Download Practice Test - Magnetic Fields File

Le Fevre High School
SACE Stage 2 Physics
Practice Test - Magnetic Fields & Motion in Magnetic Fields
1. A small current carrying wire of length 2 cm is
placed in a uniform magnetic field as shown.
The magnetic field is perpendicular and
directed into the plane of the page.
(a) Determine the direction of the force
on the current carrying wire if the
current flows from west to east.
(1 mark)
(b) The magnitude of the force on the
wire is measured as 2 x 10-7 N,
when the current flowing through
the wire is 1.5 A. Determine the
magnitude of the magnetic field.
(2 marks)
2. Sketch the magnetic field in each of the
following situations.
(a) Straight conducting wire.
(b) Cross section of a solenoid
I
Cross sectional
cut through
centre of the
solenoid.
I
I
(3 marks)
3. A particle of mass m, charge +q is
initially stationary in an electric field
of strength E. E is in the plane of the
page as shown.
(a) Explain the resulting motion of the
positive charge.
(2 marks)
(b) The electric field is now turned off
and a magnetic field of strength B
directed at right angles to the plane
of the page is now switched on.
Explain the resulting motion of the
moving charged particle.
(2 marks)
(c) Show that the period of revolution
of the resultant motion is given by
E
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2m
Bq
(3 marks)
4. (a) Draw a schematic diagram of a moving coil loudspeaker.
(4 marks)
(b) Briefly explain the action of the moving coil loudspeaker.(3 marks)
T
5. (a) What quantities determine the time an ion takes to complete a semi-circle in the
dee of a cyclotron.
(2 marks)
(b) In an experiment a magnetic field of strength 2.5 T was used, and protons were
extracted at a radius of 35 cm. What was the period of the protons in the
cyclotron?
(2 marks)
6. Ions travelling with the same velocity enter a uniform magnetic field initially at
right angles to the field.
(a) If two of the ions have equal masses but different charges, explain which ion
will move in the path with the larger radius?
(2 marks)
(b) If an ion is travelling with a velocity of 5.0 x 107 m s-1 and the magnetic
field is of intensity
3.5 x 10-3 T, determine the magnitude of the force per unit charge acting on
the ion.
(2 marks)
TOTAL MARKS: 28
Le Fevre High School
SACE Stage 2 Physics
Practice Test - Magnetic Fields & Motion in Magnetic Fields
Solution
1. (a) Using the right hand rule
I
B
FB
Force will be in the plane of the page and directed up the page.
(b)
l = 2 cm = 0.02 m,
FB = 2 x 10-7 N, I = 1.5 A
F = BIlsin at  = 90o
2 x 10-7 = B(1.5)(0.02)
B = 6.7 x 10-6 T
2.
Field lines circular, non
uniformly spaced (closer
together near the wire)
I
3.
(a) Since in an electric field and charge experiences a force, then this force on a
positive charge is directed the same as that of the field lines. Hence the charge
moves in a straight line at a constant acceleration in the direction of the electric
field lines. (to the left)
(b) Using the right hand rule, with velocity (thumb) to the left, the B field into the
page (extended fingers), then the resulting force is in the plane of the page and
vertically down.
This then changes the direction of the moving particle. However since the force is
perpendicular to the velocity, it does not alter its speed. However as the charge
changes direction, then the force also changes to remain mutually perpendicular.
This results in the particle again changing direction but not speed. This process
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continues resulting in the particle moving in a circular path (or arc of a circle) as
the force is always directed perpendicular to the velocity and hence is an
centripetal force.
(c) Since the centripetal force is supplied by the magnetic force
FB = Bqv
mv 2
Fc =
R
2
mv 2
 2 
2
As
= m R = m  R
 T
R
2
R
Bqv = Bq
T
2
2
 2 
R
m  R = Bq
 T
T
2m
Re-arranging T =
Bq
4.
(a)
Outer suspension
inner suspension
Air gap
Voice coil
Centre cap &
dust dome
Soft iron pole
pieces
Ring Magnet
4 (b) two ends of the voice coil are connected to the output terminals of an amplifier.
Across these terminals is a changing potential difference that oscillates in
proportion to the sound waveform to be reproduced. The changing potential
difference produces an oscillating current in the voice coil.
5.
(a) the time is affected by the charge & mass of the ion, and the applied magnetic
field strength.
2m
(b) T =
(from # 3)
Bq
=
2  (1.67 x 10 27 )
(1.6 x 10 19 )(2.5)
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= 2.62 x 10-8 s
6. When they enter the uniform magnetic field then magnetic force supplies the
centripetal force.
Fc = FB
mv 2
= Bqv
R
mv
hence R =
Bq
since R  1/q and they have the same
m, v the particle with the smaller charge will travel in the larger radius path.
(b)
F = Bqv
F/q = Bv
= (3.5 x 10-3)(5.0 x 107)
= 1.75 x 105 N C-1