Tag: current electricity

Questions Related to current electricity

Which of the following statements is/are incorrect for a meter bridge, which is used to compare two resistances?

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

  1. If its wire is replaced by another wire having same length, made of same material but having twice the cross sectional area, the accuracy increases.

  2. If its wire is replaced by another wire of different material, having same cross sectional area but of twice the length, accuracy increases.

  3. If its wire is replaced by another wire of same material, having half the cross sectional radius and half the length, accuracy decreases but sensitivity increases

  4. Metre bridge works on the principle of Wheat-stone bridge.

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Correct Option: A
Explanation:

The Meter Bridge works on the principle of Wheatstone bridge. The accuracy would depend on the fact that how precisely we find out the balance length. As we find the balancing length manually , the accuracy would increase if the voltage variation along the length decreases so that the balancing length is larger and the relative uncertainty associated to it is less. Changing the area won't affect the accuracy much. However, it certainly would affect the sensitivity of meter bridge.

In measuring a resistant using metre bridge, the resistance in the gaps are interchanged to minimize error due to

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

  1. the yielding of the supports

  2. nonuniformity of the bridge wire

  3. the contact or end resistance

  4. Joule heating of the bridge wire.

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Correct Option: B

If the wire in the experiment to determine the resistivity of a material using metre bridge is replaced by copper or hollow wire the balance point i.e. null point shifts

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

  1. to right

  2. to left

  3. at same point

  4. None of these

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Correct Option: C
Explanation:

The null point will remain at same point because null point depends on known resistance, $R$ and unknown resistance , $S$ only. 
Null condition: $\dfrac{R}{S}=\dfrac{l}{100-l}$ where $l=$ null point position in the wire. 

The null point should be obtained on the meter bridge wire to get maximum accuracy at

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

  1. the middle of the wire.

  2. the left end of the wire.

  3. the right end of the wire.

  4. the 1/4th distance from the left end.

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Correct Option: A
Explanation:

The null point, when obtained at the middle of wire, will give us maximum accuracy as the resistance on both the side will be equal.

In specific resistance measurement of a wire using a meter bridge, the key k in the main circuit is kept open when we are not taking readings. The reason is

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

  1. the emf of cell will decrease.

  2. the value of resistance will change due to joule heating effect.

  3. the galvanometer will stop working.

  4. none of these.

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Correct Option: B
Explanation:
The key k in the main circuit is kept open when we are not taking the readings because the value of resistance will change due to joule heating effect. When an electric current passes through the resistor, heat is generated which rises the temperature of resistor which in result changes the resistance of resistor.

In a Wheatstone bridge, three resistances P, Q and R are 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 the bridge to be balanced will be

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

  1. $\displaystyle \frac{P}{Q} = \frac{R(S _1 + S _2)}{2S _1S _2}$

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

  3. $\displaystyle \frac{P}{Q} = \frac{2R}{S _1 + S _2}$

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

Show answer
Correct Option: D
Explanation:

The balanced condition for Wheatstone bridge is  $\dfrac{P}{Q}=\dfrac{R}{S}$

Here $S=\frac{S _1S _2}{S _1+S _2}$  (as $S _1$ and $S _2$ are connected in parallel in S arm)
so, $\dfrac{P}{Q}=\dfrac{R(S _1+S _2)}{S _1S _2}$