Program for All India Ranks, State Ranks & Centums CBSE, State Boards, ICSE / ISC JEE (Main & Advanced), AIPMT CBSE XII

PHYSICS

Program for 490+/500

Marks: 70

Time : 3 Hours

General Instructions:  All questions are compulsory.  Questions 1 to 5 carry one mark each  Questions 6 to 10 carry 2 marks each  Questions 11 to 22 carry 3 marks each  Question 23 carries 4 marks  Questions 24 to 26 carry 5 marks each

1.

2.

TEST - 2 Chapter 3. CURRENT ELECTRICITY A wire is stretched to thrice its length. What happens to the drift velocity of the free electrons if the applied potential difference is the same? A 10V battery of negligible internal resistance is connected across a 38Ω resistor as shown. Find the value of current in the circuit. 10V

38Ω

200

3.

The emf of a cell is always greater than the terminal voltage. Why? Give reason.

4.

In an experiment using metre bridge if the balance length AC is x, what would be the balance length if the radius of the metre bridge wire AB is doubled? Justify your answer.

5.

Two wires of equal length, one of copper and the other of magnanin have the same resistance. Which wire is thicker?

6.

Estimate the average drift speed of conduction electrons in a copper wire of cross-sectional area 2.5 x 10-7m2 carrying a current of 1.8A. Assume the density of the conduction electrons to be 9 x 1028 m-3.

7.

A cell of emf ‘E’ and internal resistance ‘r’ is connected to a variable resistor ‘R’. Plot a graph showing the variation of terminal voltage ‘V’ of the cell versus current ‘I’. Using the plot, show how the emf of the cell and its internal resistance can be determined.

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8.

2

A cell of emf E and internal resistance r is connected to two external resistors R1 and R2 and a perfect ammeter. The current in the circuit is measured in four different situations: (i) Without any external resistance in the circuit. (ii) With resistance R1 only (iii) With R1 and R2 in series combination (iv) With R1 and R2 in parallel combination The currents measured in the four cases are 0.42A, 1.05A, 1.4A and 4.2A but not necessarily in that order. Identify the currents corresponding to the four cases mentioned above.

9.

Two metallic wires of the same material have the same length but cross-sectional areas in the ratio 1:2. They are connected in i) series and ii) parallel. Compare the drift velocities of the electrons in the two wires in both the cases.

10.

In the given circuit, assume point A to be at zero potential, use Kirchhoff’s law to determine the potential at B. 1A

2Ω 2A 11.

Two cells of emf E1 and E2 and internal resistance r1 and r2 respectively are connected in parallel. Deduce the expression for the equivalent emf and equivalent internal resistance of the combination.

12.

Answer the following (i) Why are the connections between the resistors in a metre bridge made of thick copper strips? (ii) Why is it generally preferred to obtain the balance point in the middle of the metre bridge wire? (iii) Which material is used for the metre bridge wire and why?

13.

A resistance RΩ draws a current from a potentiometer as shown in the figure. The potentiometer has a total resistance R0 Ω. A voltage V is supplied to the potentiometer. Derive an expression for the voltage across R when the sliding contact is in the middle of the potentiometer. V R0 A

B

R 14.

Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of the free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.

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3

15.

Two heating elements of resistances R1 and R2 when operated at a constant supply of voltage V, consume powers P1 and P2 respectively. Deduce the expression for the power of their combination when they are in turn connected in i) series and ii) parallel across the same voltage supply.

16.

Determine the reading of the ammeter.

17.

Find the potential difference across the terminals of each cell given E1=4V, E2=8V, r1=0.5Ω and r2=1Ω. Also find the current through each resistor.

18.

Using Kirchhoff’s laws determine the currents I1, I2 and I3.

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19.

4

In the figure a long potentiometer wire AB is having a constant potential gradient along its length. The null points for the two primary cells E1 and E2 are obtained as shown. Find E1/E2 and the position of null point with only E1. Also explain how the sensitivity of the potentiometer can be increased? E

120cm E1

300cm

E2 G

E1

E2 G

20.

With the help of a circuit diagram state the working principle of a metre bridge. Obtain the expression for determining the unknown resistance. What happens to the balance point if the cell and galvanometer are interchanged?

21.

You are required to select a carbon resistor of resistance 47kΩ±10% from a large collection. What should be the sequence of colour bands used to code it? Draw graphs showing the variation of resistivity with temperature for i) copper ii) magnanin. Hence explain why magnanin is used for making resistance coils.

22.

Six resistors each of resistance 4Ω are joined together as shown in the figure. Calculate the equivalent resistance across the points A and B. if a cell of emf 2V is connected across AB, compute the current through arms AB and DF. 4Ω

4Ω F

4Ω

C

E

D 4Ω A

4Ω 4Ω

B

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23.

5

Mrs Thakur left her car headlights on while parking. The car would not start when she returned. Seeing her struggle, Mohit went to her help. Not knowing much about cars, he ran and brought a mechanic Raju from a garage nearby. Raju realized that the battery had got discharged as the headlight had been left on for a long time. He brought another battery and connected its terminals to the terminals of the car battery to get the engine started. Once the engine was running, he disconnected this second battery. This is known as “JUMP STARTING”. Mrs. Thakur thanked both Mohit and Raju for helping her. (i) What values did Mohit have? (ii) A storage battery of emf 8.0volts and internal resistance 0.5 ohm is being charged by a 120 volt DC supply using a series resistor of 15.5 ohms. What is the terminal voltage of the battery during charging? What is the purpose of having a series resistor in the charging circuit?

24.

State Kirchhoff’s rules for an electric network. Using Kirchhoff’s rules, obtain the balance condition in terms of resistances of the four arms of Wheatstone bridge. In the metre bridge experimental set-up shown, null point is obtained 40cm from end A of the metre bridge wire. If a 10Ω resistor is connected in series with X, null point is obtained at 60cm from end A. Calculate X and Y.

25.

State potentiometer principle. With the help of a circuit diagram, explain how a potentiometer is used to compare the emf of two primary cells. Give two reasons for one side deflection in a potentiometer experiment.

26.

Is current density a vector or a scalar quantity? Deduce the relation between current density and potential difference across a current carrying conductor of length ‘l’, area of cross-section ‘A’ and number density of free electrons ‘n’. how does the current density in a conductor vary with (a) Increase in potential gradient (b) Increase in temperature (c) Increase in length (d) Increase in area of cross-section

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11212.102-12C-PHY-03.pdf

Derive an expression for the. voltage across R when the sliding contact is in the middle of the potentiometer. V. 14. Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift. velocity of the free electrons? Use this relation to deduce the expression for the electrical resistivity. of the material.

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