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gsce physics > magnetism and electromagnetism 6 > Flashcards

magnetism and electromagnetism 6 Flashcards

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1
Q

what do all magnets have?

A

north and south poles

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2
Q

what are magnetic field lines?

A

used to show the size and direction of magnetic fields
always point from north to south

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3
Q

what happens if you place two permanent bar magnets near each other (south and north)?

A

creates a uniform field between magnets

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4
Q

how can a material become magnetised?

A

by placing an object in a magnetic field
domains align

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5
Q

what are some properties of magnetically soft materials?

A

easy to magnetise and demagnetise
iron
electromagnets

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6
Q

what are some properties of magnetically hard materials?

A

hard to magnetise and demagnetise
steel
permanent magnets

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7
Q

whats the law of magnetism?

A

opposite poles attract
like poles repel

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8
Q

do field lines go in or out of north and south poles?

A

south - in
north - out

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9
Q

practical
finding magnetic field lines using a compass:

A

compass shows pattern and direction
- carry out on a blank piece of paper
- bar magnet is placed in the centre of the paper
- start at the edge of the north pole and put a dot where the arrow of compass Is pointing
- repeat this method until you get to the south pole

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10
Q

practical
finding magnetic field lines using iron fillings:

A

iron fillings show the pattern
- place magnet under piece of paper
- sprinkle iron on top of paper
- iron become induced magnets and lines up with field
- tap paper until iron fillings create a clear pattern

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11
Q

how do you create a uniform field with two bar magnets?

A

place two magnets next to each other with the north and south pole facing
must be close but not too close
field lines will be evenly spaced and parallel

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12
Q

what do magnetic field lines never do?

A

intersect

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13
Q

what is a magnetic field?

A

a region where magnetic materials experience a force

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14
Q

like poles…

A

repel each other

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15
Q

opposite poles…

A

attract each other

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16
Q

how is magnetism induced?

A
  • magnets affect magnetic materials and other magnets
  • poles attract magnetic materials that aren’t magnets
  • when magnetic materials are brought near to a magnet (into its magnetic field) that material acts as a magnet
  • the original magnet has induced this magnetism
  • the closer the magnet and the magnetic material get, the stronger the induced magnetism will be
17
Q

how does a current-carrying wire create a magnetic field?

A
  • an electric current in a conductor produces a magnetic field around it
  • the larger the electric current, the stronger the magnetic field
  • the direction of the magnetic field depends on the direction of the current
18
Q

what is the magnetic field around a straight wire?

A

the field is made up of concentric circles with the wire in the centre

19
Q

what is the magnetic field around a flat circular coil?

A

similar to that of a bar magnet
there are concentric ellipses (stretched circles) of magnetic field lines around the coil

20
Q

what is the magnetic field around a solenoid?

A

the magnetic field inside a current-carrying solenoid (coil of wire) is strong and uniform
- outside the coil, the field is just like the one around a bar magnet
- this means that the ends of a solenoid act like the north and south poles of a bar magnet
this type of magnet is called an electromagnet

21
Q

how can you increase the strength of a magnetic field around a solenoid?

A

by adding a magnetically soft iron core through the middle of the core

22
Q

how does current in a magnetic field experience a force?

A
  • when a current-carrying wire is put between magnetic poles, the two magnetic fields affect one another
  • this results in a force acting on the wire
  • this can cause the wire to move
  • this is called the motor effect
  • this is due to charged particles moving through a magnetic field experiencing a force as long as they are not moving parallel to the field lines
23
Q

explain the motor effect:

A
  • the wire has to be at 90 degrees to the magnetic field (if the wire runs along the magnetic field it won’t experience any force at all)
  • the force always acts in the same direction relative to the magnetic field of the magnets and direction of the current in the wire
  • the strength of the force increases with the strength of the magnetic field
  • the force also increases with the amount of current passing through the conductor
  • reversing the current or the magnetic field also reverses the direction of the force
24
Q

what is fleming’s left-hand rule?

A

helps you work out motor effect

  • LEFT hand
  • first finger = direction of the field
  • second finger = direction of current
  • thumb = direction of the force
25
Q

what 4 factors can speed up the motor effect?

A
  • more current
  • more turns on the coil
  • stronger magnetic field
  • a soft iron core in the coil
26
Q

what is a dc electric motor?

A
  • when a force is acting on two sides of the coil
  • the forces are the usual ones that act on any current in a magnetic field
  • the coil is a spindle and the forces act one up and one down it rotates
  • the split-ring commutator is a clever way of swapping the contacts every half-turn to keep the motor rotating in the same direction
  • the direction of the motor can be reversed either by swapping the polarity of the DC supply or swapping the magnetic fields over
27
Q

how do speakers work due to the motor effect?

A
  • AC electrical signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone
  • the coil is surrounded by a permanent magnet so that the AC signals cause a force on the coil and make it move back and forth
    -these movements make the cone vibrate and this creates sounds
28
Q

what is electromagnetic induction?

A

the creation of voltage (and maybe current) in a wire which is experiencing a change in the magnetic field

29
Q

what is the dynamo effect?

A
  • using electromagnetic induction to generate electricity using energy from kinetic energy stores is called the dynamo effect
30
Q

dynamo effect:
what are the two different situations where you get electromagnetic induction?

A
  • electrical conductor (a coil of wire is often used) moves through a magnetic field
  • the magnetic field through an electrical conductor changes (gets bigger or smaller or reverses)
31
Q

dynamo effect:
how can you test the two different situations where you get electromagnetic induction?

A

you can test by connecting an ammeter to a conductor and moving the conductor through a magnetic field
if the direction of movement is reversed then the induced voltage/current will be reversed too

32
Q

electromagnetic induction:
how can you get a bigger voltage?

A
  • increase the strength of the magnet
  • increase the number of turns of the coil
  • increase the speed of movement
33
Q

how do AC generators work?

A
  • generators rotate a coil in a magnetic field
  • their construction is pretty much like a motor
  • as the coil spins, a current is induced in the coil, this current changes direction every half-turn
  • instead of a split ring commutator AC generators have slip rings and brushes so the contracts don’t swap every half-turn
  • power stations use AC generators to produce electricity, they just get the energy needed to turn the coil or magnetic field in different ways
34
Q

how do transformers work?

A
  • transformers change the voltage of an alternating current
  • they all have two coils, the primary and secondary joined by an iron coil
  • when an alternating voltage is applied across the primary coil, the magnetically soft iron core magnetises and demagnetises quickly = this induces an alternating voltage in the secondary coil
  • the ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils
35
Q

what do step-up transformers do?

A

step-up transformers increase the voltage, they have more turns on the secondary coil than the primary coil;

36
Q

what do step-down transformers do?

A

step-down transformers decrease the voltage, they have more turns on the primary coil than the secondary

37
Q

how do you calculate the output voltage from a transformer from the input voltage and the number of turns on each coil?

A

input (primary) voltage / output (secondary) volatge = number of turns on primary / number of turns on secondary

38
Q

transformers are nearly 100% efficient so…

A

power in = power out
primary voltage x primary current = secondary voltage x secondary current

39
Q

how have transformers made transmitting mains electricity more efficient?

A

step-up and step-down transformers are used when transmitting electricity across the country

  • voltage produced by power stations is too low to be transmitted efficiently so the lower the voltage the higher the current for a given amount of power therefore the current causes wires to heat up
  • a step-up transformer is used to boost the voltage before it is transmitted
  • step-down transformers are used at the end of the journey to reduce the voltage so its safer and more useful

gsce physics (9 decks)

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