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Chapter 34: Electric Current

Chapter 34: Electric Current. Conceptual Physics Bloom High School. 34.1 Flow of Charge. Heat flow- difference in temperature exists Potential difference- when the ends of a conductor are at different electric potentials Voltage Continues until both ends are at the same potential

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Chapter 34: Electric Current

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  1. Chapter 34: Electric Current Conceptual Physics Bloom High School

  2. 34.1 Flow of Charge • Heat flow- difference in temperature exists • Potential difference- when the ends of a conductor are at different electric potentials • Voltage • Continues until both ends are at the same potential • DPE/q (Volts=Joules/Coulomb) • Potential- PE/q • Water as an analogy (Fig. 34.1) • To keep the water flowing, we need a pump • Keeps the difference in pressure different • More water pumping is like higher pressure water

  3. 34.2 Electric Current • Electric current- the flow of electric charge • Ampere (A)- 1 Coulomb per second • Coulomb (C)- 6.25x1018 electrons • For a current of 5 amps, 5 C/s flows through it • The net charge on a wire is always zero • The number of electrons moving in equals the number moving out, regardless of voltage • Think of water flowing through a hose

  4. 34.3 Voltage Sources • Voltage source- an electric “pump” is required to keep electrons moving • Established across a circuit • Dry cells (regular batteries) • Chemical reaction provides pressure • Wet cells (car batteries) • Chemical reaction provides pressure • Generators (car alternator) • Mechanical energy provides pressure • Voltage (V)- Potential energy per Coulomb • Electromotive force (emf) • “Electric pressure” • Charge- flows through a circuit

  5. Home electricity • Potential difference of 120 volts between the two holes in the outlet • 120 Joules of energy applied to each Coulomb • Charge flowing through vs across • Charge flow through (like water in a hose) • Voltage across circuit is constant (like water pressure in the hose)

  6. 34.4 Electrical Resistance • Resistance- prohibits the flow of charge • Measured in Ohms (W) • Higher resistance in thinner, longer, higher temp. wires

  7. Resistor Chart • Write down the 1st band, 2nd band, etc. • The final band represents the # of zeros • 2-5-000=25kW • 4-6-0-000=460kW • 2-7-6=276W

  8. 34.5 Ohm’s Law • Ohm- discovered that current (amps) is proportional to the voltage (volts) and inversely proportional to the resistance (W) • Current = Voltage/Resistance (I=V/R) • Amperes = Volts/Ohms • Greater resistance leads to less current • Greater voltage leads to greater current

  9. Examples • Larger resistance, greater heat generated • Toaster- 15-20W • Lightbulb- 100W • Little heat generated • Lamp cord- 1W

  10. 34.6 Ohm’s Law and Electric Shock • Volts or Amps? • Human body offers resistance of 100W-500kW • 12V on dry skin isn’t felt • 12V on wet skin is painful • See Table 34.1 • Birds can rest on a wire because there is negligible potential difference between their feet • If the resistance is 1000W, and you touched a 24V battery, how much current is drawn? • 24V/1000W=0.024A (possibly fatal) • Third prong- acts as a ground for higher voltage devices

  11. 34.7 Direct Current and Alternating Current • Direct current (DC)- charge always flows in one direction • From negative terminal toward positive terminal • Alternating current (AC)- charge moves back and forth • In North America, cycles at 60 Hz 120V • In Europe, 240V

  12. High Voltage in the home • Some appliances require 240VAC • Clothes dryer, electric furnace, tanning bed • Every home has three lines: +120VAC, 0VAC, -120VAC • 120VAC comes from +120V connected to 0V line(neutral) • Potential difference of 120VAC • 240VAC comes from +120V connected to -120V line • Potential difference of 240VAC

  13. 34.8 Converting AC to DC • Diode- allows current to flow in one direction only • Takes the back-and-forth of AC and makes it “forth” only • Because it removes half of the charge, a capacitor is used to smooth out the charge • Capacitor- acts as a short-term battery • Smoothes out the pulses from a diode

  14. 34.9 The Speed of Electrons in a Circuit • When a voltage is applied (potential difference), the speed of the electrons is slow • The random motion of the electrons is pushed along by the voltage • Net speed (drift speed)- 1 cm/s in DC • In AC, drift speed is 0 m/s

  15. 34.10 The Source of Electrons in a Circuit • When you purchase a hose, it doesn’t come with water installed • When you purchase a lamp, it does come with electrons already! • Electric utilities sell the energy needed to oscillate the electrons, not the electrons themselves

  16. 34.11 Electric Power • Moving charges expend energy and results in heat • Electric power- the rate at which mechanical energy is converted to electrical energy • Power = Current x Voltage (P=I V) • 1 Watt = Ampere x Volts • 100 W bulb = 120V x 1A • 60W bulb = 120V x 0.5A

  17. Calculations

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