Download Test 3 (Magnetic Field I)

AL (F.7) Physics Test 3
Chapter 3.4 ── Magnetic Field I
Time allowed: 35 mins
Name: ___________________(
Date: 6-10-2006 (Fri)
)
Section A  Structural Question (13 marks)
25

%
1. (a)
Figure 1
Figure 1 shows a simple current balance. A flat solenoid is connected to a
horizontal rectangular copper loop ABCD, such that the same current can pass
through them as shown. The loop is pivoted on the axis XY which is mid-way
between AB and CD, with CD inside the solenoid and perpendicular to the axis of
the solenoid. When a current, I, flows through the solenoid and the loop, a rider of
mass 104 kg has to be placed on AB to restore equilibrium. The length, l, and the
number of turns, N, of the solenoid are 50 cm and 600 respectively. The length of
CD is 20 cm.
(Permeability of free space o = 4  107 H m1 )
(a) Indicate on Figure 1 the direction of the magnetic field inside the solenoid.
(1 mark)
(b) (i) If the magnetic field strength inside the solenoid can be calculated by the
formula
o NI
l
, find, in term of I, the force acting on arm CD.
(2 marks)
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(ii)
Hence deduce the value of I.
(2 marks)
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(c) State and explain one precaution of the experiment.
(3 marks)
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(d) Is this current balance useful for measuring a.c. as well ? Explain briefly.
(2 marks)
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(e)
The solenoid is not infinitely long. What effects does this have on the value
of I obtained in (b)? Explain briefly.
(3 marks)
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Section B M.C. Questions (12 marks, @1.5marks)
Please leave the M.C. answers in the following blanks.
M.C. Ans:
1. ___
2.___ 3.___ 4.___ 5.___ 6.___ 7.___ 8.___
1.
13
12
-2
A vertical wire 0.2 m long carries a current of 0.5 A in a magnetic field of flux density 10 T,
which dips at angle of 60o to the horizontal. The force on the wire is
-4
A.
5.0 × 10 N
D.
1.5 × 10 N
-3
-4
B.
8.7 × 10 N
E.
1.7 × 10 N
C.
-3
1.0 × 10 N
-3
2.
2/4
In the figure shown, MN is a fixed long conductor, carrying a current I1. HK is another conductor
perpendicular to MN. When a current I2 is allowed to pass through HK in the direction shown, the
force on HK
A. acts in the +y direction.
B. acts in the -y direction.
C. acts in the +x direction.
D. acts in the -x direction.
E.
zero.
3.
The above figure shows two long parallel straight wires separated by a distance of 0.2 m, carrying
currents of 1 A in same directions. The magnetic field at a point X mid-way between the wire is
(Given : permeability constant o = 4   10-7 T m A-1 )
A.
D.
0T
4  10-6 T into paper
B. 2  10-6 T into paper
E. 4  10-6 T out of paper
C. 2  10-6 T out of paper
4.
A very long solenoid with a metallic core has a radius r and n turns per unit length. It carries a
current I. The magnetic flux density B on its axis is
(1)
independent of r.
(2)
proportional to n.
(3)
independent of the material of the core.
A. (1), (2) and (3) are all correct
B.
(1) and (2) only are correct
C. (2) and (3) only are correct
D. (1) only is correct
E.
(3) only is correct
5.
Which of the following graphs best represents the variation of the magnetic flux density B along the
axis of a long current-carrying solenoid, with the distance x from the centre of the solenoid along the
axis?
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6.
Two long, straight, parallel conducting wires P and Q are positioned as shown. The same current
flows through both wires and is directed into the plane of the paper. Points A, B and C on the plane
of the paper are equidistant from both wires where C is the mid-point between the wires. Which of
the following statements is/are correct?
(1) The magnetic field strength at C is zero.
(2) The directions of the magnetic field at A and at B are in y- direction.
(3) The magnetic field strength at B will increase if the current flowing in the wires increases.
A.
(1) only
B.
(3) only
C. (1) and (3) only
D.
(2) and (3) only
E.
(1), (2) and (3)
7.
An a.c. supply S of frequency 50 Hz is connected to a resistor R via two long, parallel metal wires X
and Y, as shown. The force between X and Y
A. is equal to zero.
B. always attracts.
C. always repels.
D. sometimes attracts and sometimes repels; the frequency of variation is 50 Hz.
E.
sometimes attracts and sometimes repels; the frequency of variation is 100 Hz.
8.
X and Y are two flexible conducting ribbons, where mass of Y is larger than X suspended from
points P and Q. The bottom parts of the ribbons lie in dishes of a conducting liquid at R and S.
When a current 2I is passed from P to R through X, and a current I is passed from Q to S through Y,
which of the diagrams best represents the shapes assumed by the ribbons (if the ribbons remain in
contact with the conducting liquid at R and S)?
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