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bmat > Physics - Magnetism > Flashcards

Physics - Magnetism Flashcards

1
Q

how many poles do permanent magnets have

A

two - north and south

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2
Q

what is the north pole of the magnet attracted to

A

‘north seeking’
geographical north

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3
Q

what can iron filings be used for

A

to depict the magnetic field lines

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4
Q

soft magnetic materials characteristics

A

easily magnetised but also lose their magnetism easily

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5
Q

hard magnetic materials characteristics

A

difficult to magnetise but once magnetised they are difficult to be demagnetised

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6
Q

what do electromagnets usually consist of

A

coil wrapped around a soft iron core

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7
Q

why is soft iron used in electromagnets

A

gains and loses magnetism quickly

  • can be turned on and off quickly
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8
Q

inducing magnetism

A

pole of bar magnet near an unmagnetised magnetic material, there is a force of attraction experienced (never repulsion)

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9
Q

what is induced magnetism used for

A

making permanent magnets by placing a hard magnetic field in a strong magnetic field as a solenoid

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10
Q

what links electric current and magnetism

A

as current flows through a wire, it creates a magnetic field in the surrounding area

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11
Q

what does reversing current do to the magnetic field created

A

reverses magnetic field

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12
Q

what does increasing current do to the magnetic field

A

increases magnetic strength

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13
Q

what causes the magnetic field in wires

A

charge carriers

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14
Q

how can a magnetic field be created in a vacuum

A

beam of charged particles

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15
Q

magnetic field around wire characteristics

A

consists of concentric circles

circles get further apart and further from wire

have a direction predicted by right hand grip rule

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16
Q

right hand grip rule

A

thumb in direction of current

fingers curl in direction of magnetic field lines

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17
Q

what happens to the magnetic field when the wire is wrapped in a tight coil

A

strong field running through cores centre

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18
Q

what is a solenoid

A

long coil of electrical wire with a magnetic field
narrow coils wound together

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19
Q

magnetic field in a solenoid

A

resultant = very uniform

field direction can be predicted by right hand grip rule

field is comparable to a bar magnet with poles at each end of solenoid

weak field at sides of solenoid and opposite in direction to the field inside solenoid

field strength and direction controlled by current

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20
Q

what pole is created by coils turning clockwise

A

north pole

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21
Q

what pole is created by coils turning anticlockwise

A

south pole

22
Q

magnetic field strength is dependent on

A

current

distance

surroundings (magnetic materials surrounding the wire can increase field strength)

23
Q

iron is ferromagnetic, what does that mean

A

each iron atom creates a magnetic dipole

24
Q

what happens to iron atoms when placed in an external magnetic field

A

atomic dipoles line with this field

25
Q

how to increase strength of magnetic field of solenoid

A

increasing number of turns per unit length

using a soft iron core inside coil

increasing current in coil

26
Q

key differences between permanent magnets and electromagnets

A
  • field shape
  • electromagnets can be turned on/off
  • electromagnets field strength can be varied whereas a permanent magnet’s field strength is largely constant
  • polarity of electromagnet can be reversed by reversing the current whereas a permanent magnet has a constant dipole
27
Q

how are permanent magnets made

A

placing a hard magnetic material (iron alloy, neodymium) in a strong external magnetic field

28
Q

permanent magnets field strength

A

magnets retain their strength for a long time but are weakened by impact and can be demagnetised by heating above their Curie temperature

29
Q

why are superconducting coils useful

A

current can only be increased so much before the wire gets too hot

superconducting coils (zero resistance) which require a very low temperature to function

30
Q

motor effect

A

force perpendicular to direction of current and direction of magnetic field

current carrying wire is parallel to the magnetic field, no force is experienced by the wire

current carrying wire is perpendicular to the magnetic field, then the force is at maximum

force occurs as magnetic field created by current carrying wire interacts with magnetic field of the permanent magnet creating a force on both the wire and the magnet

31
Q

flemings left hand rule

A

middle = direction of curent

index = direction of magnetic field

thumb = direction of motor force

32
Q

what happens to motor effect if you reverse current OR magnetic field

A

motor effect reversed

33
Q

what happens to motor effect if you reverse current AND magnetic field

A

motor effect unaffected

34
Q

what is the strength of the motor effect force determined by

A

current - increasing the current increases the force

magnetic field - increasing the field strength increases the force

length - increasing length of wire in field increases the force

angle - between current and magnetic field: greatest at 90 degrees and zero when parallel

35
Q

equation to determine the magnitude of the motor effect force

A

F = BIL

F = motor effect force in Newtons (N)
B = magnetic field strength in Tesla (T)
I = current in amperes (A)
L = length of wire in metres (M)

36
Q

what is a DC motor

A

motor effect used to create a turning effect on a current carrying coil arising from a pair of motor effect forces acting in opposite directions

37
Q

how does a DC motor work

A

current carrying rectangular coil is placed in a permanent magnetic field

motor effect forces on either side of the coil can produce a turning effect

maximum force occurs when the coil is in the plane of the field and the current is perpendicular to the field

force is zero when the coil is perpendicular to the field and the current is parallel to the field

38
Q

what must happen for a DC motor to rotate in the same direction and how is it achieved

A

current must be reversed every time the coil passes the vertical

using a SPLIT RING COMMUTATOR which rotates with the coil and connects to the DC power supply by two brushes and acts as a rotating switch

39
Q

split ring commutator

A

rotates with the cpil in DC motor and connects to the DC power supply by two brushes and acts as a rotating switch

made from brass or coper and with graphite brushes which are low friction but make good electrical contact (brushes need replacing when worn down)

40
Q

turning effect of coil determined by

A
  • current
  • size of coil
  • number of turns
  • angle of coil in field
  • winding the coil onto soft iron core
  • magnetic field strength
41
Q

how can voltage be induced in a conductor

A
  • passing conductor across lines of magnetic field
  • magnetic field line changes across wire

doesnt matter whether coil or magnetic field moves, if one crosses the other a voltage is induced in the wire or coil

42
Q

when does electromagnetic induction result in an induced voltage and induced current

A

induced voltage = always
induced current = only in closed circuit

43
Q

magnitude of induced voltage is directly proportional to ..

A
  • rate at which wire cuts magnetic field lines
  • rate at which the magnetic field through a conductor changes
44
Q

how does stronger magnet affect induced voltage

A

stronger magnet = greater voltage induced

45
Q

if electromagnet is used to generate a magnetic field, the induced voltage can be increased by

A
  • increasing frequency of AC supply
  • increasing amplitude of AC supply
  • increasing number of coils on electromagnet
46
Q

why does induced voltage create a dipole

A

induced voltage opposes the change that makes it in order to obey the principle of energy conservation
dipole created to repel the magnetic field that created voltage

use this rule to predict the current direction in coil

47
Q

what can induced voltage be reversed by

A
  • reversing direction of cutting the magnetic field lines
  • decreasing magnetic field strength instead of increasing it
48
Q

AC generators

A

consists of coil rotating in magnetic field causing coil to continuously cut the magnetic field and induce a voltage

voltage induced alternates as the coil rotates and cuts the magnetic field in the opposite direction, therefore generator produces AC out put

49
Q

how to increase amplitude of output of AC

A
  • rotate coil quicker
  • increase magnetic field strength
  • increase area of coil in magnetic field
  • increase number of turns of coil in magnetic field

all factors increase number of field lines being cut per unit time

50
Q

why are generators important

A

transform mechanical energy n rotating cooil into an electrical output
source of mechanical energy can be from
- chemical/nuclear
- kinetic

51
Q

why are transformers important

A

convert AC at one voltage to AC of a different voltage
This is useful as different voltages of electricity have different uses

bmat (33 decks)

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