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Physics MA > Magnetism and electromagnetism > Flashcards

Magnetism and electromagnetism Flashcards

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1
Q

what is the unit/name for current?

A

ampere (A)

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2
Q

what is the unit/name for voltage?

A

volt (V)

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3
Q

what is the unit for power?

A

watt (W)

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4
Q

what do magnets do?

A

magnets repel and attract other magnets and attract magnetic substances

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5
Q

can magnetism. be induced? how?

A

magnetism is induced in some materials when they are placed in a magnetic field

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6
Q

what happens when there is a current in a conductor

A

an electric current in a conductor produces a magnetic field around it

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7
Q

when is a voltage induced in a conductor/coil?

A

a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it and describe the factors that affect the size of the induced voltage

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8
Q

what is the relationship for input and output power? (efficiency)

A

input power = output power Vp Ip =Vs Is
for 100% efficiency
p being primary
s being secondary wire

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9
Q

what is the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer

A

input (primary) voltage/output (secondary) voltage = primary turns/ secondary turns

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10
Q

what do permanent magnets do? what are they main features

A

A permanent magnet always causes a force on other magnets, or on magnetic materials. Key features of a permanent magnet:
it produces its own magnetic field
the magnetic field cannot be turned on and off - it is there all the time

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11
Q

for a single bar magnet, where is the magnetic field strongest

A

nearest to the poles

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12
Q

summarise the magnetic domain theory

A

when placing an external domain, the atoms line u in a certain dimension

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13
Q

what is the uniform field

A

the region between the poles where there are equally spaced, parallel lines. The field strength remains constant as you move around this area. the lines of flux get further apart when moved out from the space

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14
Q

what happends to an iron nail when they are removed form a permanent magnet? what is this an example of?

A

an iron nail is not usually magnetic, it is magnetically soft. when is it removed from the permanent magnet, the domains go back to having different orientations (demagnetised)
this is an example of induced magnetism

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15
Q

what would happen if steel was put next to a permanent magnet

A

it woudl stay permanently magnetic after it was removed

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16
Q

what are the uses of electromagnets

A

uses in scrap yards - carlifts (as they are easy to control and regulate strength)
circuit breakers
door bell
the relay

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17
Q

how can electromagnets be used in scrap yards?

A

iron is a soft magnetic material. when current flows the iron becomes strongly magnetised, picking up scarp iron, steel. When the current is turned off, the iron loses its magnetised (its soft) so it releases the scrap

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18
Q

how does an electric bell work? what is it an example of?

A

it is an example of a use of an electromagnet
1. when the switch is closed, current flows around the circuit turning on the electromagnet
2. the soft armour armature is pulled towards the electromagnet and the hammer hits the gong
3. the contact switch to open cutting off the electric current
4. spring now pulls the armature back again, closing the contact switch
5. current now flows again and the hammer hits the gong again

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19
Q

how does a relay switch work?

A
  1. a relay switch is a way of using a low voltage circuit to switch remotely a high voltage (an possibly dangerous circuit)
  2. when switch A is closed, the small current provided by the cell causes the electromagnet to become magnetised
  3. the iron armature is then attracted to the electromagnet causing springy contact B to close in the high voltage circuits
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20
Q

what is s circuit breaker? how does it function?

A
  1. current normally flows between terminals A and B through the contact and the electromagnet
  2. when the current in a circuit increase, the strength of the electromagnet will also increase
  3. this will pull the soft iron armature towards the electromagnet
  4. as a result, spring 1 pulls apart the contact, disconnecting the circuit immediately, and stopping current flow
  5. the reset button can be pushed to bring the contact back to its original position to reconnect the circuit
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21
Q

what are the rules for magnetic field lines around a bar magnet?

A
  • magnetic field strength is weaker, further away from the magnet
  • the stronger the magnetic field strength, the closer the field lines at that point
  • the field lines loop around exiting the north pole and entering the south pole
  • field lines never cross
  • magnetic field lines are sometimes clled ‘lines of magnetic flux’
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22
Q

which elements are ferromagnetic?

A

iron, cobalt, nickel

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23
Q

what is ferromagnetic, and paramagnetic what can make a metal ferromagnetic

A

‘magnetic’ is short for ferromagnetic
‘paramagnetic’ means to become magnetic
a metal can become ferromagnetic if it is an alloy of one of the three ferromagnetic elements (nickel, cobalt, iron)

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24
Q

how do you make something magnetic?

A

rub a magnetic material on a non magnetic material to magnetise it

25
Q

what are permanent magnets?

A
  • made from magnetically hard materials (such as steel) which are resistant to demagnetising
  • every magnet has a magnetic field we can detect
  • iron is a soft magnetic material and not a suibtable permanent magnet
26
Q

what is an electromagnet?

A
  • a coil wrapped by a material with power pumping through it
  • iron would be more prefferable to use over steel because iron becomes demagnetised meaning it can be turned on and off and the strength can be controlled
  • this is cheaper and easeir (to make a strong magnet) rather that a hard magnet
27
Q

explain magnetic domains

A

the ferromagnetic materials have regions called domains where the magnetisation of all the atoms line up in a certain direction
- different domains usually have different orientations but the line up placed in an external magnetic field
- hard magnetic materials stay magnetised evern after magnetic field removed
- soft magnetic materials demagnetise once magnetic field removed

28
Q

how to you change the strength of an electro magnet?

A
  • increase current
  • no coils/solenoid
  • increase the mass of the soft iron core
29
Q

what happens if a magnet is split?

A

2 more magnets are made

30
Q

what is the right hand rule?

A

(thumbs up position)
the thumb points the direction of the conventional current. the way your fingers wrap around is the way the magnetic field it

31
Q

what is the circuit breaker designed to do?

A

to protect an electrical curcuit from damage caused by a short circuit

32
Q

explain how a circuit breaker works?

A

too much current passes through coil
the iron core of the electromagnet is magnetised
the switch is attracted to the core of the electromagnet
the circuit breaker switch is opened
the current is cut off

33
Q

why does a buzzer, like an electric bell, work and why does it vibrate at a higher frequency?

A

the hitting of the core causes a vibration which is the source of the buzzing. it vibrates at a higher frequency becuase it has a smalle mass than a bell and responds quicker to the on and off of the electromagnet

34
Q

what is a resultant field?

A

any moving charge in an external magnetic field feels a force

35
Q

what is the formal name for the left hand rule? what is it?

A
  • fleming’s left hand rule
  • thumb shows motion - gives motion force
  • first finger (index) gives direction of field (N-S)
    -second finger gives the current - gives direction to current in wire
36
Q

what factors affect force on current carrying conductor (wire)

A
  • current
  • strength of magnetic field
  • length of wire
37
Q

what is the expression to define magnetic field strength

A

F=BIl
B=F/Il (capital ‘i’, lowercase l)
b is the symbol for magnetic field strength (in Tesla)
‘i’ is current, l is length, f is force

38
Q

what is 1 tesla?

A

magnetic field strength in which current carrying wire of 1A, produces of force 1N for each 1 metre or length of wire in the field, at right angles to both wire and field

39
Q

how does a loud speaker work?

A
  1. current in the electrical current is varying and passes through the coil causing the coil to experience a force (inwards or outwards)
  2. The current is reversed repeatedly to reverse the force
  3. The greater the current the greater the force
  4. As the current and force vary, the coil the vibrates
  5. The coil is attached to a cone which also vibrates and causes the air molecules to vibrate
  6. This movement of the air molecules produces the pressure variations in the air needed for a sound wave producing compressions and rarefactions
40
Q

from what directions do magnetic field lines point?

A

north to south

41
Q

what is the purpose of a split ring commutator?

A

the reverse the current every half turn (if you don’t it will just wobble)

42
Q

how do you reverse the current of a DC motor?

A

reverse the current
reverse magnetic field

43
Q

how do you increase the speed of a DC motor

A

increase the current, more coils, increase mass of soft iron core

44
Q

how does fleming’s left hand rule apply to the motor?

A

shows the direction of force, current, N+S magnetic field

45
Q

name a pice of technology utilising the motor effect

A

loudspeakers

46
Q

which way does a compass point when there is no current

A

north

47
Q

which way does a compass point if the current is very large (e.g., 30 A)

A

they go the same way as the current. if there are two compass’, one above and one below the wire, and the magnetic field goes in a circular motion around the wire.
the north points to the way the current goes

48
Q

which way does the current go when there is a very small current (1A)

A

the compasses point half where earth north is and the owhere the magnetic field it

49
Q

draw the magnetic field near a long straight wire, solenoid, a circular flat coil (whatever that is)

A

check pg. 193, 194 for correct diagrams (in textbook)

50
Q

how can voltage be induced with coils

A

voltage is induced across the ends of the coil when it crosses through magnetic field lines. If the conductor is part of a complete circuit, the induced potential difference makes an electric current to flow ina. circuit

51
Q

how can induced voltage be increased?

A

increasing the number of turns on the coil of wire, increasing the strength of the magnet, making the magnet coil pass on another more quickly (increase rate at which magnetic field lines are cut)

52
Q

what is the Lorenz force?

A

when there is a magnetic field, a moving charge is produced and vice versa

53
Q

what is the main use of electromagnetic induction?

A

AC genetrator

54
Q

what happens in electromagnetic induction when the coil rotates faster?

A

larger frequency of alternating current, the bigger the peak value of induced current (more flux lines are being cute per second)

55
Q

what is the generator effect?

A

when a conductor cuts through magnetic field lines or vice versa and a current is induced

56
Q

what is ‘cutting’?

A

moving through each other at right angles

57
Q

what is an alternating current?

A
  • not a constant output the entire time
  • varies in magnitude, but can generate electrical power regardless of direction
  • direction of current is switched periodically so volatage is also switched
58
Q
A

Physics MA (2 decks)

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