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.

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Earn 100

State the working principle of potentiometer, explain with the help of circuit diagram how the potentiometer is used to determine the internal resistance of the given primary cell.

Important Questions on Current Electricity

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12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 13

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

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 14

Three identical resistors are connected in parallel and total resistance of the circuit is

\frac{R}{3}

. Find the value of each resistance.

MEDIUM

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 1

What is the condition in which the heat produced in an electric circuit is :

(a) directly proportional

(b) inversely proportional

to the resistance of the circuit?

Compute the ratio of the total quantity of heat produced in the two cases.

HARD

12th Andhra Pradesh Board

IMPORTANT

Intermediate Second Year Physics>Chapter 6 - Current Electricity>MODEL QUESTIONS (IPE PATTERN)>Q 2

Two metallic wires

A

and

B

are connected in parallel. Wire

A

has length

L

and radius

r

. Wire

B

has length

2 L

and radius

2 r

. Compute the ratio of the total resistance of the parallel combination and resistance of wire

A

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12th Andhra Pradesh Board

IMPORTANT

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

Two lamps, one rated

100 W

220 V

and the other

60 W

220 V

are connected in parallel to a

220

volt supply. What current (in

A

) is drawn from the supply line?

HARD

12th Andhra Pradesh Board

IMPORTANT

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

The average drift speed of conduction electrons in a copper wire of cross-sectional area $3.0 \times 10^{- 7} m^{2}$ carrying a current of $5 A$ is $x \times 10^{- 4} m s^{- 1}$ . Assume that each copper atom contributes roughly one conduction electron. The density of copper is $9.0 \times 10^{3} kg m^{- 3}$ and its atomic mass is $63.5 u$ . Write the value of $x$ to the nearest integer.

(Take $1 e = 1.6 \times 10^{- 19} C$ and $N_{A} = 6 \times 10^{23} {mol}^{- 1}$ )

HARD

12th Andhra Pradesh Board

IMPORTANT

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

Compare the average drift speed of conduction electrons in a copper wire of cross-sectional area

3.0 \times 10^{- 7} m^{2}

carrying a current of

5 A

with the thermal speed of copper atoms at ordinary temperatures. Assume that each copper atom contributes roughly one conduction electron. The density of copper is

9.0 \times 10^{3} kg m^{- 3}

and its atomic mass is

63.5 u

HARD

12th Andhra Pradesh Board

IMPORTANT

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

Compare the average drift speed of conduction electrons in a copper wire of cross-sectional area

3.0 \times 10^{- 7} m^{2}

carrying a current of

5 A

with the speed of propagation of electric field along the conductor which causes the drift motion. Assume that each copper atom contributes roughly one conduction electron. The density of copper is

9.0 \times 10^{3} kg m^{- 3}

and its atomic mass is

63.5 u

State the working principle of potentiometer, explain with the help of circuit diagram how the potentiometer is used to determine the internal resistance of the given primary cell.

Important Questions on Current Electricity

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