6 Magnetism and electromagnetism Flashcards
what do magnets do
attract or repel other magnetic substances
what are the properties of a magnetically hard material
not easily magnitised or demagnitised
once it is magnetised it will stay magnetised for a long time even once the external field is removed
what are the properties of a magnetically soft material
easily magnetised and unmagnetised
won’t stay magnetised once the external field is removed
examples of a magnetically hard material
steel
examples of a magnetically soft material
iron
what is a magnetic field line
The region around a magnet where a force acts on another magnet or on a magnetic material
how is magnetism induced
it is induced in certain materials when they are placed in a magentic field
practical: investigate the shape and direction of the magnetic field around a bar magnet
place the magnet on a piece of paper
Draw a dot at one end of the magnet (near its corner)
Place a plotting compass next to the dot, so that one end of the needle of the compass points away from the dot
Use a pencil to draw a new dot at the other side of the compass needle
Move the compass so that it points away from the new dot, and repeat the process above
Keep repeating the previous process until there is a chain of dots going from one end of the magnet to the other
Then remove the compass, and link the dots using a smooth curve – this will be the magnetic field line
practical: investigate the shape of a magnetic field around a bar magnet
place the magnet on a piece of paper
gently scatter iron filings around the magnet
gently tap the piece of paper and the iron will move into the shape of the field
how to use 2 permenant magnets to form a uniform magnetic field
place the 2 magnets a few centimeters apart
have the north of one magnet point to the south of the other magnet and have them aligned
how to draw a uniform magnetic field
straight evenly spaced lines pointing from north to south
what does an electric current in a conductor produce
a magnetic field
construction of an electro magnet
a soft iron core wrapped with coiled wire connected to a cell/battery
what is the magnetic field pattern around a straight wire
field lines go in circles around the wire
use the right hand rule to work out the direction that the field lines are going in
what is the magnetic field pattern around a flat circular coil
it is circles around the wire
what is the magnetic field pattern around a soleniod
the same as a bar magnet
what happens to a charged particle when it moves parallel to the field lines
nothing it doesn’t experience a force
what happens to a charged particle when it moves perpendicular to the field lines
it will experience maxium force
what is flemings left hand rule
left hand
thumb represents the direction in which the force will act on the wire
first finger - the direction the magnetic field runs in (north to south)
middle finger - the direction the current is going in (+ -> -)
why is a force exerted on a current carrying wire in a magnetic field
the 2 magnet field (existing one + wire) interact
a force will act on the wire
how is d.c. used to create simple electric motors
Current flows in the wire/coil.
This creates a magnetic field around the wire/coil.
This magnetic field interacts with the field from the permanent magnet.
This produces a force on the wire/coil which moves the wire/coil.
equal forces in opposite directions produce a turning movement so the coil spins
The split-ring commutator changes the direction of the current every half turn as it spins. This reverses the direction of the forces, allowing the coil to continue spinning.
how is a.c. used to create simple loudspeakers
a cone is placed on the south pole of a circular magnet
the cone is wrapped in wire coils which are connected to a AC power supply
as current is passed through the wire it generates an magnetic field
the two fields interact and produce a force which causes the cone to oscillate
when the current switches direction the force on the cone reverses
this causes the cone to move in and out and generate sound waves
by changing the frequency of the AC supply we can change the frequency that the cone vibrates
the vibrations create sound waves
by increasing the current we increase the amplitude and loudness
how does the force on a current carrying conductor (eg wire) in a magnetic field change with the magnitude and direction of the field and current
If you increase the magnitude of the current through a wire or the size of the magnet being used, you increase the force on the wire.
If you change the direction of the current or reverse the poles of the magnet, you change the direction of the force on the wire
if you wind more coils of wire that will also increase the force
formula with input and output voltage, primary and secondary turns
input (primary) voltage / output (secondary) voltage = primary turns / secondary turns
formula with input power and output power
input power = output power
Vp x Ip = Vs x Is
Vp is the primary voltage
Ip is the primary current
Vs is the secondary voltage
Is is the secondary current
what is electromagnet induction
when 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
what factors change the size of voltage induced in electromagnetic induction
use a stronger magnet
use more coils of wire
move the wire faster through the magnetic field
how can electricity be generated by using electromagnetic induction
by rotating a magnet within a coil of wires
by rotating a coil of wires within a magnet
structure of a transformer
A basic transformer consists of:
A primary coil
A secondary coil
An iron core (as iron is magnetically soft)`
how does a transformer work
An alternating current is supplied to the primary coil
The current is continually changing direction
This means it will produce a changing magnetic field around the primary coil
The iron core is easily magnetised, so the changing magnetic field passes through it
This changing field cuts through the secondary coil and induces a potential difference (induction)
As the magnetic field is continually changing the potential difference induced will be alternating
The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil
If the secondary coil is part of a complete circuit it will cause an alternating current to flow
how does a transformer change the amount of voltage
by having different number of turns on the input and output side
what is the use of step-up and step-down transformers in the large scale generation and transmission of electrical energy
as having a high current causes lots of resistance it is more efficient to transport electricity at high voltage with low current as it causes less resistance for the same amount of power and so less wasted energy.
A step up transformer is used to increase the amount of voltage when it is being transported in the electricity pylons as this minimises lost energy. A step down transformer is then used to decrease the amount of voltage when the electricity is going into your home to a less dangerous amount that appliances can use.
