch works independently so the total current in a parallel circuit is the sum of the currents in each branch.

  25. Calculating in current and resistance in a parallel circuit • In a series circuit, adding an extra resistor increases the total resistance of the circuit. • In a parallel circuit, more current flows so the total resistance decreases.

  26. Calculating current and resistance Calculate the total resistance, total current, and current in each branch for the circuit shown. • Looking for:…total resistance, total current, and each branch current. • Given: …resistance of each branch (5,1) and the total voltage (3 V) • Relationships:Use the formula for parallel resistance and Ohm’s law. • Solution:part 1 Rtot = 1/5  + 1/6  = 5/6  = 0.83 

  27. Calculating current and resistance Calculate the total resistance, total current, and current in each branch for the circuit shown. • Solution: part 2 Itot = (3 V) ÷ (0.83 ) = 3.6 A I1= (3 V) ÷ (1 ) = 3.0 A I5= (3 V) ÷ (5 ) = 0.6 A

  28. Parallel vs. Series • Remember: series/same/current; parallel/same/voltage. • Use Ohm’s law for both.

  29. Short circuits • A short circuit is a parallel path in a circuit with very low resistance. • A short circuit can be created accidentally by making a parallel branch with a wire.

  30. Short circuits • Each circuit has its own fuse or circuit breaker that stops the current if it exceeds the safe amount, usually 15 or 20 amps • If you turn on too many appliances in one circuit at the same time, the circuit breaker or fuse cuts off the current. • To restore the current, you must FIRST disconnect some or all of the appliances.

  31. Fuses • In newer homes, flip the tripped circuit breaker. • In older homes you must replace the blown fuse. • Fuses are also used in car electrical systems and in electrical devices such as televisions or in electrical meters used to test circuits.

  32. Unit 4: ElectricityChapter 13: Electrical Systems • 13.1 Series Circuits • 13.2 Parallel Circuits • 13.3 Electrical Power, AC and DC Electricity

  33. Key Question: How much energy is carried by electricity? 13.3 Investigation: Electrical Energy and Power Objectives: • Build parallel circuits. • Compare and contrast series and parallel circuits. • Discuss applications of parallel circuits.

  34. Electrical Power • Electrical power is measured in watts, just like mechanical power. • Power is the rate at which energy is changed into other forms of energy such as heat, sound, or light. • Anything that “uses” electricity is actually converting electrical energy into some other type of energy.

  35. Important review

  36. Electrical Power • The watt is an abbreviation for one joule per second. • A 100-watt light bulb uses 100 joules of energy every second.

  37. Power • Power is a “rate” and is measured using current and voltage.

  38. Different forms of the Power Equation

  39. Kilowatt • Most electrical appliances have a label that lists the power in watts (W) or kilowatts (kW). • The kilowatt is used for large amounts of power.

  40. Calculating power A 12 V battery is connected in series to two identical light bulbs. The current in the circuit is 3 A. Calculate the power output of the battery in watts. • Looking for:... power of the battery • Given:…voltage (12 V); current (3 A) • Relationships:UsePower: P = V x I • Solution:P = (3 A)(12 V) = 36 W

  41. Paying for Electricity • Utility companies charge customers for the number of kilowatt-hours (kWh) used each month. • A kilowatt-hour is a unit of energy. • The number of kilowatt-hours used equals the number of kilowatts multiplied by the number of hours the appliance was turned on.

  42. Paying for Electricity • There are many simple things you can do to use less electricity. • When added up, these simple things can mean many dollars of savings each month.

  43. Calculating cost of power How much does it cost to run a television and a video game console for 2 hours? Use the reference table and a price of $0.15 per kWh. • Looking for: …cost of T.V. + video game for 2 hours • Given: … price = $0.15/kWh, P = 250 W and 170 W; t = 2 h • Relationships:Use: no. of kWh= (price) x (t) and 1 kW = 1,000 W • Solution: 250W + 170W = 420W Convert watts to kW:420 W x 1 kW = .42 kW 1000 W No. of kWh = .42 kW x 2 h = .84 kWh Cost = .84 kWh x $ 0.15 = $0.126 = about 13¢ for 2 hours1 kWh

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