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Wonders of Physics

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Electromagnetic Induction

Consider these two scenarios.

Scenario 1 A wire moves vertically between the magnets. An induced current is produced when the wire cuts the magnetic lines of force. The direction of the induced current is out of paper and magnets are stationary. Is the wire moving up, A, or downwards, B?


Solutions: Since force is applied to the wire and an induced current is produced, Fleming’s Right Hand Rule (FRHR) is applied here. Using FRHR, you will be able to determine that the direction of the motion of wire (force) is downwards (towards B).

Scenario 2 If now the wire is stationary, but the magnets move vertically instead.  An induced current that flows out of paper is produced as the magnets move. Which direction does the magnets move, upwards (towards A) or downwards (towards B)?


Solutions: From Scenario 1, FRHR is applied to know that wire moves down in order to produce an induced current out of paper. Here, wire is stationary, and magnets move instead. One has to know that the effect of wire moving down (scenario 1) is the same as the magnets moving up (scenario 2). In both cases, the way the wire cuts the magnetic lines of force is the same. Hence, in this scenario, the magnets are moving up (towards A) in order to achieve induced current out of paper.

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Moving Charged Particles in Magnetic field – Fleming’s Left Hand Rule (FLHR)

a) The diagram below shows a region of magnetic field represented by crosses. At the instant shown, two charged particles are moving in the directions as shown by the arrows. Indicate, on the diagram the forces acting on the two particles due to the magnetic field.




Consider the ‘+’ charged particle. The black arrow on the ‘+’ charge represents the direction of the motion. Hence it is the direction of the current. In this case, direction of ‘+’ charge is same as the direciton of conventional current.

Using Fleming’s Left Hand Rule, magnetic field into paper (first finger), convectional current (second finger) in direction of the current (same as ‘+’ charge motion), hence the thumb will indicate the direction of the force (as shown in red).

For the ‘-‘ charged particle, the concepts are the same, just that the motion of ‘-‘ charge direction (electron flow) is opposite to conventional current.

Hence when applying FLHR, the second finger has to be opposite to the motion of the ‘-‘ charge. It is due to this force acting on the charged particle which causes it to bend.

Consider another question.

b) The diagram below shows a radioactive particle P, which can be spontaneously split into smaller particles, in a uniform magnetic field represented by the crosses.  At A, particle P splits into smaller particles Q and R.



P is neutral, Q is positive and R is negative. Refer to video tutorial to see how to apply FLHR.

<p>Applying FLHR on charged particles from evantoh on Vimeo.</p>

Path of charged particle R curved to the right, hence force acting on it is to the right. Using FLHR, magnetic field into paper (first finger), force (thumb) to the right, the conventional current (second finger) is opposite to the motion.

As mentioned above, electron flow is opposite to conventional current, hence R must be negative. Charged particle Q can be easily determined as ‘+’ charged using FLHR too.