Current Electricity  Important Questions for CBSE Class 12 Physics Kirchhoff’s Laws and Electric Devices

1.Kirchhoff has given two rules based on conservation of electric charge and of energy, these are known as  Kirchhoff’s laws
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2.Kirchhoff’s First Law or Junction Rule The algebraic sum of electric currents at any junction of electric circuit is equal to zero i.e. the sum of current entering into a junction is equal to the sum of current leaving the junction i.e.
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Junction law supports law of conservation of charge because this is a point in a circuit which cannot act as a source or sink of charge(s).
Kirchhoff’s Second Law or Voltage Rule In any closed mesh of electrical circuit, the algebraic sum of emf’s of cell and the product of currents and resistance is always equal to zero i.e.
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Kirchhoff’s second law supports the law of conservation of energy. Because the net change in the energy of a charge, after the charge complete a closed path must be zero
NOTE Sign convention for Kirchhoffs second law
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4.Wheatstone Bridge It is an arrangement of four resistance connected to form the arms of quadrilateral A battery with key and galvanometer are connected along its two diagonals respectively.
where, P,Q = ratio arms, R = known resistance
S = unknown resistance
The bridge is said to  be balanced, when
(i) Vb = Vd
(ii) There is no flow of current through galvanometer.
The Wheatstone bridge is said to be sensitive, if it gives ample deflection in the galvanometer even on slight change of resistance.
For sensitivity of galvanometer, the magnitude of four resistances P, Q, R, S should be of same order.
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5.Meter Bridge
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It is an electrical device used to determine the resistance and hence, specific resistance of material of given wire/conductor.
It is based on the principle of balanced Wheatstone bridge.
For uniform wire, resistance of wire Length of conductor At balanced situation of bridge
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Previous Year  Examination Questions

1 Mark Questions

1.A heating element is marked 210 V, 630 What is the value of the current drawn by the element when connected to a 210 V DC Source? [Delhi 2013]
Ans.
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2.In an experiment on meter bridge, if the balancing length AC is X, what would be its value, when the radius of the meter bridge wire AB is doubled? Justify your answer.[All India 2011 C]
Ans.
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3.In a meter bridge, two unknown resistances R and S when connected in the two gaps, give a null point at 40 cm from one end. What is the ratio of R and S?[Delhi 2010]
Ans.
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4.Use Kirchhoff’s rules to determine the value of the current Ix flowing in the circuit shown in the figure.
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Ans.
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5.In the given circuit, assuming point A to be at zero potential, use Kirchhoff’s rules to determine the potential at point B
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Ans.
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6.In the meter bridge experiment, balance point was observed at J with AJ =l.
(i)The values of R and X were doubled and then interchanged. What would be the new position of balance point?
(ii)If the galvanometer and battery are interchanged at the balanced position, how will the balance point get affected?
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Ans.
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7.Using Kirchhoff’s rules in the given circuit, determine
(i)the voltage drop across the unknown resistor R and
(ii)the current I2 in the arm EF
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Ans.
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8.Calculate the current drawn from the battery in the given network
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Ans.
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9. For the circuit diagram of a Wheatstone bridge shown in the figure, use Kirchhoff’s laws to obtain its balance condition.
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Ans.
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10.Obtain the formula for the power loss (i.e. power dissipated) in a conductor of resistance R, carrying a current.[Delhi 2009 C]
Ans.
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11.The adjoining graph shows the variation of terminal potently difference V, across a combination o three cells in series to a resistor vers1 the current I.
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(i)Calculate the emf of each cell.
(ii)For what current I, will the power  dissipation of the circuit be  maximum? [All India 2008]
Ans.
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3 Marks Questions

12.Answer the following
(i)Why are the connections between the resistor in a meter bridge made of thick copper strips?
(ii)Why is it generally preferred to obtain the balance point in the middle of the meter bridge wire?
(iii)Which material is used for the meter  bridge wire and why?[All India 2014]
Ans.(i)The connections between the resistors in a meter bridge are made of thick copper strips because of their negligible resistance.

(ii) It generally preferred to obtain the balance point in the middle of the meter bridge wire because meter bridge is most sensitive when all four resistances are of same order

(iii)Alloy, magnanin or constantum are used for making meter bridge wire due to low temperature coefficient of resistance and high resistivity

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Ans.
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14.Define the current sensitivity of a galvanometer. Write its SI unit.Figure shows two circuits each having a galvanometer and a battery of 3 V.When the galvanometer in each arrangement do not show any deflection, obtain the ratio R1  /R2.
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Ans.
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Ans.
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16.Calculate the value of the resistance R in the circuit shown in the figure, so that the current in the circuit is 0.2A. What would be the potential difference between points A and B ?
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Ans.
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17.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 expressions for the power of their combination when they are, in turn, connected in
(i)series and
(ii)parallel across their same voltage supply.[All India 2011]
Ans.
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Ans.
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19. State Kirchhoff’s rules. Use these rules to write the expressions for the currents Ilt I2 and I3 in the circuit diagram shown in figure below.
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Ans.
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20.State Kirchhoff’s rules. Apply Kirchhoff’s rules to the loops ACBPA  and  ACBQA   to write the expressions for the currents I1,I2 and I3 in the network.
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Ans.
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21.State Kirchhoff’s rules. Apply these rules to the loops PRSP and PRQP to write the expressions for the currents I1,  I2 and I3 in given circuit.
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Ans.
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Ans.
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Ans.
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24.In a meter bridge, the null point is found at a distance of l1 cm from A. If a resistance of X is connected in parallel with S, then null point occurs at a distance l2 cm from A  Obtain the formula for X in terms of l1,l2 and S.
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Ans.
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Ans.
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Ans.(i)
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27.(i) State the principle of working of a  meter bridge.
(ii) In a meter bridge balance point is found at a distance l1  with resistances R and S as shown in the figure. When an unknown resistance X is connected in parallel with the resistance S, the balance point shifts to a distance l2. Find expression for X in terms of l1 , l2 and S. [All India 2009]
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Ans.
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Ans.
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29.State Kirchhoff’s rules. Use Kirchhoff’s rules to show that no current flows in the given circuit
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Ans.
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Ans.
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31.Draw a circuit showing a  Wheatstone bridge. Use Kirchhoff’s rule to obtain the balance condition in terms of the values of the four resistors for the galvanometer to give null deflection.   [Delhi 2008 CI
Ans.
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32.Draw a circuit diagram for a Wheatstone bridge. Explain briefly how the balance condition for the galvanometer to give null deflection provides a practical method for the determination of an unknown resistance? [Delhi 2008]
Ans.
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5 Marks Questions

33.(i) State Kirchhoff’s rules for an  electric network. Using Kirchhoff’s rules, obtain the balance condition in terms of the resistances of four arms of Wheatstone bridge.
(ii) In the meter bridge experimental set up, shown in the figure, the null point D is obtained at a distance of 40 cm from end A of the meter bridge wire.
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Ans.
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34.(i) Use Kirchhoff’s rules to obtain  the balance condition in a Wheatstone bridge.
(ii) Calculate the value of R in the balance condition of the  Wheatstone bridge, if the carbon resistor connected across the arm CD has the colour sequence red, red and orange, as shown in the figure.
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(iii) If now the resistance of the arms BC and CD are interchanged, to obtain the balance condition, another carbon resistor is connected in place or R. What would now be sequence of colour bands of the carbon resistor?[Delhi 2012]
Ans.
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35.(i) State with the help of a circuit  diagram, the working principle of a meter bridge. Obtain the expression used for determining the unknown resistance.
(ii)What happens if the galvanometer and cell are interchanged at the balance point of the bridge?
(iii)Why is it considered important to obtain the balance point near the mid-point of the wire? [Delhi 2011 c]
Ans.(i)
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(ii)
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(iii)It is because of the fact that meter bridge is  most sensitive when null point occur nc the mid-point of wire and all the four resistances are of same order.

36.State the two rules that serves as general rules for analysis of electrical circuits.Use these rules to write the three equations that may be used to obtain the values of the three unknown currents in the branches (shown) of the circuit given below.  [All India 2008 C]
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Ans.
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