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Chapter 34: Electric Current Conceptual Physics Bloom High School

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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 – PE/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

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34.2 Electric Current Electric current- the flow of electric charge –Ampere (A)- 1 Coulomb per second Coulomb (C)- 6.25x10 18 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

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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

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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)

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

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

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

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Examples Larger resistance, greater heat generated –Toaster- 15-20 –Lightbulb- 100 Little heat generated –Lamp cord- 1

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34.6 Ohm’s Law and Electric Shock Volts or Amps? –Human body offers resistance of 100 -500k –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 1000 , and you touched a 24V battery, how much current is drawn? –24V/1000 =0.024A (possibly fatal) Third prong- acts as a ground for higher voltage devices

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34.7 Direct Current and Alternating Current Direct current (DC)- charge always flow 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

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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

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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

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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

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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

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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

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Calculations

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