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Electric Currents

Electric Currents. Physics Department, New York City College of Technology. Electric battery Electric current Resistance Ohm’s law Resistivity Electric energy Electric power Direct current. Alternating current Average power rms values. Key words. Electric battery.

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Electric Currents

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  1. Electric Currents Physics Department, New York City College of Technology

  2. Electric battery Electric current Resistance Ohm’s law Resistivity Electric energy Electric power Direct current Alternating current Average power rms values Key words

  3. Electric battery • A battery produces electricity by transforming chemical energy into electrical energy.

  4. +  Electric battery • In a diagram of a circuit, it is denoted by the symbol

  5. Electric current • Electric current is a flow of charge

  6. Electric current • Current is defined as • I is current • ∆Q is the amount of charge that passes through the conductor at any location • Δt is the time interval • The unit of I is ampere (A)

  7. Direction of electric current

  8. Electrical resistance • Defined as • R is the resistance of a wire • V is the potential difference applied across the wire • I is the current • The symbol of a resistor is

  9. Electron motion model • Disc 17, #22

  10. Resistance R is a constant independent of V, i.e., Experimentally found by Ohm Generally holds in metal conductors in room temperature Ohm’s law

  11. Ohm’s law • Disc 17, #19 • Disc 17, #20 • Disc 17, #21

  12. Resistivity • It is experimentally found that • ρ is resistivity and depends on the material • L is the length of a wire • A is the cross-sectional area

  13. Resistivity

  14. Example #1 • A flashlight bulb draws 300mA from its 1.5-V battery. (a) What is the resistance of the bulb? (b) If the battery becomes weak and voltage drops to 1.2V, how would the current change?

  15. Example #1—continued (a) (b)

  16. Connect stereo to speakers. (a) If each wire must be 20m long, what diameter copper wire should be used to keep the resistance less than 0.10 Ω? (b) If the current is 4.0A, what is the potential difference across each wire? Example #2

  17. Example #2—continued (a) (b)

  18. Electric energy • Energy transformation between electric and other forms are very common • Motors transform electric energy into mechanical energy • In electric heaters and hair dryers, electric energy is transformed into thermal energy • In a lightbulb, electric energy is transformed into light and thermal energy

  19. Electric power • The electric power transformed by any device is • P is electric power • I is current • V is potential difference • The unit of electric power is Watt (W) • For a resistance R, we have

  20. Calculate the resistance of a 40-W automobile headlight designed for 12V. Example #3

  21. Example #3—continued

  22. Alternating current • The voltage and current produced by an electric generator are sinusoidal • V0 is the peak voltage • I0 is the peak current • The frequency f is the number of complete oscillations per second, and

  23. DC and AC

  24. AC power • The power transformed in a resistance R at any instant is • The average power is calculated as or

  25. AC power—continued

  26. rms values and peak values The average power in rms values: rms (root-mean-square) values

  27. Example #4 • (a) Calculate the resistance and the peak current in a 1000-W hair dryer connected to a 120-V line. (b) What if it is connected to a 240-V line in Britain?

  28. Example #4 (a) (b)

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