Transcript r 34 lec
R Ch 34 Electric Current pg 1 Text Qs pg 545 RQ 1, 3-19,21 ,22 R 34.1 Flow of Charge pg 2 • Charges flow in a conductor when there is a difference in voltage (another name for voltage is potential). The flow will continue until both locations have the same voltage R 34.2 Electric Current pg 3 • Electric current is the flow of electric charge. • Solids only electrons flow • Liquids electrons & positive ions flow • Gasses electrons & positive ions flow • The overall or net charge of a conductor is zero, as a new charge enters another charge leaves R 34.3 Voltage Sources pg 4 • Cells are what we call batteries. Cells convert chemical energy to electrical energy • Dry cell = flashlight battery • Wet cell = car battery • Two cells hooked together = battery • Generators like the alternator in your car & power plant convert mechanical energy to electrical energy R 34.3 Voltage Sources pg 5 • The electrons have potential energy leaving the negative terminal heading to the positive terminal. • The electrons feel pressure to move. This pressure is called the voltage or EMF (electromotive force) R 34.3 Voltage Sources pg 6 • At home the power company provides a 120 volts of potential difference or pressure between the two holes in the socket. • This means there is 120 Joules of energy given to each Coulomb of charge (6 e18 electrons) R 34.3 Voltage Sources pg 7 R 34.4 Electrical Resistance pg 8 • Electrical Resistance is a measure of how difficult it is for the electrons to flow through a wire & depends on; • • • • 1) material type, different metal has diff. resistanc 2) wire thickness, thicker wire = less resistance 3) length of wire, longer wire = more resistance 4) temperature, higher temp = more resistance R 34.5 Ohm’s Law pg 9 • Ohm’s law is the V = I x R R 34.5 Ohm’s Law & Electric Shock pg 10 • Skin, shoes & clothes all provide a good deal of resistance to current electricity. Dry skin has about 500,000 ohms of resistance. • If current travel through your body two things can happen; • 1) overheating (cooking) tissue • 2) disrupt nerve function, stopping the heart & or breathing R 34.6 Ohm’s Law & Electric Shock pg 11 • To help someone who is being electrocuted; • 1) think • 2) use an insulator (probably wood or plastic) to break contact between the body and the wire • Birds contact wires all the time but do not get shock because; • 1) their scaly feet are good insulators • 2) both feet are on the same wire, there is no difference in voltage so no current flows from one foot to the other R 34.7 Direct Current & Alternating Current pg 12 • Direct Current or DC current the charges only flow in one direction. • Alternating Current or AC the charges vibrate back & forth and never move out of the wire R 34.7 Direct Current & Alternating Current pg 13 • In North America the electron vibrates back and forth 60 times each second (due a change in voltage polarity) with a 120 volts of EPE • Europe uses a 220 volt system because it is more efficient. R 34.7 Direct Current & Alternating Current pg 14 • The 60 vibrations a second is called 60 cycle or 60 Hertz electricity. • It means that for 1 / 120th of a second the wire is positive and for the next 1 / 120th of a second the wire is negative. R 34.7 Direct Current & Alternating Current pg 15 • At your house there is a circuit breaker box call the electric service drop. • A 120 volt negative wire and a 120 volt positive wire enter your house from the neighborhood electric line. • Normally these are kept separate but they can be combined to make 240 volts for the air conditioning etc. R 34.8 Converting AC to DC pg 16 • To convert the AC current that comes out of the wall socket to DC needed to run household devices requires three things; • 1) a transformer, to lower or raise the voltage • 2) a diode a one-way valve to eliminate the vibration back and forth • 3) a capacitor to fill in the missing electrons R 34.8 Converting AC to DC pg 17 R 34.9 The Speed of Electrons in a Circuit pg 18 • DC current electrons moves about 0.01 cm/sec because the electrons keep bouncing off the positive nucleus which are in the way. • AC current electrons just vibrate back and forth and don’t go anywhere • An electric field is what really moves through the wire at close to the speed of light R 34.10 The Source of electrons in a Circuit pg 19 • So the truth is electrons do not come out of a wire into a light bulb. An electric field comes out of the wire and pushes and or pulls on the electrons in light bulb. • The electric field caused the electrons to move and do work. Just like a magnet would push or pull a metal ball bearing. R 34.10 The Source of electrons in a Circuit pg 20 • When you get an electric “shock” what you feel is the electrons in your body being pushed & pulled 60 times each second with 120 Joules of EPE. R 34.11 The Source of electrons in a Circuit pg 21 • Electric power is the rate at which electric energy is converted to heat, light or mechanical energy. • The electric company sells you Kilowatthours of energy, (not Joules of energy) • So 10 100 watt light bulbs burning for one hour is 1 kw-hr of electric energy.