Edexcel amp-clamp question

I was wondering how an amp-clamp worked. Its in the 2013 june 6ph04 paper, but the mark scheme doesn't really help explain it.

I would imagine an AC current produces a magnetic field which is changing and moving in magnitude and direction, thus inducing an emf in the coil, and so a current.

However, from what I gather from the mark scheme, the magnetic field induces a second(?) magnetic field from the iron core, which then induces an emf in the coil.

Why was the second step necessary? I feel as if I have a fundamental misunderstanding of magnetic flux and magnetic flux linkage.

I would be very grateful if somebody could explain how the amp clamp worked and so how an emf is induced in a coil wrapped around an iron core?

Thank you very much :smile:

Reply 1

uberteknik

Original post by ThatWasHard!

If you post a copy of the full question and the mark scheme answer, you will get a faster response. :smile:

Reply 2

ThatWasHard!

Original post by uberteknik

If you post a copy of the full question and the mark scheme answer, you will get a faster response. :smile:

yep, I've done it :smile:

Original post by ThatWasHard!

yep, I've done it :smile:

Thank's, that makes it a lot easier to answer you. :smile:

Your understanding is not flawed but you are missing out some parts.

The amp clamp is nothing more than a transformer. Each part of the amp clamp is directly equivalent to the parts of a standard transformer.

The jaws act as a transformer iron core to both intensify and channel the magnetic field generated by the wire-under-test. i.e. the magnetic field generated by the wire-under-test may be quite weak dependent on the current flowing in the wire.

The coil of wires wrapped around the jaws, allows the stronger magnetic field in the core to induce a larger e.m.f and hence current in the in the secondary coil circuit. One which can now be more easily measured by a voltmeter.

Relating this diagram to the amp-clamp:

a) the 'wire under test' forms the primary winding of the transformer;

b) the 'coil of wires' inside the jaws, form the secondary winding of the transformer;

c) the moveable jaws form the iron core of the transformer.

You should now be able to relate the operation of a transformer to the operation of the amp-clamp and also to the mark scheme:

1) According to Ampere's law, a current in a conductor will generate a magnetic field proportional to the magnitude of the current. The alternating current in the 'wire under test' will therefore produce an alternating magnetic field.

2) The amp-clamp jaws form an iron core placed in the magnetic field of the wire-under-test. The magnetic domains within this iron-core, align with the external alternating magnetic field to sum together and create a much stronger alternating magnetic field in the core.

3) Faraday's law of induction tells us that a changing magnetic field will induce an e.m.f. in a conductor placed inside the changing magnetic field. The coil of wires inside the 'jaws' (iron core) are wrapped around the core to form a secondary coil winding in which the strong alternating magnetic field within the core is coupled to this secondary coil and induces an e.m.f. within it.

i.e. a more accurate reading is obtained because the stronger magnetic field in the jaw iron-core induces a larger e.m.f. in the secondary winding than simply the direct magnetic coupling between the wire-under-test and the secondary winding on their own.

Edexcel amp-clamp question

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