# 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.

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:

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

B⊥ denotes the field component normal to the loop. The flux linkage is zero when loop and field are parallel. It is highest when the loop is perpendicular to the field, i.e. cosθ=1, thus, ε = NA dB/dt.

The search coil is always used to measure the magnetic fields. It consists of N turns of the coil enclosing an area A. When exposed to a changing magnetic field B, an e.m.f. is induced across the ends of the coil. The induced e.m.f. (ε) is directly proportional to the rate of magnetic field, i.e. ε = NA dB/dt .When the search coil is connected to a CRO, the corresponding induced e.m.f. and hence magnetic field magnitude can be determined.

Precautions for magnetic field around straight wire

1. The ...