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Download 34.1 Flow of Charge
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34.1 Flow of Charge • Potential difference – difference in potential voltage until both ends reach common equilibrium • Sustain a difference in potential 34.2 Electric Current • Charge = through = Amps • Flow of electric charge through wire = amperes • Solids – electrons carry charge because they are free to move • Protons are fixed inside nucleus • Measured in amperes – SI unit = I • One coulomb of charge per second • One coulomb = 6.24 billion electrons • Current carrying wire normally has a charge of zero. # of electrons and protons coming and going are equal 34.3 Voltage Sources • Voltage = across = Volts = pressure differential • Whatever provides the potential difference is known as the voltage source • Dry cells & wet cells (chemical energy converted to electrical energy), generators (batteries) • Generators (like alternators in cars) convert mechanical energy to electrical energy • Voltage provides electric pressure to move electrons between terminals in a circuit (emf) • *Voltage causes current • *Charges/current flows through a circuit because of applied voltage across a ciruit. 34.4 Electric Resistance • How well a material conducts charge (current) or how much resistance it offers – Conductivity – silver less resistant – Thickness – thicker is less resistant – Length – shorter is less resistant – Temperature – lower temp is less resistant • Superconductors – Ch 32. • SI unit for resistance is ohms • Georg Simon Ohm – German physicist 34.5 Ohm’s Law • Amp = I for current – “Intensite de courant” “current intensity” • Resistance = R for resistance • Voltage = V for voltage 34.6 Ohm’s Law and Electric Shock • Human body – current passing through it causes damage, depends on amount of voltage • Dry skin - 24V barely a tingle/ .001A • Moist – 24V painful/ .005A • 120V – cause death – especially if bare feet in a wet bathtub, reducing resistance • To receive shock there has to be a difference in potential 34.7 DC & AC • DC – direct current – Flows in one direction only – Batteries produce this • AC – alternating current – Flows in alternating directions at rate of 60 cycles/second (in USA) – Voltage in USA is 120V (in Europe is 220 -240V) • Power transmission more efficient at higher V • Transfer of energy quietly from 1 place to another 34.8 Converting AC to DC • To convert AC to DC must use transformer, diode and capacitor • Lower voltage with transformer • Diode acts a one way valve to allow electron flow in only one direction • Only ½ the cycle passes through the diode • To maintain smooth current, use a capacitor. Also used for storage of energy. 34.9 Speed of Electrons in a Circuit • • • • Signal is moving, not the electrons Electrons vibrate at rate of 1 million km/hr The electrons are provided by the metal wire Drift speed – speed at which an electron moves due to the electric field 34.10 The Source of Electrons in a Circuit • Source of electrons is the wire itself. We provide this. • Power companies sell energy, NOT electrons • The energy causes vibrations, free electrons to vibrate in unison. • Large vibrations can be fatal. 34.11 Electric Power • Power = current x voltage • Watt = amps x volts • Power = rate at which electrical energy is converted into another form; such as mechanical energy, heat, or light.