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Electric currents Chapter 18

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Electric Battery Made of two or more plates or rods called electrodes. – Electrodes are made of dissimilar metals Electrodes are immersed in an electrolyte – The electrolyte will react with one metal in a way to dissolve positive ions into the solution thus becoming negatively charged. The other metal loses electrons to the electrolyte becoming positively charged and a potential difference (pressure difference) is created. The electrodes in an electrolyte is a cell Cells put together form a battery.

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battery simulation

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Electrical potential and Voltage Electric potential (V)– the potential energy per unit charge – This is caused by the electrical force of repulsion. Voltage- a difference in electrical potential – Considered electrical pressure – This is measured in volts – Flashlight batteries are 1.5V, Household outlets are 110V or 220V

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Electric current Current is the flow of charge through a circuit. I = ΔQ/Δ t – Rate at which charge flows Measured in Amperes – 1A = 1Coulomb/second Electron Current – Negative electron flows from (-) to (+) circuit simulation Conventional current – Positive charge flows from (+) to (-)

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Ohm’s Law/ Resistance For a current (flow of charge) to occur there must be a Voltage (electric pressure) difference. This can be produced by a battery. How much current there is depends on how easily the charge can move through a circuit (resistance). Current, Voltage, and resistance are all related by Ohm’s Law

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RESISTORS Electrical resistance is the opposition of the flow of electricity by some object or substance. Resistors are used to control the flow of electricity in a circuit. Every conductor has some resistance depending on the material. resistance simulation Resistance is measured in Ohms ( ) 1 = 1V/1A The bands on the resistor show how much resistance the resistor has Uses for resistance – Electronic circuits (control flow of electricity) – Toaster, heating element, variable resistors – Lightbulbs

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Ohm’s Law Current, Voltage, and resistance are all related by Ohm’s Law V = I *R If you graph the relationship between voltage and current, the slope represents the resistance. Current is consistent through a circuit but voltage changes.

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Practice Ohm’s Law A small flashlight bulb draws 300mA from its 1.5V battery. – What is the resistance of the bulb? – If the voltage dropped to 1.2V, how would the current change. How many electrons would pass through the bulb in one second?

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Resistivity How resistive a material is – ρ = constant of proportionality/resistivity Measured in Ohms/meter Temperature dependent (α) ρ T = ρ o [1+ α(T-T o )] Measure of the resistance in a wire. Depends on? – Length, Cross-sectional area, resistivity R= ρ(L/A)

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Resistors in series and parallel (19.1) When resistors are connected end to end they are said to be in series – Resistors can be bulbs, heating elements, …etc. – Voltage = I R eq – For a series Circuit: R eq = R 1 +R 2 +R 3 … When resistors are on their own wire they are in parallel. – Voltage = I R eq – For a series Circuit: 1/R eq = 1/R 1 +1/R 2 +1/R 3 …

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Practice Find the equivalent resistance. R = 8 ohms R eq = 4 ohms solution R eq =11.1 ohms

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Electric Power Electric energy is useful because it can be transformed into mechanical energy to do work. To find the power transformed by an electrical device: Power = energy transformed/time Measured in Watts Electric companies charge in kW*hrs

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Practice Your parents yell at you for leaving the lights on in the living room overnight. You want to find out how much you cost them. If there were 2 75Watt bulbs running for 7 hours and the electric company charges $0.09 per kW*hr, how much do you owe them?

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