# Homework Help: Solenoid (magnetising current)

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1. Aug 6, 2017

### Suyash Singh

1. The problem statement, all variables and given/known data

Part d of attached question
2. Relevant equations
Solenoid equations
B=uo n i

3. The attempt at a solution
B (absence)=B (presence)
Uo n (i+im)=u n i
im=798A

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2. Aug 6, 2017

### Staff: Mentor

I don't understand what you calculated, but such a small difference looks like a rounding error.

3. Aug 6, 2017

### rude man

The answer should be M/H x 2A.

4. Aug 7, 2017

### Suyash Singh

Can you guys give a formula for magnetising current for solenoid?Every book gives for transformer but not for solenoid

5. Aug 7, 2017

### rude man

I just did, in post 3.

But you raise a good point: magnetizing current in a transformer is fundamentally different from magnetizing current in a solenoid. The latter term is actually rarely encountered. Why? Because it's a fictitious current whereas for a transformer it's very real.

For a solenoid, magnetizing current is defined to be the extra current that would be needed to restore the flux if the high-permeability core were removed.

For a transformer it's the actual primary current with the secondary open. It's the current needed to establish the flux given by Faraday's emf = -N d phi/dt. Note that emf = applied voltage - primary losses voltage drops.

Last edited: Aug 7, 2017