Physics question, PLEEEEASE HELP :) thank you :)

advanced higher physics

A d.c circuit consists of a switch, lamp and inductor.
The inductor is replaced with an inductor with twice the number of turns in the coil of wire, nothing else about it is changed.
State the effect on
A) maximum current B) the time to reach max current

I thought increasing the turns would increase the current
and also increase the time taken since the back emf would also be greater

but the answer is
max current decreases
and time to reach it increases

can anyone explain why?


for question 8b)
http://mrmackenzie.wikispaces.com/file/view/AHpastpaper2006.pdf
answers are here

http://www.sqa.org.uk/files_ccc/06miPhysicsAH.pdf
I don’t understand what they’ve done in the answers because I thought
back emf = supply voltage – voltage across inductor
so from the graph I used when 6V is across inductor the back emf is 3V and dI/dt is 8 (from graph)
then I used back emf = -L dI/dt
subbing in the values above I got L = 0.375H

but the answers have just used values straight from the graph – when 9V is across the inductor then dI/dt = 12 and subbed these in
I don’t understand this

any help is appreciated



please quote me if you reply or I'll miss it :P
thanks v much in advance

Current is lower.
A)Because if you increase the length of the coil then there will be more time for the charge carriers (electrons) to collide with atoms so in total, more collisions occur. More collisions means more resistance. Because of the equation V=IR, the increase of resistance will mean that the Voltage will also increase but the components don't want that so the current will decrease to make up for the increase in resistance. So the current decreases because the resistance increases.

B)It takes longer because of the increase in collisions between electrons and atoms in the coil.

Edit: I may have read this wrong... Never really came across an inductor (I do OCR)

haha ok

I hope you're right :P

do you know, Is the back emf in an inductive circuit at any given time equal to the voltage across the inductor?
or do you have to subtract the voltage across the inductor from the supply voltage to find back emf?

thanks in advance


any chance you know the answer to my other question posted above?