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 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 wire should be long
2. The distance(r) should much smaller than the length of the wire.

Procedure

A. Magnetic field around straight wire
1. The circuit as shown in Fig.C15.1 and a lateral type search coil to a CRO was connected.

2. The signal generator was turned on and was set to 0.5A and 5kHz. 3. The centre of the search coil was placed 1 cm away from the straight wire. The search coil was at the same level and perpendicular to the straight wire. The CRO setting was adjusted to display a whole trace on its screen. 4. The time base of the CRO was switched off. The length of the vertical trace shown on the CRO was recorded, which represents the induced peak-to-peak e.m.f. (V) in the search coil and also the magnetic field around the straight wire. 5. The steps 2 to 4 were repeated with the other values of current (I) from the signal generator in steps of 0.1A. Then, the results were tabulated. 6. A graph of the induced e.m.f.(V) against the current(I) was plotted. 7. The steps 2 to 4 were repeated with the others values of distances (r) of the search coil away from the straight wire. The results were tabulated. 8. A graph of the induced e.m.f.(V) against the reciprocal of distance([pic]) is plotted. 9. The frequency of the signal generator was varied to change...
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