# Physics Alevel question :- electromagnetism induction and alternating current

Q:-An alternating current is passed through an air-cored solenoid.
An iron core is inserted into the solenoid and then held stationary within the solenoid. The current in the solenoid is now smaller. Explain why the root-mean-square (r.m.s.) value of the current in the solenoid is reduced as a result of inserting the core.

ans:-
(for same current) iron core gives large(r) (rates of change of) flux (linkage) e.m.f induced in solenoid is greater (for same current)
induced e.m.f. opposes applied e.m.f. so current smaller/acts to reduce current

now i understand that inserting iron increases Emf in the solenoid but what does it mean that "the induced emf opposes applied emf so current smaller/acts to reduce current"
is this an application of lenzs law if yes how does lenz law here tell us that current should be decreases when i thought lenzs law only tells us direction of current
I would have tried to explain it with reactance

The inductance was increased by the addition of the core so the reactance also increased. Reactance has the same relationship to voltage and current that resistance does in a DC circuit.

What exam board is this?
Original post by Joinedup
I would have tried to explain it with reactance

The inductance was increased by the addition of the core so the reactance also increased. Reactance has the same relationship to voltage and current that resistance does in a DC circuit.

What exam board is this?

Cambridge A-level
Original post by Joinedup
I would have tried to explain it with reactance

The inductance was increased by the addition of the core so the reactance also increased. Reactance has the same relationship to voltage and current that resistance does in a DC circuit.

What exam board is this?

hmm it might make sense but unfortunately this is not taught at my level
Q:-An alternating current is passed through an air-cored solenoid.
An iron core is inserted into the solenoid and then held stationary within the solenoid. The current in the solenoid is now smaller. Explain why the root-mean-square (r.m.s.) value of the current in the solenoid is reduced as a result of inserting the core.

ans:-
(for same current) iron core gives large(r) (rates of change of) flux (linkage) e.m.f induced in solenoid is greater (for same current)
induced e.m.f. opposes applied e.m.f. so current smaller/acts to reduce current

now i understand that inserting iron increases Emf in the solenoid but what does it mean that "the induced emf opposes applied emf so current smaller/acts to reduce current"
is this an application of lenzs law if yes how does lenz law here tell us that current should be decreases when i thought lenzs law only tells us direction of current

Hi Joinedup, do you have any ideas or comments on why engineering books or articles such as the one that you have linked, do not like to place the minus in writing the induced voltage across the inductor?
In a typical physics text, we would see

$\varepsilon = - L \dfrac{di}{dt}$

but in several electric circuits texts, they would ignore the minus.

I have asked this question to several engineers before.
The answers vary and would like to hear from you as you did not caution OP that there is a missing minus sign.
Original post by Eimmanuel
Hi Joinedup, do you have any ideas or comments on why engineering books or articles such as the one that you have linked, do not like to place the minus in writing the induced voltage across the inductor?
In a typical physics text, we would see

$\varepsilon = - L \dfrac{di}{dt}$

but in several electric circuits texts, they would ignore the minus.

I have asked this question to several engineers before.
The answers vary and would like to hear from you as you did not caution OP that there is a missing minus sign.

No I didnt notice that missing minus on the webpage but I think I do know what you're talking about.

I'd guess electrical and electronic engineers dont worry about +/- very much cos you can often just fix it by switching the leads on your meter or scope over...

We'd talk about the effect of Lens law in this question as 'back EMF' so the direction is sort of baked in to the name and so we're not likely to go wrong with it.

Also any answer that means you've created a perpetual motion machine is obviously wrong so we are able to spot errors and end up going wrong less often than might be expected.

Hope that was the sort of answer you were looking for.
Original post by Joinedup
No I didnt notice that missing minus on the webpage but I think I do know what you're talking about.

I'd guess electrical and electronic engineers dont worry about +/- very much cos you can often just fix it by switching the leads on your meter or scope over...

We'd talk about the effect of Lens law in this question as 'back EMF' so the direction is sort of baked in to the name and so we're not likely to go wrong with it.

Also any answer that means you've created a perpetual motion machine is obviously wrong so we are able to spot errors and end up going wrong less often than might be expected.

Hope that was the sort of answer you were looking for.

I am sorry for the late reply, I have overlooked this.