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Science

Physics Task: Magnets DC motors, Electromagnetic induction-Task and Questions

PART 1
The diagram below shows the cross-sections of two wire conductors M and N, both
perpendicular to the page and both carrying equal currents into the page. In these questions
ignore any contribution from the Earth’s magnetic field. The arrows A-D and letters E-G
represent options from which you are to choose in answering questions 1, 2 and 3.
A
P
M
Q
Conductors
N
D
E out of the page
F into page
G zero field
B
C
1.
What is the direction of the magnetic field due to the two currents at point P?
Answer + force diagram + explanation
(2
marks)
2.
What is the direction of the magnetic field due to the two currents at point Q?
Answer + force diagram + explanation
(2
marks)
3.
The current in conductor N is reversed so that the current is flowing out of the page.
What is the direction of the magnetic field due to the two currents at point P now?
Answer + force diagram + explanation
(2
marks)
4.
A wire MN experiences a force which acts down between the jaws of a horseshoe
magnet (see below). Current flows in the wire from M to N.
Downward
force
Current
Flow
Which is the north pole of the magnet (X or Y), and which is the south pole of the
magnet (X or Y) to cause this downward force?
Answers + one sentence explanation
(2 marks)
North Pole =
South Pole =
Questions 5-8 refer to the following information.
A DC motor consisting of a pair of magnets and a rotor coil is shown below at t = 0 seconds.
Note: assume that there is acommutator attached to the rotor’s coil.
The magnets produce a uniform magnetic field of 0.80T. The rotor coil consists of 20 turns of
wire, each of which are squares of side length 4cm. The rotor coil is rotating with a frequency
of 5.0Hz, and has a current of 1.5A flowing through it.
5.
Calculate the magnitude of the magnetic force acting at t = 0 seconds on
(a) side AB of the coil
(b) side BC of the coil
Formula + calculations
(2 marks)
ForceAB =
N
ForceBC =
N
6.
Which one of the following describes the initial rotation of the coil in the figure
above:
A. clockwise B. anti-clockwise C. it doesn’t rotate D. cannot be determined
Answer + explanation
(2 marks)
7.
At which time(s) will the rotor coil experience maximum torque? (1 or more answers)
At = 0s Bt = 0.05s Ct = 0.10s Dt = 0.15s E t = 0.2s
Answer + explanation
(2 marks)
8.
Describe what would happen if there is no commutator attached to the coil.
Explanation required
(1 mark)
9.Which of the following actions best describes the situation in which the coil is caused to
rotate faster?
A increasing the current
B increasing the magnetic field strength
C increasing the cross-sectional area of the coil
D all of the above.
Answer + explanation
(2 marks)
Questions 10-12 refer to the following information.
An electromagnet consists of a coil of wire wrapped around a soft-iron core. An electric
current is then passed through the wire. A student uses a magnetic compass to investigate the
direction of the magnetic field around the electromagnet. Assume that the magnetic field of
the Earth can be ignored.
10.
Which end of the coil of wire is the North Pole: the left end or the right end?
Answer only
(1 mark)
Use the answer key above right to indicate the direction of the magnetic field of the
electromagnet at each of the following locations. If there is no magnetic field write zero
11.
12.
At point C which is inside the coil of wire.
Answer only
2
(1 mark)
W
m
At point B which is directly underneath the coil of wire
Answer only
(1 mark)
A current-carrying wire is placed to the right of a current-carrying solenoid (see below).
I2
I1
13.
W
m
State the direction of the magnetic force on the wire at P. Use the key given below.
Answer
(1 mark)
2
W
m
2
14.
A to the right
B to the left
C up the page
E into the page
F out of the page
G no force
D down the page
A moving coil loudspeaker contains a coil of wire of 1000 turns, each of radius 5.0
cm, placed in a radial magnetic field of 0.40T. The wire of the coil is at right angles to
the magnetic field at all times.
When there is a current of 0.25A in the wire what is the magnitude of the force on the
coil?
Formula + working
(2 marks)
N
15.
Two particles X and Y are travelling through a magnetic field which is directed into
the page. See below.
What is the charge on each particle X and Y: positive or negative? If none, write “no
charge”. Explain your answers.
W
m
2
1.
2.
PART 2
A circular loop of wire of area 0.40 m2 is in a magnetic field of 2.5 10-2 T. What is the
magnetic flux through the loop if the plane of the loop is (a) perpendicular (b) parallel to
the direction of the magnetic field?
Answers + formulas and calculations
(2 marks)
(a)
Wb
(b)
Wb
An emf of 1.5 millivolts is induced in a solenoid consisting of ten turns of wire when
the magnetic flux threading the solenoid increases from zero to a value of X in 1.5
milliseconds.
What is the magnitude of X?
Use a formula and calculations to explain your answer
(2
marks)
Wb
A square loop of wire of side length 5.0 10-2m rotates at 50Hz between the poles of a
large electromagnet. The strength of the magnetic field of the electromagnet is 0.80 T.
3.
4.
How much flux cuts through the loop when the plane of the loop is (a) paralleland (b)
perpendicular to the direction of the magnetic field?
Use a formula and calculations to explain your answer
(3 marks)
(a)
Wb
(b)
Wb
Calculate the average EMF induced as the loop turns 90 from the parallel position
relative to the magnetic field, to the perpendicular position relative to the magnetic
field, as described in Question 3 above?
Use a formula and calculations to explain your answer
(3 marks)
V
5.
If the resistance of the loop is 4.0, what current flows in the loop during this 90
rotation?
Use a formula and calculations to explain your answer
(2 marks)
A
The figure below shows the north pole of a bar magnet being pushed into a solenoid.
For each of the situations in Questions 6, 7 and 8 state whether the current flows
through the galvanometer from X to Y, from Y to X, or neither.
6.
7.
The north pole is moved towards the solenoid.
Answer + vector diagram.
marks)
The north pole of the magnet is withdrawn from the solenoid.
Answer + vector diagram.
(2
(2 marks)
8.
The magnet is held stationary in the solenoid.
Answer + vector diagram or explanation.
(2 marks)
A physics student pulls a rectangular loop of wire at a constant speed through a uniform
magnetic field. The magnetic field has a strength of 0.30 T and the loop itself is 4.0 cm
long and 3.0 cm wide.
9.
Calculate the magnetic flux threading the loop when it is completely immersed in the
magnetic field.
Use a formula and calculation to support your answer
(2 marks)
10. The student keeps pulling the loop with a constant speed until it completely exits the
magnetic field.
Which of the graphs below best demonstrates the variation of the magnetic flux through
the loop with time?
Answer + explanation
(2 marks)
Wb
11. Copy below the flux-time graph you chose for Question 10. Using this graph sketch a
graph of the (a) EMF induced across the ends of the loop and (b) the current I in the
loop with respect to time. Hint: the induced voltage is given by the negative slope of a
flux-time graph.
(3 marks)
EMF
t
t
I
t
PART 3
To investigate electromagnetic induction a student uses the equipment shown below.
The student moves the coil vertically out of the field at a constant speed. The entire coil is
within the field from t = 0 until t = 0.40s. It leaves the field between t = 0.40 s and t = 0.60 s.
The graph below shows how the magnetic flux through the coil varies with time.
Magnetic
Flux
0
1.
M
a
g
n
e
t
i
0.2
c
F
l
u
following
x
0.4
0.6
0.8
Time (s)
Which of the
graphs on the next page best shows the variation with time of the
voltage across the ends of the coil? Write your answer here with an explanation.
Answer + explanation
(2 marks)
The coil has 200 turns and the area of each turn is 0.020 m2. The uniform magnetic field is
0.080 T.
2.
What is the average voltage produced across the ends of the coil for the time interval from
t = 0.40s to t = 0.60s?
Include a formula and calculations in your answer
(2 marks)
V
The coil is now rotated counterclockwise at constant speed about its axis of rotation, and the
variation of voltage with time is shown below.
V
2
2
1
1
X
Y
Axis of rotation
10
20
30
40
50
60
Time (ms)
5
3.
During the first half (180) of the coil’s rotation, does the induced current flow from X,
through the coil, to Y; or from Y, through the coil, to X?
Answer + explanation
(2 marks)
X to Y?
4.
or Y to X?
What is the frequency of rotation of the coil?
Formula + calculations
(2 marks)
Hz
5.
The student then increases the rate of rotation of the coil to twice the initial rate.
On the voltage-time graph (see previous page) draw another voltage-time curve showing the
variation of voltage with time which would be observed when the coil is turned at twice the
initial rate.
Make sure your curve is accurately drawn.
(2 marks)
A transformer for a transistor radio steps down 240V AC to 9.0V AC, which in turn is
converted by diodes to 9.0V DC. The secondary coil of the transformer consists of 60 turns and
the radio draws 400 mA of electric current.
6.
Calculate the number of turns on the primary coil of the transformer.
Use a formula and calculation in your answer
(2 marks)
turns
7.
Calculate the current in the primary coil of the transformer. Answer in mA.
Use a formula and calculation in your answer
(2 marks)
mA
Questions 8-12 refer to the following information.
A student uses a laboratory transformer to power a small heating coil. The transformer runs off
240V RMS mains electricity and has a secondary voltage of 12V RMS. The heating element is
using 6W of power.
8.
Calculate the peak-to-peak voltage in the secondary coil.
Use a formula and calculation in your answer
(2 marks)
V
9.
Determine the value of the ratio: Turns in Secondary Coil
Turns in Primary Coil
Use a formula and calculation in your answer
(2 marks)
10. Calculate the current in the (a) secondary coil (in A)and the (b) primary coil (in mA) of the
transformer.
Use a formula and calculation in your answer
(3 marks)
Current SECONDARY =
A
Current PRIMARY =
mA
11. Which side of the transformer (primary or secondary) should have the thicker wires? Explain.
Answer + explanation required.
(2 marks)
Primary or
secondary?
12. During the practical activity the student mistakenly switches to 240V DC. Would the heating
element still operate? Explain.
(2 marks)
PART 4:Yenka Activity
Week 4Yenka: Power Transmission activity
Purpose
In this activity you will compare the voltage loss in two circuits: one without a transformer (Circuit 1)
and one with a transformer (Circuit 2). In both circuits the signal generator represents a power station,
the 1kresistor represents the resistance of the transmission lines and the 10 resistor represents the
load at the consumer.
Circuit 1
Open, in the following order: Yenka, New Model. From the Parts Library (on the left of screen)
click on Electronics, Analog. Click on Signal Generators and drag the generator on to the
simulation window. Click on the (green) signal generator and in the Propertiespane at left change the
frequency to 50Hz and the peak voltage to 45V.In the Parts Libraryclick on Passive Components,
and drag two Resistors on to the simulation window.Click on each resistor in turn and in the
Properties pane change one to 1K and the other to 10. To rotate resistor click on the ‘circle’
above the resistor and rotate. To wire components together, click and draw between them with your
mouse. From the Power Supplies folder drag the Ground (Earth) to the position shown in Circuit 1
below.
From the Parts Library, click Presentation (scroll down) and drag the Graph icon onto the
simulation window.Click and drag on the Target toolnext to the red dash on the graph window so that
a line connects the target tool (a crosshair will appear next to the cursor)to a point between the signal
generator and the 1K resistor. The connecting line goes solid when the connection has been made,
and when the mouse button is released it turns a red-dashed colour. In the Traces folder in the
Properties pane find Trace 2 in the drop-down menu. Check the Show trace box. Go back to your
graph and drag on the new (blue) Target tool so that a line connects the tool (crosshair) to the circuit
between the 1k and the 10 resistor. Click on Property…to the left of the y-axis on the graph and
click on Voltage from the drop down menu for both traces. The Properties pane should appear on left
of screen. Click on th Y-axis tab and adjust the Range from -50V (Min) to +50V (Max)with major
gridlines every 5V. Set the X-axis range from 0 to 200 ms (milliseconds), with major gridlines every
50ms. Remember to use the Answer Questions 1 and 2 before drawing Circuit 2.
Circuit 2
Click on Signal Generators and drag another generator on to the simulation window. Click on the
(green) signal generator and in the Properties pane at left change the frequency to 50Hz and the peak
voltage to 45V.In the PartsLibrary click on Passive Components and drag two transformers on to
the simulation window. Click on each transformer in turn and in the Properties pane change one (left)
ratio to 0.4: 1 and the other (right) to 20:1. Drag two resistors,with the same values as you did with
Circuit 1, on to the simulation window, and wire the components together. Add threeGroundsto the
circuit as shown in Circuit 2 below.
Placing the cursor over the red target tool (crosshair), drag the tool to a point on Circuit 2 to the left of
the 1k resistor in the Transmission Circuit. Repeat for the blue target tool (crosshair), and drag the
tool to a point to the right of the 1k resistor in the Transmission Circuit. Now go back to the Traces
folder in the Properties pane. Find Trace 3 in the drop-down menu (green). Check the Show trace
box. Go back to your graph and drag on the new (green) Target tool so that a line connects the tool
(crosshair) to a point to the left of the 10 resistor in the Load Circuit.
Adjust the Y-axis scale on the graph to ± 120V with major gridlines every 5V so that you’ll see all
three traces. See diagram below. Now answer Questions 3 and 4 on the next page.
Remember: Click on the Start simulation button in the toolbar to start the simulation. You may need
to click on the Restart button (right of graph window) first to reset. Also, use the Pause simulation
button again to capture your traces on your graph.
Refer back to the Sliding Box activity in Unit 3 Week 3 to familiarise yourself with Yenka.
Week 4Yenka activity: Power TransmissionThis is a compulsory activity
Circuit 1
Question 1.Copy (or copy and paste) your graph showing the two voltage-time traces: a red trace and
a blue trace. Clearly show which is which by using red and blue ink, and clearly label and number
your axes.
Question 2. Use your graph to find (a) the voltage drop across the transmission lines (1Kresistor)
and (b) the voltage left for the consumer (10load). Explain your answers.
Circuit 2
Question 3.Copy your graph showing the three voltage-time traces: a red, a blue and a green trace.
Clearly show which is which by using red, blue and green, and clearly label and number your axes.
Question 4. Use your graph to find (a) the voltage drop across the transmission lines (1K resistor)
and (b) the voltage left for the consumer (10 load). Explain your answers.
PART 5
A remote cattle station in the Northern Territory uses a generator to provide its electricity.
The generator produces 10.0A of current at 300V.
Initially the electricity was carried direct to the farmhouse through long conductors with a
total resistance of 20.
The owner of the cattle station notices that the lights and appliances in the farmhouse are not
operating to their full capacity and so uses a voltmeter to measure the voltage being delivered
to the farmhouse.
1.
What percentage of the power being produced by the generator is lost in the transmission
lines using the arrangement above?
Use formulas and calculations in your answer
(3 marks)
%
2.
The owner decides to buy a pair of transformers to step the voltage up to 3000V for
transmission, and then down to 200V for use in the house. Assume that the transformers
are 100% efficient.
Calculate the amount of power lost in the transmission lines using the arrangement
shown above.
Use formulas and calculations in your answer
(3 marks)
W
A remote lighthouse operates on power supplied by a diesel generator located 500m
away. The power from the generator (1200W, 60V) is carried to the lighthouse through a
pair of wires having a total resistance of 2.0.
3.
Calculate the size of the current that flows in the wires leading to the lighthouse.
Use a formula and calculation in your answer
(2
marks)
A
4.
Calculate the amount of power lost in the wires.
Use a formula and calculation in your answer
(2 marks)
W
5 . Calculate the voltage that is actually delivered to the lighthouse.
Use a formula and calculation in your answer
marks)
(2
V
A power station produces 200 MW of power. It is found that 20MW of this power is lost in
the transmission lines before reaching a distant town. The voltage input to the transmission
system at the power plant is 500 kV.
6. What is the current in the transmission lines?
Use a formula and calculation in your answer
(2 marks)
A
7.
What is the total resistance of the transmission lines?
Use a formula and calculation in your answer
(2 marks)
8.
What is the voltage delivered to the town?
Use a formula and calculation in your answer
(2 marks)
V
9.
At peak periods the power use in the town increases significantly. However, the power
plant continues to produce 500kV. Which of the following is the most likely effect on
the voltage being delivered to the town?
A.
B.
C.
D.
The voltage would increase slightly.
The voltage would decrease slightly.
The voltage would remain the same.
The voltage would halve.
(If you don’t know the answer please don’t guess!! Feel free to do so in the exam
though!!)
(2 marks)
10. Why is the power tranmitted at such high voltages?
At least two dot point explanations required. You must include formulas
marks)
(2
11. The graph below shows the average demand for electricity in Melbourne over a 15 hour
period on a particular day.
300
250
Power (MW)
200
150
100
50
0
9.00
am
12.00
3.00
Time of day
6.00
9.00
12.00
pm
(hours)
Calculate the total amount of electrical energy consumed between 12.00pm (mid-day)
and 12.00am (midnight) on this day. Give your answer in (a) Joules and (b) kWh.
Show all calculations
(3
marks)
(a)
J
(b)
kWh
...

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