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Wonders of Physics

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N2007P1Q33 – Electromagnetism

A copper wire is taped to two wooden blocks which stand on a sensitive balance. When there is a current in the wire and a magnet is held in the position shown, the balance reading increases.


Which other arrangement of magnet and current will give the same increased reading?



Solutions: Option D

This question is rather direct. Applying FLHR, the force acting on the copper wire is downwards. Hence using FLHR again on the 4 options, only D creates the same downward force.

But what I wish to highlight in this question is for you to compare this with the other question which I posted earlier. They seems similar but they are different. View the other related question.

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A U-shaped magnets sits on top of a electronic beam balance. A wire is placed horizontally between the poles of the magnets as shown in the diagram below.


Initially, when there is no current flowing through the wire, the balance reads 170.05 g. When a steady current of 1.50 A (flowing out of the paper) is passed through the wire, the balance reads 180.25 g as shown above.

Solutions: As a direct current is provided through the wire, Fleming’s Left Hand Rule (FLHR) is applied here. As magnetic field is from N to S (left to right) and current is out of the paper, the force on the wire will be acting upwards (in a direction from strong magnetic field to weaker magnetic field). We are all very familiar to this type of question.

But one has to remember that the forces always come in a pair (action is equal to reaction – Newton’s 3rd Law).

Hence due to the combined magnetic field between the magnets and wire, a force is acting on the wire upwards, hence there must be an equal and opposite force acting downwards on the magnets.


That explains why the balance reads a higher value. On the other hand, if current is into the paper, the principle applies here too. The force acting on wire will be downwards, and hence there is a equal and opposite force acting on the magnet upwards.

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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.