Tuesday, June 2, 2015

Physics 9702 Doubts | Help Page 160

  • Physics 9702 Doubts | Help Page 160

Question 793: [Electric field]
A dipole is a pair of one negative charge and one positive charge of equal magnitude. Electric field of a dipole is shown below.
In which direction does the force act on an electron when at point X?

Reference: Past Exam Paper – June 2012 Paper 11 Q31 & Paper 13 Q31

Solution 793:
Answer: D.
The direction of the electric field lines indicates the electric force on a positive charge. For an electron, which has a negative charge, the direction of the force would be in the opposite direction than that shown by the arrow. [A and B are incorrect]

At any specific point, the electric force acts along the tangent to the electric field at a point. At point X, the direction of the force is along D.

Question 794: [Matter > Deformation]
Diagram shows the force-extension graphs for two materials, of the same dimensions, loaded to fracture.

What describes the behaviour of the materials?
A Both materials are brittle.
B Both materials obey Hooke’s law.
C Both materials are plastic.
D Both materials have the same ultimate tensile stress.

Reference: Past Exam Paper – November 2012 Paper 12 Q26

Solution 794:
Answer: D.
Both of these wires break at the same force (or almost). This can be seen by drawing a horizontal line at the maxima of both curves. This would correspond to the same value of force on the vertical axis.

The wires have identical diameters (dimensions) and thus identical cross-sectional areas. So they both have the same ultimate tensile stress.
Stress = Force / Area

Hooke’s law: F = ke, giving a curve with a straight line
Choice B is incorrect since one of the graphs is a curve, and thus does not vary linearly (does not obey Hooke’s law).

A material is brittle if, when subjected to stress, it breaks without significant deformation (strain). In a graph, a brittle material is indicated by the fact that it does not extend much. That is, the graph would not extend along the horizontal axis. As soon as a significant force is applied, the material breaks (if it breaks, it can no longer extend). Only one of the graphs corresponds to this.

Plastic materials undergo permanent deformation, but does not break easily. The other curve corresponds to this.

Question 795: [Vectors]
What is the component of this displacement vector in the direction XY?

A 3.0 km                     B 4.0 km                     C 5.0 km                     D 6.6 km

Reference: Past Exam Paper – November 2010 Paper 12 Q3

Solution 795:
Answer: A.
Horizontal component = 5cos(53) = 3.0km

Question 796: [Matter > Young modulus]
A steel string on an electric guitar has following properties.
diameter = 5.0 × 10–4 m
Young modulus = 2.0 × 1011 Pa
tension = 20 N
The string snaps, and contracts elastically.
By what percentage does a length l of a piece of the string contract?
A 5.1 × 10–4 %            B 5.1 × 10–2 %            C 1.3 × 10–4 %            D 1.3 × 10–2 %

Reference: Past Exam Paper – November 2009 Paper 11 Q 22 & Paper 12 Q21

Solution 796:
Answer: B.
Young modulus, E = stress / strain = (F/A) / (e/l)
(e / l) = F / EA

Percentage by which a length l of piece of string contract = (e/l) × 100%
(e/l) × 100% = F/(EA) × 100%

Cross-sectional area. A = πd2 / 4
(e/l) × 100% = 20 / ([2.0×1011] × [π{5.0×10-4}2/4]) × 100% = 0.0509% = 5.1×10-2 %

Question 797: [Nuclear Physics]
An isotope of thorium has nucleon number of 232 and a proton number of 90. It decays to form another isotope of thorium with nucleon number of 228.
How many alpha particles and beta particles are emitted by a nucleus of thorium during this decay?
alpha particles              beta particles
A         0                                  4
B         1                                  0
C         1                                  2
D         2                                  1

Reference: Past Exam Paper – June 2014 Paper 13 Q39

Solution 797:
Answer: C.
Since they are both isotopes of thorium, the proton number of the isotope formed should also be 90. The total proton number on both sides of the equation should be equal.

Alpha particle is 42α and beta particle is 0-1β.

Choice A is incorrect since the proton number of final isotope = 90 – 4(-1) = 94.
For choice B, proton number of final isotope = 90 – 2 = 88.
Choice C is correct since proton number of final isotope = 90 – 2 – 2(-1) = 90.
Choice D is incorrect since proton number of final isotope = 90 – 2(2) – 1(-1) = 87. 

Question 798: [Electric field]
Diagram shows two metal plates connected to a constant high voltage.

Which graph shows variation of the electric field strength E midway between the two plates as the distance d between the two plates is increased?

Reference: Past Exam Paper – November 2014 Paper 11 & 12 Q29

Solution 798:
Answer: A.
Electric field strength, E = V / d

The plates are connected to a power supply, so the p.d. V across them is constant.

The electric field strength E is inversely proportional to the distance d between the 2 plates. So, as d increases, the electric field strength E decreases. [B and C are incorrect]

A graph of E = V / d at constant values of V is a curve. This can be done by assigning some values of d.

Question 799: [Matter > Hooke’s law]
A steel spring has spring constant of 150 N m–1. When a 25 N weight is hung from the spring, it has a stretched length of 55 cm.
What was original length of the spring?
A 0.38 m                     B 0.49 m                     C 0.61 m                     D 0.72 m

Reference: Past Exam Paper – November 2013 Paper 11 & 12 Q24

Solution 799:
Answer: A.
Hooke’s law: F = kx
That is, the force, F needed to extend or compress a spring by a distance x is proportional to the distance.

Let the original length = l.

When F = 25N, extension x = 0.55 – l
F = k (0.55 - l) 
(0.55-l) = F / k
Original length, l = 0.55 – (F/k) = 0.55 – (25/150) = 0.38m

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