Investigation of Magnetic Fields by Search Coil

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Physics Lab Report – C15

Title: Investigation of magnetic fields by search coil

Objective:

To use a search coil and a CRO to investigate the magnetic fields generated by alternating currents through a straight wire and a slinky solenoid.

Apparatus:

|Search coil 1 |Slinky solenoid 1 |

|CRO 1 |Slotted bases 2 |

|Signal generator 1 |Metre rule 1 |

|a.c. ammeter 1 |Crocodile clips 2 |

|PVC-covered copper wire 26 s.w.g. 1 m long |Connecting leads. 2 |

Theory:

When there is a change of the magnetic flux Φ linked with a wire loop, it induces an electromotive force (emf) ε between the loop ends, but a constant magnetic flux or a non-linked flux does not. This is the basic fact of electromagnetic induction, expressed by Faraday’s law for a wire loop,

ε = －dΦ/dt

The induced emf, ε is equal to the negative rate of change of the magnetic flux Φ linked with the loop. If we replace the wire loop by a short coil of N turns, the induced voltage is N times that of a single loop, so Faraday’s law becomes ε = －NdΦ/dt

When loop ends are connected, ε produces a current which yields its own magnetic field. Its direction always opposes the flux change dΦ/dt. This fact is known as Lenz’s law and is expressed by the negative sign.

For a circular loop of radius r and area A = π r2 in a constant magnetic field B (Fig. 36.2), the magnetic flux linkage Φ is Φ = B⊥A = BA cosθ...