topic 12 - magnetism and the motor effect Flashcards
like poles
repel
unlike poles
attract
what are the 3 main magnetic materials
iron
cobalt
nickel
(some alloys and compounds of these metals are also magnetic)
what is the magnetic force between a magnet and magnetic material
always attractive (due to magnetic induction)
permanent magnet
magnets that produce their own magnetic field all of the time
induced/temporary megnats
only produce a magnetic field when they are IN another magnetic field
made by placing any magnetic material inside a magnetic field
once the field is removed, they lose their magnetism
magnetically soft (material example and meaning)
pure iron and nickel-iron alloys
lose magnetism very quickly
magnetically hard (material example and meaning)
steel
lose magnetism more slowly - used to make permanent magnets
magnetic field
the region around a magnet where other magnets or magnetic materials experience a force
field lines flow in what direction
north -> south
field lines show
which way a force would act on a north pole at that point in the field
where are the magnetic field and magnetic forces the strongest
at the poles
uniform magnetic field
a magnetic field where the field is the same strength everywhere
how can you investigate magnetic field of an object using plotting compasses (5)
1) place magnet on a piece of paper, draw around it
2) place the compass on the paper near the magnet - the needle will point in the direction of the field line in this position
3) mark direction of compass needle with two dots
4) move compass so that the tail end is where the tip is where it was in the previous position ; put a dot at the new tip
5) repeat and join up the dots to have one drawing of a field line around the magnet
6) repeat at different points around the magnet to get several field lines
compass
an object containing a needles that always lines up with the magnetic field it is in
why do compasses point north and how do we know our earth’s core is magnetic
the earth generates its own magnetic field
compasses point to follow the field lines from north to south
our geographical north is our magnetic south pole
this shows that the Earth’s core must be magnetic
what happens when a current flows through a long straight conductor
a magnetic field is created around it
the field consists of concentric circles around and perpendicular to the wire
right hand thumb rule (method)
point thumb in current direction and curl fingers
what does the right hand thumb rule tell you
the direction of the magnetic field
how can you increase the strength of the field around a long straight conductor (2)
- increase the size of the current
- go closer to the wire
where is the force on the wire strongest (motor effect)
when the wire is at 90° to the magnetic field
what happens if the wire runs along the field (motor effect)
no force is experienced at all
what does a ● or ✖ mean in diagrams about a wire
● means that the wire is coming towards you
✖ means that the wire is going away from you
motor effect
when a current-carrying conductor is place between magnetic poles, the two magnetic field interact to produce a force on the wire
the force from the motor effect always acts in the same direction relative to what
the magnetic field and the current direction of the wire
what does each finger in Fleming’s left hand rule indicate
First finger - Field
seCond finger - Current
thuMb - force/ Motion
size of force (motor effect) formula
force = magnetic flux density x current x length
what is magnetic flux density and what is it measured in
number of field lines in a given region
Tesla (T) or newtons per ampere per metre (N/Am
what situation does the size of force (motor effect) formula apply to
when the current is at 90° to the magnetic field it is in
how does a d.c MOTOR work
forces act on either side of a coil/wire carrying a current
these forces act in opposite directions on either side so the coil rotates
the split ring commutator swaps the contacts every half turn to keep the wire spinning
the direction of the motor can be reverse by swapping polarity or swapping poles over
solenoid
a long coil of wire
how can you increase the magnetic field strength of a single piece of wire
wrap it in a long coil with lots of loops
electromagnet + example
a magnet that can be turned on and off using electric current
eg. solenoid
what is the field like between individual wires in a solenoid
the field lines cancel each other out so the field is very weak apart from at solenoid ends
what is the field like inside the solenoid loops
the field lines add together to form a stronger and almost uniform field
what does direction of field in a solenoid depend on
direction of current
how can you increase the magnetic field strength of a solenoid and why
by putting a block of iron in the centre of the coil
this becomes an induced magnet whenever current is flowing
