CBSE Class 12 Physics HOTs Current Electricity

     UNIT 02

CURRENT ELECTRICITY

ONE MARK QUESTIONS

Question. Define the terms conductivity of a conductor?On what factors do it depend? 
Answer : Conductivity is the reciprocal of resistivity,It depends upon no density of electrons and Average relaxation time.

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

Answer : For metallic wire R=V/I increases with temperature, hence T1 will be higher.

Question. A 10 V battery of negligible internal resistance is connected across a 200 V battery and a resistance as shown in the figure find the value of current in the circuit.

Answer : I=200-10/38=190/38=5Amp.

Question. When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?
Answer : No, Maximum no of electrons will drift opposite to the direction of higher potential

Question. Show on a graph the variation of resistivity with temperature for a typical semiconductor?
Answer : 

Question. Two metallic resistors are connected first in series and then in parallel across a d.c. supply. Plot of I – V graph is shown for the two cases. Which one represents a parallel combination of the resistors and why?

Answer : In parallel combination less resistance will be the equivalent resistance, therefore A will represent parallel combination.

TWO MARKS QUESTIONS

Question. Calculate the current drawn from the battery by the network of resistors shown in figure 

Answer : As the circuit is similar to a whaetstone bridge 5 Ω resistor can be removed ,then R=2Ω
Hence i=2 amp.

Question. Using the concept of drift velocity of charge carriers in a conductor, deduce the relationship between current density and resistivity of the conductor.
Answer :  j= I/A = n e V d j =σ E =1/ρE

Question. A variable resistor R is connected across a cell of emf E and internal resistance r as shown in the figure. Draw a plot showing the variation of (i) terminal voltage V and (ii) the current I, as a function of R. (Image 4)
Answer : 

Question. Two electric bulbs P and Q have their resistances in the ratio of 1: 2. They are connected in series across a battery. Find the ratio of the power dissipation in these bulbs.
Answer : P = V2/R hence the ratio will be 2:1

THREE MARKS QUESTIONS

Question. A 16 Ω resistance wire is bent to form a square. A source of emf 9 V is connected across one of its sides as shown. Calculate the current drawn from the source. Find the potential difference between the ends C and D.
If now the wire is stretched uniformly to double the length and once again the same cell is connected in the same way, across one side of the square formed, what will now be the potential difference across one of its diagonals?
Answer : 

Question. A cell of emf ‘E’ and internal resistance ‘r’ is connected across a variable load resistor R. Draw the plots of the terminal voltage V versus (i) R and (ii) the current I.
It is found that when R = 4 , the current is 1 A and when R is increased to 9 , the current reduces to 0.5 A. Find the values of the emf E and internal resistance r.
Answer : E=1(4+r)=0.5(9+r), Solving r=1Ω,E=5V

Question. A potential difference V is applied across a conductor of length L and diameter D. How is the drift velocity, Vd, of charge carriers in the conductor affected when (i) V is halved, (ii) L is doubled and (iii) D is halved? Justify your answer in each case.
Answer : Drift velocity halved
Drift velocity halved
Drift velocity halved

Question. A resistance R draws current from a potentiometer, the potentiometer AB, has a total resistance R, a voltage V is applied to the potentiometer.
Derive an expression for the voltage across R when the sliding contact is in the middle of potentiometer wire. 

Answer : Vac=2VR/4R+Ro

FIVE MARKS QUESTIONS

Question. Use Kirchhoff’s rules , calculate the current in the arm AC of the given circuit
On what principle does the meter bridge work? Why are the metal strips used in the bridge?
Answer : I1 = -12/13 A
I2 = -8/13 A
I1+ I2 = -8/13 + -12/13=-20/13A

Question. In the figure a long uniform potentiometer wire AB is having a constant potential gradient along its length. The null points for the two primary cells of emfs E1and E2 connected in the manner shown are obtained at a distance of 120 cm and 300 cm from the end A.
Find E1 / E2 and position ofnull point for the cell E1.
How is the sensitivity of a potentiometer increased?
Answer : 7/3,
210cm

Question. Draw a circuit diagram of a potentiometer .State its working principle. Derive the necessary formula to describe how it is used to compare the emfs of the two cells.
Write two possible causes for one sided deflection in a potentiometer experiment.
Answer : E1/E2 = l1/l2 (derive it)
The emf of the driver cell may be less than the secondary cell
Very high resistance connected in series with the drver cell circuit

One mark questions

 

Q9. Why the terminal Potential is always less than EMF of a cell, while in use?

 

Q10. The conductivity of a semi conductor increases with the rise of temperature. Give reason. Two Marks Questions

 

Q11.N number of identical resistors each of resistance R is combined to get the maximum and minimum resistances, what is the ratio of the maximum to minimum resistance.

 

Q12.Two wires of equal length one of copper and other of manganin have same resistance. Which of the two wires will be thicker? Justify your answer with the help of suitable formula.

 

Q13.A resistor of 24 ohm resistance is bent in the form of a loop as shown in the figure. Calculate effective resistance between points A and B?

 

Q14.V-I graph for metallic wire at two different temperature ‘a’ and ‘b’ is shown in fig. Which of the two temperatures is higher and why?

 

Q15. Explain why the I-V characteristics of a resistor are obtained to deviate from a straight line from higher values of current as shown below:

 

Three Marks Question-

Q16.Two resistances are in the ratio 1:4 if these are connected in parallel their total resistance becomes 20 ohm. Find the value of each resistance.

 

Q17.A household circuit has a fuse of 5A rating. Calculate the maximum number of bulbs of rating 60W-220V each which can be connected in the household circuit.

 

Q18. Calculate current in each branch.

 

Q19.Two identical cells of EMF E and internal resistance r whether joined in series or in parallel give the same current, when connected to external resistance of 1 ohm. Find internal resistance of each cell.

 

Q20. 4 cells of identical EMF E, internal resistance r are connected in series to a variable resistor. The following graph shows the variation of terminal voltage of the combination with the current output:

1.What is the EMF of each cell?

2.calculate the internal resistance of each cell.

 

ANSWERS-

1. during charging

2. uncharged

3. black, brown, black (gold)

4. decreases

5. ML³T^-4A^-1

6. Low temp. Coefficient of resistance high Resistivity.

7. drift velocity halved

8. k_b<k_a (k is the potential gradient)

9. T.P. =EMF - Ir

10. Due to increase in carrier density

 

Two Marks-

11. N²:1

12. MANGANIN

13. 10/3 OHM

14. a>b

15. temp. increases

slope decreases

ohm law is disobeyed

 

Three Marks-

16. R1=25 ohm

R2=100 ohm

17. no. of bulb=18

18 .I1=5/2A

I2=5/8A

I3=7/8A

19 r=1 ohm

20. (1)E=1.4 V

(2)r=0.7 ohm.

 

28. Two metallic wires of the same material and same length but of different cross sectional areas are joined together.

1) in series

2) in parallel to a source of emf.

In which of the two wires will the drift velocity of electron be more in each of the two cases and why?

 

29. The resistivity of a material of a conductor of uniform cross section varies along its length as e=ε (1+αx). deduce the expression for the resistance of the conductor, if its length is “L” and area of cross section is “A”.

 

30. A uniform wire of resistance “R” is shaped into a regular “n” sided polygon, where “n” is even. Find the equivalent resistance between

1) opposite corners of the polygon

2) adjacent corners of the polygon.

 

31. Under what condition is the heat produced in an electric circuit

1) directly proportional

2) inversely proportional to the resistance of the circuit?

 

32. A 4Ω non insulated wire bent 180⁰ in the middle and the two halves are twisted together. What will be its new resistance?

 

33. What is the safest voltage you can safely put across a 98Ω, 0.5 watt resistor?

 

34. A student has two wires of iron and copper of equal length and diameter. He first joins the two wires in series and passes electric current through the combination which increases gradually. After that he joins the two wires in parallel and repeats the process of passing current. Which wire will glow first in each case and why?

 

35. A series combination of a 2KΩ  resistor and 1KΩ resistor is connected across a battery of emf 6V and negligible internal resistance. The potential drop across the 2Ω resistor is measured by 

1. a 30KΩ voltmeter

2. a 1KΩ voltmeter

3. both these voltmeters connected across it.

 

If the voltmeter readings in the three cases are V₁, V₂ and V₃ respectively,arrange these readings in descending order.

How will the three readings be compared with one another if the potential drop were measured across the series combination of the 2 K Ω and 1 K Ω resistor?i.e. across A and B?

CURRENT ELECTRICITY

1. Magnesium is used for making standard resistors, why?

2. The sequence of bands marked on a carbon resistor are: Red, Red, Red, Silver. Write the value of resistance with tolerance.

3. A wire of resistively ρ is stretched to three its initial length, what will be its new resistively.

4. If p.d.v applied across a conductor is increased to 2v, how will the draft velocity of the electrons change?

5. A 10Ω thick wire is stretched so that its length becomes three times. Assuming thatthere is no change in its density on stretching. Calculate the resistance of new wire.

6. You are given 8 Ω resistor. What length of wire of resistance 120 Ωm-1 should be joined in parallel with it to get a value of 6 Ω ?

7. Three resistance 3Ω,6Ω and 9Ω are connected to a battery. In which of them will the powerdissipation be maximum if

    a) They are all connected in parallel

    b) They are all connected in series Give reason.

8. A silver wire has a resistance of 2.1Ω at 27.5˚c and a resistance of 2.7Ω at 100˚c. determine the temperature coeff. of resistivity of silver.

9. Give any two applications super conductors.

10. Two wire of equal length one copper and manganin have same resistance , which wire is thicker?.

11. Why manganin is used for making standard resistor?

12. A copper wire of resistivity r is stretched to reduce its diameter to half of its previous value .What  will be the new resistances?