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123 What do the following terms mean? Current Resistance Parallel Circuit Series Circuit

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121 Draw a circuit that has 3 capacitors in series to a 10V cell. Draw three capacitors in parallel to a 10V cell.

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Electric Currents AP Physics Chapter 18

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Electric Currents 18.1 The Electric Battery

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The Electric Battery Alessandro Volta – produces the first battery made of zinc, silver, and cloth soaked in salt solution Electric Cell – two plates or rods (electrodes) of dissimilar metal placed a solution (like dilute acid) Battery – several cells connected together 18.1

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The Electric Battery The potential difference (voltage) between the terminals of a cell depend on what the electrodes are made of. Remember half-cell reactions from chemistry When two cells are placed end to end, or in series, their voltages add up 18.1

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Electric Currents 18.2 Electric Current

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Electric Current Circuit – continuous conducting path between terminals of a battery (or other source of EMF) Electric Current – flow of charge (electrons) I – current (amperes) Q – charge (coulomb) T – time 18.2

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Electric Current Ampere (for Andre’ Ampere) Usually called an amp Open Circuit – break in the circuit, no current flow 18.2

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Electric Current Short Circuit – when the load is bypassed Current increase Ground – allows for a continuous path for charge flow 18.2

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Electric Current For historical reasons, current is defined as being in the direction that positive charge flows 18.2

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Electric Currents 18.3 Ohm’s Law

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Ohm’s Law George Simon Ohm The actual values depend on the resistance of the conductor Called Ohm’s Law R – resistance measured in Ohms ( ) 18.3

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Ohm’s Law Only true for Ohmic materials Vacuum Tubes, Transistors, Diodes are nonohmic 18.3

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Ohm’s Law A graph of current vs. potential difference The metallic conductor is ohmic The diode and filament are not 18.3

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S-122 A squirrel is working on a high tension line when he accidentally touches both wires. If the wires have a potential difference of 10,000V and the squirrel experiences a current of 2850A, what is the resistance of the squirrel?

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Ohm’s Law Resistor – anything that uses electric energy Resistor – device used to control current The symbol for a resistor is 18.3

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Ohm’s Law The resistance value of a resistor is indicated by the colored bands on the resistor 18.3

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Ohm’s Law Misconceptions 1.Cells (batteries) do not put out a constant current. They maintain a constant potential difference. 2.Current passes through a wire and depends on the resistance of the wire. Voltage is across the ends of the wire. 3.Current is not a vector, it is always parallel to the conductor. The direction is from + to -. 18.3

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Ohm’s Law Misconceptions 4. Current or charge do not increase or decrease. The amount of charge in one end of the wire comes out of the other end. 18.3

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Electric Currents 18.4 Resistivity

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Resistivity Resistance is found to be directly proportional to its length and inversely proportional to its cross sectional area. is called the resistivity ( m) Longer extension cords must be thicker to keep resistance low 18.4

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Resistivity Some common resistivity values 18.4 MaterialResistivity (Wm) Temperature Coefficient (C o-1 ) Silver1.59x10 -8 0.0061 Copper1.68x10 -8 0.0068 Gold2.44x10 -8 0.0034 Aluminum2.65x10 -8 0.00429 Tungsten5.6x10 -8 0.0045 Platinum10.6x10 -8 0.00651 Nichrome100x10 -8 0.0009

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Electric Currents 18.5 Electric Power

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Electric Power The rate of energy flow for an electric circuit That is more commonly written as Combining with Ohm’s Law it can also be written 18.5

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Electric Power The power company charges by the kilowatt- hour (kWh) Just a cool picture 18.5

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Electric Currents 18.6 Power in Household Circuits

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Power in Household Circuits Household circuits – wires will heat up as current increases In a 20A household circuit In a 15A household circuit Circuits are typically designed to run at 80% of the rated power output Different circuits have different gauge wires (diameter) 18.6

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Power in Household Circuits Circuit Breakers and Fuses Break the circuit 18.6

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Resistivity Best Conductor is Silver, but Copper is close and much cheaper Tungsten is used in filaments Nichrome Apparently an Anime character 18.4

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