# Homework Help: Primary solenoid with secondary coil effect? if any?

1. Oct 23, 2008

### alphaomega

primary solenoid with secondary coil...effect?? if any??

A primary solenoid of 1200 evenly spaced turns is wound on a cylindrical former 800mm long and 30mm in diameter. A secondary coil of 10 000 turns is wound around the central portion of the solenoid. A steady current of 2A is flowing in the primary solenoid

a) What is the magnetic flux in the solenoid

b) If the current is now steadily reduced to zero over a perio of .01s, what average emf is induced in the second coil?

c) A cylindirical core of iron of relative permeability 50 is now slid in to fill the former, and then the current is steadily raised back to 2A over 0.01s.

What emf is induced in the secondary coil during the increase?

Magnetic field in a solenoid = B =$$\mu$$ nI
magnetic flux = $$\phi$$ = $$\pi$$ =$$\int$$ B dA =BA
emf - $$\epsilon$$ = change in magnetic flux over change in time

a and b are easy for a single solenoid. But is there any effect due to the secondary coil? that is what is really screwing with me.

Im also not too sure about c)

Cheers guys

I can find the magnetic flux for the solenoid easily enough using the given formula, use the magnetic field to find B, and use that to find the flux.
B= 0.00376T
so magnetic flux is 2.6x10^-6 Wb.

so then the emf is 0.26mV

BUT...I'm not sure if this is right because I'm not sure if that extra coil has any effect on it

Last edited: Oct 23, 2008