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Lecture One Resistance, Ohm ’ s Law series and parallel

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Resistance The property of an electrical circuit that opposes the flow of charges (electric current) The SI unit for resistance (R) is the Ohm ( )

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What determines resistance? Conductivity – how well it conducts or what it is made out of Length – long wires have more resistance Cross-section area – (thickness) Thick wires are less resistant than thin wires Temperature – for most wires increased temperature means increased resistance.

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Ohm ’ s Law Current is directly proportional to the voltage across the circuit an inversely proportional to the resistance of the circuit. Current = voltage/resistance I = V/R

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Problems for Practice What is the resistance of an electric frying pan that draws 12 amperes of current when connected to a 120 volt circuit? How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 30 volts is impressed across it?

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Ohm ’ s Law and Simple Series Circuit Current has only one way to go through the resistors. The current through each resistor is the same. To get the total (or equivalent) resistance, add up the resistance of all the resistors. The current of the circuit is equal to the supply voltage divided by the total resistance.

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+ - VR2R2 R3R3 R1R1 Parallel Circuit

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Ohm ’ s Law and Simple Parallel Circuit The voltage is the same across each resistor. The current in each branch is inversely related to the resistance. The total current in the circuit equals the sum of the currents in the branches. As the number of parallel branches increases, the overall resistance decreases. 1/R equivalent = 1/R 1 + 1/R 2 + 1/R 3 + 1/R 4 + ….

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Lecture two!! Complex circuits

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R2R2 R3R3 R4R4 + - V R1R1 Complex Circuit

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Simplifying Complex Circuits If two or more resistors are in series, combine them in an equivalent resistance. If two or more resistors are in parallel, combine them in an equivalent resistance. Repeat steps 1 and 2 until the circuit has been simplified into a single resistance. Determine the current of the simplest circuit.

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AC/DC Direct Current and Alternating Current

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Direct Current Charge always flows in one direction. Terminals on the battery have the same sign as the charge. Charge moves from repelling negative to attracting positive terminal.

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Alternating Current Household current is alternating (AC) Current cycles between positive and negative 60 cycles per second (60 hertz) 110 volts was chosen because it made light bulbs as bright as gas lamps Voltage is now officially 120 volts

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240 volts Many big appliances like stoves, water heaters, and air conditioners need more electricity. They run on 240 volts 120 volts connects positive lead to ground 240 volts connects +120V to -120V

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Alternating Current Electrons alternate back and forth about a relatively fixed position. Occurs by alternating the polarity of voltage at the source. The frequency in North America is 60 hertz or 60 cycles per second. Typical voltage is 120 V. AC energy can be transmitted great distances without a lot of loss.

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Lecture Three Power

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Electric Power The rate at which electrical energy is transformed into another form: –Mechanical energy –Heat –Light Electrical power = Current x voltage –1 watt = 1 ampere x 1 volt

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Problems and Formulas The Basics for Direct Current Circuits

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Current and Resistance Current = charge/time I = q/ t Potential difference = current x resistance V = IR

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Household current in a circuit cannot generally exceed 15 A for safety reasons. What is the maximum amount of charge that could flow through this circuit in a house during the course of a 24.0-h day?

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What is the resistance of the heating element in a car lock de-icer that contains a 1.5 V battery supplying a current of 0.5-A to the circuit?

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Power The amount of work done in a given time. Power = work/elapsed time P = E/ t = ( qV)/ t = IV

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The lighter in TJ ’ s car has a resistance of 4.0 . How much current does the lighter draw when it is run off the car ’ s 12 V battery? How much power does the lighter use?

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Which has more resistance when plugged into a 120 V line, a 1400 W microwave oven or a 150 W electric can opener?

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