r ch 34 electric current pg 1 text qs pg 545 rq 1, 3-19,21,22
TRANSCRIPT
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R Ch 34 Electric Current pg 1
Text Qs pg 545
RQ 1, 3-19,21 ,22
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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
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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
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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
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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)
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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)
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R 34.3 Voltage Sources pg 7
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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
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R 34.5 Ohm’s Law pg 9
• Ohm’s law is the V = I x R
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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
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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
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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
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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.
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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.
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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.
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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
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R 34.8 Converting AC to DC pg 17
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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
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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.
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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.
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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 Kilowatt-hours of energy, (not Joules of energy)
• So 10 100 watt light bulbs burning for one hour is 1 kw-hr of electric energy.