Tag: current electricity

Questions Related to current electricity

In a meter bridge, a standard resistor of R ohm is connected in the left gap and two wires A and B are connected one after the other in the right gap. The balancing length measured from the left is 50 cm for either of them. If the two wires are connected is series and put in the right gap, the balancing length measured from the left would be (in cm)

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. 25

  2. 33.3

  3. 66.7

  4. 75

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Correct Option: B
Explanation:

let the resistance of wire A be a and that of  wire B and  b,then
$\dfrac{R}{50}=\dfrac{a}{100-50}$$\therefore$  a$=R$
also,$\dfrac{R}{50}=\dfrac{b}{100-50}$ $\therefore$  b$=R$
when both are connected in series$\dfrac{R}{i}=\dfrac{2a}{100-i}$
or $100-i=  \ 2i$
or $i=\dfrac{100}{3}$

In the metre bridge experiment of resistances, the known and unknown resistances are inter-changed. The error so removed is:

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. end correction

  2. index error

  3. due to temperature effect

  4. random error

Show answer
Correct Option: A
Explanation:

$ \alpha, \beta$ are the end correction on left and right side.


case 1:- Without interchanging.

$ \dfrac{P}{Q} = \dfrac{R}{S} = \dfrac{X+\alpha+l _{1}P}{(Y+\beta+(100-l _{1})P)}$       ..........( 1 )

case 2:- After interchanging.

$\dfrac{P}{Q} = \dfrac{R}{S} = \dfrac{Y+\propto + l _{2}P}{X + \beta+(100-l _{2})P}$    ............( 2 )

on simplification of eq.  (1) and (2) we get
$X = Y +(l _{2}-l _{1})P$
$\therefore$ By interchanging the end correction is removed.

A lamp of 6 V and 30 W is used in a laboratory but the supply is of 120 V. what will be done to make use of the lamp?
(1) A resistance may be used
(2) A resistance may be used in series with lamp.
(3) The resistance should be of 18 $\Omega$. 

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. 1, 2 and 3 are correct

  2. 1 and 2 are correct

  3. 1 and 3 are correct

  4. 2 and 3 are correct

Show answer
Correct Option: B
Explanation:

Here, the resistance should be used in series with lamp in order to divide the voltage or to reduce potential drop across the lamp.

Lamp can withstand a maximum of 6 V , and a current of $\dfrac { 6 }{ { R } _{ lamp } } $ where ${R} _{lamp}$ is $\dfrac { { 6 }^{ 2 } }{ 30 } =1.2\Omega $
hence maximum current that lamp can with stand is  $\dfrac { 6 }{ { 1.2} }= 5A $
so the resistance which is to be connected in series would also get same current of 5 A hence if it will be of $18 \Omega$ then voltage drop accros it will be 90 V and maximum of 6 V across lamp but both voltage drop does not make 120 V . hence the resistance value should be more than $18 \Omega$.  So 1 and 2 is correct but 3 is incorrect.

Why is the Wheatstone bridge better than the other methods of measuring resistances?

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. It does not involve Ohm's law

  2. It is based on Kirchoff's law

  3. It has four resistor arms

  4. It is a null method

Show answer
Correct Option: D
Explanation:

The Wheatstone bridge is used to measure the unknown resistance by using null method. i.e, when the bridge is balanced, no current through the galvanometer. Using this null method, we can easily measure the unknown resistance if the other three arm's resistor are given.  

In a metre bridge experiment null point is obtained at $40$cm form one end of the wire when resistance X is balanced against another resistance Y. If X $<$ Y, then the new position of the null point from the same end, if one decides to balance a resistance of $3$X against Y, will be close to.

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. $80$ cm

  2. $75$ cm

  3. $67$ cm

  4. $50$ cm

Show answer
Correct Option: C
Explanation:

$\dfrac{x}{40}=\dfrac{y}{60}$
$\Rightarrow \dfrac{x}{y}=\dfrac{2}{3}$
$\therefore \dfrac{6k}{l}=\dfrac{3k}{100-l}$
$\Rightarrow l=67$.

In a Wheatstone's bridge, there resistances P, Q and R connected in the three arms and the fourth arm is formed by two resistances $S _1$ and $S _2$ connected in parallel. The condition for bridge to be balanced will be :

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. $\dfrac{P}{Q}=\dfrac{R}{S _1+S _2}$

  2. $\dfrac{P}{Q}=\dfrac{2R}{S _1+S _2}$

  3. $\dfrac{P}{Q}=\dfrac{R(S _1+S _2)}{S _1S _2}$

  4. $\dfrac{P}{Q}=\dfrac{R(S _1+S _2)}{2S _1S _2}$

Show answer
Correct Option: C
Explanation:

$\dfrac{P}{Q}=\dfrac{R}{S}$

$=\dfrac{R(S _1+S _2)}{S _1S _2}$. wher S is the equivalent resistance of $S _1 and S _2$

In a meter bridge an unknown resistance P is connected in the left gap and a $50 \Omega$ resistance in the right gap. Null point is obtained at x cm from the left end. The unknown resistance now shunted with an equal resistance. Find the value of the resistance in the right gap so that the null point is not shifted.

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. $60 \Omega$

  2. $38 \Omega$

  3. $25 \Omega$

  4. $50 \Omega$

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Correct Option: C

In a metre bridge experiment, the null point is obtained at $20\ cm$ from one end of the wire when the resistance $X$ is balanced against another resistance $Y$, where $Y>X$. What will be the new position of the null point, from the same end, if one decides to balance a resistance of $12/7$ against $Y$?

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. $30 cm$

  2. $40 cm$

  3. $50 cm$

  4. $60 cm$

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Correct Option: D

Two resistors $R _1$ and $R _2$ are connected in the left gap and right gap of a meter bridge, and the null point is obtained at $20\;cm$ from the left. On interchanging the resistors in the two gaps. the null point shift by.

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. $20\;cm$

  2. $40\;cm$

  3. $60\;cm$

  4. $80\;cm$

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Correct Option: C

In the measurement of resistance by a metre bridge, the known and unknown resistance are interchanged to eliminate 

physics current electricity meter bridge meter bridge and problems on it galvanometer

  1. end error

  2. index error

  3. random error

  4. error due to thermoelectric effect

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Correct Option: B