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CH 7 Electricity

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Section 1: Electric Charge Static electricity is the accumulation of excess electric charges on an object.

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According to the law of conservation of charge, charge can be transferred from object to object, but it cannot be created or destroyed. Remember—opposite charges attract and like charges repel.

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An electric field surrounds every electric charge. The electric field exerts the force that causes other electric charges to move. Any charge that is placed in an electric field will be pushed or pulled by the field. Electric Field

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Conductors and Insulators An excess of electrons can move more easily through conductors. What are the best conductors? –metals An insulator is a material that doesn’t allow electrons to move through it easily.

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Transferring Electric Charge The process of transferring charge by touching or rubbing is called charging by contact. The rearrangement of electrons on a neutral object caused by a nearby charged object is called charging by induction.

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Lightning and Thunder Lightning is a large static discharge— transfer of charge through the air between two objects because of a buildup of static electricity. Thunder is powerful sound waves generated by lightning. It is estimated that Earth is struck by lightning more than 100 times every second. It can cause power outages, injury, loss of life, and fires.

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Section 2: Electric Current Charges flow from high-voltage areas to low-voltage areas. Voltage is like an electrical pressure that pushes charge. A voltage difference causes charges to flow and is measured in volts (V).

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The flow of charges through a wire or any conductor is called electric current. SI unit for current (I)—amperes (A)

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Closed and Open Circuits A circuit is a closed, conducting path. Electric charge flows only when the wire makes a closed loop. When any part of a circuit is disconnected, no current flows through the circuit (called an open circuit).

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Sources of Voltage Difference In order to keep the current moving continuously through a circuit, a device must be used to maintain a voltage difference. One common source of this is a battery. Another source is wall sockets.

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Dry-cell and Wet-cell Batteries Each cell of a battery has an electrolyte (a chemical that conducts charge) and two electrodes (terminals). One electrode provides electrons to the electrolyte. The other electrode takes electrons from the electrolyte. In dry cell batteries, the electrolyte is the paste. The electrodes are the carbon rod and zinc container. In wet cell batteries, the electrolyte is a liquid. The electrodes are the lead dioxide and lead plates.

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Resistance Electric current loses energy as it moves through the filament because the filament resists the flow of electrons. Resistance is the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. SI unit for resistance = ohms (Ω)

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Ohm’s Law Ohm’s Law is related to the flow of charges, or current, in a circuit. Ohm’s Law: current (A) = voltage difference (V) resistance (Ω) If the resistance in a circuit increases, the current decreases. If the voltage difference increases, the current increases.

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Section 3: Electrical Energy Circuits typically include a voltage source, a conductor such as a wire, and one or more devices that use the electrical energy to do work.

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Series Circuit In a series circuit, the current has only one loop to flow through. Series circuits are used in flashlights and some holiday lights.

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Parallel Circuits Parallel circuits contain two or more branches for current to move through. The current splits up to flow through the different branches.

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Fuses and Circuit Breakers Fuses –contain a small piece of metal that melts if the current becomes too high –When it melts, it causes a break in the circuit, stopping the flow of current through the overloaded circuit. Circuit Breaker –contains a piece of metal that bends when it gets hot –The bending causes a switch to flip and open the circuit.

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Electrical Power The rate at which electrical energy is converted to another form of energy is electrical power. power = current x voltage difference P (watts) = I (amperes) x V (volts) SI unit for power: watts (W)

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Calculating Energy energy = power x time E (kWh) = P (kW) x t (h) unit of electrical energy: kilowatt-hour (kWh) One kilowatt is equal to 1000 watts. You can calculate your energy bill, by multiplying the energy used by the cost per kilowatt-hour.

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Example Problems A dryer draws 40 A of current at a voltage difference of 240V. What is the power rating for the dryer? –9600 W A 100 W lightbulb has a power rating of 100 W. How much energy in kWh is used when you leave it on for 5 h? –0.5 kWh Determine the cost of using a 100 W lightbulb for 20 h at a cost of $0.09/kWh. –$0.18

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