2 coils question..

[nabliat6]

i know that V1/V2=1/a
I1/I2=-a

how they interpret the left resistor
to be like this ?

 

[Diver300]

That is basic transformer theory.

The transformer has a ratio of 100:1 so the secondary voltage (on Rb) is 1/100 of the primary voltage. The secondary current is 100 times the primary current.

Ignoring Ra for now, if the primary voltage is Vp, the secondary voltage is Vp/100. That means the secondary current is Vp/100/Rb.

The primary current is Vp/100/Rb/100 = Vp/(Rb * 10000)

which is the same current as you would get if there was no transformer and the load resistance were 10000 * Rb

Then add Ra back in.

 

[nabliat6]

I1/I2=-a

"The secondary current is 100 times the primary current."
where is the minus?

you forgot the minus

 

[Diver300]

I1/I2=-a

"The secondary current is 100 times the primary current."
where is the minus?

you forgot the minus

Yes, the secondary current (flowing into the transformer) is -100 times the primary current (also flowing into the transformer).

That means that the secondary current (flowing out of the transform) is +100 times the primary current (flowing into the transformer)

 

[nabliat6]

those two lines contradict each other

the secondary current is -100 times the primary current,
the secondary current is +100 times the primary current

 

[Diver300]

those two lines contradict each other

the secondary current is -100 times the primary current,
the secondary current is +100 times the primary current

The direction of current flow is entirely a matter of convention.

If you put back the bits I put in brackets it makes more sense:-

the secondary current (flowing into the transformer) is -100 times the primary current
the secondary current (flowing out of the transformer) is +100 times the primary current

Now that is what you would expect. If you put two ammeters in series, but in different directions, one will read the negative of the other.