CBSE Class 12 Physics VBQs Potential and Capacitance

POTENTIAL AND CAPACITANCE

ONE MARK QUESTIONS

Question. Define electric potential. Is it a vector or a scalar quantity?
Answer : Electric potential at a point is equal to amount of work required to bring a unit positive charge from infinity to that point in the electric field. It is a scalar quantity.

Question. What is the electrostatic potential due to an electric dipole at an equatorial point?
Answer : Zero

Question. What would be the work done if a point charge +q is taken from a point at the circumference of a circle to another point at the circumference if another point charge is at the centre of the circle.
Answer : Zero, because direction of motion is perpendicular to the force acting at the charge.

Question. A and B are two conducting sphere of the same radii and same material. A being solid and another hollow. Both are charged to same potential. What will be relation between the capacitances of them ? which will assume more charge?
Answer : Both will have same charges because capacitances in these cases are same.

Question. Which orientation of an electric dipole in a uniform electric field would correspond to stable equilibrium?
Answer : When it is placed parallel to the electric field.

Question. If the radius of the Gaussian surface enclosing a charge is halved, how does the electric flux through the Gaussian surface change?
Answer : Remain unchanged.

Question. A glass rod when rubbed with silk acquires a charge +1.6 × 10–12C. What about silk ?
Answer : Silk cloth with acquire equal and opposite charge i.e. –1.6 × 10–12 C 

Question. If Coulomb’s law involved 1/r3 dependence instead of 1/r2 , will the Gauss theorem be applicable?
Answer : Silk cloth with acquire equal and opposite charge i.e. –1.6 × 10–12 C 

Question. Define the electric dipole moment of a dipole. Write its SI unit.
Answer : It is defined as the product of either charge and the length of the dipole.
Mathematically, P= q (2a). Its SI unit is Cm.

Question. What is the work done in moving a test charge q through a distance of 1 cm along the equatorial axis of an electric dipole
Answer : Zero

TWO MARKS QUESTIONS

Question. A point charge Q is placed at the point O in following figure. Is the potential difference VA – VB positive, negative or zero if Q is (i) positive (ii) negative?
Q----------------- A----------B
Answer : (positive , negative)

Question. During lightning, the safest way to protect is to be inside the house or car. Why?
Answer : electrostatic shielding. Explain.

Question. Two capacitors of capacitances 6μF and 12μF are connected in series with a battery. The voltage across 6μF capacitor is 2 V. compute the total supply voltage.
Answer : (3 V)
Solution- 1/CS= 1/6 + 1/12 i.e. Cs = 4 μF.
charge across 6μF= CV= 6x2= 12μC. It is equal to total charge in the circuit as both are connected in series.
Voltage across 12μF= q/c= 12/12= 1 V
Total supply voltage= 2+1= 3 V.

Question. A slab of material of dielectric constant k has the same area as that of the plates of a parallel plate capacitor but has the thickness d/2, where d is the separation between the plates. Find out the expression for its capacitance when the slab is inserted between the plates of the capacitor.
Answer : We know
Capacitance with dielectric of thickness ‘t’
C = ɛoA /d-t+t/K
After putting the value of t=d/2 the value of C will be
C= 2ɛoAK / d(K+1)

Question. A parallel plate capacitor of capacitance c is charged to a potential V. If it is then connected to another uncharged capacitor having the same capacitance. Find out ratio of the energy stored in the combined system to that stored initially in the single capacitor.
Answer : Let q be the charge on the charged capacitor.
So, energy stored in it is given by U= q2/2C
When another uncharged similar capacitor is connected, then the net capacitance of the system is given by
C’= 2C
The charge on the system remains constant. So, the energy stored in the system is given by
U’= q2/2C’ = q2/4C as C’= 2C
Thus the required ratio is given by
U’/U= ½

Question. Derive an expression for the work done in rotating a dipole from the angle ɵo to ɵ1 on a uniform electric field E.
Answer : Let an electric dipole be rotated in electric field be rotated in electric field from angle ɵo to ɵ1 in the direction of electric field.
Torque on dipole = PEsinɵ
The work done i rotating the dipole further through a small angle dɵ is
dW = PEsinɵdɵ
total work done in rotating the dipole from angle ɵo to ɵ1 is given by integrating the expression within the limits and the result obtained is given by
w = PE(cosɵo - cosɵ1)

Question. Draw the equipotential surfaces corresponding to a field that uniformly increases in magnitude but remain constant along Z-direction. How are these surfaces different from that of a constant electric field along Z-direction?
Answer :  (Image)

Question. Define electric flux. Write its SI unit.
A charge q is enclosed by a spherical surface of radius R. If the radius is reduced to half, how would the electric flux through the surface change?
Answer : Electric flux linked with any surface is the total number of electric field lines passing through the surface normally.
Its SI unit is Nm2C-1 or JmC-1 or Vm.
Remain unchanged.

Question 1. An elderly woman went alone to the Registrar’s office to disburse her property. When she enquired in the office she was asked to get a Xerox copy of the document which works under electrostatic induction. The Xerox shop was far away and across the road. She took the help of the passer-by and got her Xerox done. 

a. What values did the passer-by have?

b. How does a neutral body get charged by electrostatic induction?

Answer:

a. Helping, sharing, respect for elderly people.

b. For a body to get positively charged, a negatively charged body has to be brought close to the neutral body which after earthing gets charged uniformly.

Question 2 . Ram and Shyam went to the trade fair. They were busy in a crowded corner. Balloons were sold. A child was seen troubling his parent and crying for something. On seeing this, Ram went to the child and said that he would perform a trick with balloons. Ram took two balloons and Shyam helped him to inflate and tie. When the balloons were rubbed with the sweater he was wearing, they were attracted. When taken nearer to wall, the balloons got stuck. The child enjoyed and stopped crying.

a. Give two values of Ram and Shyam.

b. How did the balloons get attracted? Will they repel also?

Answer:

a. Presence of mind and knowledge of static electricity.

b. When balloons were rubbed with woollen sweater, it becomes negatively charged. When taken nearer the wall, positive charges are induced by electrostatic induction on that part of the wall, so gets attracted. ie, when the bodies are similar charged they repel.

Question 3. Arun had to repaint his car when he was reminded by the car company for his regular car service. He told them to do spray painting of mountain dew colour. The company also replied that they usually perform spray painting only as wastage is minimized and even painting is achieved.

a. What values did the car service company have?

b. If spray painting is done by electrostatic induction, how is even painting achieved?

Answer:

a. Customer care, commitment, concern and truthfulness

b. Droplets of paint are charged particles which get attracted to any metallic objects by electrostatic forces.

Question 4 . In Akash’s classroom the fan above the teacher 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 for an electrician who came and changed the capacitor only after which the fan started running fast.

a. What values did Akash and his classmates have?

b. What energy is stored in the capacitor and where?

Answer: 

a. Team work, concern, respect to teacher and responsibility

b. Electrical energy in the dielectric of the capacitor.

Question 5. Aswin asked his Physics teacher why are voltmeter, galvanometer and ammeter kept in a cavity inspite of its outer covering being made of an insulator

a. What values did Aswin have? KVS Class XII PHYSICS 139 

b. Explain Faraday’s cage or electrostatic shielding

Answer: 

a. inquisitiveness, to improve his knowledge in Physics, willingness to gain knowledge

b. These sensitive instruments should not be disturbed by external electric field.

Question 6. Vijay was preparing an electronic project for science exhibition. He required a capacitance of 2μF having a capacity to operate under 1 kV potential. He went to a shop to purchase it. Shopkeeper was having only 1μF capacitors of 400 V rating. Vijay calculated minimum number of capacitors of 1 μF so that he could arrange them to form a capacitor of 2μF value.

(a) What values do you judge in Vijay?

(b) Show the calculations done by Vijay

Answer: 

a. Scientific attitude and hard working nature

Question 7. During a thunderstorm Rohan and his friends were in the middle of a jungle. His friends were standing under a tree. Rohan advised them not to stand there. Rather he suggested they should move back to the bus. He explained them that they are safe inside the bus due to electrostatic shielding.

(a) What are the values exhibited by Rohan ?

(b) What is electrostatic shielding?

Answer: 

a. Scientific temper and care and concern

b. explanation of electrostatic shielding.

Value Based Questions

Question 1. In Predeep’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) What values did the classmates have? 

Answer: 

(i) Electrical energy is stored in the capacitor. It is stored in the dielectric.

(ii)Team work, concern, respect to teacher and responsibility.

Question 2. An old woman who had suffered from a heat stroke was taken to hospital by her grandson who is in class XII. The grandson has studied in physics that, how to save person who is suffering from a heat stroke, regular beating of the heat is to be restored by delivering a jolt to the heart using a defibrillator, whose capacity is 70 μF and charged to a potential of 5000 V and energy stored is 875 J , 200J of energy is passed through a person’s body in a pulse lasting 2 ms.The old woman gets paniced and refuses to be treated by the defibrillator. Her grandson then explains to her the process that would be adopted by the medical staff and how the result of that would bring her back to normalcy. The old woman was treated and was back to normal.

Answer the following questions based on the above information.

(i) What according to you are the values displayed by the grandson?

(ii) What will be the net charge of the capacitor in defibrillator?

Answer: 

(i) Presence of mind, knowledge of subject, concern for his grandmother, empathy helping and caring

(ii) The net charge of the capacitor in defibrillator , Q = CV
Q = 70x10-6 x 5000
= 35x10^-2 C

Question 3. Shivani was working on a project for science exhibition. She consider a capacitance of 2μF having a capacity of operating under 1 KV potential. When she reached to shop , she found that the shopkeeper is having a capacitors of 1μF of 400 V rating. Shivani
calculated minimum number of capacitance of 1μF each , so she could arrange to form a capacitor of 2μF.

Answer the following questions based on the above information.

(i) What are the calculation done by Shivani ?

(ii) What do you think of Shivani ?

Answer: 

(i) Total potential difference across each row =1000V
Potential difference across each capacitor = 400V
No. of capacitor in series, n = 1000/400 =2.5 3
Capacitance of capacitor in series = μF
Let m be rows of capacitors ,for an equivalent capacitance of 2 μF, we have
m X = 2
m = 6
Hence , total number of capacitance required = m X n = 6 X 3 = 18
Total 6 rows of capacitor in parallel with three capacitors in each row .

(ii) Presence of mind , ability to calculate scientifically in daily life .

Question 4. An elderly woman went alone to the Registrar’s office to disburse her property. When she enquired in the office she was asked to get a Xerox copy of the document which works under electrostatic induction. The Xerox shop was far away and across the road. She took the help of the passer-by and got her Xerox done.

a) What values did the passer-by have?

b) How does a neutral body get charged by electrostatic induction?

Answer:

a) Helping, sharing, respect for elderly people.

b) For a body to get positively charged, a negatively charged body has to brought close to the neutral body which after earthing gets charged uniformly.

Question 5. Ram and Shyam went to the trade fair. They were busy in a crowded corner. balloons were sold. A child was seen troubling his parent and crying for something. On seeing this, Ram went to the child and said that he would perform a trick with balloons. Ram took two balloons and Shyam helped him to inflate and tie. When the balloons were rubbed with the sweater he was wearing, they were attracted. When taken nearer to wall, the balloons got stuck. The child enjoyed and stopped crying.

a) Give two values of Ram and Shyam.

b) How did the balloons get attracted? Will they repel also?

Answer:

a) Presence of mind and knowledge of static electricity.

b) When balloons were rubbed with woollen sweater, it becomes negatively charged. When taken nearer the wall, positive charges are induced by electrostatic induction on that part of the wall, so gets attracted. Yes, when the bodies are similar charged they repel.

Question 6. Arun had to repaint his car when he was reminded by the car company for his regular car service. He told them to do spray painting of mountain dew colour. The company also replied that they usually perform spray painting only as wastage is minimized and even painting achieved.

a) What values did the car service company have?

b) If spray painting is done by electrostatic induction, how is even painting achieved?

Answer:

a) Customer care, commitment, concern and truthfulness.

b) Droplets of paint are charged particles which get attracted to any metallic objects by electrostatic forces