unit six: magnetism and electromagnetism Flashcards
magnets definition
a metal solid that is able to have magnetic charges through north and south poles
magnetic materials def
metal substance able to be attracted or repelled by a magnet
non - magnetic material
metal substance unable to be attracted or repelled by a magnet.
give some magnetic metals
iron, nickel, cobalt and steel
steel contains iron so is also magnetic
what is a magnetic field
the region around a magnet where a force acts on another magnet or on a magnetic field
what are the two poles on a magnet
north and south
what are hard magnetic materials used for
for things like magnets that we want to stay magnetised
what are soft magnetic materials used for
when we need something to magnetise and demagnetise easily
what are permenant magnets usually made of
steel or a range of modern magnetically ‘hard ‘materials
like poles…
repel
unlike poles…
attract
what is a magnetic field
the area where a magnetic material experiences force
what are magnetic field lines
they point from north to south (always)
how does the electric motor work
the current going through a loop of wire makes one side go up and one side go down so it spins. a split ring changes the direction of the current so that the rotation continues.
what machines make electricity
generators and alternators
how does a dynamo work
a little dynamo wheel presses up against the big bicycle wheel which makes it spin around. the dynamo wheel is attached to a magnet which spins. a solenoid is next to the magnet, and the moving magnetic field induces a current in the solenoid, which powers the lightbulb.
input voltage / output voltage =
transformer equation
number of turns on primary coil / number of turns on secondary coil
why are powerlines high voltage
if you have a large voltage, you can transfer the electrical energy with a small current. having a small current minimises heat loss.
electric power equation =
power = voltage x current P = V I
voltage =
V =
current x resistance
V = I x R
power =
P =
current squared x resistance
P = I^2 x R
which direction do magnetic field lines point
from north to south
if you place the north and south end of two permanent bar magnets near each other what do you create
a uniform magnetic field
what do compasses and iron align themselves with
magnetic fields
how can you set up an experiment to see magnetic field lines
put the magnets under a piece of paper, scatter with iron filings, and then tap until the iron filings form a clear pattern. or put the compass at the pole of a permanent magnet and follow the north arrow.
magnets affect…
magnetic materials and other magnets
both poles attracct…
magnetic materials
when magnetic materials are brought near to a magnet (into it’s magnetic field), that material acts as a…
magnet
what is it called when a magnetic material is brought near a permanent magnet and acts as a magnet
induced magnetism by the original magnet
does magnetism get stronger or weaker when you bring the magnets together
stronger
what does an electric current produce
a magnetic field
the larger the electric field, the stronger the…
magnetic field
what does the direction of the magnetic field depend on
the direction of the current
what is a solenoid
a coil of wire
describe the magnetic field inside a current-carrying solenoid
strong and uniform.
what is the magnetic field like outside a solenoid
like a bar magnet’s
so the ends of a solenoid act as the
north and south poles
what is the current-carrying solenoid called now as the ends act like a bar magnet
an electromagnet
soft magnetic material
if it loses it’s induced magnetism quickly
hard magnetic material
if it keeps it’s induced magnetism quickly
example of a soft magnetic material
iron
example of a hard magnetic material
steel
how do you increase the strength of an electromagnet
increase the number of coils, increase the current or add soft iron core through the middle,
what does your thumb represent in the right-hand grip rule
direction of current
what do your fingers represent in the right-hand grip rule
direction of the magnetic field
when a charged particle moves through a magnetic field and it’s motion is not parallel, what does it experience
a force
explain the motor effect
when a current is passed through a wire placed in a magnetic field, a force is produced which acts on the wire.
what does a dot as a wire mean
the current is coming towards you or ‘out of the page’
what does a cross as a a wire mean
the current is leaving the page, and going away from you.
how do you make the force created using the motor effect stonger
increase the strength of the magnetic field or increase the current
what does the thumb represent in fleming’s left hand rul
motion/force
what does the index finger represent in fleming’s left hand rule
direction of magnetic field
what does the middle finger represent in fleming’s left hand rule
current
if you reverse the current or the magnetic field what happens
the direction of the force reverses.
how does a loudspeaker work (1)
electric currents from a source, such as radio, pass through the coils of a speaker.
how does a loudspeaker work (2)
these currents, which represent sounds, are always changing in size and direction, like vibrating sound waves.
how does a loudpeaker work (3)
the field of the coil and the permanent magnet are therefore creating magnetic field patterns which are also changing in strength and direction.
how does a loudspeaker work (4)
these fields in turn apply rapidly changing forces to the wires of the coil, which cause the speaker cone to vibrate..
how does a loudspeaker work (5)
these vibrations create the soundwaves we hear.
does a loudspeaker uses d.c. or a.c.
alternating current
what does a loudspeaker consist of
electrical signals produced by an amplifier fed into a coil of wire in the speaker, which is wrapped around the base of a cone.
what does a simple electric motor consist of
a single turn coil of wire that is free to rotate in a magnetic field about an axle. carbon brushes make contact with the ends of the coil that are connected to a commutator so that current can be passed through the coil.
what speeds up an electric motor
more current
more turns on the coil
stronger magnetic field
a soft iron core in the coil
explain why the electric motor’s coil rotates
when there is current in the loop of wire, one side of it will experience a force pushing it upwards, and the other will experience a force pushing it downwards, so the loop will begin to rotate.
explain what happens as the loop in an electric motor reaches its vertical position
the momentum takes it past vertical. if the rotation is to continue, the forces on the wires must be reversed so that the wire at the top is pushed down and the wire at the bottom must be pushed up.
how do we continue rotation of the loop in an electric motor
use a split ring to connect the loop of wire to the electrical supply. now each time the loop of wire passes the vertical position, the connections change, the direction of the current changes, and the forces on the different sides of the loop change direction.
what is electromagnetic induction
when a conductor is in a changing magnetic field a voltage will be induced in the conductor
how do you increase the size of the induced voltage
move the wire faster
use a stronger magnet so that there are more field lines ‘cut’
wrap the wire into a coil so that more pieces of wire move through the magnetic field
explain electromagnetic induction
if a wire is moved across a magnetic field at right angles, a voltage is induced or generated in the wire. if the wire is part of a complete circuit there is current.
how ELSE can you generate a voltage and current using induction
push a magnet into a coil. the cutting action between the field lines of the moving magnet and of the coil occurs.
a voltage is induced when a conductor…
cuts through magnetic field lines
a voltage is induced when magnetic field lines…
cut through a conductor
the faster the lines are cut…
the larger the induced voltage.
what type of current does a loudspeaker have to use
an alternating current
describe a bicycle dynamo
as the cyclist pedals, the wheel rotates and makes a small magnet within the dynamo turn around. as this magnet turns, its magnetic field turns too. the field lines cut through the coil inducing a current in it. this current can be used to work the cyclists lights.
how do larger a.c. generators work
generators rotate a coil in the magnetic field. as the coil spins, a current is induced tht changes direction every half turn. instead of a commutator, they have slip rings and brushes so that the contacts don’t swap every half turn. this means they produce a.c. voltage, which results in higher overall voltage and more peaks.
what does a transformer consist of
a core made from thin sheets of a magnetically soft material clamped together. two separate coils of wire, insulated from one another, are tightly wound onto the core.
how does a transformer work
it can be a step up or a step down. when an alternating voltage is applied across the primary coil (from an a.c. supply), the rapidly changing magnetic field will cause the magnetically soft core to magnetise and demagnetise quickly. this induces an a.c. current in the secondary coil.
step up transformer:
increase the voltage
(decrease the current)
more coils on secondary coil than primary coil.
step down transformer:
decrease the voltage
(increase the current)
they have more turns on the primary coil than on the secondary.
input (primary) voltage / output secondary voltage =
number of turns on primary / number of turns on secondary
Vp / Vs =
Np / Ns
power =
voltage x current
P =
V x I
how efficent are transformers
100%
input power =
output power
Vp Ip =
Vs Is
why does the national grid use transformers
to minimise energy losses through heat as current heats the wires
P =
I squared x R
describe the transmission of mains electricy by the national grid
power stations (25 kV) –> step up transformer –> power lines (132 kV) –> step down transformer –> homes and factories (0.23 kV)
how do transformers increase voltage and decrease current
P = VI so the higher the voltage, the lower the current. (so the lower the voltage the higher the current).
why are step down transformers used at the end of the national grid
to reduce the voltage so it’s safer and easier to use.
why iron
it’s a soft magnetic material which is easily magnetised but does not retain it’s magnetism
what is meant by magnetic field line
a line that shows the direction of the magnetic force/field
how do you know from a data logger if the voltage is alternating/varying
it switches from being positive to negative
describe the changes in a data logger when instead of one magnet being bounced through coil, it is two magnets
larger amplitude
lower frequency
alternate trace that diminishes
how would curved magnets and a piece iron improve the performance of an electric motor
the force will be increased by a stronger field
radial magnetic field
coil remains in the field for a longer time
when there is a current in the wire what is created around it
a magnetic field
describe the magnetic field formed around a current carrying wire
quite weak and circular
if the wire is made itnto a flat, single turn coil the magnetic field around the wire…
changes shape to go around the wire
how do you increase the strength of a magnetic field around a current carrying wire
increasing the current in the wire
wrapping the wire into a coil or solenoid
