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Concept Summary Batesville High School Physics

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Potential Difference Charges can “lose” potential energy by moving from a location at high potential (voltage) to a location at low potential. Charges will continue to move as long as the potential difference (voltage) is maintained.

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Current A sustained flow of electric charge past a point is called an electric current. Specifically, electric current is the rate that electric charge passes a point, so Current = or I = q/t Charge time

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Measuring Current If 1 Coulomb of charge (6.25 x 10 18 electrons) passes a point each second, the current is 1 Ampere. So, 1 Ampere = 1 Coulomb/sec

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Voltage Source A battery or electrical outlet is a source of electric potential or voltage - not charge. The electrons that move in a conductor are supplied by the conductor - not the voltage source. The net charge on a current-carrying conductor is zero.

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Electromotive Force An old-fashioned term for electric potential or voltage is “electromotive force” or “emf”.

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Electrical Resistance Most materials offer some resistance to the flow of electric charges through them. This is called electrical resistance.

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Resistance Resistance of a conductor depends on: Material - Gold is best Length - longer conductors have more resistance. Cross section - thick wires have less resistance than thin wires Temperature - higher temperature means more resistance for most conductors

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Ohm’s Law For many conductors, current depends on: Voltage - more voltage, more current Current is proportional to voltage Resistance - more resistance, less current Current is inversely proportional to resistance

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Ohms’ Law In symbols: V = IR V IR

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Direct Current If the voltage is maintained between two points in a circuit, charge will flow in one direction - from high to low potential. This is called direct current (DC) Battery-powered circuits are dc circuits.

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Alternating Current If the high & low voltage terminals switch locations periodically, the current will flow “back and forth” in the circuit. This is called alternating current (AC). Circuits powered by electrical outlets are AC circuits.

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AC in the US In the US, current changes direction 120 times per second, for a frequency of 60 cycles per second or 60 Hertz. Normal outlet voltage in the US is 110- 120 volts, although some large household appliances run on 220-240 volts.

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Converting AC to DC AC is converted to DC using devices called diodes, which allow charges to move in only 1 direction.

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Speed of Electrons Electrons in a circuit do not move quickly - they actually “drift” at about 1 mm/s. It is the electric field that moves quickly - at about the speed of light - through the circuit and carries the energy.

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Electric Power Power = energy/time = current x voltage P = IV = I 2 R 1 Watt = (1 Amp)(1 Volt) 1 kilowatt = 1000 Watts A kilowatt-hour is a unit of energy

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The End

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