Name: ______________________________
PHYSICS
9 September 2005
2.5 hours
Level 2
2.6 Demonstrate understanding of electricity and electromagnetism
Credits: Five
Answer ALL the questions in the spaces provided.
For all numerical answers, full working should be shown and the answer should be rounded to the correct number of significant figures and given with an SI unit.
For all ‘describe’ or ‘explain’ questions, the answer should be in complete sentences with all logic fully explained.
For Assessor’s use only
Achievement Criteria
Achievement
Achievementwith Merit
AchievementWith Excellence
Identify or describe aspects of phenomena, concepts or principles.
Give descriptions or explanations in terms of phenomena, concepts, principles and/or relationships.
Give concise explanations that show clear understanding, in terms of phenomena, concepts, principles and/or relationships.
Solve straightforward problems.
Solve problems.
Solve complex problems.
Overall Level of Performance (all criteria within a column are met)
You are advised to spend 50 minutes answering the questions in this booklet.
Assessor’s use only
QUESTION ONE: HOUSEHOLD ELECTRICITY SUPPLY
(a) Kimberly and Jamie connected a 100W lamp to a 220V power supply and a current of 0.455A was measured passing through it. Show that the resistance of the lamp is 484W.
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Gi Na and Yun Jae helped Herenia connect a toaster (780W), a heater (2400W) and a lamp (100W) together in parallel to a 220V power source.
(b) Draw a circuit diagram to illustrate this. (Use the symbol for resistors to represent each component).
220V Power Supply
(c) Calculate the resistance of the toaster.
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____________________________________ RTOASTER = ____________________
(d) Use the information in (b) to calculate the total resistance of the three appliances in the parallel circuit.
Assessor’s use only
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Resistance = _________________________________
(e) Explain why the power companies do not state the current supplied to household electrical equipment.
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Natalie suggests to Danielle that the three electrical devices should be connected in series to save money on wire. The total resistance in this series circuit is 566W.
220V Power Supply
(f) Calculate the voltage across the lamp.
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Voltage = ____________________________________
(g) Explain why the power output of the electrical equipment in the series circuit will not be the same as the power output of the electrical equipment in the parallel circuit. Recall that the resistors and power supply are equal for both types of circuits.
Assessor’s use only
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QUESTION TWO: KAHURANGI POWER COMPANY
Ashleigh, Jeanine and Tarra-Leigh visit the Kahurangi Power Company. The learn that the Power Company charges its customers 18 cents per kWh. This means that it costs 18 cents to use a 1kW device for one hour.
(a) Calculate how much it would cost to run a 2400W heater for one hour.
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Cost = ______________________________________
(b) Calculate the cost to run a TV, of resistance 60W, for 1.0 hour from a 220V source.
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Cost = ______________________________________
Power companies supply electricity in the form of alternating current. In an alternating current, electrons are moved first in one direction and then in the opposite direction. The graph below shows an example of this.
Assessor’s use only
(c) State the function of a diode.
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output
AC voltage source(d) Draw the graph of current versus time for the AC circuit with a diode connected to it, as shown in the circuit diagram.
The Kahurangi Power Company transmits electricity over great distances. To do so it uses power cables held up by large pylons.
(e) A power cable carries a 5.0A current between two pylons that are 450 metres apart. The Earth’s magnetic field in this region measures 5.5 X 10-5T and is at right angles to the power cables. Calculate the magnetic force on the cable between the pylons.
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Force = _____________________________________
Assessor’s use only
Assessor’s use only
The source of energy for the Kahurangi Power Company is a dam. Water flowing downhill turns a turbine which turns a generator which produces electricity. Below is a simplified diagram of a generator.
(f) State the direction of conventional current in the coil. Use the letters A and B, at the corners of the coil, to assign a direction.
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(g) State the design changes that could be made to the generator to increase the current produced by it.
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Assessor’s use only
QUESTION THREE: AIRBUS A380
Marsha and Stephanie watch as Wendy and Christina board an airplane-the AIRBUS A380. It is the largest commercial jet ever. Its wingspan is 79.8m. The body and wings of the plane are made from electrically conducting material. Later, the AIRBUS A380 is flying over the north magnetic pole at 250ms-1.
Diagram showing the direction of the magnetic field lines at the North Magnetic Pole
(a) Calculate the size of the induced voltage across the length of the wingspan when it is travelling over the north magnetic pole through a region where the Earth’s magnetic field is 7.0 x 10-5 T. State your answer to the correct number of significant figures.
Assessor’s use only
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_________________________________ Voltage = ________________________
(b) Trinity says that there would be no induced voltage across the wingspan if the Airbus A380 was flying along the equator. Explain why.
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Assessor’s use only
Assessor’s use only
QUESTION FOUR: CHARGE IT!
A 20V battery supplies the charge to two parallel metal plates held 0.2m apart. An electron, of charge 1.6 x 10-19C, is fired from an electron gun into the space between the plates. The mass of an electron is 9.10956 X 10-31kg.
(a) On the simplified diagram below, draw lines to show the correct shape and direction of the electric field formed between the plates
20V
(b) Show that the strength of the electric field is 100 between the two charged plates. State the correct SI unit with your answer.
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___________________________________________________Unit ___________
(c) Calculate the electric force acting on the electron anywhere between the charged plates.
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Electric Force = ________________________________
Assessor’s use only
(d) An electron enters the electric field half way between the two plates. Calculate the kinetic energy of the electron the instant before it makes contact with one of the plates.
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Kinetic Energy = ______________________________________
Formulae that you may find useful are given below
Assessor’s use only
Name:___________________________________
ASSESSMENT SCHEDULE
Achievement Standard 90257
2.6 Demonstrate understanding of electricity and electromagnetism
Note: Minor computational errors will not be penalised. A wrong answer will be accepted as correct provided there is sufficient evidence that the mistake is not due to a lack of understanding. Such evidence includes:
the last written step before the answer is given has no unexpanded brackets or terms and does not require rearranging.
the power of any number that is multiplied by a power of 10 is correct.
Correct units and significant figures are required only in the questions that specifically ask for them.
Italics indicate replacement evidence.
Question
Evidence
Achievement
Merit
Excellence
One
(a)
V = IR
220 = 0.455xR
R = 483.5W
2 Correct working
(b)
1 Resistor symbol (box or jagged line) and correct parallel circuit drawn
(c)
OR
220 = 3.45 x R
R = 62.05 = 62Ω
2 Correct current
2 Correct Answer
One (cont)
Evidence
Achievement
Merit
Excellence
(d)
PTotal = 780w + 2400w + 100w = 3280w
OR
OR other reasonable method.
2 Correct calculation of current.
2 Correct calculation of Resistance, either way
(e)
Electrical equipment has a range of resistances. Since the voltage is constant then the current depends on the resistance of the appliance being used.
1 Mentions a connection between resistance and current.
1 Clearly links resistance as the cause for different electrical devices having different currents.
(f)
Vtotal = IRtotal
I = 220/566
I = 0.38869
Vlamp = IRlamp
Vlamp = (0.38869A)(490Ω)
Vlamp = 188.13 = 188 OR 190V
2 Current for the circuit calculated
2 All Correct
(g)
Power is directly related to current and voltage (P = IV). The decreased total resistance in the parallel circuit results in greater current. The voltage across each component is also greater in parallel circuits because they are linked directly to the power source. The current and voltage are greater for parallel circuits and so must be the power output.
1 Power output for parallel circuits is greater.
1 Explanation of achieve answer includes a correct and relevant statement about resistance or current or voltage.
1 Clearly links the relationship between resistance and current, and voltage to the power output.
Question
Evidence
Achievement
Merit
Excellence
Two
(a)
(2.4kW)(18cents/1kWh)(1 hours)
= 43.2cents
= 43 cents or $0.43
2 Correct
(b)
V = IR
220 = I (60)
I = 3.6666666666
P = IV
P = (3.66666666)(220)
P = 806.67
Cost = (0.81kW)(18cents/kW)
Cost = 14.5199
Cost = 15cents
2 Power of the TV is calculated correctly
2 All Correct
(c)
Allows current in one direction only
1 Correct
(d)
Or upside down
1 Correct
(e)
F = BIL
F = (5.5X10-5T)(5.0A)(450m)
F = 0.12375N
F = 0.12N
2 Correct
(f)
A to B
1 Correct
(g)
iron centre
stronger magnetic field
more wires per coil
more coils
1 One correct
1 Two correct
1 Three or more correct
Question
Evidence
Achievement
Merit
Excellence
Three
(a)
V = BvL
V = (7.0X10-5T)(250ms-1)(79.8m)V = 1.3965V
V = 1.4V
2 Correct
Sf
2 sf
1 correct
(b)
The wingspan of the AIRBUS (charge carrying conductor) would be parallel to the magnetic field. This means there would be no change in the magnetic field and no resulting induced voltage.
1 Some understanding of the relationship between induced voltage and direction of the magnetic field.
1 Clear understanding of the relationship between induced voltage and the direction of the magnetic field.
Question
Evidence
Achievement
Merit
Excellence
Four
(a)
20VDiagram contains evenly spaced lines with arrows pointing down.
1 Correct answer
(b)
E = V/d
E = 20V / 0.2m
E = 100Vm-1
2 Correct answer
Unit
Vm-1 .
1 Correct answer
(c)
F = Eq
F = (100Vm-1)(1.69X10-19)
F = 1.69X10-17N
F = 2X10-17 N
2 Correct answer
(d)
ΔEp = Eqd
ΔEp = (1.69X10-17N)(0.1m)
ΔEp = 1.69X10-18J
1 Energies same stated OR
2 Correct calculation.
2 Energies the same stated andcorrect calculation.
Achievement
Merit
Excellence
Total Opportunities: Criterion 1
Criterion 2
4
5
4
3
3
3
Suggested Sufficiency Statement
Achieved
Achieved with Merit
Achieved with Excellence
Criterion 1, explain.
3 x AME1’s
2 ME1’s in 5 AME1’s
2 E1’s, 4 ME1’s in 7 AME1’s
Criterion 2, solve
3 x AME2’s
2 ME2’s in 5 AME2’s
2 E2’s, 4 ME2’s in 7 AME2’s
Both criteria must be met at a particular level in order to achieve at that level overall.
