Electromagnetic induction type q Watch

Presto
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#1
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Can't seem to grasp this part. Can someone please clear my doubts?
-Why does an alternating current produce an alternating magnetic field?
-Why does the iron core become magnetised?
-How and why does a change in flux linkage cause emf to be induced in the coil?


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Joinedup
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Do you remember the right hand grip rule that shows the direction of the magnetic field around a current carrying wire... well if the direction of current alternates what happens to the magnetic field?

I don't think you need a lot of detail for A level - it's sufficient to know that magnetic fields 'like' going through iron and iron channels magnetic flux - this type of meter has a lot in common with electrical transformers so revise your notes on transformers and their iron cores.

Changing flux linkage causes an emf as stated by Faraday's law (as mentioned in the MS), this is the 'how' - the why is I think beyond the scope of A level.
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Presto
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(Original post by Joinedup)
Do you remember the right hand grip rule that shows the direction of the magnetic field around a current carrying wire... well if the direction of current alternates what happens to the magnetic field?

I don't think you need a lot of detail for A level - it's sufficient to know that magnetic fields 'like' going through iron and iron channels magnetic flux - this type of meter has a lot in common with electrical transformers so revise your notes on transformers and their iron cores.

Changing flux linkage causes an emf as stated by Faraday's law (as mentioned in the MS), this is the 'how' - the why is I think beyond the scope of A level.
Prsom.
Thank you!
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Presto
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Presto
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Need help with part b and d part i) please.
- Why does constant current mean no change of flux?
-Why does current affect flux like this? Is there an equation which relates current and magnetic flux?
- I was thinking F = BIL, therefore B= F/IL so since flux = BA, shouldn't increase in current lead to a decrease in magnetic flux density B so a decrease in flux?
Last edited by Presto; 3 weeks ago
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Joinedup
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(Original post by Presto)
Need help with part b and d part i) please.
- Why does constant current mean no change of flux?
-Why does current affect flux like this? Is there an equation which relates current and magnetic flux?
- I was thinking F = BIL, therefore B= F/IL so since flux = BA, shouldn't increase in current lead to a decrease in magnetic flux density B so a decrease in flux?
Things the marker is supposed not to credit look a bit weird IMO - looks like you've had it if you talk about 'cutting' and that was exactly the way teachers talked about it when I was learning this stuff at A level. but otoh mentioning 'linkage' is optional.

anyway
constant current means a constant amount of flux, which since we're considering an iron ring clamped around the conductor means a constant amount of flux linked - seems we're forbidden to talk about 'cutting' but you only get induction of emf when the amount of flux linked changes over time... there's some formulae for this in the formula book
ϕ=BA (amount of flux is the product of field strength and cross sectional area)
ε = N Δϕ/Δt (Faraday's law for the magnitude of induced emf... might also be written as ε = -N Δϕ/Δt to make it incorporate Lenz's law)
Δϕ/Δt is the rate change of flux linked over time... and it can be zero as it will be in case of a constant DC
N is the number of 'turns' and though it's not always really obvious the current carrying wire is always making 1 turn through the iron ring - even if the diagram shows a cross section or a short length of wire... because the whole circuit *has* to look like this..
Name:  220219ampclmp.jpg
Views: 6
Size:  14.7 KB
... so the amp clamp is working like a transformer with 1 turn on the primary and however many turns are on it's internal coil on the secondary...

F=BIL is for the force on a current carrying conductor in a magnetic field - not sure how that would apply here tbh
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Presto
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(Original post by Joinedup)
Things the marker is supposed not to credit look a bit weird IMO - looks like you've had it if you talk about 'cutting' and that was exactly the way teachers talked about it when I was learning this stuff at A level. but otoh mentioning 'linkage' is optional.

anyway
constant current means a constant amount of flux, which since we're considering an iron ring clamped around the conductor means a constant amount of flux linked - seems we're forbidden to talk about 'cutting' but you only get induction of emf when the amount of flux linked changes over time... there's some formulae for this in the formula book
ϕ=BA (amount of flux is the product of field strength and cross sectional area)
ε = N Δϕ/Δt (Faraday's law for the magnitude of induced emf... might also be written as ε = -N Δϕ/Δt to make it incorporate Lenz's law)
Δϕ/Δt is the rate change of flux linked over time... and it can be zero as it will be in case of a constant DC
N is the number of 'turns' and though it's not always really obvious the current carrying wire is always making 1 turn through the iron ring - even if the diagram shows a cross section or a short length of wire... because the whole circuit *has* to look like this..
Name:  220219ampclmp.jpg
Views: 6
Size:  14.7 KB
... so the amp clamp is working like a transformer with 1 turn on the primary and however many turns are on it's internal coil on the secondary...

F=BIL is for the force on a current carrying conductor in a magnetic field - not sure how that would apply here tbh
Thank you so soo much for such a well explained detailed reply!
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