CBSE Class 12 Physics Current Electricity Worksheet Set C

(a) 4 rows, 6 cells in a row

(b) 12 rows, two cells in a row

(c) 2 rows, 12 cells in a row

(d) 3 rows, 8 cells in a row

 

(a) Parallel

(b) Series

(c) Mixed grouping

(d) Depends upon the relative values of internal and external resistances

 

(a) High specific resistance and high melting point

(b) High specific resistance and low melting point

(c) Low specific resistance and high melting point

(d) Low specific resistance and low melting point

Question. What happens if the galvanometer and cell are interchanged at the balance point of the bridge? Would the galvanometer show any current?
Answer. No change . The galvanometer will not show any current

Question. Two wires A and B of the same material and having same length have their cross-sectional areas in the ratio 1:4. What would be the ratio of heat produced in the wires when same voltage is applied?
Answer. 𝑃=𝑉₂/𝑅 since voltage is same power is inversely proportional to resistance
𝑃1/𝑃2=𝑅2/𝑅1=𝐴1/𝐴2=1:16

Question. A potential difference V is applied across a conductor of length L. How is the drift velocity affected when V is doubled and L is halved?
Answer. 𝑉 =𝑒𝜄𝑉/𝑚𝐿 and hence becomes 4 times

Question. Plot a graph showing the variation of resistance of conducting wire as a function of its radius, keeping the length of the wire and its temperature as constant
Answer. 𝑅 𝛼1/𝑟2 graph is rectangular hyperbola

Question. Two wires A and B of the same material having length in the ratio 1:2 and radii in the ratio 2:1. What is the ratio of the resistance?
Answer. 𝑅 𝛼𝐿𝐴 𝑅1𝑅2=(𝐿1𝐿2)𝑥 (𝐴2𝐴1)= 2:1

SHORT ANSWR QUESTIONS

Question. Three identical cells each of emf 2V and internal resistance 0.2Ω are connected in series to an external resistor of 7.4 Ω. Calculate the current in the circuit
Answer. E effective= 6
r effective = 0.6
Total resistance 7.4+0.6=8
I = 6/8=.75A

Question. Distinguish between emf and terminal voltage
Answer. Definition given in summary

Question. Why alloys like constantan are used for making standard resistors?
Answer. Low value for temperature coefficient of resistance. Hence resistance wont vary much with change in temperature

Question. Distinguish between ohmic and non ohmic devices. Give one example each
Answer. Given in summary

Question. The thickness of a conductor continuously decreases from its one end A to another end B. It is connected across the terminals of a battery. What will be the effect on the value of?
(I) electric field
(ii) current density (iii) mobility of the electron
at a point on the conductor as one moves from end A to end B
Answer. 𝐸 = 𝑉 /𝐿 𝐼𝑅 /𝐿=𝐼/𝐿 (𝜌𝐿/𝐴)=𝐼𝜌/𝐴
E is proportional to 1/A hence decreases from end A to end B
J =I/A
Hence j decreases from end A to B
μ= 𝑒𝜄/𝑚 hence constant

 

CASE BASED QUESTIONS

1. An electric cell is source of energy that maintains a continuous flow of charge in a circuit. it changes chemical energy into electrical energy. It has two electrodes – positive and negative. Electric cell has to do some work in maintaining the current through a circuit. The work done by the cell in moving unit positive charge through the whole circuit is called the emf of the cell.

Question. When two electrodes of a cell are immersed in an electrolytic solution the charges are exchanged between
(i) Positive electrode and electrolyte only
(ii) Negative electrode and electrolyte only
(iii) Both electrodes and electrolyte
(iv) Directly between two electrodes
Answer. C

Question. The current flowing in the sell is
(i) I = E /(R+r)
(ii) I = E/R
(iii) I=E/r
(iv) I = (R+r)/E
Answer. A

Question. The maximum current that can be drawn from a cell is for
(i) R = infinity
(ii) R = finite non-zero resistance
(iii) R = 0
(iv) R =r
Answer. D

Question. When R is infinite, then potential difference V between P and N is
(i) E
(ii) 2E
(iii) E/2
(iv) E/4
Answer. A

 

Assertions & Reasons Questions Following questions consider two statements: one labelled as statement-1 and another as statement-2. Study both statements and mark your answer using the codes given blow.
A. Both Statement-1 and Statement-2 are true and Statement-2 is correct explanation of Statement-1.
B. Both Statement-1 and Statement-2 are true and Statement-2 is not a correct explanation of Statement-1.
C.Statement-1 is true but statement-2 is false.
D.Statement-1 is false and statement-2 is true. E. Statement-1 is false and also statement-2 is true.

Question. Statement-1 Free electrons in conductor are always in state of continuous random motion.
Statement-2 The random motion of free electrons is due to thermal energy of the conductor.
Answer. A

Question. Statement-1 Drift velocity of electrons in a conductor increase on increasing the temperature of the conductor.
Statement-2 On increasing the temperature of a conductor, resistivity of the conductor increases.
Answer. D

Question. Statement-1 For metals relaxation time for electrons decreases with increase in temperature.
Statement-2 With the increase in temperature, number of collision per unit time that electrons made with lattice ion also increases.
Answer. B

Question. Statement-1 Fuse wire has high resistance and low melting point.
Statement-2 Fuse wire is for small current flow only.
Answer. C

Question. Statement-1 Power rating of resistance is not so important when used in a circuit.
Statement-2 The resistance value changes with temperature.
Answer. B

Question. Statement-1 After a long use, a potentiometer is declared to be less sensitive
Statement-2. The potential gradient of a potentiometer depends on the area of cross section of the wire used.
Answer. A

Question. Statement-1 In meter bridge experiment, a high resistance is always connected in series with a galvanometer.
Statement-2 As resistance increases current through the circuit increases.
Answer. C

Question. Statement-1 In a balanced Wheatstone’s bridge no current runs across the galvanometer.
Statement-2. The terminals of the path across which the galvanometer is connected are at the same potential.
Answer. A

Question. Statement-1 Kirchhoff’s junction law follows from conservation of charge.
Statement-2. Kirchhoff’s loop law follows from conservative nature of electric field.
Answer. B

 

 

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

 

2 A wire of resistance 8R is bent in the form of a circle.What is the effective resistance between the ends of a diameter AB?

 

3 A wire of resistance 5 ohm is drawn out so that its length is increased to twice its original length. Calculate its new resistance.

 

4 Two identical slabs, of a given metal, are joined together, in two different ways, as shown in figures (a) and (b). What is the ratio of the resistances of these two combinations?

 

 

5 A cylindrical metallic wire is stretched to increase its length by 10%. Calculate the percentage change in its resistance.

 

6 A steady current flows in a metallic conductor of non-uniform cross-section.Which of these quantities is constant along the conductor: current, current density, electric field, drift speed?

 

7 A low voltage supply from which one needs high currents must have very low internal resistance. Why?

 

8 A high tension (HT) supply of, say, 6 kV must have a very large internal resistance. Why?

 

9 The electron drift speed is estimated to be only a few mm s–1 for currents in the range of a few amperes? How then is current established almost the instant a circuit is closed?

 

10 Two conducting wires X and Y of same diameter but different materials are joined in series across a battery. If the number density of electrons in X is twice that in Y, find the ratio of drift velocity of electrons in the two wires?

 

11 In an experiment on metre bridge, if the balancing length AJ is ‘ l ’, what would be its value, when the radius of the metre bridge wire AB is doubled? Justify your answer.

 

12 The sequence of coloured bands in two carbon resistors R1 and R2 is (i) brown, green, blue and (ii) orange, black, green. Find the ratio of their resistances.

 

13 Can terminal potential difference be greater than emf of a cell? Justify.

 

14 A (i) series (ii) parallel combination of two given resistors is connected, one-byone, across a cell. In which case will the terminal potential difference, across the cell, have a higher value?

 

15 V- I graph for a metallic wire at two different temperatures T₁ and T₂ is shown in the figure. Which of the two temperatures is higher and why?

 

16 A cell of emf E and internal reistance r is connected across a variable resistor R. Plot a graph showing the variation of terminal potential difference V with resistance R. Predict from the graph the condition under which V becomes equal of E. Also plot a graph showing the variation of ε with R.

 

17 A heating element is marked 210V, 630W. What is the value of the current drawn by the element when connected to a 210 V DC source?

 

18 Two bulbs of same wattage, one having a carbon filament and the other having a metallic filament, are connected in series to the mains. Which one will glow more?

 

19 Of the bulbs in a house, one glows brighter than the other. Which of the two has a large resistance?

 

20 Two electric bulbs of 50W and 100W are given. When they are (i) connected in series (ii) connected in parallel, which bulb will glow more?

 

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

 

22 A conductor of length ‘l’ is connected to a dc source of potential ‘V’. If the length of the conductor is tripled by gradually stretching it, keeping ‘V’ constant, how will (i) drift speed of electrons and (ii) resistance of the conductor be affected? Justify your answer.

 

23 Two materials Si and Cu, are cooled from 300 K to 60 K. What will be the effect on their resistivity?

 

24 Plot a graph showing the variation of resistance of a conducting wire as a function of its radius, keeping the length of the wire and its temperature as constant.

 

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

 

26 A potential difference V is applied to a conductor of length L, diameter D. How are the electric field E, drift velocity v and resistance R are affected when (i) V is doubled, (ii) L is doubled, (iii) D is doubled.

 

27 Answer the following:

(i) Why are the connections between resistors in a meter bridge made of thick copper strips?

(ii) Why is it generally preferred to obtain the balance point near the middle of the bridge wire in meter bridge experiments?

(iii) Which material is used for the meter bridge wire and why?

 

28 A cell of emf (ε) and internal resistance (r) is connected across a variable external resistance (R) Plot graphs to show variation of (i) ε with R, and (ii) terminal potential difference of the cell (V) with R.