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Physics > Magnetic Fields > Flashcards

Magnetic Fields Flashcards

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1
Q

Magnetic field facts

A 
All magnets produce magnetic fields
Always North to South
Closer lines = stronger field
Further from magnet = weaker field
Magnetic field is strongest at poles
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2
Q

Magnetic field definition

A

Region where other magnets or magnetic materials experience a force

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3
Q

What happens when 2 unlike poles are placed together

A

Attraction
Uniform field in between (same strength everywhere between the poles)
Draw at least 3 field lines with the same distance between each line

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4
Q

What happens when 2 like poles are placed together

A

Repulsion

Field lines curve away from each other

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5
Q

All magnetic materials

A

Anything with:

  • Iron (e.g. steel)
  • Nickel
  • Cobalt
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6
Q

Permanent magnets definition

A

Produces magnetic field all the time

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7
Q

Induced magnets definition

A

Only produces magnetic field when in another magnetic field

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8
Q

Type of force between magnet and magnetic material

A

Always attractive because poles of magnetic induces unlike poles in magnetic material

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9
Q

How quickly induced magnets lose their magnetism

A

Magnetically “soft” e.g. pure iron, nickel-iron alloys = quickly
Magnetically “hard” e.g. steel = slowly
Permanent magnets are made from magnetically hard material

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10
Q

How magnets are used in fridge doors

A

Permanent magnetic strip to keep it closed

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11
Q

How magnets are used in cranes

A

Induced electromagnets to attract and move magnetic materials e.g. scrap metal

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12
Q

How magnets are used in doorbells

A

Electromagnet is turned on and off rapidly
Arm is attracted and released repeatedly
Arm strikes bell to produce ringing noise

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13
Q

How magnets are used in magnetic separators

A

Magnet attracts scrap metal in recycling plants

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14
Q

How magnets are used in Maglev trains

A

Magnetic repulsion used to make train float above track (reduce friction)
Also used to propel train along

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15
Q

How magnets are used in MRI scans

A

Magnetic fields used to create images inside of body

No ionising radiation required (less hazardous)

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16
Q

Magnetic fields and current

A

When current flows, a magnetic field is made
The field is made up of concentric circles perpendicular to the wire
Changing direction of current changes direction of field

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17
Q

Factors of magnetic fields made by current

A

Larger the current = stronger field

Closer to wire = stronger field

18
Q

Right hand thumb rule

A 
Thumb = direction of current
Fingers = direction of magnetic field
19
Q

Motor effect

A

When a current-carrying conductor is put between magnetic poles, the magnetic fields interact and create a force
Wire has to be at 90° to experience full force
Force acts the same relative to direction of magnetic field and current

20
Q

Fleming’s left hand rule

A

thuMb = motion
First finger = field
seCond finer = current

21
Q

Force action on a conductor formula

A

Force (N)= magnetic flux density (T)x current(A) x length(m)
Current has to be 90° to magnetic field for formula to be accurate

22
Q

How motors work

A

Current flows through current in a magnetic field
Forces act up or down on either side of coil, causing it to turn
Current is then temporarily cut off due to split ring commutator
Momentum causes coil to keep turning and current flows but in the opposite direction
Forces up and down are oppositely re-established so coil keeps turning in the same direction

23
Q

How direction of rotation in a motor can change

A

Swapping polarity of d.c. current (reversing current)

Swapping magnetic poles over (reversing field)

24
Q

How to work out direction of rotation in a motor

A

Current always goes positive to negative
Use Fleming’s left hand rule to work out direction of force
Work out direction of turning

25
Q

Solenoid definition

A

Long coil with lots of loops

Type of electromagnet

26
Q

Solenoid magnetic field facts

A

Inside of coil, lots of lines are pointing in same direction
Outside of coil, overlapping field lines cancel each other our so field is weak part from at the ends of solenoid
Field strength can be increased using an iron core inside the solenoid, as it is induced

27
Q

Electromagnetic induction definition

A

Creating potential difference in a wire experiencing a change in magnetic field

28
Q

How to cause electromagnetic induction

A

Rotating / moving a magnetic in a coil of wire
Rotating / moving a conductor in a magnetic field
Magnetic field through an electrical conductor changes
Induced p.d. / current always opposes the change that made it

29
Q

How to increase size of induced potential difference

A

Increasing strength of magnetic field
Increasing speed of movement / change of field
Increasing turns per length on coil of wire

30
Q

Purpose of transformers

A

Uses induction to change the potential difference in an alternating current

31
Q

How transformers work

A

Two coils of wire are joined with an iron core
Alternating p.d. is applied across primary coil
Produced alternating magnetic field alternates magnetisation in iron core
Changing magnetic field induces p.d. in secondary field

32
Q

Coils in step-up transformers

A

More coils on secondary than on primary

Increases p.d.

33
Q

Coils in step-down transformers

A

More turns on primary than on secondary

Decreases p.d.

34
Q

Power formula for transformers

A

potential difference across primary coil x current through primary coil = potential difference across primary coil x current through secondary coil

35
Q

How most power stations generate energy

A

Fuels burned to turn heat into steam, which turns a turbine
Turbine spins powerful magnet inside of generator (cylinder wrapper with coils of copper wire)
Large p.d. and alternating current induced in the coils
Coils joined together in parallel to create a single output from generator

36
Q

Why electricity is transferred with high voltage

A

Power = voltage x current
High voltage or high current needed for high power
High current means more energy lost through heat stores so high voltage is used to transfer lots of power

37
Q

How transformers are used in the national grid

A

Step-up transformers boost p.d. (400,000 V) at power stations
Step-down transformer lower p.d. to safer levels at consumers’ end

38
Q

Ratio between voltage and number of turns in transformers

A

input voltage / output voltage = number of turns on primary coil / number of turns on secondary coil

39
Q

How dynamos work

A

Force rotates a coil in a magnetic field
As coil spins, current is induced in the coil
Current changes direction every half-turn
Split-ring commutator swan connection every half-turn so current keeps flowing in the same direction

40
Q

How alternators work

A

Force is applied to rotate a coil in a magnetic field
As coil spins, current is induced in the coil
Alternators have slip rings and brushes so contacts don’t swap every half-turn
Alternating potential difference is induced, as well as a.c. current if in a complete circuit

41
Q

How microphones work

A

Sound waves hit flexible diaphragm attached to coil of wire
Coil of wire surrounds one pole of permanent magnet and is surrounded by other pole
As diaphragm moves, current is generated in the coil
Movement of coil depends on vibration of diaphragm

42
Q

How loudspeakers work

A

coil is attached to paper cone
Coil is wrapped around one pole of a permanent magnet so a.c. signal causes force on coil, moving cone
When current is reversed, force acts in opposite direction
Movements make come vibrate, causing variations in air pressure and creating a sound wave

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