Magnetism Flashcards
(15 cards)
-They trace out invisible lines of influence in the magnetic field. These lines are called lines of flux or lines of force.
-Lines of flux of a magnet emerge from the N pole and re-enter at the S pole.
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-Although, in diagrams, some lines of flux are shown incomplete they are in fact always ……………
-Lines of flux never …………. .
- When two magnets are brought close together their resultant field ……………………………….
-When lines of flux from the two magnets oppose one another they tend to ……………….
- What are poles of a magnet ?
-Although, in diagrams, some lines of flux are shown incomplete they are in fact always continuous.
-Lines of flux never cross .
- When two magnets are brought close together their resultant field is modified by the fact that lines of flux cannot cross. Where lines of flux from the two magnets are in the same direction they reinforce one another and the flux density is increased.
-When lines of flux from the two magnets oppose one another they tend to cancel each other out.
-Magnetic effects are most powerful at two points, usually near the ends of the magnet, called
the poles of the magnet.
- What is permeability ?
- What is retentivity ?
- What is residual magnetism ?
- If an unmagnetised piece of soft iron is placed in a magnetic field, the lines of flux concentrate to flow through the iron. The iron itself becomes magnetised and produces additional lines of flux.
- This property of increasing the flux density is called permeability.
- If it is removed from the magnetic field, the soft iron loses most of its magnetism. Soft iron is said to have low magnetic retentivity. The little magnetism which remains is called its residual magnetism.
THE CORKSCREW RULE
THE RIGHT HAND GRASP RULE
–If a right-handed corkscrew is turned so as to move in the direction of the conventional current
in the conductor, the direction of rotation of the corkscrew is the direction of the lines of flux.
–If a solenoid is held in the right hand so that the fingers are curled round it pointing in the
direction of the conventional current, the outstretched thumb points to the North pole of the
solenoid.
The strength of the field of a solenoid can be increased by:
The strength of the field of a solenoid can be increased by:
– increasing the number of turns on the coil.
– increasing the current
– using a soft iron core.
Difference between a solenoid and a Relay
The solenoid has a moving core whereas the relay has a stationary core and an attracted
armature.
The wires that form the coil of the solenoid or relay are insulated and have no physical or
electrical contact with the circuit which is controlled by the contacts.
The area of force around a magnet is termed:
a. conductance.
b. stable.
c. magnetic resistance.
d. magnetic field.
D
When a magnet is unable to accept any further magnetism it is termed:
a. reluctance.
b. saturation.
c. active.
d. reactance.
B
Permanent magnets are manufactured from:
a. steel.
b. plastic.
c. liquid.
d. glass.
A
Magnetic lines of force flow externally from:
a. one main line station to another.
b. the master station.
c. the north to the south pole.
d. in a random direction.
C
Which of the two poles has the greatest strength:
a. north seeking pole.
b. south seeking pole
c. both poles have the same strength.
d. the saturated pole.
C
Electromagnetism is a product of:
a. voltage.
b. current.
c. resistance.
d. engine resistance.
B
To increase electromagnetic force one would:
a. increase coil resistance.
b. reduce current flow.
c. lower EMF.
d. increase current flow.
D
If you bring two magnets together:
a. like poles will attract.
b. unlike poles will attract.
c. over heating will occur.
d. their magnetic fields will adjust to avoid overcrowding.
B
A soft iron core in an electromagnet:
a. increases flux density.
b. decreases flux density.
c. reduces arcing.
d. increases the lines of strength.
A
