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Electric Current

Electric Current. Chp. 20 PreAP Physics. ELECTRIC CURRENT The electric current ( I ) is rate at which electric charges (q) pass through a conductor each second. SI Units: C/s = Ampere (A) Named after French Mathematician -- Andre Ampere. How do you get charges to flow?.

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Electric Current

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  1. Electric Current Chp. 20 PreAP Physics

  2. ELECTRIC CURRENT The electric current(I) is rate at which electric charges (q) pass through a conductor each second. SI Units: C/s = Ampere(A) Named after French Mathematician -- Andre Ampere

  3. How do you get charges to flow? • It needs a push (a force) provided by an Electrical Potential Difference • A.K.A. Voltage Difference • The charge flows from high potential energy to low potential energy. • If a voltage difference is joined together with a conductor the charge will always moveuntil the force acting on it is reduced to a minimum or until the voltage becomes the same.

  4. If power lines have 10000 V how much voltage does the bird have when it is on the wire? 10000V Why doesn’t the bird get electrocuted? No Voltage (potential) Difference Why can a bird sit on the power line?

  5. What if the high voltage wire is near the ground? • A chicken steps on a high voltage wire with one foot on wire and one foot on the ground. What happens to the Chicken? • Electrocuted • WHY??? • Voltage Difference between the ground and the wire.

  6. In more recent centuries we have learned that in a wire, electrons are the only charged particles moving in an electrical current. Use Conventional Current • The direction is the same as the direction in which positive charges would move • Positive terminal toward the negative terminal • Historically it was believed that positive charges moved through metal wires. • Electricians still use it today even though its NOT how charge really flows.

  7. The source of the emf in this case is chemical energy. ELECTROMOTIVE FORCE () A source of electromotive force (emf) is a device that converts, chemical, mechanical, or other forms of energy into the electric energy necessary to maintain a continuous flow of electric charge.

  8. emf continued • A.K.A. Maximum Voltage (potential difference) • Example: Car Battery • Positive terminal has a max. voltage of 12 V higher then the negative terminal • So emf = 12 V

  9. 2 Types of Current • Direct Current (DC) • Charges move in an electric circuit in one direction • Ex. Batteries • Alternating Current (AC) • Charges change direction several times a second • In USA 60 Hz • Ex. Generators at power companies, wall outlets

  10. Thomas Edison Supporter of DC current Nikola Tesla Inventor of AC current War of the CurrentsLate 1800’s • Battle over which current system would be used at the 1893 World’s Fair in Chicago and ultimately the USA • Became a Brutal Rivalry • Edison went to great lengths to demonstrate that AC current was dangerous • Electrocuted animals on stage and film • 1st criminal killed by electric chair hooked up to an AC motor • Tesla insisted that his AC current was safe and more efficient then DC.

  11. Who Won? • Tesla & Westinghouse won the bid for the World Fair in 1893 • Claimed they could light the fair for half of what Edison was going to charge • However, Edison refused to sell them any light bulbs so they had to create their own • Ultimately AC became the wave of the future • More efficient • Cost effective • Fewer wires • Less bulky • Could be sent great distances

  12. Resistance • Which offers more resistance for water to flow? • Narrow or wide pipe? • Long or short pipe? • Could electric current flow be affected by resistance similar to water flow? • YES • Relationship between current, voltage and resistance was discovered by German Physicist, George Simon Ohm

  13. OHM’S LAW "For a given resistor at a particular temperature, the current is directly proportional to the applied voltage." OR R = Resistance (V/A) = ohm(Ω) V = Voltage (V) I = Current (A)

  14. 20.1 The voltage between the terminals of an electric heater is 80 V when there is a current of 6 A in the heater. What is the current if the voltage is increased to 120 V? V1 = 80 V I = 6 A V2 = 120 V = 13.3 Ω = 9 A

  15. Ammeter measures current through the battery, the filament, and itself.Current must pass through it so must be in the circuit. Low Resistance. Voltmeter measures voltage drop across a battery, filament or resistor. The current does not travel through a voltmeter, has high resistance. Four devices are commonly used in the laboratory to study Ohm’s law: the battery, the voltmeter, the ammeterand a resistance or load.

  16. The following symbols are used in electric circuits:

  17. ELECTRIC POWER AND HEAT LOSS • Change in Energy per unit Time • How fast electrical work is done. • Measured in Watts (W) • The rate at which heat is dissipated in an electric circuit is referred to as the power loss. P = I2 R P = V I

  18. 20.3 A current of 6A flows through a resistance of 300 Ω for 1 hour. a. What is the power loss? I = 6 A R = 300 Ω t = 1 hour P = I2R = (6A)2(300) = 10,800 W b. How much heat is generated in loss? Remember Work = Energy and heat is a form of energy. W = Pt = 10800W (3600s) = 3.89x107 J

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