Magnetic Fields Flashcards
Magnetic field facts
All magnets produce magnetic fields Always North to South Closer lines = stronger field Further from magnet = weaker field Magnetic field is strongest at poles
Magnetic field definition
Region where other magnets or magnetic materials experience a force
What happens when 2 unlike poles are placed together
Attraction
Uniform field in between (same strength everywhere between the poles)
Draw at least 3 field lines with the same distance between each line
What happens when 2 like poles are placed together
Repulsion
Field lines curve away from each other
All magnetic materials
Anything with:
- Iron (e.g. steel)
- Nickel
- Cobalt
Permanent magnets definition
Produces magnetic field all the time
Induced magnets definition
Only produces magnetic field when in another magnetic field
Type of force between magnet and magnetic material
Always attractive because poles of magnetic induces unlike poles in magnetic material
How quickly induced magnets lose their magnetism
Magnetically “soft” e.g. pure iron, nickel-iron alloys = quickly
Magnetically “hard” e.g. steel = slowly
Permanent magnets are made from magnetically hard material
How magnets are used in fridge doors
Permanent magnetic strip to keep it closed
How magnets are used in cranes
Induced electromagnets to attract and move magnetic materials e.g. scrap metal
How magnets are used in doorbells
Electromagnet is turned on and off rapidly
Arm is attracted and released repeatedly
Arm strikes bell to produce ringing noise
How magnets are used in magnetic separators
Magnet attracts scrap metal in recycling plants
How magnets are used in Maglev trains
Magnetic repulsion used to make train float above track (reduce friction)
Also used to propel train along
How magnets are used in MRI scans
Magnetic fields used to create images inside of body
No ionising radiation required (less hazardous)
Magnetic fields and current
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
Factors of magnetic fields made by current
Larger the current = stronger field
Closer to wire = stronger field
Right hand thumb rule
Thumb = direction of current Fingers = direction of magnetic field
Motor effect
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
Fleming’s left hand rule
thuMb = motion
First finger = field
seCond finer = current
Force action on a conductor formula
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
How motors work
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
How direction of rotation in a motor can change
Swapping polarity of d.c. current (reversing current)
Swapping magnetic poles over (reversing field)
How to work out direction of rotation in a motor
Current always goes positive to negative
Use Fleming’s left hand rule to work out direction of force
Work out direction of turning
Solenoid definition
Long coil with lots of loops
Type of electromagnet
Solenoid magnetic field facts
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
Electromagnetic induction definition
Creating potential difference in a wire experiencing a change in magnetic field
How to cause electromagnetic induction
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
How to increase size of induced potential difference
Increasing strength of magnetic field
Increasing speed of movement / change of field
Increasing turns per length on coil of wire
Purpose of transformers
Uses induction to change the potential difference in an alternating current
How transformers work
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
Coils in step-up transformers
More coils on secondary than on primary
Increases p.d.
Coils in step-down transformers
More turns on primary than on secondary
Decreases p.d.
Power formula for transformers
potential difference across primary coil x current through primary coil = potential difference across primary coil x current through secondary coil
How most power stations generate energy
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
Why electricity is transferred with high voltage
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
How transformers are used in the national grid
Step-up transformers boost p.d. (400,000 V) at power stations
Step-down transformer lower p.d. to safer levels at consumers’ end
Ratio between voltage and number of turns in transformers
input voltage / output voltage = number of turns on primary coil / number of turns on secondary coil
How dynamos work
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
How alternators work
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
How microphones work
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
How loudspeakers work
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
