# Homework Help: Basic inductors question

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1. Feb 13, 2015

### ah4p

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?
thanks v much in advance

2. Feb 13, 2015

### cellurl

voltage= inductance * di/dt

since its dc, the voltage ends up zero.
Inductors "oppose" change in current.
More inductance, more opposition, eg slower change in current.
Thus it takes more time to reach the same dc current (whatever that is).

3. Feb 13, 2015

### Pythagorean

A) Well what do turns do to L? and what does L do to current?

B) what is the expression for the time constant? how does L affect the time constant?

4. Feb 13, 2015

### ah4p

[moderator note: Removed and moved new question to new thread]

L opposes current and I think more turns creates a bigger back emf but I don't know why and I don't know what more turns does to L

I have never heard of time constant (I'm doing SQA Advanced Higher Physics) but I assume it's how long it take to reach max. current so I'd say a bigger L means the time taken for current to reach its maximum is longer?

tjanks again

Last edited by a moderator: Feb 13, 2015
5. Feb 13, 2015

### cellurl

6. Feb 13, 2015

### Staff: Mentor

If they are claiming that the maximum current decreases then they are implying that adding turns to the coil not only adds inductance, but also resistance. This will be true for a real-life coil made of wire with inherent resistivity, as opposed to a theoretically ideal inductor which has no resistance.

7. Feb 13, 2015

### ah4p

ah ok thank you very much for your quick reply :)

8. Feb 13, 2015

### Pythagorean

9. Feb 13, 2015