Suggest an explanation for each of the following observations.

Question 4

Suggest an explanation for each of the following observations.

(a) Two wires are laid side-by-side and carry equal currents I in opposite directions, as shown in Fig. 6.1.

Fig. 6.1

The total magnetic flux density due to the current in the wires is negligible. [3]

(b) An air-cored solenoid is connected in series with a battery, as shown in Fig. 6.2.

Fig. 6.2

As an iron core is inserted into the solenoid, an e.m.f. that opposes the e.m.f. of the battery is induced in the solenoid. [4]

Reference: Past Exam Paper – November 2015 Paper 43 Q6

Solution:

(a)

{The direction of the magnetic field is given by the right hand grip rule.}

The magnetic flux densities are equal in magnitude and opposite in direction. The fields cancel out to give a negligible resultant field.

(b)

Inserting the iron core into the solenoid causes an increase in the magnetic flux in the solenoid. (Poles (or field) are induced in the core). The changing magnetic flux cuts the turns in the solenoid. From Faraday’s law, an e.m.f. is induced in the solenoid and from Lenz’s law, this e.m.f. opposes the e.m.f. of the battery.