CBSE Class 12 Physics VBQs Current Electricity

 VBQ‐Current Electricity

Question. That night Vaikunth was preparing for his physics exam. Suddenly the light in his room went off and he could not continue his studies. His cousin brother Vasu who had come to visit him was quick to react. Vasu using the torch (an android application) installed in his mobile phone found that the fuse had blown out. He checked the wiring and located a short circuit. He rectified it and put a fuse wire. The light came to life again. Vaikuth heaved a sigh of relief, thanked Vasu and continued his studies.

(1) What are the values projected by Vaikunth and Vasu? 

(2) Why did Vasu have to check the wiring?

(3) What is an electric fuse? What characteristics you would prefer for a fuse wire?
Answer:
(1) Vaikunth: acknowledging the help from others with gratitude.
Vasu: awareness of the technology, helping tendency, practical knowledge of the subject.
(2) Vasu checked the wiring to know whether it is short circuit or broken.
(3) An electric fuse is a wire used as a safety device which melts when current exceeds the limit. Low melting point, high resistivity.

Question. Kumaran wanted to pay electricity bill that day. He realized that the consumption shown by the meter was unbelievably low. He thought that the meter must have been faulty. He wanted to check the meter. But unfortunately he did not have any idea as to how to do this. There came his friend Subhash to help him. He told Kumaran to run only the electric heater rated 1kW in his house for some time keeping other appliances switched off. He also calculated the power consumed in kilowatt hour and compared the value with the meter. . Kumaran was happy and thanked Subhash for his timely help and the knowledge.
(1) What are the values displayed by the friends?
(2) Express kWh in joules.
(3) Find the resistance of the heater.
Answer:
(1) Honesty, sharing of knowledge, willingness to help
(2) 1kWh= 1000 watt x 3600 s =3.6x106 J.
(3) R = V2 /P=220 x220 /1000=48.4 Ω

Question 3. Raghav is lives in an area where birds in large groups play around producing pleasing humming sounds. One day he notices that the high power lines soon after a strong wind have come too close which may prove fatal for the birds that would sit on them and flutter their wings for some reason or other. He complained to the authorities and the lines were set at the proper distance once again.
(1) What are the values possessed by Raghav and the authorities? 
(2) What is the danger that could happen to the innocent birds in Raghav’s view?
(3) How did distancing the lines solve the problem?
Answer:
(1) Raghav: affection towards birds, taking appropriate action Authorities: duty conscious
(2) The bird may get electrocuted; avoid sparking as shown in the diagrams below.
(3) The spacing between the wire stops the short circuiting.

Question. Supraja was doing an experiment (Comparison of emfs ) using potentiometer in Physics lab. She could not take the readings because the galvanometer showed same side deflection. She checked the circuit and the connections were correct. Her friend Manasa who was doing her experiment nearby came to help Supraja. Manasa increased the voltage of the eliminator (by turning the knob) supplying current to the potentiometer. Supraja tried the experiment again and got the readings. She thanked Manasa for her help.
(1) What are the values displayed by both Supraja and Manasa?
(2) State one reason why the galvanometer showed same side deflection.
(3) Distinguish between emf and terminal pd.
Answer:
(1) Sharing of knowledge, caring for and helping others.
(2) The emf of the driving cell should be greater than the emf of the experimental cells.
(3) The potential difference between the terminals of a cell when the cell is in the open circuit and the potential difference between the terminals of a cell when the cell is in the closed circuit.

Question. Rama was a childless widow. She ran her life only by the pension for the Sr. citizens from the Government. When she switches of one bulb in her house, all the other appliances get switched off. She could not even spend for an electrician. Sujatha living nearby decided to do something about this. She referred to Physics books and learnt that the series combination for the household connection should be the reason. She called an electrician and had the circuit changed to parallel combination. The problem was solved and Rama was happy. She thanked Sujatha for her help to solve the problem.
a. What are the values possessed by Sujatha?
b. Why for household a parallel combination used? What are its advantages.
Answer: 
a. care for elderly people, empathy, willingness to gain knowledge
b. same voltage for all appliances, even if one appliance is not working the others can work.

VALUE BASED QUESTIONS

Question.​​​​​ While performing an experiment on determination of unknown resistance using a metre bridge, Rahul obtained deflection in the galvanometer in the same direction even after repeated adjustments in the circuit and thus could not get any results. In order to avoid getting noticed and scoldedby the teacher, he pretended having performed the experiment and copied the readings by another student.

Answer the following questions based on the above information :

(a) Write the possible reasons for getting the deflection in the galvanometer in the same direction.
Answer: Connections may be wrong or loose)

(b) Which two values is Rahul violating in copying the readings from another student ?
Answer: honesty , loyalty etc)

(c) What is your opinion should have Rahul done in the given circumstances
Answer: Rahul must consult his teacher about the problem he faced )

Question. Prof Kumar conducts an interview to select a physics teacher and asks the following two questions from every candidate :
(a) Why should a potentiometer be preferred over a voltmeter for measurement of emf of a cell?
(b) Why should a ten wire potentiometer be preferred over a four wire potentiometer?
There was a strong recommendation for candidate X who could not answer many questions including the above two. However, another candidate Y did not have any recommendations but replied most of the questions correctly. Prof Kumar recommended the selection of candidate Y ignoring completely the recommendation for the other candidate.

Answer the following questions on the basis of given information:

(i) Write in your own words , the answer to the above two questions asked by Prof Kumar.
Answer: 
(a) because potentiometer is based on null point method)
(b) because ten wire potentiometer is more sensitive)

(ii) Which values are displayed by Prof Kumar in the selection of teacher?
Answer: Loyal to his profession, honesty)

(ii) Suggest one activity to promote any one of the values displayed by Prof Kumar.
Answer: Role play, Poster making, Painting competition)

Question. During an experiment on determination of internal resistance of a primary cell using a potentiometer, Sohan obtained null point beyond the length of potentiometer wire, even after repeated adjustments in the circuit and thus could not get any results. He then went to his teacher and carefully understood the reason and then completed the experiment.

Answer the following questions based on the above information :

(a) Write the possible reasons for getting balance point beyond the length of potentiometer wire
Answer:  The emf of driving cell may be less than emfs to be measured )

(b) Write two values displayed by Sohan
Answer: Sincerety, scientific attitude etc.)

(c) What may be the possible corrections by the teacher.
Answer: The teacher may advice him to use a driving cell whose emf is more than experimental emfs)

Question. Two Brothers Sachin and Arjun , purchased an electric iron. Sachininsisted on using the new iron with a two pin plug which was available in the house. Arjun advised him to use it with a 3-pin plug only. Sachin got angry. But Arjun calmed down Sachin and patiently explained him the importance of using 3-pin plug.

Answer the following questions on the basis of given information :

(a) Write one values each , being displayed by the Arjun and Sachin .
Answer: Arjun–Knowledge of electrical appliances use while Sachin is enthusiastic and flexible

(b) Why we should use 3-pin plug instead of 2-pin plug ?
Answer: For safety purpose)

(c) Why a high tension source (HT) supply of 6kV must have a very large resistance.
Answer: A high tension supply has large r ,otherwise circuit get shorted and exceeds the safety limit)

Question. Mrs. Sharma parked her car and forgot to switch off the car headlights. When she returned, she could not start the car Rohit a passerby, came to her for help. After knowing about the problem, he went to nearby garage and call mechanic Ramu. Ramu noticed that the car battery has been dis charged as headlight were left on for a long time. He brought another battery from his garage and connected its terminals to the terminals of the car battery. He succeeded in starting the engine and then disconnected his battery. This is called ‘jump starting’, Mrs. Sharma felt happy and thanked both rohit and ramu.

Answer the following questions based on the above information:
(a) What values were displayed by rohit?
Answer: Helpful, aware of his limits, Ability to take quick decision)

(b) A storage battery of emf 12V and internal resistance 0.5ohm Is to be charged by a battery charger which supplies 110V dc. How much resistance must be connected in series with the battery to limit the charging current to 5A.what will be the p.d. across the terminals of the battery during charging? What is the purpose of having a series resistor in the charging circuit?
Answer: R = 19.1 ohm , V= 14.5 V, to protect from high charging current)

Question. Naveen had to take his ailing friend to hospital for treatment .he started his bike but it did not start . he checked the battery ,replaced it with another battery of dyr cells giving output 6 volt but could not start his bike . Naveen’s friend, Anoop suggested him to take his bike’s storage battery to Naveen so that he is able to reach the hospital in time .

a. what is your opinion are the human values displayed by Anoop?

b. if each dry cell is of 1.5v and internal resistance 0.3ῼ, find the current delivered by the pack of dry cell to 200ῼ spark plug.

c. why did Anoop suggest Naveen to replace the dry cell pack with storage battery?

d. find the terminal potential difference of the dry cell. How is it different from emf of the cell?
Answer: (a) Anoop is very helpful and courteous

(b) I = E/R+r = 6/0.200+1.2 =6/1.400
(c) Anoop knew that the dry cell cannot deliver large starting current required by the bike
(d) E = V+Ir V=E- Ir =6-4.2x1.2 =6-5.4=0.6v

ONE MARK QUESTIONS

Question. I – V graph for two identical conductors of different materials A and B is shown in the figure. Which one of the two has higher resistivity?

Answer : B will be of higher resistivity

Question. The plot of the variation of potential difference across the combination of three identical cells in series is shown in the figure: what is the emf and internal resistance of each cell.

Answer : When I=0 emf of three identical cells in series is equal to 6V, hence E=2V,r=2Ω

Question. The variation of potential difference V with length l for two potentiometrs P and Q are as shown , which one of the two will you prefer to comparing emf of two primary cells and why?

Answer : As Potential Gradient k=V/l, Potentiometer Q will be preferred.

Question. State the underlying principle of the potentiometer?
Answer : The potential difference across any two points of uniform current carrying conductor is directly proportional to the length of the conductor between those two points.

Question. Two identical cells each of emf E having negligible internal resistance are connected in parallel with each other across an external resistance R, what is the current through this resistance? 
Answer : I=E/R

Question. Define the term ‘mobility’ of charge carriers. Write its S.I. unit.
Answer : μ=(V d)/E= eτ/m, m2C/sJ

TWO MARKS QUESTIONS

Question. In a potentiometer arrangement for determining the internal resistance of a cell, the balance point of the cell in open circuit is 350 cm. When a resistance of 9 Ω is used in the external circuit of the cell, the balance point shifts to 300 cm. Determine the internal resistance of the cell.
Answer :  r= ( E/V- 1)R
r = ( l_1/l_2 - 1)R=(350/300- 1)9=1.5Ω

Question. In the potentiometer circuit shown, the null point is at X. State with reason, where the balance point will be shifted when
(a) Resistance R is increased, keeping all other parameters unchanged;
(b) Resistance S is increased, keeping R constant.
Answer : Potential gradient will decrease ,hence shift towards B 

No Change

Question. Define the term ‘power loss’ in a conductor of resistance R carrying a current I. In what form does this power loss appear? Show that to minimize the power loss in the transmission cables connecting the power stations to homes, it is necessary to have the connecting wires carrying current at enormous high values of voltage.
Answer : P = VI =I2 R = V2/R
Power loss will be less.

Question. You are required to select a carbon resistor of resistance 47 kΩ10% from a large collection. What should be the sequence of colour bands used to code it?
Answer : Violet, Yellow, Orange, Silver.

THREE MARKS QUESTIONS

Question. Two identical cells of emf 1.5V each joined in parallel supply energy to an external circuit consisting of two resistances of 7Ωeach joined in parallel. A very high resistance voltmeter reads the terminal voltage of cells to be 1.4 V, calculate the internal resistance of each cell.
Answer : Net emf=1.5 V, Net external resitance=3.5Ω,V=1.4V,I=1.4/3.5=0.4A
Solve with the equation V=E-IR, r=0.25 Ω

Question. Two wires X, Y have the same resistivity, but their cross-sectional areas are in the ratio 2: 3 andlengths in the ratio 1: 2. They are first connected in series and then in parallel to a d.c.source. Find out the ratio of the drift speeds of the electrons in the two wires for the two cases.
Answer : In series 3:2, In parallel 2:1

Question. Write two factors by which sensitivity of a potentiometer can be increased.
Why is a potentiometer preferred over a voltmeter for measuring the emf of a cell?
Answer : By increasing the length of the potentiometer wire, decreasing the emf of the driver cell.
It does not draw any current since it uses the null deflection method.

Question. State Kirchhoff’s rules. Use these rules to write the expressions for the currents I1, I 2 and I 3 inthe circuit diagram shown.

Answer :  I1=2/13A, I 2=7/13A and I 3 =9/13A(solve using Kirchoff’s law)

Question. A potentiometer wire of length 1 m is connected to a driver cell of emf 3 V as shown in the figure. When a cell of 1.5 V emfis used in the secondary circuit, the balance point is found to be 60 cm. On replacing this cell and using a cell of unknown emf, the balance point shifts to 80 cm.
(i) Calculate unknown emf of the cell.
(ii) Explain with reason, whether the circuit works, if the driver cell is replaced with a cell of emf 1 V.
(iii) Does the high resistance R, used in the secondary circuit affect the balance point? Justify your Answer.
Answer : .2.0V
Do not work
No it does not effect the null point