# For magnetic field produced by a toroid we have two formulae:
1)B=permeability of free spacexnxi-for a normal toroid
2)B=(permeability of free spacexnxi)/2xpixr-for a toroid made from infinitely long solenoid.
Which formula is to be used when the nature of the toroid i,e,.whether finite or infinite is not
clearly mentioned in the question?

### Asked by Chandropal Parashor | 11th Jul, 2014, 06:55: AM

A toroid is nothing but a solenoid bent into a circular shape.

Also the term "n" in the two formulae that you have provided are not the same.

For a toroid, the ideal formula for any question is

(1)

Where, N is the total number of turns of the toroid.

The second formula is a comparison with the solenoid which is given as

(2)

Where, is the number of turns per unit length.

Hence, ideally both the formulae are correct.

If in the question the nature is not mentioned, then look for the words "Number of turns" or "Number of turns per unit length", and then use the appropriate formula.

If it says "Number of turns", then use the formula (1) and if it says "Number of turns per unit length", then use the formula (2).

### Answered by Romal Bhansali | 11th Jul, 2014, 09:51: AM

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