Presentation on theme: "R Ch 34 Electric Current pg 1 Text Qs pg 545 RQ 1, 3-19,21,22."— Presentation transcript:
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 / 120 th of a second the wire is positive and for the next 1 / 120 th 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 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 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 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 Kilowatt- hours of energy, (not Joules of energy) So watt light bulbs burning for one hour is 1 kw-hr of electric energy.