Magnetism and Electromagnetism Question

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1. A group of students investigating electromagnetism make the electromagnet shown in the Figure below. The core of the magnet is a U-shaped piece of iron. Only a few turns of each of the coils P and Q are shown. An iron plate is also placed below the electromagnet.

                  

(a) Explain which of the two ends of the core is the N-pole of the electromagnet.
(b) When the electromagnet is lowered until the gap between the ends of the core and the iron plate is reduced to a millimeter or so, the plate is rapidly attracted to the ends of the core. Explain how this happens.
(c) Explain how, without changing the coils or core, the iron plate could be made to jump a larger vertical gap.

(a) Analysis:
To determine which end is North pole, we have to know the direction of current in the solenoid, then use Right hand grip rule to determine it.

Answer:
P using Right Hand Grip Rule.
(b) Analysis:
Remember that induction happens before attraction.

Answer:
When the electromagnet is near to the iron plate, magnetic induction takes place. The left side of the plate next to P will be South pole and that next to Q will be North pole. Hence, the iron plate is attracted to the core as unlike poles attract.
(c) By increasing the size of the current flowing in the coils P and Q. So as to increase the strength of the magnetic field so that the field at the plate is strong enough to enable induction and subsequent attraction to take place.

2. The Figure below shows a solenoid. Two pieces of soft iron are placed in it. When the switch is closed, the compass needles point in the direction of the magnetic field produced at each position.

                

(a) On the Figure, mark with arrows, in compasses A, B and C, to show the direction of the magnetic field produced.
(b) When the switch is closed, the two soft iron pieces become magnets and move.
      (i) On the Figure, mark the poles formed on each piece of soft iron.
      (ii) State and explain how the pieces move.
(c) The experiment is then repeated with the following changes. State the effect on the magnetic field and the movement of the two iron pieces when,
      (i) the direction of the current is reversed.
      (ii) the size of the current of the current is reduced.
      (iii) the current is replaced with a.c. current with 10 Hz frequency.

(a) Analysis:
A magnetic field is created when a current flowing through the solenoid. Using Right Hand Grip Rule, you should be able to determine that right end of the solenoid is North pole. Recall that magnetic field is from North pole to South pole. Thus, compass C points to right, B to left and A to right.

Answer:
         

(b)(i) Analysis:
From part (a), right end of the solenoid is North pole. Thus the right end of the iron is also magnetised to be North pole.

Answer is shown in part (a)
      (ii) They will attract each other at the centre as unlike poles attract.
(c)(i) Magnetic field and the poles will reverse but the irons pieces will still be attracted to each other.
      (ii) Magnetic field strength will reduce/weaken. Iron pieces will still be attracted to each other but attraction will be weaker / move towards each other with smaller force.
      (iii) Magnetic field will change its direction 10 times each second. Iron pieces will still be attracted to each other as each time it alternates, unlike poles are still produced at the centre.

3. The Figure below shows the structure of a simple d.c. motor.

                  

(a) Explain briefly why the coil will rotate when the current flows in it.
(b) Explain the purpose of the part labelled X in the Figure.
(c) Suggest a reason why the motor sometimes does not start when switched on, even if there is no friction.

(a) When current flowing through the coil, by Fleming’s Left Hand Rule, the direction of force (indicated by Thumb) is perpendicular to both the direction of magnetic field from North pole to South pole (indicated by Index Finger) and the direction of current (indicated by Middle Finger), each side of the coil will experience an equal but opposite force. Thus the coil rotates.

OR

When current flows through the coil, a magnetic field would be produced around the coil. The magnetic field due to the current in the coil will interact with the magnetic field of the magnet. Each side of the coil will experience an equal but opposite force. Thus the coil rotates.
(b) The split ring commutator reverses the direction of current in the coil every half a revolution, which to ensure the coil to rotate in the sae direction.
(c) Sometimes, contact between the split-ring commutator and the carbon brush is broken at the space between the half rings.Hence the motor will not start because there is no current in the coil.

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