Assignment 5 - Mag & Electromag - Ans Key

8/9/2019 Assignment 5 - Mag & Electromag - Ans Key

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DHP Year 4 Physics Assignment 5Magnetism & Electromagnetism

Magnetism1a) Like poles repel, but unlike poles attract.

1b)

1ci)

ndu ed

m gnet

ii) Magnetic Induction

iii) Electromagnets for separating magnetic materials from non-magneticmaterials, magnetic door catch, magnetic door stopper, magnetic paper clips

holder, fridge door,

2a) The soft iron would be induced to become a magnet with its lower end being aNorth pole hence attraction between soft iron and magnet takes place.

2b) Iron is a soft magnetic material and almost all (some field lines may not reachthe iron) the magnetic flux will reach the iron to induce it into anelectromagnet. Since the cross section of the iron is the same as the originalmagnet, the magnetic field strength (magnetic flux per unit area) at the otherend of the iron where the magnetic field exit is only slightly weaker than the

magnet (leakage). Hence the iron is able to attract almost the same numberof tacks.

2c) As the magnet is moving away, less of magnetic flux is used to induce the iron(leakage). Hence the iron gradually induced with weakening magneticstrength. The tacks will fall off one-by-one until eventually the iron become tooweak to attract any tack. (note that even when the magnet is not in contactwith the iron but is near proximity, the iron may still be able to attract sometacks due to magnetic induction).

2d) The magnetic flied line pattern will not be affected much by the wood, since it

is a poor magnetic material (not much difference compare to the case of air).Hence the magnetic field pattern is diverging once the field leave the pole of




the magnet. At the other end of the wood, it is a distance away from themagnet and the field strength is much weaker due to lower concentration ofmagnetic flux. Very few tacks can be attracted.

3a) Invert one of the bar and if the two bars still attract, then one of the bar is iron

while another is a magnet. If repulsion occurs, then both bars are magnet.

3b) Use one end of the first bar to move along the long side of the other bar fromone end to another. If the first bar experience stronger attraction at the twoends and weaker attraction at the middle, then the first bar is the iron.If constant attraction is experienced, then the first bar is the magnet.

4a, b)

ci) If the nails are able to cause a deflection of the compass needle, they are stillmagnetised.

cii) Withdraw nail from solenoid along east-west direction while a.c. is still on

*5a)

*5b)

*5c Magnetic screening/shielding for sensitive navigation device

S

nail

N NSS




Electromagnetism6a)

6b) The soft iron is magnetised with B being a north pole and A being a southpole.

7 a) b)

8

9a, b, c)

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wire carryinga current

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wire carryinga current

beam of electrons

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magnetic field directed into paper

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soft ironcore

A BN S

F




10a)

10b) The forces are equal in magnitude and acts in opposite direction at the twoends of the coil. These forces produce a moment (couple) about the axle andcause the coil to rotate clockwise.

10c)#1) Increase the current in the coil. The magnetic field produced by a larger

current is stronger. This stronger magnetic field will interact more with theexisting magnetic field from the magnet to produce a larger resultant force,and hence turning moment on the coil.

#2) Increase the number of turns in the coil. The same current flows through eachturn of the coil and each turn will produce its own magnetic effect. This resultsin a stronger magnetic field in the coil which will interact more with theexisting magnetic field from the magnet to produce a larger resultant force,and hence turning moment on the coil.

#3) Place a soft iron core within the coil. Soft iron core concentrate the magneticflux through the coil to produce a stronger magnetic field which will interactmore with the existing magnetic field from the magnet to produce a largerresultant force, and hence turning moment on the coil.

#4) Increase the distance between the two ends of the coil. With the same forceproduced, a larger turning moment about the axle will be produce with alarger moment arm.

11a, c)

11b) As the rotating coil ACDB passes the vertical, the split-ring commutatorchanges contact with the brushes. This results in a reversal of the current inthe coil. Forces exerted on each side is still in the same direction and the coilwill continue to rotate in the same anti-clockwise direction.

N S. +

F

F

C D

N S

A

B




12ai, ii)

12bi) The interaction of the magnetic fields due to the current in the coil and that ofthe current in the solenoid result in a force between the coil and the solenoid.The forces, acting on each side of the coil, are in opposite directions (the sidenearer to the solenoid is stronger). These forces create turning moments inthe anti-clockwise direction about the axis XY.

12bii) Due to the presence of the slip rings, the current in the coil flows in the samedirection even after the coil passes the vertical. The force on each side of thecoil causes the coil to remain in the vertical orientation. The coil may oscillatea few times before coming to a stop at the vertical.

13a, ci)

bi) Strength: No changeDirection: Opposite direction

bii) Magnetic field lines are closer than before

cii) The Law of Magnetism is used to determine the direction of force producedon a wire with current placed in a magnetic field. The force produced movesthe wire in a direction to reduce the tension of the combined magnetic field.

QP QP




14ai) Force acts vertically upwards

14aii) Magnetic field of the permanent magnet interacts with the magnetic field ofthe current. A force is produced in the upward direction to move the wire fromthe stronger to the weaker field in order to reduce the tension of the uneven

combined field

14bi) Force acts vertically upwards

14bii) Position of permanent magnets and direction of current remain unchanged.The force produced on AB also remains vertically upwards so continuousrotation is not sustained

14c) Vertical force is only produced on AB when bottom half of wire makes contactwith clip. The force produced allows coil to rotate half a revolution but whenthe insulator makes contact with clip, no current flows through. Continuous

rotation is sustained by the inertia

*15 d.c. motor depends on the momentum of its coil to overcome the cross-overposition. When near to the cross-over, there is little or no turning effect (forcex perpendicular distance) to enable the commutators to come into contactwith the carbon brushes again. Hence at low speed, and high load, the coilmay be stopped in the cross-over position because there is not enough forceto create a large enough turning moments to overcome the load at nearcross-over position.

*16 The torque (or turning effect) fluctuates. It is strongest when the coil plane isparallel to the permanent magnets’ magnetic field and zero when the plane isperpendicular to the magnetic field. With multiple poles, each set of coil isactivated at maximum torque position and cut off when the torque is still highto allow another coil to be activated to continue operating at high continuoustorque so that the coil is kept rotating at relatively constant speed underloaded conditions.

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Assignment 5 - Mag & Electromag - Ans Key