101. Two rods are joined end to end, as shown. Both have a cross-sectional area of 0.01 cm². Each is 1 meter long. One rod is of copper with a resistivity of 1.7 × 10^–6 ohm-centimeter, the other is of iron with a resistivity of 10^–5 ohm-centimeter. How much voltage is required to produce a current of 1 ampere in the rods?
     ​
     

1. 1. 0.117 V ​

   
   
   

   2. 0.00145 V ​

   
   
   

   3. 0.0145 V ​

   
   
   

   4. 1.7 × 10^–6 V

   
   
   

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1. View Hint View Answer Discuss in Forum

   Copper rod and iron rod are joined in series.
   
   From ohm’s law V = RI ​
   = (1.7 × 10^–6 × 10^–2 + 10^–5 × 10^–2) 0.01 × 10^–4 volt ​
   = 0.117 volt (∵ I = 1A)
   

   Correct Option: A

   Copper rod and iron rod are joined in series.
   
   From ohm’s law V = RI ​
   = (1.7 × 10^–6 × 10^–2 + 10^–5 × 10^–2) 0.01 × 10^–4 volt ​
   = 0.117 volt (∵ I = 1A)
   

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102. A wire of resistance 4 Ω is stretched to twice its original length. The resistance of stretched wire would be​​

1. 1. ​4 Ω ​

   
   
   

   2. 8 Ω ​

   
   
   

   3. 16 Ω

   
   
   

   4. ​2 Ω

   
   
   

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1. View Hint View Answer Discuss in Forum

   Resistance R =	ρl
   	A

   
   ∵ l' = 2l
   

   ∴ A' =	A
   	2

   
   

   ∴ R'ρ	2l	= 4R = 4 × 4Ω = 16Ω
   	A

   
   Therefore the resistance of new wire becomes 16 Ω

   Correct Option: C

   Resistance R =	ρl
   	A

   
   ∵ l' = 2l
   

   ∴ A' =	A
   	2

   
   

   ∴ R'ρ	2l	= 4R = 4 × 4Ω = 16Ω
   	A

   
   Therefore the resistance of new wire becomes 16 Ω

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103. Across a metallic conductor of non-uniform cross section a constant potential difference is applied. The quantity which remains constant along the conductor is :​​

1. 1. current

   
   
   

   2. drift velocity ​

   
   
   

   3. ​electric field ​

   
   
   

   4. current density

   
   
   

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1. View Hint View Answer Discuss in Forum

   Here, metallic conductor can be considered as the combination of various conductors connected in series. And in series combination current remains same.
   
   

   Correct Option: A

   Here, metallic conductor can be considered as the combination of various conductors connected in series. And in series combination current remains same.
   
   

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104. ​The resistance of a wire is 'R' ohm. If it is melted and stretched to 'n' times its original length, its new resistance will be :-​​

1. 1. R/n

   
   
   

   2. n²R

   
   
   

   3. R/n²

   
   
   

   4. nR

   
   
   

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1. View Hint View Answer Discuss in Forum

   We know that, R =	ρl
   	A

   
   

   or R =	ρl²	⇒ R ∝ l²
   	Volume

   
   According to question l₂ = nl₁
   

   	R2	=	n²l1²
   	R1		l1²

   
   

   or,	R2	n²
   	R1

   
   ⇒ R₂ = n²R₁

   Correct Option: B

   We know that, R =	ρl
   	A

   
   

   or R =	ρl²	⇒ R ∝ l²
   	Volume

   
   According to question l₂ = nl₁
   

   	R2	=	n²l1²
   	R1		l1²

   
   

   or,	R2	n²
   	R1

   
   ⇒ R₂ = n²R₁

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