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igcse physics > magnetism > Flashcards

magnetism Flashcards

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1
Q

what do magnetic field lines show

A
  • strength of the field: closer the lines the stronger the field
  • direction of the field: magnetic field lines always run from the north pole to the south pole
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2
Q

how to find the direction of a magnetic field lines using iron fillings

A

-place a piece of paper on top of the magnet
-gently place iron fillings on top of the paper
now carefully tap the paper to allow the iron fillings to settle on the field lines. This is to spread them and break the inertia of the fillings

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3
Q

how to find the direction of magnetic field lines using plotting compasses

A
  • place the magnet on top of a piece of paper
  • draw a dot at one end of the magnet (near its corner)
  • place a compass next to the dot, so that one end of the needle of the compass points towards the dot
  • use a pencil to draw a new dot at the other side of the compass needle
  • now move the compass so that it points towards the new dot, and repeat the above process
  • keep repeating until you have a chain of dots going from one end of the magnet to the other. Then remove the compass and link the dots using a smooth curve - the magnetic field line
  • the direction of the field line is the same as the direction of the plotting compass
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4
Q

what are soft magnetic materials

A
  • easy to magnetise and demagnetise

- used to make electromagnets and temporary magnets

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5
Q

what are hard magnetic materials

A
  • hard to magnetise and demagnetise

- used to make permanent magnets because they retain their magnetism for a long time

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6
Q

what is induced magnetism

A
  • when a magnet is held close to a magnetic material, the side of the material close to the magnet becomes the opposite pole and the material temporarily becomes magnetised
  • this is known as magnetic induction and results in a force being exerted on the material
  • induction always precedes attraction
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7
Q

methods of magnetisation

A
  • stroking with a magnet in one direction repeatedly
  • place the material in a coil carrying a DC current
  • if the material is placed in a magnetic field and then hit with a hammer the material will also become magnetised
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8
Q

methods of demagnetisation

A
  • placing the magnet in a coil carryign AC current
  • heating the magnet strongly
  • hammering the magnet
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9
Q

how to make electromagnets

A
  • a coil carrying d.c. current produces a magnetic field which can be made much stronger by placing a soft magnetic material such as soft iron in it
  • these electromagnets can be turned on and off by switching the current on and off
  • their strength can be increased by (i) passing more current through the coil or (ii) increasing the number of turns on the coil
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10
Q

how is electric charge measured

A

Coulombs (C)

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11
Q

what is the direction of an electric field

A

direction of an electric field at a point is the direction of the force on a positive charge at that point

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12
Q

what is an electric field

A

the space around a charged particle where another electric charge experiences a force

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13
Q

simple experiment to show the production and detection of electrostatic charges

A
  • rub one end of the material using a cloth (in order to give it a charge)
  • now take a second piece of insulating material and charge that by rubbing it with a cloth
  • hold the charged end of the second piece close to the charged end of the first piece
  • if the first piece rotates away (is repelled) from the second piece then the materials have the same charge
  • if the first piece moved towards (is attracted to) the second piece then they have opposite charge
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14
Q

what does charging a body always involve

A

the addition or removal of electrons

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15
Q

electric fields characteristics

A
  • always go from positive to negative

- have several forms (around a single charge, between two parallel conducting planes, between two opposite charges,

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16
Q

what is charging by induction

A

-when a charged object is kept close to a conductor, electrons in the conductor move towards or away from the object depending on the change of the object. The conductor is then connected to the earth. After the earth connection is removed, the conductor also gets charged, Positive charges cannot move and only electrons move

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17
Q

what is Faraday’s Law of electromagnetic induction

A

when there is relative motion between a conductor and a magnetic field, there is a change in the magnetic flux linkage associated with the conductor and thus an e.m.f is induced in the conductor. This can be achieved by either moving a conductor inside a magnetic field or by moving a magnet inside a coil

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18
Q

what are the factors affecting magnitude of induced e.m.f:

A
  • moving the wire or magnet faster
  • increasing the number of turns on the coil
  • using a stronger magnet
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19
Q

What does Lenz’s law state

A

the direction of the induced e.m.f will be such that it opposes the cause that produces it. This means that any magnetic field created by the EMF will act so that it tries to stop the wire or magnet from moving

20
Q

how to find direction of induced e.m.f

A

Fleming’s right hand rule

21
Q

what is a.c. (alternating current)

A

a current that continuously changes its direction, going back and forth around a circuit

22
Q

what is d.c. (direct current)

A

a current that is steady, constantly going the same way around a circuit, from positive to negative

23
Q

what is an a.c. generator

A

-a coil made to rotate inside a magnetic field. As this happens, there is a change in the magnetic flux linkage associated with coil which induces a current in the coil.
This generator produces an alternating current

24
Q

what are slip rings

A

an a.c. generator makes use of slip rings which not only connect the coil to the rest of the external circuit but also ensure that the coil is rotating freely

25
Q

when is the induced EMF greatest in an ac generator

A

when the coil is horizontal, as in this position it cuts through the field at the fastest rate

26
Q

when is the induced EMF smallest in an ac generator

A

The EMF is smallest when the coil is vertical, as in this position it will not be cutting through field lines

27
Q

what happens when the coil in an ac generator is rotated faster

A

the frequency and amplitude of the current both increase because the magnitude of the induced current depends on rate of change of magnetic flux linkage

28
Q

what is a transformer

A

an electrical device that can be used to increase or decrease the voltage of an alternating current

29
Q

what is the principle of operation of an ac generator

A
  • when an alternaating current is supplied to the primary coil, a changing magnetic field is produced by the primary coil
  • this field passes through the soft iron core and through the secondary coil
  • the changing field in the secondary coil induces an EMF
30
Q

formula for voltage/number of turns on the coil

A

(Vp/Vs) = (Np/Ns)

31
Q

what is a step-up transformer

A
  • increases the voltage of a power source

- has more turns on the secondary coil than on the primary coil

32
Q

what is a step-down transformer

A
  • decreases the voltage of a power source

- has fewer turns on the secondary coil than on the primary coil

33
Q

formula for voltage transformers

A

IpVp = IsVs

34
Q

transformer efficiency

A

if a transformer is 100% efficient, then the input power is equal to the output power

35
Q

why are there step up transformers near power stations

A
  • to increase the voltage for high-voltage transmission of electricity
  • this reduces power loss because current is lower so less power is lost as heat to the surroundings
  • it is cheaper because current is less so thinner cables can be used
36
Q

why are there step-down transformers

A

to reduce voltage for use in homes and industries

37
Q

how to find direction of magnetic field around a current wire

A

right hand thumb rule

38
Q

how to find direction of magnetic field around a solenoid

A

right hand grip rule

39
Q

uses of magnetic effects of current

A

can be used in relay circuits which can then be used in doorbells and electronic door locks

40
Q

direction of a magnetic field line at a point

A

the direction of the force on the N pole of a magnet at that point

41
Q

factors affecting strength and direction of the field

A
  • as the field lines spread out, the field gets weaker
  • the field lines around a wire get further apart the further they are from the wire
  • if the current is increased, the strength of the field increases
  • if the direction of the current is changed, the direction of the magnetic field changes
42
Q

what is flemings left hand rule

A
  • a current-carrying conductor produces a magnetic field around it and if it is placed in another magnetic field, the current carrying conductor will experience a force due to the interaction of the two magnetic fields
  • the direction of this force can be found using Fleming’s left hand rule
  • a beam of charged particles will experience a similar force because a beam of charged particles is equivalent to a current
43
Q

what does the direction of a force caused by a current carrying conductor in another magnetic field depend on

A
  • the direction of the field
  • the direction of the current
  • reversing either of the above will reverse the direction of the force
44
Q

turning effect in a current-carrying coil

A

a current-carrying coil in a magnetic field experiences a turning effect and that the effect is increase by

  • increasing the number of turns on the coil
  • increasing the current
  • increasing the strength of the magnetic field
45
Q

how does a dc motor work

A
  • a dc current is passed through a coil
  • this current produces a magnetic field which interacts with the magnetic field it is kept in
  • this exerts a force on the coil causing it to rotate
  • a d.c. motor has a split ring commutator which ensures that the direction of the current does not change after every half-cycle and makes sure that the coil keeps on rotating in one direction only
  • this means that electrical energy is converted to mechanical energy

igcse physics (12 decks)

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