66. A straight wire of diameter 0.5 mm carrying a current of 1 A is replaced by another wire of 1 mm diameter carrying same current. The strength of magnetic field far away is​​

1. 1. twice the earlier value ​

   
   
   

   2. same as the earlier value ​

   
   
   

   3. one-half of the earlier value

   
   
   

   4. one-quarter of the earlier value

   
   
   

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1. View Hint View Answer Discuss in Forum

   B =	μ0i
   	2πr

   
   and so it is independent of thickness. ​
   The current is same in both the wires, hence magnetic field induced will be same.

   Correct Option: B

   B =	μ0i
   	2πr

   
   and so it is independent of thickness. ​
   The current is same in both the wires, hence magnetic field induced will be same.

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67. At what distance from a long straight wire carrying a current of 12 A will the magnetic field be equal to 3 × 10^-5 Wb/m² ?​

1. 1. 8 × 10^-2m

   
   
   

   2. 12 × 10^-2m

   
   
   

   3. 18 × 10^-2m

   
   
   

   4. 24 × 10^-2m

   
   
   

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1. View Hint View Answer Discuss in Forum

   Current (I) = 12 A and magnetic field (B) = 3 × 10^–5 Wb/m². Consider magnetic field ^→_B at distance r.
   

   Magnetic field , B =	μ0I
   	2πr

   
   

   ⇒ r =	μ0I	=	(4π × 10-7) × 12	= 8 × 10-2m
   	2πB		2 × π × (3 × 10-5)

   Correct Option: A

   Current (I) = 12 A and magnetic field (B) = 3 × 10^–5 Wb/m². Consider magnetic field ^→_B at distance r.
   

   Magnetic field , B =	μ0I
   	2πr

   
   

   ⇒ r =	μ0I	=	(4π × 10-7) × 12	= 8 × 10-2m
   	2πB		2 × π × (3 × 10-5)

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68. The magnetic field at a distance r from a long wire carrying current i is 0.4 tesla. The magnetic field at a distance 2r is​​

1. 1. 0.2 tesla ​

   
   
   

   2. 0.8 tesla ​

   
   
   

   3. 0.1 tesla ​

   
   
   

   4. 1.6 tesla

   
   
   

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1. View Hint View Answer Discuss in Forum

   B =	μ0i	or B ∝	1
   	2πr		r

   
   ​When r is doubled, the magnetic field becomes half, i.e., now the magnetic field will be 0.2 T.

   Correct Option: A

   B =	μ0i	or B ∝	1
   	2πr		r

   
   ​When r is doubled, the magnetic field becomes half, i.e., now the magnetic field will be 0.2 T.

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69. The magnetic induction at a point P which is at a distance of 4 cm from a long current carrying wire is 10^-3 T. The field of induction at a distance 12 cm from the current will be

1. 1. 3.33 × 10^-4 T ​

   
   
   

   2. 1.11 × 10^-4 T

   
   
   

   3. ​3 × 10^-3T ​

   
   
   

   4. 9 × 10^-3 T

   
   
   

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1. View Hint View Answer Discuss in Forum

   B =	μ0I	⇒ B ∝	1
   	2πr		r

   
   As the distance is increased to three times, the magnetic induction reduces to one third. Hence,
   

   B =	1	× 10-3 tesla = 3.33 × 10-4 tesla
   	3

   Correct Option: A

   B =	μ0I	⇒ B ∝	1
   	2πr		r

   
   As the distance is increased to three times, the magnetic induction reduces to one third. Hence,
   

   B =	1	× 10-3 tesla = 3.33 × 10-4 tesla
   	3

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70. Energy in a current carrying coil is stored in the form of​

1. 1. electric field

   
   
   

   2. ​magnetic field

   
   
   

   3. dielectric strength

   
   
   

   4. heat

   
   
   

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1. View Hint View Answer Discuss in Forum

   Energy is stored in magnetic field.

   Correct Option: B

   Energy is stored in magnetic field.

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