SP12: magnetism and the motor effect Flashcards
where is a magnet’s force the strongest?
near the magnets pole
rules of magnetism
opposites attract, likes repel
what type of force is a magnetic force?
a non-contact force
magnetic material examples
iron, steel, nickel and cobalt
what does it mean if something is magnetic?
able to be magnetised/attracted to a magnet
what is a permanent magnet?
-magnet made from a magnetic material
-its magnetism cannot be turned on or off
key features of a magnetic material:
-it produces its own magnetic field
-the magnetic field cannot be turned on and off – it is there all the time
examples of permanent magnets:
bar magnets and horseshoe magnets
what are induced magnets?
-a temporary magnet
-made from a magnetic material placed in a magnetic field
-the induced magnetism is lost when moved out of the magnetic field
examples of induced magnets:
iron filings
how do you test for magnetism?
a permanent magnet can:
-attract or repel another permanent magnet
-attract a magnetic material (but not repel it)
this means that you can only show that an object is a permanent magnet by checking if it repels another magnet
magnetic field
area around a magnet where it’s force can act on another magnet or magnetic material
can a magnetic field be seen?
no, it is invisible
how can you detect a magnetic field?
-using a magnetic compass
-a compass contains a small bar magnet on a pivot so that it can rotate
-the compass needle points in the direction of the earth’s magnetic field, or the magnetic field of a magnet
how to map out a magnetic field using a plotting compass:
- place the plotting compass near the magnet on a piece of paper
- mark the direction the compass needle points
- move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
- join the points to show the field lines
what produces the earth’s magnetic field?
the earth’s core, which is made from iron and nickel
direction of the magnetic field of a bar magnet:
from the north pole of the magnet to its south pole
direction of the earth’s magnetic field
from south to north
what happens when current flows in a wire?
it creates a circular magnetic field around the wire
what can the magnetic field created by a wire do?
-deflect the needle of a magnetic compass
where is the strength of a magnetic field created by a wire the strongest?
closer to the wire, it increases if the current increases
right hand rule
-the direction of the current and magnetic field can be found using the right hand grip rule
-coil the fingers of the right hand as if holding the handlebars of a bicycle, with the thumb pointing away from the hand
-the thumb indicates the direction of the current, and the fingers then indicate the direction of the magnetic field
what is a solenoid?
a long, thin coil of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it (the shape of the magnetic field is very similar to the field of a bar magnet)
what is the field inside of a solenoid like?
-strong and uniform
-the small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field
what is the field outside a solenoid like?
-the small magnetic fields from each wire cancel each other out and the outside field is much weaker than inside
the motor effect
-a wire carrying a current creates a magnetic field
-this can interact with another magnetic field, causing a force that pushes the wire at right angles
-there is no motor effect force if the current and magnetic field are parallel to each other
equation for the motor effect force
force = magnetic flux density × current × length
units for the equation for the motor effect force
-force in newtons (N)
-magnetic flux density in tesla (T)
-current in amperes/amps (A)
-length in metres (m)
2A flows through a 50 cm wire. calculate the force acting on the wire when it is placed at right angles in a 0.4 T magnetic field
F = BIl
F = 0.4 x 2 x 0.5
F = 0.4N
when does the force on a wire in a magnetic field increase?
-when the current in the wire increases
-when the strength of the magnetic field increases
when is the force on a wire in a magnetic field the strongest?
when the direction of the current is 90° to the direction of the magnetic field
fleming’s left hand rule
hold your thumb, forefinger and second finger at right angles to each other:
-the thumb shows the direction of the motor effect force on the conductor carrying the current (F)
-the pointer finger is lined up with magnetic field lines pointing from north to south (B)
the middle finger is lined up with the current pointing from positive to negative (I)
electric motor (detailed explanation using cognito video)
-a wire/coil with current entering at the positive terminal and exiting at the negative terminal
-one side of the coil experiences an upwards force while another experiences downwards force, this causes the coil to spin (either clockwise or anti-clockwise)
-when the coil has spun 180 degrees, the current is going in the opposite direction, so the forces also swap direction
-the coil then spins in the opposite direction to how it originally spun and goes back to where it stated
-to fix the fact that the coil can never turn 360 degrees, the direction of the current must be changed every half turn, so that the coil can continue to rotate in the same direction
-a split ring commutator is used to do this (it swaps the negative and positive connections every half turn, so that the direction of the current also swaps every half turn, this means the forces acting on the coil will always act in the same direction)
how can the speed of an electric motor be increased?
-increase the current that is passing through the wire
-add more turns to the coil
