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Electricity

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**Unit 4: Electricity Chapter 13: Electrical Systems**

13.1 Series Circuits 13.2 Parallel Circuits 13.3 Electrical Power, AC and DC Electricity

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**13.1 Investigation: Series Circuits**

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

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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? Electrical Systems

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

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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.

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**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.

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**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.

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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+R3 and Ohm’s Law I = V ÷ R Solution: Rtot = 6 Ω and I = 12 V ÷ 6 Ω = 2 amps

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**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).

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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.

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**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.

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**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.

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**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 = 3 Ω I = 9 V ÷ 3 Ω = 3 amps

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**Calculating voltage drop**

Solution: - part 2 Use resistance to find current: I = 9 V ÷ 3 Ω = 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

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**Unit 4: Electricity Chapter 13: Electrical Systems**

13.1 Series Circuits 13.2 Parallel Circuits 13.3 Electrical Power, AC and DC Electricity

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**13.2 Investigation: Parallel Circuits**

Key Question: How do parallel circuits work? Objectives: Build parallel circuits. Compare and contrast series and parallel circuits. Discuss applications of parallel circuits.

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

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**Kirchhoff’s Current Law**

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

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**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.

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**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.

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**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.

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**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:

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**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.

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**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.

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**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

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**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

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

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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.

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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.

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**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.

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**Unit 4: Electricity Chapter 13: Electrical Systems**

13.1 Series Circuits 13.2 Parallel Circuits 13.3 Electrical Power, AC and DC Electricity

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**13.3 Investigation: Electrical Energy and Power**

Key Question: How much energy is carried by electricity? Objectives: Build parallel circuits. Compare and contrast series and parallel circuits. Discuss applications of parallel circuits.

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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.

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Important review

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**Electrical Power The watt is an abbreviation for one joule per second.**

A 100-watt light bulb uses 100 joules of energy every second.

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

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**Different forms of the Power Equation**

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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.

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**Looking for:... power of the battery **

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: Use Power: P = V x I Solution: P = (3 A)(12 V) = 36 W

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**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.

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**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.

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**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 = kW 1000 W No. of kWh = .42 kW x 2 h = .84 kWh Cost = .84 kWh x $ 0.15 = $ = about 13¢ for 2 hours kWh

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**Alternating and direct current**

Although the letters “DC” stand for “direct current” the abbreviation “DC” is used to describe both voltage and current. DC flows in one direction as in a battery.

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**Distributing electricity**

Many electronic devices, like cell phones or laptop computers, use DC electricity. An “AC adapter” is a device that changes the AC voltage from the wall outlet into DC voltage for the device.

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AC and DC The electrical system in your house uses alternating current or AC. Alternating current constantly switches direction.

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**Distributing electricity**

Electricity is a valuable form of energy because electrical power can be moved easily over large distances. Alternating current is easier to generate and transmit over long distances.

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120 VAC The 120-volt AC (VAC) electricity used in homes and businesses alternates between peak values of +170 V and – 170 V. This kind of electricity is called 120 VAC because +120V is a type of average positive voltage.

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**Electricity, power and heat**

Wires are made in different sizes to carry different amounts of current. A large diameter wire has less resistance and can safely carry more current than a smaller, thinner wire.

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Electricity in homes Electricity comes into most homes or buildings through a control panel which protect against wires overheating and causing fires.

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**Electricity in homes Each wall socket is connected to three wires.**

One wire, called the “hot” wire, carries 120 volts AC. The neutral wire stays at 0 volts. A third wire is connected to the ground (0 V) near your house.

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Electricity in homes Electrical outlets in bathrooms, kitchens, or outdoors are now required to have ground fault interrupt (GFI) outlets. GFI outlets are excellent protection against electric shocks, especially in wet locations.

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Plugged In Most of the vehicles on US roads today are powered by gasoline. Concerns about global climate change, rising oil prices, and dependence on imported oil have spurred automotive engineers to look for alternatives. Some of the most promising new technologies involve “plugging in”—connecting our vehicles to the electricity grid.

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