Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit...
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Transcript of Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit...
![Page 1: Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit must be zero. If you get 10 Volts at the battery…you.](https://reader036.fdocuments.net/reader036/viewer/2022062801/56649e385503460f94b2867d/html5/thumbnails/1.jpg)
Chapter 18Electric Currents
![Page 2: Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit must be zero. If you get 10 Volts at the battery…you.](https://reader036.fdocuments.net/reader036/viewer/2022062801/56649e385503460f94b2867d/html5/thumbnails/2.jpg)
Loop Rule
• The sum of all the potential differences around a circuit must be zero.• If you get 10 Volts at the battery…you have to give up 10 volts somewhere
around the circuit.
![Page 3: Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit must be zero. If you get 10 Volts at the battery…you.](https://reader036.fdocuments.net/reader036/viewer/2022062801/56649e385503460f94b2867d/html5/thumbnails/3.jpg)
Lightbulb Lab
• Do it.• http://www.youtube.com/watch?v=8ve23i5K334
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Electric Batteries
• A battery generates a potential difference by producing a separation of charge.
• This separation of charge occurs from chemical energy being transformed into electrical energy.
• In our example of our single cell "battery", the acid solution dissolves the Zinc electrode, which leaves behind two electrons upon the electrode as it dissolves, thus negatively charging it.
![Page 5: Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit must be zero. If you get 10 Volts at the battery…you.](https://reader036.fdocuments.net/reader036/viewer/2022062801/56649e385503460f94b2867d/html5/thumbnails/5.jpg)
Electric Batteries
• A standard every day "dry cell" battery, works off the same principle as our "wet" battery above, only it uses an electrolyte paste instead of a acidic liquid.
• Batteries do work on charges, causing them to separate.
• Batteries are a source of potential difference.
![Page 6: Chapter 18 Electric Currents. Loop Rule The sum of all the potential differences around a circuit must be zero. If you get 10 Volts at the battery…you.](https://reader036.fdocuments.net/reader036/viewer/2022062801/56649e385503460f94b2867d/html5/thumbnails/6.jpg)
Electric Batteries
• The only battery with an acceptable name is Energizer.
• A “rechargeable” battery is a lie.
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Stop
• Finish Lab• Paper Throw
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Hudson River
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Seneca River
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St. Lawrence River
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Niagara River
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Oswego River
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Current in a Wire• In a river:
• How much water/time• In a circuit:
• Current is measured in Amperes (A)• Ampere = Coulomb/second
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Practice Problem
• 5 Coulombs of charge pass by a certain point in a wire in a period of 2 seconds. What is the current in this section of wire?
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Practice Problem
• 5 x 10 20 electrons pass by a certain point in a wire in a period of 3 seconds. What is the current in this section of wire?
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Whitewater Rafting
• http://www.youtube.com/watch?v=28wbI73IVvQ• The filament in a light bulb is similar to the rapids in a river.• How?
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Ohm’s Law
• Proportionalities
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Sample Problem
• A small flashlight bulb draws 300 mA from its 1.5 V battery.• What is the resistance of the bulb?• How would the current change if the battery was 1/3rd as big?
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Stop
• Paper Throw
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Plinko
• http://www.youtube.com/watch?v=E7DKxe_m1AM• Things we could change about the board?• Act it out
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Microscopic View of Charges
• The potential difference (V) in a battery sets up an electric field.
• Electrons are forced in one direction by the electric field.
• The amount of stuff electrons run into, resistance (R), determines the current (I) in the circuit.
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Resistance in a Wire
• The unit for Resistance is Ohms (Ω)• Resistance in a wire is:
–Directly proportional to the resistivity “rho” (Ω * m)– How bad of a conductor the wire is.
–Directly proportional to the length “l” (m)– How much wire there is.
– Inversely proportional to the cross sectional area “A” (m2)– How thick the wire is.
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Resistance Practice Problems
• Determine the overall resistance of a 100-meter length of copper wire with a cross sectional area of .1 m2.
• Determine the overall resistance of a 2-meter length of nichrome wire with a radius of 2 cm.
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STOP
• Throw paper
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Sample Question
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Electrical Power
• Mechanical Power was how much energy we transformed in a certain amount of time. Electrical Power is the same thing. Derive:
• Power is measured in Watts.• Current is measured in Amps (C/s)• Potential difference is measured in Volts (J/C)
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Sample Problem
• Calculate the resistance of a 40W automobile headlight designed for 12V.
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Sample Problem
• How much power does my roommate Brian’s bedroom light generate if it draws 0.5 amps of current and is plugged into a 120 Volt wall socket?
• He leaves his light on 40 hours a week. If electricity costs $0.12/kWh, how much less money should I pay in rent each month?
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Conventional Current
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Conventional Current Flow
• Much like the E field lines, current travels from positive to negative.• This is our convention even though protons do not move.
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Why do we Physics?
• So our house doesn’t burn down.
• Fuses and circuit breakers are designed to “switch off” when too much current passes through.
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Why do we Physics?
• So you can get places.• Maglev trains literally levitate
ontop of the track.• Work is being done to create
“superconducting” materials which have no resistance.
• This will make maglev trains incredibly cheap and efficient.
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Why do we Physics?
• To help people better (and make boatloads of cash)
• Superconductors are used in MRI’s, as well as SQUID’s.
• Each of these can be used to create images of brainwaves.
• SQUID’s are currently being used to search for oil.
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Why do we Physics?
• So you can be alive…and help other people be alive.
• Your nervous system works using charged ions that create potential differences.
• Those potential differences send electrical impulses to different parts of your body which allow you move, react and function.
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HW Due Friday
• Ch 18: 5, 9, 11, 13, 20, 34, 37
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STOP
• Throw paper