Physics Workshop for the Month of July

PHYSICS WORKSHOP FOR THE MONTH OF JULY

VALUE BASED QUESTIONS

ELECTROSTATICS

Q1 .Aneesha has dry hair. She runs plastic comb through her hair and finds that the comb attracts small bits of paper. But her friend Manisha has oily hair. The comb passed to Manisha hair could not attract small bits of paper. Aneesha goes to her Physics Teacher and gets an explanation of this phenomenon from her. She then goes to different junior classes and demonstrates this experiment to the students. The junior students feel very happy and promise her to join her science club set up for searching such interesting phenomena of nature.

(a)What are the values displayed by Aneesha?

(b)A comb run through one’s dry hair attracts small bits of paper. But it does not attract when run through wet hair.

Why?

Ans: (a)Curiosity, leadership and compassion

(b)When the comb runs through hair, it gets charged by friction and attracts small small bits of paper. The comb does not get charged when run through wet hair due to less friction and so it does not attract bits of paper

Q2. Neeta’s grandmother, who was illiterate, was wrapping her satin saree. She found some sparks coming out from it. She was frightened and called Neeta. Neeta calmed down her grandmother and explained to her the scientific reason behind these sparks.

(a)What according to you, are the values displayed by Neeta?

(b)Why do sparks appear when a satin saree cloth is folded?

Ans: (a)Awareness and sensitivity.

(b)The different portions of the cloth get charged due to friction. Then the flow of charge gives rise to sparks.

Q3. While travelling back to his residence in the car, Dr. Pathak was caught up in a thunderstorm. It became very dark. He stopped driving the car for the thunderstorm to stop. Suddenly he noticed a child walking alone on the road. He asked the boy to come inside the car till the thunderstorm stopped. Dr. Pathak dropped the boy at his residence. The boy insisted that Dr. Pathak should meet his parents. The parents expressed their gratitude to Dr. Pathak for his concern for safety of the child.Answer the following questions based on the above information:

(a)Why is it safer to sit inside a car during a thunderstorm?

(b)Which two values are displayed by Dr. Pathak in his actions?

(c)Which values are reflected in parents’ response to Dr. Pathak

(d)Give an example of a similar action on your part in the past from everyday life.

Ans: (a)It is safer to sit inside a car during a thunderstorm because the metallic body of the car becomes an electrostatic shielding from lightening.

(b)Safety of human life, helpfulness, empathy and scientific temper.

(c)Gratefulness and indebtedness.

(d)Neeta’s grandmother was wrapping her satin saree. She found some sparks coming out from it. She was frightened and called Neeta. Neeta calmed down her grandmother and explained to her the scientific reason behind these sparks.

Q4.One evening, Pankaj outside his house fixed a two metre high insulating slab and attached a large aluminium sheet of area 1m2 over its top. To his surprise, next morning when he incidentally touched the aluminium sheet, he received an electric shock. He got afraid. He narrated the incident to his Physics Teacher in the school who explained him the reason behind it.

(a)What are values being displayed by Pankaj?

(b)What maybe the reason behind the electric shock received by Pankaj?

Ans:(a)Keen observer and curiosity.

(b)The aluminium sheet and the ground form a capacitor along with the insulating slab. The discharging current of the atmosphere charges the capacitor steadily and raises its voltage. So, when Pankaj touches the aluminum sheet, he receives an electric shock.

Q5. 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?

Ans: 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 earthling gets charged uniformly.

VALUE BASED QUESTIONS.

CURRENT ELECTRICITY

1. 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 scolded by 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.

(b) Which two values is Rahul violating in copying the readings from another student ?

(c) What in your opinion Rahul should have done in the given circumstances?

ANS:

(a) Connections may be wrong or loose

(b) Honesty , loyalty etc

(c) Rahul must consult his teacher about the problem he faced

1. Prof Kumar conducts an interview to select a physics teacher and asks the following two questions from everycandidate :

(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.

(a) Ans : Because potentiometer is based on null point method.

(b) Ans : Because ten wire potentiometer is more sensitive.

(ii) Which values are displayed by Prof Kumar in the selection of teacher?

Ans : Loyal to his profession, honesty

(iii) Suggest one activity to promote any one of the values displayed by Prof Kumar.

Ans: Role play, Poster making, Painting competition

1. 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.

Ans : The emf of driving cell may be less than emfs to be measured.

(b) Write two values displayed by Sohan

Ans :Sincerity, scientific attitude etc.

(c) What may be the possible corrections by the teacher?

Ans : The teacher may advice him to use a driving cell whose emf is more than experimental emfs

1. Two Brothers Schin and Arjun , purchased an electric iron. Sachin insisted 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 .

Ans : 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 ?

Ans : For safety purpose

(c) Why a high tension source (HT) supply of 6kV must have a very large resistance.

Ans : A high tension supply has large resistance ,otherwise circuit get shorted and exceeds the safety limit.

1. 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 called mechanic Ramu. Ramu noticed that the car battery has been discharged as headlight was 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?

Ans : 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 potential difference across the terminals of the battery during charging? What is the

purpose of having a series resistor in the charging circuit?

Ans : R = 19.1 ohm , V= 14.5 V, to protect from high charging current.

1. 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 dry 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 are the 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?

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.

HOTS ON ELECTROSTATICS

1. Four point charges are placed at the four corners of a square in the two ways (i) and (ii) as shown below. Will the (i) electric field (ii) Electric potential, at the centre of the square, be the same or different in the two configurations and why?

ANS:

(I)Electric field is a vector quantity in the first case electric field at the centre due to charges at A and due to C add up and also due to charges at B and D are added up. There exists electric field at the centre. Where as in the second case field due to A and C are equal and opposite and also due to B and D are also equal and opposite, so the resultant field is zero.

(II)Potential is a scalar quantity and is positive due to positive charge and negative due to negative charge. Hence resultant potential at the centre is zero in both the cases.

1. Find the ratio of potential difference that must be applied across the parallel and series combination of two capacitors C1 and C2 with their capacitance in the ratio 1:3 so that the energy stored in the two cases is same.

ANS:

Given

Substituting CS-And Cp= C1+C2

1. Find the capacitance of the infinite ladder between the points X and Y

ANS:

Let the equivalent capacitance of the network be C Since it is an infinite network addition one such more unit will not affect equivalent capacitance. The network will appear like

The equivalent capacity of the new arrangement must be C.

Or C2- C- 2=0

Therefore C=2μF or -1μF

As capacitance cannot be negative therefore C=2μF

4.Three points A, B and C lie in uniform electric field E of 5x103N/C as shown in the figure. Find potential difference between A and C.

ANS:

Points B and C lie on equipotential surface. So VC = VB

Potential difference between A and C = Potential difference between A and B

5Three charges –q, +Q and –q are placed at equal distance on a straight line. If the potential energy of the system of three charges is zero, find the ratio of Q/q.

ANS:

As total potential energy is zero

6. .A metallic sphere placed between two charged metallic plates. A student draws the lines of force as shown in figure. Is he correct?

ANS:

No, because within a metal the electric field is zero. Therefore, no lines of force should exist between the spheres.

1. Two charged spherical conductors, each of radiuses R, are distance d apart such that d is slightly greater than 2R. They carry charge +q and -q. Will the force of attraction between them be exactly ?

ANS:

No, the force of attraction between spherical conductors will be more than K.q2/d2, due to

attractive of opposite charges; these will be redistribution of charges on spheres. Obviously, the

effective distance between the charges will be reduced and hence effective force will be increased.

1. Two identical charges, Q each are kept at a distance r from each other. A third charge q is placed on the line joining the two charges such that all the three charges are in equilibrium. What is magnitude, sign and position of the charge q?

Ans : Q/4, Positive, r/2

1. Two dipoles, made from charges q and Q, respectively, have equal dipolemoments. Give the (i) ratio between the ‘separations’ of these two pairs ofcharges (ii) angle between the dipole axis of these two dipoles.

Ans : q a=Q A or a/A=Q/q θ =

Current Electricity

Answersof HOT Questions

Current Electricity