Download L1 Science 2010

L1 Science 2010
Level 1 Electricity
Main references:
New Directions in Science, NCEA Level 1(NDS)
Year 11 Science Study Guide (SSG)
1. Electricity or an Electric Current
 Is defined as a flow of electrons
 We can measure the number of electrons moving and how much
(electrical) energy each electron has.
 There are 3 types of electricity;
1. Static,
2. Direct Current (DC)
3. Alternating Current (AC)
Static is made by friction, such as rubbing material on a plastic rod or in
a van der Graf generator. Examples of static electricity can be found on;
holding the dust on a TV screen, lightning, sparks and noise when you
pull off a dry fleece top and the practical’s you need to do and describe
below.
Direct Current (DC) is made by batteries, DC generators or AC/ DC
transformers. Examples of direct current electricity can be found on:
torches, digital cameras, iPods, car and small boat electric systems,
hunter’s spot lights.
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Alternating Current (AC) is made by commercial electric generating
plants such as the Clyde and Roxbrough dams. Examples of alternating
electricity can be found on: most household, school and other building
electric systems.
Revise the structure of a typical atom.
Draw and label an atom e.g. Sodium (Na) and include: neutrons,
protons, electrons, electron ring or shell, positive charge, negative
charge, no charge. NDS p42 or SSG p194
Draw the internal atomic structure of a good conductor (metal) and label
positive nucleus and negative electrons and a poor conductor (insulator).
NDS p195
Do static electric practical NDS p24 P84, also try charged plastic rod
near thin flow of water, polystyrene balls, with small pieces of ripped
paper and the electroscopes.
As a class spend some time trying to get the van der Graf generator
working properly i.e. hair standing on end, lighting a Bunsen burner with
a human spark!
Understand terms: like and unlike electric charges
e.g. - and -, + and +, - and +, + and -.
Draw diagrams and make notes to represent what 2 like electric charges
do to each other and what 2 unlike electric charges do, include terms:
attractive and repulsive forces. NDS p195
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2. Electrical Conductors and Insulators
A good electrical conductor is
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material than electricity can pass through easily
has low resistance
also known as a poor insulator
examples include: metal elements and alloys, carbon and many
metal salts dissolved in water.
A good electrical insulator is
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material that does not let electricity pass through easily or at all
has high to very high resistance
also known as a poor conductor
examples include: wood, glass, ceramics, most plastics
A rheostat or variable resistor can be adjusted using a slider or dial to
change from being a good to a poor conductor.
In this unit on electricity we will only be concerned with; DC electricity, good
conductors, insulators and with building, testing, drawing and trying to
understand simple circuits and the concepts associated with them.
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The circuit symbols you must know and understand are:
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positive charge
negative charge
wires,
lamps,
batteries, cells or other power supply,
switches (open and closed),
voltmeters,
ammeters,
resistors
rheostat
fuses
diodes
Glue in a copy of circuit symbols to your notes now or copy NDS p195
Review the structure of the atom and structure of metals and how it relates
to conductors and insulators.
Set up your first simple circuit including; a 12v power supply, wires,
ammeter and lamp to test a selection of materials as conductors or
insulators.
Before you do this experiment you must show know what a touch test is
and explain why it is important!
Write this experiment up with; title, aim, method (you must draw the circuit
diagram using a pencil and ruler), results and conclusion.
Test at least 4 conducting and 4 insulating materials.
Select from; tap water, salty water, carbon, copper, wax, plastic, iron,
sulphur, aluminium, zinc, paper, wool, wood, glass or rubber.
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3. Resistance
Resistance is
 defined as how difficult it is for electrons to flow through a conductor
Resistance in a wire experiment
Write up as a full experiment with: title, aim, method, results, conclusion
and discussion.
Set up the same simple circuit you used last time with an ammeter, lamp,
12v power supply, wires and replace the test material with a length of
nichrome wire mounted on a one metre wooden ruler.
The lamp is important to make sure you don’t fry the wire!!! Also touch test
your ammeter.
Collect a valid and reliable set of results that tests the relationship between
the length of nichrome wire and current in the wire (measured with the
ammeter).
Use a copy of the table below for your results and also graph the length of
wire verses current (amps).
Results:
Length of wire (IV)
Amps (DV)
In your conclusion discuss the relationship between the length of wire and
current in terms of the resistance of the wire (Current increases/decreases
as length of wire increases/decreases?). Read NDS p206 – 207.
In your discussion explain the other things other than length of wire that can
affect resistance in a conductor and give one example of where resistance
is a good thing and one example of where it is a bad thing.
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4. Ohm’s Rule
Ohm’s Rule is
 A rule that relates the Voltage (V) in volts to the Current (I) in amps.
Voltage – Current relationship experiment
Carry out NDS Ex12H p208 or Example E p187 Year 11 Science Study
Guide.
Write up as T, A, M, R, C and D
Graph your results of Current verses Voltage, and draw a line of best fit.
Remember the IV usually goes on the x axis and the DV on the y axis.
Results table
Current (amps) (IV)
Voltage (volts) (DV)
Conclusion: As I (current measured in amps with an ammeter)
increases/decreases then V (voltage measured in volts with a voltmeter)
increases/decreases.
In your discussion explain why the nichrome wire was placed in a beaker of
cold water and also explain proportionality in a relationship (complete the
exercise below)
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If a graph gives a straight line of best fit we say X is proportion to Y,
meaning that each time X increases by a fixed amount then Y increases by
a fixed amount as well.
Example: Graph the data below:
Mass of apples (IV)
1kg
2kg
3kg
5kg
10kg
Cost of apples (DV)
$1.20
$2.40
$3.60
$6
$12
The gradient or slope of the graph gives us a constant e.g. X/ Y =constant
(it doesn’t matter what X we choose, we will get the same constant answer).
Calculate the constant for the apples. (constant = cost/ mass)
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From our graph in the Voltage – Current relationship experiment,
the line of best fit is a straight line and this tells us:
 that the resistor in the water is an ohmic resistor
 that voltage is proportional to the current for this resistor
 if we divide any value of volts with its corresponding value for
amps we will get a constant number
 this constant number is called resistance
 Resistance = Volts divided by Amps ( R=V/I )
 The units for resistance are Ohms
 Resistance is not measured directly but is always calculated
from the volt and amp measurements.
 Ohm’s Rule
R= V/I
Resistance = Voltage / Current
In Ohms
In Volts
In Amps
Draw the triangle for this equation.
Write out the other 2 equations from the triangle.
Example:
Calculate the resistance of a resistor that allows 4 amps of current to pass
through in a 12 volt circuit?
R = V/I
R = 12/4 = 3.0 ohms
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Now you try:
1. Calculate R, if a circuit has 240 volts and the resistor passes 10
amps?
2. Calculate V, if resistance is 12 ohms and current is 4.5amps?
3. Calculate I, if voltage is 12 volts and resistance is 2.3 ohms?
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5. Series and Parallel Circuit revision
Do NDS P85 p24.
Write up with TAMRCD.
Include tasks # 1, 2, 3, 4, 5, 6, 7.
The Famous Xmas Tree Light Thought experiment
Xmas tree lights can be wired up in either series or parallel.
Draw circuit diagrams for Xmas lights in series and in parallel, joining at
least 6 lights in each circuit.
Each of these 2 types of wiring systems for Xmas lights have advantages
and disadvantages, list the main advantages and disadvantages for series
and parallel Xmas lights.
Xmas Lights….
Advantages
Disadvantages
In series
In parallel
One of these systems is considered a “cheaper” and less user friendly
system than the other. Using your list of advantages and disadvantages
above, explain which system is the “cheaper” and which is the “dearer” and
why? Feel free to test your ideas by wiring up the real systems in class.
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6. Current in a Series Circuit
Current is
 a measure of the number of electrons passing a given point at any
one time
 measured in amps or amperes, symbol A
A
 measured with an ammeter, symbol
 1 amp = 1 coulomb of electrons pass a given point every second
 1 coulomb = 6 X1018 electrons!
 a good analogy for electric current (flow of electrons) is a line of
people tramping on a single track that can’t pass each other
Ammeters are always set up in series with the lamp or resistor being
measured and this is why touch tests must always be done before setting
up any permanent circuits. Ammeters only work in one direction.
Ammeters measure the number of electrons passing a point every second.
Current in a Series Circuit experiment
Aim: To investigate the current in simple circuits with 1, 2 and 3 lamps in
series.
Method:
1. Set up a simple circuit with a 12v power supply, one ammeter, one
lamp and some wires.
2. By shifting the wires and rearranging the ammeter measure and
record the current in both positions in the circuit.
3. Add one more lamp in series and repeat part 2, you will have 3
ammeter readings to do
4. Add a third lamp in series and repeat part 2 with 4 ammeter readings.
5. Draw all 3 circuits and label ammeter positions A1 to A4.
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Results:
1 lamp
A1=
2 lamps
A1 =
3 lamps
A1=
A2=
A2 =
A2=
A3 =
A3=
A4=
AT=
AT=
AT=
Conclusions:
 Current is the same anywhere in a series circuit. In a series
circuit all the electrons must pass around the whole circuit as
they have no choice about where they can go! The electrons
always start and finish at the same place - at the power supply.
 Current decreases as more lamps are added. It is harder for the
electrons to get around the circuit so they slow down, i.e. more
resistance. (Think of the lamps as hills, electrons slow down
with more lamps as do trampers with more hills in their journey)
 Total Current (AT) = A1 = A2 = A3 in a Series Circuit only !!!
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7. Current in a Parallel Circuit
Revise what current is, how it is measured and what units it is measured in.
Remember touch tests on ammeters!!!
Current in Parallel Circuit experiment
Aim: To investigate the current in simple circuits with 1, 2 and 3 lamps in
parallel.
Method:
1. Set up a simple circuit with a 12v power supply, one ammeter, one
lamp and some wires.
2. By shifting the wires and rearranging the ammeter, measure and
record the current in both positions in the circuit.
3. Add one more lamp in parallel and repeat part 2, you will have 6
ammeter readings to do
4. Add a third lamp in parallel and repeat part 2 with 8 ammeter
readings.
5. Draw all 3 circuits and label ammeter positions A1 to A8.
6.
Results:
1 lamp
2 lamps
3 lamps
A1=
A1=
A1=
A2=
A2=
A2=
A3=
A3=
A4=
A4=
A5=
A5=
A6=
A6=
A7=
A8=
AT=
AT=
AT=
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Conclusions:
 Total current increases as more lamps are added. It is easier for
the electrons to flow so they move faster i.e. less resistance.
(The number of the tramping tracks has increased so the
trampers can move faster)
 The current in each branch of the circuit adds up to equal the
total current. Current splits at each junction. (The total number
of trampers is the same but they have a choice about which way
they go)
 The brightness of each lamp in each loop is the same as the
brightness of a single lamp.
 Total Current (AT) = A1 + A2 + A3 only in a parallel circuit !!!
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8. Voltage in a Series Circuit
Voltage is
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a measure of the electrical energy each electron has in a circuit.
measured with a voltmeter
units are volts (V)
V
also known as potential difference
supplied by the power supply and used up in the resistors and lamps
a good analogy for voltage and electrical energy is a pack of food that
each electron or tramper gets before leaving the power supply and is
all used up around the circuit as they travel back to the power supply.
Voltmeters are always set up in parallel with the resistor or lamp being
tested.
Remember your touch test for all electric meters!
Ammeters in series and voltmeters in parallel when setting up simple
electrical circuits.
Voltage in Series Circuits Experiment
Do NDS P86 p24 part 1 and 2 with 1, 2, 3 lamps in series separately.
Draw circuit diagrams for each of your 3 circuits. Label all voltmeter
positions.
Record with TAMRCD
Results:
1 lamp series circuit
Lamp 1 V=
VT=
2 lamp series circuit
Lamp 1 V=
Lamp 2 V=
VT=
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3 lamp series circuit
Lamp 1 V=
Lamp 2 V=
Lamp 3 V=
VT=
Conclusion:
The voltage of each lamp/ bulb can be added together to equal the total
voltage supplied by the power supply in a series circuit. All lamps or
resistors share the electrical energy (volts) supplied by the power supply.
 Total Voltage (VT)= V1 + V2 + V3 for a series circuits only !!!!
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9. Voltage in a Parallel Circuit
Voltage in Parallel Circuits experiment
Do part 3 and 4 of P86 p24 NDS with 1, 2, 3 lamps in parallel separately.
Draw circuit diagrams for each of your 3 circuits. Label all voltmeter
positions.
Record with TAMRCD
Results:
1 lamp parallel circuit
Lamp 1 V=
2 lamp parallel circuit
Lamp 1 V=
Lamp 2 V=
VT=
VT=
3 lamp parallel circuit
Lamp 1 V=
Lamp 2 V=
Lamp 3 V=
VT=
Remember
 Voltmeters are always set up in parallel with the lamp being tested
 Ammeters in series and voltmeters in parallel.
 Touch tests!
Conclusion:
The voltage in each lamp is the same (equal) as the total voltage of the
power supply.
The current splits up in a parallel circuit but each electron or tramper has
the same amount of electrical energy (volts) at each lamp or resistor
 Total Voltage (VT) = V1 = V2 = V3 for a parallel circuit only !!!
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Do NDS Exercises 12D p201 # 1, 2, 3, 4, 5, 6
Summarise the differences and list the advantages and disadvantages of
series and parallel circuits in general.
List common uses of parallel and series circuits i.e. cars, houses, Christmas
lights, etc…
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10. Power
Power is
 the rate we use or supply electrical energy
 units of power are Watts (W)
 One Watt (1 W) = 1 Joule of electrical energy is used or supplied per
second (1 J/s = 1 W)
 Power is calculated by multiplying the voltage (V) in volts by the
current (I) in amps
Power = Voltage x Current
P = V.I
in Watts
in Volts
in Amps
Write in triangle notation.
Write the family of equations from the triangle.
Remember, from earlier work the other Power equation:
 Power (P) = E/t = Energy/time = Work/time
Example: Calculate the power of a 12 volt electricity supply with 4 amps of
current?
P = V.I
P = 12 volts x 4 amps = 48 watts
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Now you try:
1. Calculate the power, if a circuit has 240 volts and 10 amps?
2. Calculate the voltage, if the power is 36 watts and current is 4.5
amps?
3. What current will flow in a lamp, if the voltage is 12 volts and power
supplied is 55 watts?
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11. The Total Current (IT) and Total Resistance (RT)
relationship (extension)
We already know that if we increase resistance (R) then the current (I)
decreases and if we decrease resistance (R) the current (I) increases (see
the resistance in a one metre length of nichrome wire lesson).
We also know that:
V = I.R
I = V/R
and R= V/I (see Ohm’s rule lesson)
Complete the following tables using the data from your earlier experiments
and graph ITotal against the number of bulbs for both series and parallel
circuits, on the same axis.
Results:
# bulbs
VTotal
ITotal
RTotal
# bulbs
VTotal
ITotal
RTotal
1
12v
Bulbs in series
2
12v
3
12v
1
12v
Bulbs in parallel
2
12v
3
12v
Graph the change in the Total Current (IT) with the change in the number of
bulbs for series and parallel circuits.
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Conclusion:
The graph shows:
Series Circuits
 Increasing the # of bulbs in series decreases IT because RT
increases.
 Decreasing the # of bulbs in series increases IT because RT
decreases.
Parallel Circuits
 Increasing the number of bulbs in parallel increases IT because RT
decreases.
 Decreasing the number of bulbs in parallel decreases IT because RT
increases.
Discussion:
Think of the electrons as trampers and they know their route from the start.
They can see the hills or bulbs or resistors as they get out of the car or
powerpack door.
The electrons and trampers act in a similar way, they know what type of
tramp or circuit (series or parallel) they will be travelling around and act
accordingly, right from the start.
In series; the more bulbs = more total resistance (= more hills for the
trampers), therefore slower and less electrons flow (or slower and less
trampers) which means the total current is less.
In parallel; the more bulbs = more choice = less total resistance, therefore
faster and more electrons flows (or faster and more trampers) which means
the total current is more.
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12. Fuses
Fuses
 are devices that protect electrical circuits and components from
damage from unexpected increases in current or voltage
 are made with fuse wire or reset switch mechanisms
 are usually rated for maximum current (amps) to flow through them
 work by failing, burning out or switching off if the electricity becomes
too high
 can be wired externally in series in a circuit leading to a component or
internally inside a component
Examples are readily seen in all houses and buildings switch boards, in all
cars, motorbikes, trucks, boats and other powered vehicles fuse boxes and
in the back of power packs
Draw the modern symbol for a fuse
WB page 241 Investigating Fuses (see teacher)
Use iron wool for a fuse
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13. Diodes
Diodes are
 A type of resistor that allows current to flow in one direction only
 examples include; LED’s used in standby lights in household
appliances (LED = light emitting diodes) and semiconductor diodes
 electrons move from the negative terminal on the power supply to the
positive (remember “like” charges have attractive forces acting on
them)
Draw the circuit diagrams for simple circuits containing semiconductor
diodes and light emitting diodes.
Do NDS P90 p25
Do NDS 12F p205 # 1, 2, 3, 4, 5, 6, 7
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