There is Case Study Questions in class 12 Physics in session 2020-21. The first two questions in the board exam question paper will be based on Case Study and Assertion & Reason. Case Study Questions will have 5 MCQs out of which students will have to attempt any 4 questions. Here are the questions based on case study.
Case Study Question 1:
Electric field strength is proportional to the density of lines of force i.e., electric field strength at a point is proportional to the number of lines of force cutting a unit area element placed normal to the field at that point. As illustrated in given figure, the electric field at P is stronger than at Q.
(i) Electric lines of force about a positive point charge are
(a) radially outwards
(b) circular clockwise
(c) radially inwards
(d) parallel straight lines
(ii) Which of the following is false for electric lines of force?
(a) They always start from positive charge and terminate on negative charges.
(b) They are always perpendicular to the surface of a charged conductor.
(c) They always form closed loops.
(d) They are parallel and equally spaced in a region of uniform electric field.
(iii) Which one of the following patterns of electric line of force is not possible in field due to stationary charges?
(iv) Electric field lines are curved
(a) in the field of a single positive or negative charge
(b) in the field of two equal and opposite charges.
(c) in the field of two like charges.
(d) both (b) and (c)
(v) The figure below shows the electric field lines due to two positive charges. The magnitudes EA, EB and EC of the electric fields at point A, B and C respectively are related as
AnswerAnswers: (i) a
Case Study Question 2:
Smallest charge that can exist in nature is the charge of an electron. During friction it is only the transfer of electron which makes the body charged. Hence net charge on any body is an integral multiple of charge of an electron (1.6 x 10-19 C) i.e., q=±ne
where r= 1, 2, 3, 4 ….
Hence no body can have a charge represented as 1.8e, 2.7e, 2e/5, etc.
Recently, it has been discovered that elementary particles such as protons or neutrons are elemental units called quarks.
(i) Which of the following properties is not satisfied by an electric charge?
(a) Total charge conservation.
(b) Quantization of charge.
(c) Two types of charge.
(d) Circular line of force.
(ii) Which one of the following charges is possible?
(a) 5.8 x 10-18 C
(b) 3.2 x 10-18 C
(c) 4.5 x 10-19 C
(d) 8.6 x 10-19 C
(iii) If a charge on a body is 1 nC, then how many electrons are present on the body?
(a) 6.25 x 1027
(b) 1.6 x 1019
(c) 6.25 X 1028
(d) 6.25 X 109
(iv) If a body gives out 109 electrons every second, how much time is required to get a total charge of 1 from it?
(a) 190.19 years
(b) 150.12 years
(c) 198.19 years
(d) 188.21 years
(v) A polythene piece rubbed with wool is found to have a negative charge of 3.2 x 10-7C. Calculate the number of electrons transferred.
(a) 2 x 1012
(b) 3 x 1012
(c) 2 x 1014
(d) 3 x 1014
AnswerAnswers: (i) (d)
Case Study Question 3:
When electric dipole is placed in uniform electric field, its two charges experience equal and opposite forces, which cancel each other and hence net force on electric dipole in uniform electric field is zero. However these forces are not collinear, so they give rise to some torque on the dipole. Since net force on electric dipole in uniform electric field is zero, so no work is done in moving the electric dipole in uniform electric field. However some work is done in rotating the dipole against the torque acting on it.
(i) The dipole moment of a dipole in a uniform external field Ē is B. Then the torque τ acting on the dipole is
(a) τ=p x E
(b) τ = P. Ē
(c) τ = 2(p + Ē)
(d) τ = (P + E)
(ii) An electric dipole consists of two opposite charges, each of magnitude 1.0 μC separated by a distance of 2.0 cm. The dipole is placed in an external field of 105 NC-1. The maximum torque on the dipole is
(a) 0.2 x 10-3 Nm
(b) 1x 10-3 Nm
(c) 2 x 10-3 Nm
(d) 4x 10-3 Nm
(iii) Torque on a dipole in uniform electric field is minimum when θ is equal to
(d) Both (a) and (c)
(iv) When an electric dipole is held at an angle in a uniform electric field, the net force F and torque τ on the dipole are
(a) F= 0, τ = 0
(b) F≠0, τ≠0
(c) F=0, τ ≠ 0
(d) F≠0, τ=0
(v) An electric dipole of moment p is placed in an electric field of intensity E. The dipole acquires a position such that the axis of the dipole makes an angle with the direction of the field. Assuming that potential energy of the dipole to be zero when 0 = 90°, the torque and the potential energy of the dipole will respectively be
(a) pEsinθ, -pEcosθ
(b) pEsinθ, -2pEcosθ
(c) pEsinθ, 2pEcosθ
(d) pEcosθ, – pEsinθ
AnswerAnswers: (i) (a)