Page 1 CBSE | Physics Board Paper â€“ 2014 CBSE Board Class XII Physics (Theory) Board Question Paper 2014 â€“ Set 3 1. Anticlockwise 2. Metal A , because ?? ?? ' > ?? 0 3. The ampere is the value of that steady current which, when maintained in each of the two very long, straight, parallel conductors of negligible cross-section, and placed one metre apart in vacuum, would produce on each of these conductors a force equal to 2 × 10 -7 newtons per metre of length. 4. The lens will behave as a diverging lens. The lens is of refractive index lesser than the medium in which it is placed. Hence, its nature does not change. 5. Electric field line is a curve drawn in such a way that the tangent to it at each point is in the direction of the net field at that point. Two fields can never cross each other. If they did ,it means the field at the point of intersection will not have a unique direction, which is meaningless) 6. The frequency belongs to microwave region of the electromagnetic spectrum. 7. Neutrinos interact very weakly with other particles. Since they can penetrate large quantity of matter without interacting they remain undetected throughout the experiments. 8. a.c. voltage is preferred over d.c. because (i) a.c. voltages can be easily stepped up or down with the help of transformers. (ii) Electrical energy can be transmitted efficiently and economically over long distances with higher voltage and lower current. Page 2 CBSE | Physics Board Paper â€“ 2014 CBSE Board Class XII Physics (Theory) Board Question Paper 2014 â€“ Set 3 1. Anticlockwise 2. Metal A , because ?? ?? ' > ?? 0 3. The ampere is the value of that steady current which, when maintained in each of the two very long, straight, parallel conductors of negligible cross-section, and placed one metre apart in vacuum, would produce on each of these conductors a force equal to 2 × 10 -7 newtons per metre of length. 4. The lens will behave as a diverging lens. The lens is of refractive index lesser than the medium in which it is placed. Hence, its nature does not change. 5. Electric field line is a curve drawn in such a way that the tangent to it at each point is in the direction of the net field at that point. Two fields can never cross each other. If they did ,it means the field at the point of intersection will not have a unique direction, which is meaningless) 6. The frequency belongs to microwave region of the electromagnetic spectrum. 7. Neutrinos interact very weakly with other particles. Since they can penetrate large quantity of matter without interacting they remain undetected throughout the experiments. 8. a.c. voltage is preferred over d.c. because (i) a.c. voltages can be easily stepped up or down with the help of transformers. (ii) Electrical energy can be transmitted efficiently and economically over long distances with higher voltage and lower current. CBSE | Physics Board Paper â€“ 2014 9. Consider the charging of a capacitor. The electric field between the plates of the capacitor is shown below If the plates of the capacitor have an area A and a total charge Q, the magnitude of the electric field between the plates is 0 Q E A ? ? The field is perpendicular to the surface S as shown in the figure. Thus, using Gaussâ€™s law the electric flux through the surface is E 00 QA Q EA A ? ? ? ? ?? Now, if the charge Q on the capacitor is changing with time, there is a current dQ i dt ? associated with it, so we have E 00 E 0 d d Q 1 dQ dt dt dt d i dt ? ?? ?? ? ?? ? ? ? ?? ?? This term is the current due to changing electric field and is called displacement current. Thus, the Ampereâ€™s Circuital law is modified to give E 0 c 0 0 d B dl i dt ? ? ? ? ? ? ? ? Page 3 CBSE | Physics Board Paper â€“ 2014 CBSE Board Class XII Physics (Theory) Board Question Paper 2014 â€“ Set 3 1. Anticlockwise 2. Metal A , because ?? ?? ' > ?? 0 3. The ampere is the value of that steady current which, when maintained in each of the two very long, straight, parallel conductors of negligible cross-section, and placed one metre apart in vacuum, would produce on each of these conductors a force equal to 2 × 10 -7 newtons per metre of length. 4. The lens will behave as a diverging lens. The lens is of refractive index lesser than the medium in which it is placed. Hence, its nature does not change. 5. Electric field line is a curve drawn in such a way that the tangent to it at each point is in the direction of the net field at that point. Two fields can never cross each other. If they did ,it means the field at the point of intersection will not have a unique direction, which is meaningless) 6. The frequency belongs to microwave region of the electromagnetic spectrum. 7. Neutrinos interact very weakly with other particles. Since they can penetrate large quantity of matter without interacting they remain undetected throughout the experiments. 8. a.c. voltage is preferred over d.c. because (i) a.c. voltages can be easily stepped up or down with the help of transformers. (ii) Electrical energy can be transmitted efficiently and economically over long distances with higher voltage and lower current. CBSE | Physics Board Paper â€“ 2014 9. Consider the charging of a capacitor. The electric field between the plates of the capacitor is shown below If the plates of the capacitor have an area A and a total charge Q, the magnitude of the electric field between the plates is 0 Q E A ? ? The field is perpendicular to the surface S as shown in the figure. Thus, using Gaussâ€™s law the electric flux through the surface is E 00 QA Q EA A ? ? ? ? ?? Now, if the charge Q on the capacitor is changing with time, there is a current dQ i dt ? associated with it, so we have E 00 E 0 d d Q 1 dQ dt dt dt d i dt ? ?? ?? ? ?? ? ? ? ?? ?? This term is the current due to changing electric field and is called displacement current. Thus, the Ampereâ€™s Circuital law is modified to give E 0 c 0 0 d B dl i dt ? ? ? ? ? ? ? ? CBSE | Physics Board Paper â€“ 2014 10. Given: 28 -3 19 72 I 2.7 A n 9 10 m e 1.6 10 C A 2.5 10 m ? ? ? ?? ?? ?? The drift speed is given as ? ? ? ? ? ? 4 d 28 19 7 I 2.7 v 7.5 10 m/s neA 9 10 1.6 10 2.5 10 ? ?? ? ? ? ? ? ? ? 11. The terminal voltage across a cell is given as V E Ir ?? Here, E is the e.m.f. of the cell and r is its internal resistance. The above equation is of the form y = mx + c where y = V, m = -r, x = I and c = E Hence, the graph of V vs I will be as shown below. From the graph, it is clear that the y-intercept, that is, the intercept on V-axis gives the value of e.m.f. (E) of the cell. Also, the negative of slope of the graph gives the internal resistance (r) of the cell. 12. The capacitance of two capacitors is same, i.e. C. The voltage across charged capacitor is V 1 = V and that across uncharged capacitor is V 2 = 0. Thus, the initial energy stored in the capacitor is 22 1 1 1 11 U C V CV 22 ?? When the charged capacitor is connected across the uncharged capacitor, the two capacitors form a parallel combination. Thus, the resultant capacitance is Câ€™ = C + C = 2C Page 4 CBSE | Physics Board Paper â€“ 2014 CBSE Board Class XII Physics (Theory) Board Question Paper 2014 â€“ Set 3 1. Anticlockwise 2. Metal A , because ?? ?? ' > ?? 0 3. The ampere is the value of that steady current which, when maintained in each of the two very long, straight, parallel conductors of negligible cross-section, and placed one metre apart in vacuum, would produce on each of these conductors a force equal to 2 × 10 -7 newtons per metre of length. 4. The lens will behave as a diverging lens. The lens is of refractive index lesser than the medium in which it is placed. Hence, its nature does not change. 5. Electric field line is a curve drawn in such a way that the tangent to it at each point is in the direction of the net field at that point. Two fields can never cross each other. If they did ,it means the field at the point of intersection will not have a unique direction, which is meaningless) 6. The frequency belongs to microwave region of the electromagnetic spectrum. 7. Neutrinos interact very weakly with other particles. Since they can penetrate large quantity of matter without interacting they remain undetected throughout the experiments. 8. a.c. voltage is preferred over d.c. because (i) a.c. voltages can be easily stepped up or down with the help of transformers. (ii) Electrical energy can be transmitted efficiently and economically over long distances with higher voltage and lower current. CBSE | Physics Board Paper â€“ 2014 9. Consider the charging of a capacitor. The electric field between the plates of the capacitor is shown below If the plates of the capacitor have an area A and a total charge Q, the magnitude of the electric field between the plates is 0 Q E A ? ? The field is perpendicular to the surface S as shown in the figure. Thus, using Gaussâ€™s law the electric flux through the surface is E 00 QA Q EA A ? ? ? ? ?? Now, if the charge Q on the capacitor is changing with time, there is a current dQ i dt ? associated with it, so we have E 00 E 0 d d Q 1 dQ dt dt dt d i dt ? ?? ?? ? ?? ? ? ? ?? ?? This term is the current due to changing electric field and is called displacement current. Thus, the Ampereâ€™s Circuital law is modified to give E 0 c 0 0 d B dl i dt ? ? ? ? ? ? ? ? CBSE | Physics Board Paper â€“ 2014 10. Given: 28 -3 19 72 I 2.7 A n 9 10 m e 1.6 10 C A 2.5 10 m ? ? ? ?? ?? ?? The drift speed is given as ? ? ? ? ? ? 4 d 28 19 7 I 2.7 v 7.5 10 m/s neA 9 10 1.6 10 2.5 10 ? ?? ? ? ? ? ? ? ? 11. The terminal voltage across a cell is given as V E Ir ?? Here, E is the e.m.f. of the cell and r is its internal resistance. The above equation is of the form y = mx + c where y = V, m = -r, x = I and c = E Hence, the graph of V vs I will be as shown below. From the graph, it is clear that the y-intercept, that is, the intercept on V-axis gives the value of e.m.f. (E) of the cell. Also, the negative of slope of the graph gives the internal resistance (r) of the cell. 12. The capacitance of two capacitors is same, i.e. C. The voltage across charged capacitor is V 1 = V and that across uncharged capacitor is V 2 = 0. Thus, the initial energy stored in the capacitor is 22 1 1 1 11 U C V CV 22 ?? When the charged capacitor is connected across the uncharged capacitor, the two capacitors form a parallel combination. Thus, the resultant capacitance is Câ€™ = C + C = 2C CBSE | Physics Board Paper â€“ 2014 The initial charge on the capacitor is q = CV The final potential across the combination will be 12 qq q CV V V' C' 2C 2C 2 ? ? ? ? ? Hence, the final energy in the combination of capacitors is ? ? 2 22 2 2 1 1 V 2CV 1 CV U C'V' 2C 2 2 2 8 2 2 ?? ? ? ? ? ?? ?? Thus, the ratio of energy stored in the combined system to that in the initial single capacitor is given as 2 2 2 1 1 CV U 1 22 1 U2 CV 2 ?? 13. The Rutherford model of atom considers the atom as an electrically neutral sphere consisting of a very small, massive and positively charged nucleus at the centre surrounded by the revolving electrons in their respective dynamically stable orbits. The electrostatic force of attraction F e between the revolving electrons and the nucleus provides the centripetal force F c to keep them in stable orbits. ? ? ?? ?? ?? ec 22 2 0 2 2 0 FF mv 1 e r 4 r e r 4 mv The kinetic energy and electrostatic potential energy of the electron in hydrogen atom are 2 2 0 2 0 1e K mv 2 8 r e U 4r ?? ?? ?? ?? Thus, the total energy E of the electron in a hydrogen atom is 22 00 2 0 ee E K U 8 r 4 r e 8r ? ? ? ? ? ? ? ? ?? ?? The total energy is negative. Page 5 CBSE | Physics Board Paper â€“ 2014 CBSE Board Class XII Physics (Theory) Board Question Paper 2014 â€“ Set 3 1. Anticlockwise 2. Metal A , because ?? ?? ' > ?? 0 3. The ampere is the value of that steady current which, when maintained in each of the two very long, straight, parallel conductors of negligible cross-section, and placed one metre apart in vacuum, would produce on each of these conductors a force equal to 2 × 10 -7 newtons per metre of length. 4. The lens will behave as a diverging lens. The lens is of refractive index lesser than the medium in which it is placed. Hence, its nature does not change. 5. Electric field line is a curve drawn in such a way that the tangent to it at each point is in the direction of the net field at that point. Two fields can never cross each other. If they did ,it means the field at the point of intersection will not have a unique direction, which is meaningless) 6. The frequency belongs to microwave region of the electromagnetic spectrum. 7. Neutrinos interact very weakly with other particles. Since they can penetrate large quantity of matter without interacting they remain undetected throughout the experiments. 8. a.c. voltage is preferred over d.c. because (i) a.c. voltages can be easily stepped up or down with the help of transformers. (ii) Electrical energy can be transmitted efficiently and economically over long distances with higher voltage and lower current. CBSE | Physics Board Paper â€“ 2014 9. Consider the charging of a capacitor. The electric field between the plates of the capacitor is shown below If the plates of the capacitor have an area A and a total charge Q, the magnitude of the electric field between the plates is 0 Q E A ? ? The field is perpendicular to the surface S as shown in the figure. Thus, using Gaussâ€™s law the electric flux through the surface is E 00 QA Q EA A ? ? ? ? ?? Now, if the charge Q on the capacitor is changing with time, there is a current dQ i dt ? associated with it, so we have E 00 E 0 d d Q 1 dQ dt dt dt d i dt ? ?? ?? ? ?? ? ? ? ?? ?? This term is the current due to changing electric field and is called displacement current. Thus, the Ampereâ€™s Circuital law is modified to give E 0 c 0 0 d B dl i dt ? ? ? ? ? ? ? ? CBSE | Physics Board Paper â€“ 2014 10. Given: 28 -3 19 72 I 2.7 A n 9 10 m e 1.6 10 C A 2.5 10 m ? ? ? ?? ?? ?? The drift speed is given as ? ? ? ? ? ? 4 d 28 19 7 I 2.7 v 7.5 10 m/s neA 9 10 1.6 10 2.5 10 ? ?? ? ? ? ? ? ? ? 11. The terminal voltage across a cell is given as V E Ir ?? Here, E is the e.m.f. of the cell and r is its internal resistance. The above equation is of the form y = mx + c where y = V, m = -r, x = I and c = E Hence, the graph of V vs I will be as shown below. From the graph, it is clear that the y-intercept, that is, the intercept on V-axis gives the value of e.m.f. (E) of the cell. Also, the negative of slope of the graph gives the internal resistance (r) of the cell. 12. The capacitance of two capacitors is same, i.e. C. The voltage across charged capacitor is V 1 = V and that across uncharged capacitor is V 2 = 0. Thus, the initial energy stored in the capacitor is 22 1 1 1 11 U C V CV 22 ?? When the charged capacitor is connected across the uncharged capacitor, the two capacitors form a parallel combination. Thus, the resultant capacitance is Câ€™ = C + C = 2C CBSE | Physics Board Paper â€“ 2014 The initial charge on the capacitor is q = CV The final potential across the combination will be 12 qq q CV V V' C' 2C 2C 2 ? ? ? ? ? Hence, the final energy in the combination of capacitors is ? ? 2 22 2 2 1 1 V 2CV 1 CV U C'V' 2C 2 2 2 8 2 2 ?? ? ? ? ? ?? ?? Thus, the ratio of energy stored in the combined system to that in the initial single capacitor is given as 2 2 2 1 1 CV U 1 22 1 U2 CV 2 ?? 13. The Rutherford model of atom considers the atom as an electrically neutral sphere consisting of a very small, massive and positively charged nucleus at the centre surrounded by the revolving electrons in their respective dynamically stable orbits. The electrostatic force of attraction F e between the revolving electrons and the nucleus provides the centripetal force F c to keep them in stable orbits. ? ? ?? ?? ?? ec 22 2 0 2 2 0 FF mv 1 e r 4 r e r 4 mv The kinetic energy and electrostatic potential energy of the electron in hydrogen atom are 2 2 0 2 0 1e K mv 2 8 r e U 4r ?? ?? ?? ?? Thus, the total energy E of the electron in a hydrogen atom is 22 00 2 0 ee E K U 8 r 4 r e 8r ? ? ? ? ? ? ? ? ?? ?? The total energy is negative. CBSE | Physics Board Paper â€“ 2014 The significance of this negative energy is that the electron is bound to the nucleus. If it is positive, then the electron will not follow a closed orbit around the nucleus. OR To find the expression of radius, Bohrâ€™s second postulate is used. The expression of angular momentum is L mvr ? Bohrâ€™s second postulate of quantisation says electron revolves only in those orbits for which the angular momentum is integral multiple of h/2 ?. nh L 2 ? ? Thus, for electron in n th orbit nn nh L mv r 2 ?? ? â€¦â€¦ (1) The relation between velocity and radius for electron in n th orbit is n 0n e v 4 mr ? ?? â€¦â€¦ (2) From equations (1) and (2), we get ? ? n 0 n 2 2 n n 0 e v nh 4m 2 mv ev v h 4n 2 ? ?? ? ?? ?? ? ? ? 2 n 0 e1 v h 4n 2 ?? ?? ? â€¦â€¦ (3) Substituting (3) in (1), we get 2 2 0 n 2 4 nh r m 2 e ?? ?? ?? ? ???? ? ?? ?? â€¦â€¦ (4) This gives the radius of electron in the n th orbit From equation (4), the radius of innermost orbit n = 1 is found as 2 0 1 2 h r me ? ? ? This is called the Bohr radius and is denoted as a 0. 14. (i) Field lines around a paramagnetic substanceRead More

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