CBSE 12th Standard Physics Subject Value Based Questions 5 Mark Questions With Solution 2021

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Questions:

Generic 5

15 x 5 = 75

The surface integral of electrostatic field E produced by any source over any closed surface S enclosing a volume V in vacuum, i.e. total electric flux over the closed surface S in vacuum is \(1/{\epsilon}_{0}\) times the total charge Q contained inside S, i.e.
\({\phi}_{E}=\oint E.dS={{Q}\over{{\epsilon}_{0}}}\)
The charges inside S may be point charges or even continuous charge distributions. There is no contribution to "total electric flux from the charges outside S. Further, the location of Q inside S does not affect the value of surface integral.
Read the above passage and answer the following questions
What are the SI units and dimensions of electric flux?
A closed surface in vacuum encloses charges -q, + 3q and +5q. Another charge + 4q lies outside the surface. What is total electric flux over the surface?
A point charge a lies inside a spherical surface of radius r. How will the electric flux be affected, if, radius of the sphere is doubled?
What values of life do you learn from this theorem?

When puja, a student of 10th class, watched her mother washing clothes in the open, she observed coloured soap bubbles and was curious to know why the soap bubbles appear coloured. In the evening when her father, an engineer by profession, came home, she asked him this question. Her father explained to her basic phenomenon of physics due to which bubbles appear coloured.
(a) What according to you are the values are the displayed by puja and her father?
(b) State the phenomenon of light involved in the formation of coloured soap bubbles.

Raj was cycling from his village to a neighbouring village.On the way, he met an old man who was walking in the direction of the same neighbouring village. Raj invited the old man to sit on his cycle and took him to the neighbouring village.The old man got down from the cycle, thanked Raj and gave him blessing.
Read the above passage and answer the following questions:
(i) What are the values displayed by Raj?
(ii) Can you relate the given paragraph with the process of modulation?

Kritika Singh was enjoying TV programme at her home with her younger brother Surya at night. Suddenly, the light went off causing darkness all over. Surya asked her to bring candle along with matchstick to put the TV switch OFF. Kritika at once picked the mobile phone and pressed the button lighting up the surrounding. Her younger brother was surprised and asked, where the light was coming. Kritika proudly showed her mobile.
Read the above passage and answer the following questions:
(i) Which value is displayed by Kritika?
(ii) Name the material used in the formation of LED.
(iii) LED works, in which biasing?

A teacher has given three lenses of power 0.5D, 4D and 10D to a studet. He is not sure as to which lens should he use for constructing a good astronomical telescope. So, he consults his seniors and the teacher, and then constructs a telescope. Later, he shows this telescope to the junior classes and ex[plains about the choice of lenses.
Read the above passage and give the answer of the following questions:
(i) What values has he shown by doing these?
(ii) Which lenses are used as objective and which one as eyepiece?

One day, Chetan's mother developed a sever stomach ache all of a sudden. She immediately rushed to the doctor, who suggested for an immediate endoscopy test and gave an estimate of expenditure for the same. Chetan immediately contacted his class teacher and shared the information with her. The class teacher arranged the money and reached to the hospital. On realising that Chetan belonged to a below average incom group of family, even the doctor offered concession for the test fee.The test was conducted successfully.
Read the above passage and answer the following questions:
(i) Which principle of optics is used in endoscopy?
(ii) Briefly explain the values reflected in the action taken by the teacher.
(iii) In what way do you appreciate the response of the doctor on the given situations?

Kritika's mom is finding difficult to cook in the kitchen as there was power-cut and she told the same to Kritika. She immediately took a plane mirror from her shelf made it stand against a wall such that sun rays were focussed into the Kitchen due to the reflection of the mirror. There was some lighting and her mother was able to finish her work.
Read the above passage and answer the following question:
(i) What are the values shown by Kritika?
(ii) Name the types of mirror used in periscope.
(iii) Give the nature of image in case of plane mirror.

When an electric dipole of moment \(\left| P \right| \) = q \(\times\) 2a is held at an angle \(\theta\), with the direction of uniform external electric field E, a torque \(\tau \) = pE sin\(\theta\) acts on the dipole. This torque tries to align the electric dipole in the direction of the field. When p is along E, \(\theta=0^{o}\), \(\tau \) = pE sin\(0^{o}\) = zero.

Read the above passage and answer the following questions.
(i) What is the direction of torque acting on an electric dipole held at a angle with uniform external field?
(ii) An electric dipole of length 10 cm having charge \(\pm 6\times { 10 }^{ -3 }\), C placed at \(30^{o}\) with respect to a uniform electric field experiences a torque of magnitude \(6\sqrt { 3 } \) N-m. Calculate magnitude of electric field.
(iii) What is the physical significance of this concept in our day-today life?

Two friends went on an excursion on a hill top where temperature was quite low. One of them fell ill.The other one put a blanket on him, collected the pieces of dry wood and ignited fire in his vicinity. After sometime the ill friend felt better.
(i) What are the type of EM waves coming from fire?
(ii) Why did the ill friend feel better after sitting rear the fire?
(iii) What basic values do you learn from this study?

10.

In Pradeep's classroom, the fan was running very slowly. Due to which, his teacher was sweating and was restless and tired.All his classmates wanted to rectify this. They called an electrician who came and changed the capacitor only, after which the fan started running fast.
Answer the following questions based on the above information:
(i) What energy is stored in the capacitor and where?
(ii) A thin metal sheet is placed in the middle of a parallel plate capacitor. What will be the effect on the capacitance?
(iii) What values did the classmates have?

11.

Vishwajeet purchased cells for his transistor. He felt that cells are not working properly. He wanted to check their emf. So, he took the cells to the physics lab and with the help of potentiometer found their emf. To his surprise, emf was less than the value claimed by the manufacturer. He lodged the complaint with consumer forum and received the deserving response.
Read the above passage and answer the following question.
(i) What values are displayed by Vishwajeet?
(ii) Why do you think Vishwajeet used potentiometer instead of voltmeter to find out emf of the cell? For more precise measurement, the potential gradient of the potentiometer should be high or low?

12.

Saniya and Priya are friends. Both of them know that a small compass needle point always along north-south direction. One day Saniya is plotting field due to a bar magnet in the laboratory. She discovers a point where compass needle does not point along N-S. Rather, it sets itself in any arbitrary direction. Saniya thinks first that compass needle has become faulty. Priya then explains to her the real situation.
Read the above passage and answer the following questions:
(i) How did Priya justify the situation?
(ii) If a bar magnet is placed along the N-S direction with its north pole pointing north, what is the position of neutral points?
(iii) If a bar magnet is placed along N-S direction with its north pole pointing South, What is the position of neutral points?
(iv) What values of life do you learn from this piece of knowledge?

13.

According to Plank's constant quantum theory of light, every source of radiation emits photons(i.e., packets of energy) which travel in all directions with the same speed (i.e., speed of light). Each photon is of energy \(E=hv=\frac { hc }{ \lambda } \) where h is plank's constant, v is the frequency and \(\lambda \) is wavelength of radiation emitted. The photons emitted from different sources of radiation are different.
Read the above passage and answer the following questions:
(i) On what factors does the number of photons emitted per second from a source of radiation depend?
(ii) Why are the high energy photons not visible to eye?
(iii) Which basic values do you learn from this study?

14.

Poonam's mother is diagnosed cancer. The attending physician told her that she has to undergo radiotherapy. While telling her the side effects of the treatment, the doctor told that her beautiful hair may fall and she may become bald. Poonam's mother refuses to get the treatment.
Read the above passage and answer the following questions:
(i) What would you do if you were in Poonam's place?
(ii) What values are associated with your attitude?

15.

According to Bohr's theory of hydrogen atom, total energy of electron in a stationary orbit is \(E=-\frac { 13.6 }{ { n }^{ 2 } } eV,\) where n is the number of orbit. Clearly, total energy of electron in a stationary orbit is negative, which means the electron is bound to the nucleus and is not free to leave it. An n increases, value of negative energy decreases, i.e., energy is progressively larger in the outer orbits.
Read the above passage and answer the following questions :
(i) What is total energy of electron in ground state of hydrogen atom? What does it imply?
(ii) Energy required to remove an electron is smaller when atoms is in any one excited state. Comment.
(iii) How is this concept translated in day to day life?

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Answers Key:

Generic 5

15 x 5 = 75

SI units of electric flux is N-m²C^-1.
Dimensional formula of electric flux is [ M¹L¹T^-3A^-1].
\(\phi={{\sum {q}_{inside}}\over{{\epsilon}_{0}}}={{-q+3q+5q}\over{{\epsilon}_{0}}}={{7q}\over{{\epsilon}_{0}}}\)
As \(\phi={{q}\over{{\epsilon}_{0}}},\) electric flux is not affected by area/shape of the surface. So, the electric flux remains unaffected.
This theorem emphasis that total normal flux from the surface depends only on algebraic sum of charges enclosed by the surface, irrespective of their location. The charges outside the surface do not affect the electric flux. In day-to-day life, the theorem implies that the knowledge you can import depends only on what you have stored within you. You
cannot emanate what you have not absorbed or what lies outside your reach?
In the examination, you can write what you have learnt by heart? Extraneous help from outside by unfair means will never serve your purpose.

(a) Any one of the values displayed by puja-curiosity/observation etc.
Any one of the values displayed by father-concern/knowledge/sense of duty etc.
(b) Interference of sunlight due to the soap bubbles.

(i) Raj showed concern for the stranger and respect for the old age of the person.
(ii) Just as the old man sat on the cycle, the message signal is superposed on the carrier wave.The modulated carrier wave travels some distance.The message signal is then recovered from the modulated wave in the same way as the old man got down from the cycle.

(i) Kritika is a creative and intelligent girl. She has good knowledge of Physics.
(ii) The semiconductor material is used in the formation of LED.
(iii) LED works in forward biasing i.e. n-type is connected to a negative terminal of the battery and p-type is connected with positive terminal of the battery.

(i) The values shown by him are as follows:
(a) Consulting others in case of need
(b) Curosity
(c) Sharing knowledge.
(ii) From these three lenses, he will use a lens of power 0.5D for objective and lens of power 10D for eyepiece.

(i) Endoscopy is based on Total Internal Reflection (TIR) principle. It has tubes which are made up of optical fibres which are used for transmitting and receivingelectrical signals which are converted to light by suitable transducer.
(ii) Humanity and charity.
(iii) Doctor gave monetary help to Chetan by understanding his poor financial condition.

(i) Kritika is intelligent and has good knowledge of physics. She is helpful and can apply her mind at required time.
(ii) Plane mirror is used in periscope.
(iii) Nature of image in plane mirror is
(a) Virtual
(b) object distance = image distance
(c) same size as that of object.

(i) \(\tau \) = pE sin\(\theta\) can be rewritten in vector form as \(\tau \) = p \(\times\) E
Therefore, the direction of torque \(\tau \) is perpendicular to both p and E. It is determined by right handed screw rule.
(ii) Here, 2a = 10 cm = 10^-1 m,
q = \(6\times { 10 }^{ -3 }\) C, \(\theta=30^{o}\)
\(\tau \) = \(6\sqrt { 3 } \) N-m,E = ?
From \(\tau \) = pE sin\(\theta\) = q(2a)E sin\(\theta\)
\(6\sqrt { 3 } \) = \(6\times { 10 }^{ -3 }\left( { 10 }^{ -1 } \right) \) Esin\(30^{o}\)
\(6\sqrt { 3 } \) = \(6\times { 10 }^{ -4 }\times E \times\frac { 1 }{ 2 }\Rightarrow6\sqrt { 3 }=3\times{ 10 }^{ -4 }E\)

\(\Rightarrow\) E = \(\frac { 6\sqrt { 3 } }{ 3\times { 10 }^{ -4 } } =2\sqrt { 3 } \times { 10 }^{ 4 }\) NC^-1
(iii) From the given paragraph, we find that electric dipole held in an external electric field will be in stable equilibrium only when it is aligned along the external electric field. The same is true in day-today life.
At your work place, your boss always tries to align you along his/her paln. Your job/position is secured/stable after you are perfectly aligned and your personal whims are at the lowest ebb.

(i) The EM waves coming from the fire are infrared rays.
(ii) The infrared rays coming from the fire provide heating (soothing) effect to the body muscles of ill friend sitting near the fire. Due to it, he feels better after sometime.
(iii) We learn that the presence of mind and correct use of things available help to deal with difficult situation.

10.

(i) Electrical energy is stored in the capacitor.It is stored in the dielectric.
(ii) No effect. When the metal sheet is placed in the middle the new arrangement is equivalent to a old combination of two capacitors each of plate separation \(\frac{d}{2}\) and hence capacitance 2C.
\({ C }_{ S }=\frac { 2C\times 2C }{ 2C+2C } =C\)
(iii) Team work, concern, respect to teacher and responsibility.

11.

(i) Values displayed by Vishwajeet are as follows
(a) General awareness
(b) Presence of mind
(c) Use of scientific knowledge
(ii) Potentiometer is based on null point method. Voltmeter draws current at the time of measurement of potential difference across the cell. So, it does not give the correct value of emf. For more precise measurements, it should be low.

12.

(i) Priya tells Saniya that while plotting field due to a bar magnet, we obtain the resultant of the magnetic field of magnet and that of earth. At certain points, field due to the magnet becomes equal and opposite to the horizontal component of earth's field. Therefore, net magnetic field due to magnet and due to earth becomes zero at these points.
(ii) When noth pole of bar magnet is pointing north, two neutral pointa are along E-W line passing through centre of magnet.
(iii) When north pole of magnet is pointing south, two neutral points are obtained along the axis of the magnet on opposite sides from the center of the magnet.
(iv) From this piece of knowledge, we learn that staying neutral in a situation is the best. If you are aligned to one group or person, you are forced to go by the dictates of that group, right or wrong. You are not independent. If you are non aligned, you can choose your own course of action without killing your conscience.

13.

(i) The number of photons emitted per second from a source of radiation on its temperature, nature and area of radiating surface.
(ii) The ratina of our eye is sensitive to visible light whose frequency range is \(4\times { 10 }^{ 14 }Hz\ to\ 8\times { 10 }^{ 14 }Hz\) High energy photons are frequency much higher than \(8\times { 10 }^{ 14 }Hz\). They do not produce any sensation on our ratina.Hence, they cannot be seen by our eyes.
(iii) We have learnt that every source emits radiations (i.e., photons) The different source emits radiations of different energies, which have different effects on the exposed bodies. The radiations of holy persons cultivate in us the good qualities and radiations of criminals develop bad qualities.Therefore, we must live in a company of good people for better and peaceful life.

14.

(i) If I were in Poonam's place, I would tell my mother that treatment of cancer is a must. Hair fall is a temporary effect and the hair would grow slowly after the therapy. Convincing my mother to get the treatment, using all sorts of reasoning will be my top priority.
(ii) Concern for my mother's health is of utmost importance. I will not mind taking help from specialists to convince my mother for getting the treatment.

15.

(i) For ground state, n = 1
\(\therefore E=\frac { -13.6 }{ { n }^{ 2 } } eV= \ \frac { -13.6 }{ { I }^{ 2 } } eV= \ -13.6 \ eV\)
It implies that 13.6 eV energy is required to remove an electron from hydrogen atom in its ground state.
(ii) In first excited state, n = 2,
\(\therefore \ E=-\frac { 13.6 }{ { 2 }^{ 2 } } eV \ = \ -3.4eV\)
It means that energy required to remove an electron from hydrogen atom in first excited state is 3.4 eV, which is less than 13.6 eV. Therefore, the statement is true.
(iii) Negative energy of electron indicates that it is bound to the nucleus and cannot leave it until energy equal to its negative energy is supplied from outside. The same is true in day to day life. A person intending to leave the country has to show that nothing is pending against him in a court of law, and he has cleared income tax/sales tax payments due from him. A person from whom any type of payment is due, is not free to leave the country. He is bound till he clears all his dues.