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Chapter 19

Chapter 19. Electric Current. Terms to Know. electric current: a flow of charged particles convention: Positive charges flow flows from high potential difference (= voltage = “+” terminal of a battery) to low potential difference = “ − ” terminal). More on Electric Current, I.

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Chapter 19

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  1. Chapter 19 Electric Current

  2. Terms to Know • electric current: a flow of charged particles • convention: Positive charges flow • flows from high potential difference (= voltage = “+” terminal of a battery) to low potential difference = “−” terminal)

  3. More on Electric Current, I • Specifically it is the flow of charge per unit time • I = ΔQ / Δt = charge passing through a given area / time interval • Units: C/s = A = ampere • 1 A = 1C/s

  4. Sample Problem, 19A The amount of charge that passes through the filament of a certain light bulb in 2.00 s is 1.67 C. Determine the current in the light bulb.

  5. Sources and Types of Current • Sources • Batteries • Generators – power plants • Types • Direct • Alternating – 60 Hz in US

  6. Ohm’s Law • Ohm`s Law (9 min) • I = V/R • V (or ΔV) = potential difference (V) = PEelectric /q • Consider as the PE of 1 C of charge carries • R = resistance (Ω read ohm) • Controls the amount of current • I = current (A) • Consider V as the pressure pushing charges along a wire, then R is the hindrance to the flow of charge

  7. Sample Problem 19B, Pg 702 The plate on a steam iron states that the current in the iron is 6.4 A when the iron is connected across a potential difference of 120 V. What is the resistance of the steam iron?

  8. I = V/R • I directly proportional to V • I inversely proportional R • Ohmic vs. Nonohmic material

  9. Factors Affecting Resistance • length (of wire) • diameter (of wire) • material • temperature

  10. Silver 1 .59 x 10-8 • Copper 1.7 x 10-8 • Gold 2.4 x 10-8 • Aluminum 2.8 x 10-8 • Tungsten 5.6 x 10-8 • Iron 10 x 10-8 • Platinum 11 x 10-8 • Lead 22 x 10-8 • Nichrome 150 x 10-8 • Carbon 3.5 x 105 • Polystyrene 107 - 1011 • Polyethylene 108 - 109 • Glass 1010 - 1014 • Hard Rubber 1013

  11. Electric Circuit Symbols • connected wire • unconnected wire • battery • switch • resistor • grounded • voltmeter • parallel connection • ammeter • series connection

  12. Example Problem 2, Pg 598 A 30.0-V battery is connected to a 10.0- Ωresistor. (a) Draw the circuit. (b) What is the current in the circuit?

  13. Electric Power, P • P = energy / time • P = power • unit: watt = W = J/s • I = charge / time • unit: C/s • V = PE / charge • unit: J/C • The product of I and V = I·V • IV = (charge/time) (PE/charge) = PE/time = Power

  14. Same Formulas, Different Look • Can get all these formulas from: V = IR, P = IV, P = energy / t • Get • P = I2R • P = V2/R • An appliance consuming a power (P) over t, its energy use is: energy = Pt • Electrical energy converts to thermal energy or light • energy = I2Rt • energy = V2t / R • Units: J, wh = watt·hr • 1 wh = 3600 J

  15. Sample Problem 19C, Pg 710 An electric space heater is connected across a 120 V outlet. The heater dissipates 3.5 kW of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater.

  16. Example A heater has a resistance of 10.0 Ω. It operates on 120.0 V. • What is the power dissipated by the heater? • What thermal energy is supplied by the heater in 10.0 s?

  17. Example A 6.0-V battery delivers a 0.50-A current to a electric motor connected across its terminals. • What power delivered to the motor? • If the motor runs for 5.0 min, how much (electric) energy is delivered?

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