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A coil of one turn is made of a wire of certain length and then from the same length a coil of two turns is made. If the same current is passed in both the cases, then the ratio of the magnetic inductions at their centres will be
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- 2 : 1
- 1 : 4
- 4 : 1
- 1 : 2
Correct Option: B
Let ℓ be length of wire.
| Ist case : ℓ = 2πr ⇒ r = | |
| 2πr |
| B = | = | = | [∵ n = 1]......(1) | |||
| 2r | 2ℓ | ℓ |
| 2nd Case : ℓ = 2(2πr') ⇒ r' = | |
| 4π |
| B' = | = | = 4 |  |  | = 4B , using (1)(where n = 2) | ||||
| 2 | ℓ | ||||||||
| 4π | 2 |
