Current Electricity
Introductory Physics
Canadian Academy
Group Members:
A
Is this plausible?
Could you really power a house with static electricity?
http://www.youtube.com/watch?v=scUosAkxlvo
Current Electricity
Construct and explain.
Work in pairs or by yourself for these tasks.
With each question:
•Build it in the PhET simulation
•Build it in the lab if possible
•Draw the circuit diagram andanswer the questions on the slide.
http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc
You should be able to:
•Define current electricity
•Define resistance and state the factors thataffect resistance in a metal wire
•Define potential difference (voltage)
•Explain the effect of potential differenceand resistance on a current
•Draw basic circuit diagrams involvingbatteries, lamps, switches and wires
•Define electrical power including therelationship to voltage and current
Your finished work should beuploaded to SlideShare (orGoogle Docs if it works) andembedded into a blog post.
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cell
battery
wire
junction
bulb/ lamp
cathode
anode
switch
resistor
V
A
voltmeter
ammeter
Some basic circuit symbols
You can use these to build the circuits on the next slides.
What do these two components measure?
electron flow
conventional current
Voltmeter measures the potential difference in chargebetween two points on a circuit, while ammeter is used tomeasure the electrical current of a circuit.
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A simple series circuit
Define current electricity.
• It is a flow of electrons/electrical charge andcan be used to power objects (bulbs, etc.)
Label the direction of flow of electrons and thedirection of the conventional current.
•Conventional currents incorrectly assumeelectrons flow from positive to negative.(Shown in green arrow)
Use the non-contact ammeter to measure the currentin the circuit.
• 0.90 amps
What happens if the cell is not included in the circuit?Explain.
•Bulb is not lit because battery provides electron flow. Chemicalreactions in the battery causes electrons to build up, andelectrolyte prevents electrons from directly moving out fromanode. Instead, when wires connect a path from cathode toanode, electrons flow from the cathode (negative) to anode(positive).
Electrons flow fromnegative to positive butconventional current isthe opposite. (Shownwith red arrow)
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Switches and current
Build this circuit.
Measure the current with the switch in the openposition.
• 0 amps
Close the switch and measure the current. Explainyour answer.
•0.9 amps
By closing the switch, the circuit is completed andallows electrons to flow as a current.
Move the ammeter to different positions in the circuitand measure the current. Does position matter?
•No, electric flow remains constant
Are electrons ‘used up’ in the circuit?
Are electrons ‘created’ in the cell?
Electrons can neither be created nor destroyed, as reference to the Law ofConservation of Energy.
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Potential Difference (voltage)
Modify the circuit to increase the potentialdifference by including two, then three, cells.
What happens to the bulb?
•The more cells, the brighter the bulbbecomes
What happens to the current measurements?
1 cell: 0.9 amps
2 cells: 1.8 amps
3 cells: 2.7 amps
Explain your answers.
• Batteries provide the electrical flow, so with lessbatteries there are less electrons to overcomeresistance of bulb, therefore reducing the ampreading.
Define potential difference (voltage).
•It is the amount of energy per charge unit that isneeded for moving electrons to move through circuit
Complete the circuitdiagram for three cells.
Bulb
Resistance (incandescent bulbs or lamps)
Go to the following applet and see resistance at amolecular level and how a light bulb works.
Explain in your own words how moving charges cause abulb to glow.
The electrical flow moving from the battery through thelight bulb creates collisions between the electrons of theflow and those of the bulb itself, as well as atoms. Theseimpacts create resistance, and as the electrons flowthrough the bulb this energy is used to light up the bulb(and also produce heat).
Resistance (incandescent bulbs or lamps)
Build this circuit.
Add bulbs and record your observations.
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Bulbs
(A)
Observations
1
1.8
Bulb is very bright, only one source of resistance.
2
0.9
Bulbs create twice as much resistance, thereforeresulting amp reading is 1.8 divided by two.
3
0.6
Bulbs create thrice as much resistance, againcausing the original (1.8) amp reading to bedivided by THREE now.
4
0.45
Resistance of four bulbs create quadruple theamount of resistance, so the amps generated forone bulb is divided by four.
5
0.36
Patternpatternpattern, five times resistance=amps reading decreased by five times. (1.8divided by 5)
Explain your findings in detail.
There is an obvious pattern in that with each
additional bulb, the amp reading would decrease
proportionally to the increased resistance from bulbs.
Therefore, the formula A=1.8/B, in which A equals the amp reading and B equals the number of bulbs, can be used todemonstrate this inverse relationship. So with increase of bulbs, electron flow needs to overcome more resistance,causing amps reading decreases.
Remove all the bulbs to create a short circuit and measure the current if you can. Explain.
There is very little resistance in the circuit, therefore with the speedy movement of electrons, it goes back to cellwithout being used in light bulbs, etc. therefore causing the electrons to heat up and cause a short circuit. Resistancetransforms energy to different types, and uses some of it. With no resistance, no energy is used up and eventuallyheats up. When it hits point of resistance in battery, it causes frictional heat energy. The amps reading was 14248.61amps.
Conclusions
Use your findings in the circuits so far to write your own Laws.
Law of current in a loop.
“Electrons flow from negative to positive through the battery, circuit must be complete.“
Law of voltage and current. (what’s the relationship?)
“ As the voltage increases within circuit, current of electrons flows faster, voltage is potential difference. (what is themathematical relationship?) Current = Voltage / Resistance“
Law of resistance and current. (what’s the relationship?)
“ Potential difference (Voltage) is needed so that electrons can overcome resistance to flow in a circuit, therefore thegreater the resistance the less the current until resistance causes potential difference to reach 0. Resistance = Voltage /Current. A good conductor lets electrons flow through smoothly, while an insulators would cause greater resistance.“
Why is it dangerous to have too little resistance in a circuit?
This would cause a short circuit. See previous slide for more details. This excessive electrical current cancause a rapid heat up of batteries, fire, and even explosion. Someone could really hurt themselves..
Parallel Circuits
How many different routes can current takethrough this circuit?
•2 routes
Close the lower switch only.
Observe the bulb and measure the current.
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Close the upper switch only.
Observe the bulb and measure the current.
• 1.8 Amps
Close both switches. Observe the bulbs andmeasure the current at different positions.
•3.6 Amps
Observe the animation carefully.
What happens to the electrons at junctions?
-At junctions, electron amps are divided by twoto be equally distributed as energy to light thetwo bulbs, therefore slowing electron flow untilthe second junction speeds electron flow again.
Draw a circuit diagram forthis set-up below:
Law of Parallel Circuits
Law of parallel circuits.
“With each additional parallel path (and bulb) created, the electron amps before the first junctionoccurs gains 1.8 additional amps.“
Write your own Law, based on observations.
Now test your Law using a third bulb in parallel.
Draw the circuit diagram below and write your observations of the bulbs and of current.
As my law predicted, the number of ampsbefore the first junction occurred was a 1.8amp increase with the addition of 1 parallelpath with a bulb. As in my observations inthe previous slide, the electron amps wereevenly distributed along the three circuitpaths, so that each bulb was fueled withthe 5.4 amps divided by three, 1.8 amps.
More Parallel Circuits
Close one switch at a time and record yourobservations.
• Bottom switch closed: 0.9 amps
•Top switched closed: 1.8 amps
Close both switches and record your observations.
•Both switches closed: 2.7 amps (before firstjunction occurs)
Carefully observe the junctions.
What is happening? Explain with reference toresistance and junctions. Instead of 3.6 amps of electricalcurrent as when two parallel circuits were made (with two bulbs oneach path) only 2.7 amps were recorded. This is because the first path(with the two bulbs) consists of a series circuit, which means that bothbulbs create greater resistance. Originally, 2 batteries can power a bulbusing 1.8 amps of electrical current, however in a series circuit theelectrons need to overcome greater resistance with each added bulb.Therefore with each additional bulb added in a series for this circuit,the increasing resistance causes electrons to flow slower/decreasedamps. The top path remains unaffected because of unchangedresistance, thus the electrical current in this path is 1.8 amps. Thus atthe first junction, the 2.7 amps of current split to provide the seriespath with 0.9 amps while providing the parallel path with 1.8 amps. Inthis context, parallel circuits are more efficient in using electricity.
Do you need to modify your Law of Parallel Circuits?
Draw a circuit diagram forthis set-up below:
Law of Parallel Circuits
Law of parallel circuits.
“ In the event that there is a series circuit occurring in one or more parallelpaths of a parallel circuit, those series paths will experience of a decrease ofelectrical flow proportional to the number of bulbs added in the series, whileparallel paths with no series of bulbs always are charged by 1.8 amps (orproportional to how many batteries are being used.) “
Modify your Law.
As mentioned much earlier, the inverse relationship between number of bulbs in a seriescircuit and amps of electrical charge can be expressed with the formula
A=1.8/B , in which the number of amps are represented with variable A and B representsthe number of bulbs. However, 1.8 assumes that the circuit contains two batteries.
More Resistance
Use two cells and two bulbs in a circuit. Use CTRL-click to adjust the resistance of thebulbs (one is 20 ohms (Ω), the other is 10Ω).
What is the difference between these two bulbs on a molecular level?
•The bulb with 10 ohms of resistance does not resist the flow of electrons in a circuit as effectively as thebulb with 20 ohms of resistance. Therefore the electrons of the current can move faster through the bulband collide with the electrons and atoms of the bulb itself. However in the bulb with 20 ohms ofresistance, there much more resistance experienced because electrons are more scattered. This meansthat electrons from the current in a circuit cannot overcome the resistance as easily as the bulb with 10ohms of resistance.
Wire up the bulbs in two different circuits: series and parallel. Draw the circuits below.
Under each circuit, record and explain your observations.
Series:
Parallel:
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In the series circuit, the bulbwith greater resistancegenerated a brighter light thanthe bulb with 10 ohms less ofresistance.
Because the current remainsconstant between both bulbs,the bulb with greaterresistance needs to have alarger potential difference.
In the parallel circuit, the bulbwith less resistance generateda brighter light than the bulbwith 10 ohms more ofresistance.
Because the potentialdifference between bothbulbs remains constant,greater bulbs with lowerresistance need to draw alarger current.
Series:
Parallel:
Electrical Power
Define electrical power and state its unit.
•It is the rate that electric circuit transfers electric energy. Unit is in watts.
What is the relationship between electrical power and ‘power’ as we havestudied in the previous unit?
•Both are rates of using energy in a certain way. Electrical power is the rate that an electric circuittransfers electric energy, while the power studied in the previous unit refers to the rate thatenergy is used over time.
Compare two methods of generating electrical power: one fossil-fuel based and onerenewable. How do they work? What are the benefits/ disadvantages of each?
(See next page)
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Fossil-fuel based energy generation
PROS:
• Cheaper than some renewable energy sources
• For example: price for generatingelectricity with black coal is around 2 yenper kilowatt hour compared to wind farmelectricity which costs around 18 yen
•Compact generators for easy portable use
•Natural gas, oil, coal, etc.
•Ability to generate massive amounts of energy ata single place
•Power plants that use gas tend to be superefficient
CONS:
•Emission of carbon dioxide which pollutesthe world causing the greenhouse effectand contributing greatly to global warming
•Burning coal (also) causes sulfur to bereleased into the air which created acidrain
•Extraction of crude oils can potentially behazardous to underwater environment andcreate massive pollution if something likean oil spill were ever to happen. (BP oil spillcough cough)
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Renewable energy generation
PROS:
•Renewable energy sources can be easily replenishedand are not for one-time-use only
•(Mostly) clean source for energy with no direct harm toenvironment like fossil fuel energy
•Power plants generally require very few workers tooperate, which means low cost to operate and cheapmaintenance
•No additional fuel costs such as for solar power andwind power
CONS:
•Not always reliable and consistent in producingenergy
•I.e. wind turbines may experience lack ofwind, solar panels on a cloudy day
•Investment in setup before creation is high andcostly
•Limits to placement/portability of generator
•Hydropower near water, solar powerneeding sunny climate, etc.
Extension
If you finish with extra time:
•Check the Laws you have written against published information. Do they concur?
•Find out more about circuits and their components.
•Find out about the difference between AC and DC.
Direct currents (DC) describe currents that have a constant flow of electricity in one direction. This kind of electricity can beproduced from a battery in a circuit, or a source with both positive and negative non-changeable terminals for electronsto travel through. However, alternating currents (AC) are currents with a changing flow of electrons. This means that thesource of electricity can either contain voltages that change from positive to negative polarity over time, or can producevoltages with the ability to change polarity. Therefore, direct currents sustain current flow in one direction whilealternating currents can change direction due to shifting polarity.
In some aspects of using electricity, AC and DC can both be used so AC may seem less useful. For example, when we useelectricity to lessen a heat source, the direction or charge of the current does not matter as long as the voltage andcurrent are sufficient to produce the necessary heat. But one important aspect of AC is that these currents are crucial tobuild electric generators, motors, or systems responsible for the distribution of power. These devices run much moreefficiently using AC than DC, therefore AC is most commonly applied to projects requiring great amounts of electricity.
•Build your own circuits and draw the circuit diagrams below.
