State Kirchhoff's law for an electrical network. Using these laws deduce the condition for balance in a Wheatstone bridge.

Kirchhoff's two rules are as under: (a) Junction rule: At any junction, the sum of the currents entering the junction is equal to the sum of currents leaving the junction. (b) Loop rule: The algebraic sum of changes in potential around any closed loop involving resistors and cells in the loop is zero. The circuit diagram for Wheatstone bridge is as under: This bridge has four resistors R 1 , R 2 , R 3 and R 4 . Across one pair of diagonally opposite points, that is points A and C , a source is connected. This is called the battery arm. Between the other two vertices, B and D , a galvanometer G is connected. This is called the galvanometer arm. There will be currents flowing across all the resistors as well as a current I g through G . In the case of a balanced bridge, the resistors are such that I g = 0 . In this case, Kirchhoff's junction rule is applied to junctions D and B . Now, we apply Kirchhoff's loop rule to closed loops A D B A and C B D C . The loop A D B A gives, - I 1 R 1 + 0 + I 2 R 2 = 0 ( ∵ I g = 0 ) ∴ I 1 R 1 = I 2 R 2 or I 1 I 2 = R 2 R 1 ...(a) And the loop C B D C gives, I 4 R 4 + 0 - I 3 R 3 = 0 ( ∵ I g = 0 ) ∴ I 3 R 3 = I 4 R 4 or I 3 I 4 = R 4 R 3 ....(b) Now, let the current entering A be I then it divides into I 1 and I 2 such that, I 2 = I - I 1 . The current I 2 further divides into I g and I 4 such that I 4 = I 2 - I g . In the case of a balanced bridge, I g = 0 therefore, I 4 = I 2 . Similarly, we get I 3 = I 1 . Substituting I 3 = I 1 and I 4 = I 2 in the equation (b) we get, I 1 I 2 = R 4 R 3 ...(c) From the equations (a) and (c), we get, I 1 I 2 = R 2 R 1 = R 4 R 3 Hence, we obtain the condition, R 2 R 1 = R 4 R 3 which is the equation relating the four resistors called the balance condition for the galvanometer to give zero or null deflection.

HARD

12th Andhra Pradesh Board

IMPORTANT

Earn 100

Derive an expression for the effective resistance when three resistors are connected in series.

Important Questions on Current Electricity

HARD

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 3

Derive an expression for the effective resistance when three resistors are connected in parallel.

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 4

' m'

cells each of emf

E

and internal resistance

' r'

are connected in parallel. What is the total emf and internal resistance? Under what conditions is the current drawn from mixed grouping of cells a maximum?

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 5

Define electric resistance and write its SI unit. How does the resistance of a conductor vary if the conductor is stretched to

4

times of its length?

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12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 5

How does the resistance of a conductor vary if temperature of conductor is increased?

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12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 6

When the resistance connected in series with a cell is halved, the current is equal to or slightly less or slightly greater than double. Why?

HARD

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 7

Two cells of EMF's

4.5 V

and

6.0 V

and internal resistance

6 Ω

and

3 Ω

respectively have their negative terminals joined by a wire of

18 Ω

and positive terminals by a wire of

12 Ω

resistance. A third resistance wire of

24 Ω

connects middle points of these wires. Using Kirchhoff's laws, find the potential difference (in

V

) at the ends of this third wire.

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 8

Three resistors each of resistance

10

ohm are connected, in turn, to obtain (i) minimum resistance (ii) maximum resistance. Compute the effective resistance in each case.

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 8

Three resistors each of resistance

10

ohm are connected, in turn, to obtain (i) minimum resistance (ii) maximum resistance. Compute the ratio of minimum to maximum resistance so obtained.

Derive an expression for the effective resistance when three resistors are connected in series.

Important Questions on Current Electricity

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