A
Current Electricity
Introductory Physics
Canadian Academy
Group Members:
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 and
answer the questions on the slide.
http://phet.colorado.edu/en/simulation/
circuit-construction-kit-dc
Your finished work should be
uploaded to SlideShare (or
Google Docs if it works) and
embedded into a blog post.
You should be able to:
• Define current electricity
• Define resistance and state the factors that
affect resistance in a metal wire
• Define potential difference (voltage)
• Explain the effect of potential difference
and resistance on a current
• Draw basic circuit diagrams involving
batteries, lamps, switches and wires
• Define electrical power including the
relationship to voltage and current
Some basic circuit symbols
You can use these to build the circuits on the next slides.
battery
cell
wire
junction
+
cathode
bulb/ lamp
anode
+
-
electron flow
resistor
conventional current
What do these two components measure?
switch
V
A
voltmeter
ammeter
Voltmeter measures the potential difference in charge
between two points on a circuit, while ammeter is used to
measure the electrical current of a circuit.
A simple series circuit
Electrons flow from
negative to positive but
conventional current is
the opposite. (Shown
with red arrow)
A
Define current electricity.
• It is a flow of electrons/electrical charge and
can be used to power objects (bulbs, etc.)
Label the direction of flow of electrons and the
direction of the conventional current.
• Conventional currents incorrectly assume
electrons flow from positive to negative.
(Shown in green arrow)
Use the non-contact ammeter to measure the current
in 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. Chemical
reactions in the battery causes electrons to build up, and
electrolyte prevents electrons from directly moving out from
anode. Instead, when wires connect a path from cathode to
anode, electrons flow from the cathode (negative) to anode
(positive).
Switches and current
Build this circuit.
A
Measure the current with the switch in the open
position.
• 0 amps
Close the switch and measure the current. Explain
your answer.
• 0.9 amps
By closing the switch, the circuit is completed and
allows electrons to flow as a current.
Move the ammeter to different positions in the circuit
and 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 of
Conservation of Energy.
Potential Difference (voltage)
Modify the circuit to increase the potential
difference by including two, then three, cells.
A
What happens to the bulb?
• The more cells, the brighter the bulb
becomes
Bulb
Complete the circuit
diagram for three cells.
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 less
batteries there are less electrons to overcome
resistance of bulb, therefore reducing the amp
reading.
Define potential difference (voltage).
•
It is the amount of energy per charge unit that is
needed for moving electrons to move through circuit
Resistance (incandescent bulbs or lamps)
Go to the following applet and see resistance at a
molecular level and how a light bulb works.
http://micro.magnet.fsu.edu/electromag/java/filamentr
esistance/
Explain in your own words how moving charges cause a
bulb to glow.
The electrical flow moving from the battery through the
light bulb creates collisions between the electrons of the
flow and those of the bulb itself, as well as atoms. These
impacts create resistance, and as the electrons flow
through the bulb this energy is used to light up the bulb
(and also produce heat).
Resistance (incandescent bulbs or lamps)
A
Build this circuit.
Add bulbs and record your observations.
Explain your findings in detail.
Bulbs
(A)
Observations
1
1.8
Bulb is very bright, only one source of resistance.
2
0.9
Bulbs create twice as much resistance, therefore
resulting amp reading is 1.8 divided by two.
3
0.6
Bulbs create thrice as much resistance, again
causing the original (1.8) amp reading to be
divided by THREE now.
4
0.45
Resistance of four bulbs create quadruple the
amount of resistance, so the amps generated for
one bulb is divided by four.
5
0.36
Patternpatternpattern, five times resistance=
amps reading decreased by five times. (1.8
divided by 5)
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 to
demonstrate 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 cell
without being used in light bulbs, etc. therefore causing the electrons to heat up and cause a short circuit. Resistance
transforms energy to different types, and uses some of it. With no resistance, no energy is used up and eventually
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 the
mathematical 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 the
greater 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 can
cause a rapid heat up of batteries, fire, and even explosion. Someone could really hurt themselves..
“
Parallel Circuits
How many different routes can current take
through this circuit?
• 2 routes
Close the lower switch only.
Observe the bulb and measure the current.
•
Draw a circuit diagram for
this set-up below:
Close the upper switch only.
Observe the bulb and measure the current.
• 1.8 Amps
Close both switches. Observe the bulbs and
measure 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 two
to be equally distributed as energy to light the
two bulbs, therefore slowing electron flow until
the second junction speeds electron flow again.
Law of Parallel Circuits
Write your own Law, based on observations.
Law of parallel circuits.
“With each additional parallel path (and bulb) created, the electron amps before the first junction
occurs gains 1.8 additional amps.“
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 amps
before the first junction occurred was a 1.8
amp increase with the addition of 1 parallel
path with a bulb. As in my observations in
the previous slide, the electron amps were
evenly distributed along the three circuit
paths, so that each bulb was fueled with
the 5.4 amps divided by three, 1.8 amps.
More Parallel Circuits
Close one switch at a time and record your
observations.
• 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 first
junction occurs)
Draw a circuit diagram for
this set-up below:
Carefully observe the junctions.
What is happening? Explain with reference to
resistance and junctions. Instead of 3.6 amps of electrical
current as when two parallel circuits were made (with two bulbs on
each 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 both
bulbs create greater resistance. Originally, 2 batteries can power a bulb
using 1.8 amps of electrical current, however in a series circuit the
electrons 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/decreased
amps. The top path remains unaffected because of unchanged
resistance, thus the electrical current in this path is 1.8 amps. Thus at
the first junction, the 2.7 amps of current split to provide the series
path with 0.9 amps while providing the parallel path with 1.8 amps. In
this context, parallel circuits are more efficient in using electricity.
Law of Parallel Circuits
Modify your Law.
Law of parallel circuits.
“ In the event that there is a series circuit occurring in one or more parallel
paths of a parallel circuit, those series paths will experience of a decrease of
electrical flow proportional to the number of bulbs added in the series, while
parallel paths with no series of bulbs always are charged by 1.8 amps (or
proportional to how many batteries are being used.) “
As mentioned much earlier, the inverse relationship between number of bulbs in a series
circuit 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 represents
the 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 the
bulbs (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 the
bulb with 20 ohms of resistance. Therefore the electrons of the current can move faster through the bulb
and collide with the electrons and atoms of the bulb itself. However in the bulb with 20 ohms of
resistance, there much more resistance experienced because electrons are more scattered. This means
that electrons from the current in a circuit cannot overcome the resistance as easily as the bulb with 10
ohms 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:
Parallel:
In the parallel circuit, the bulb
with less resistance generated
a brighter light than the bulb
with 10 ohms more of
resistance.
Because the potential
difference between both
bulbs remains constant,
greater bulbs with lower
resistance need to draw a
larger current.
Series:
In the series circuit, the bulb
with greater resistance
generated a brighter light than
the bulb with 10 ohms less of
resistance.
Because the current remains
constant between both bulbs,
the bulb with greater
resistance needs to have a
larger potential difference.
16
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 have
studied in the previous unit?
•
Both are rates of using energy in a certain way. Electrical power is the rate that an electric circuit
transfers electric energy, while the power studied in the previous unit refers to the rate that
energy is used over time.
Compare two methods of generating electrical power: one fossil-fuel based and one
renewable. How do they work? What are the benefits/ disadvantages of each?
(See next page)
Fossil-fuel based energy generation
PROS:
• Cheaper than some renewable energy sources
• For example: price for generating
electricity with black coal is around 2 yen
per kilowatt hour compared to wind farm
electricity which costs around 18 yen
•Compact generators for easy portable use
•Natural gas, oil, coal, etc.
•Ability to generate massive amounts of energy at
a single place
•Power plants that use gas tend to be super
efficient
CONS:
•Emission of carbon dioxide which pollutes
the world causing the greenhouse effect
and contributing greatly to global warming
•Burning coal (also) causes sulfur to be
released into the air which created acid
rain
•Extraction of crude oils can potentially be
hazardous to underwater environment and
create massive pollution if something like
an oil spill were ever to happen. (BP oil spill
cough cough)
18
Renewable energy generation
PROS:
•Renewable energy sources can be easily replenished
and are not for one-time-use only
•(Mostly) clean source for energy with no direct harm to
environment like fossil fuel energy
•Power plants generally require very few workers to
operate, which means low cost to operate and cheap
maintenance
•No additional fuel costs such as for solar power and
wind power
CONS:
•Not always reliable and consistent in producing
energy
•I.e. wind turbines may experience lack of
wind, solar panels on a cloudy day
•Investment in setup before creation is high and
costly
•Limits to placement/portability of generator
•Hydropower near water, solar power
needing sunny climate, etc.
19
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 be
produced from a battery in a circuit, or a source with both positive and negative non-changeable terminals for electrons
to travel through. However, alternating currents (AC) are currents with a changing flow of electrons. This means that the
source of electricity can either contain voltages that change from positive to negative polarity over time, or can produce
voltages with the ability to change polarity. Therefore, direct currents sustain current flow in one direction while
alternating 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 use
electricity to lessen a heat source, the direction or charge of the current does not matter as long as the voltage and
current are sufficient to produce the necessary heat. But one important aspect of AC is that these currents are crucial to
build electric generators, motors, or systems responsible for the distribution of power. These devices run much more
efficiently 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.
Stop SOPA and
PIPA!! 
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