 # Concept Summary Batesville High School Physics. Potential Difference  Charges can “lose” potential energy by moving from a location at high potential.

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Concept Summary Batesville High School Physics

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.

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

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

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.

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

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

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

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

Ohms’ Law  In symbols:  V = IR V IR

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.

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.

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.

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

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.

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

The End 

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