electricity and magnetism- topic 4 Flashcards
what happens when magnets are free to move
they line up with the earths poles
what are magnetic materials
iron and steel, nickel and cobalt
(materials that can be used to make magnets, can be magnetised easily)
what does it mean to be magnetically soft/ hard
soft= loses its magnetisim easily
hard= retains its magnetism
what are permenant magnets made of
steel, because its magnetically hard
attraction and repulsion law
oposites attract, like poles repulse
explain induced magnetism
stroke a steel nail with a permanent magnet
steel becomes a magnet
how may a magnet be demagnetised
heating and/ or hitting with a hammer
more effective if placing magnet inside ac coil and slowly removing it
magnetic field
region where a magnetic pole experiences a force
always from north to south
how are iron fillings used
to show the magnetic field lines around a magnet (from n to s)
what is the only test for magnets
repulsion
how do you make a permenant magnet
place steel rod inside coil use high dc current for some time, turn current off and then remove rod
how do you increase the strength of an electro magnet
use iron core
more current
more turns in wire
what are examples of electromagnet
electric bells
loud speakers
some microphones
scrap car lift
properties of permenant magnets vs temporary
permenant- permenantly mgnetised, made of steel, magnetism doesnt vary in strength, magnetic poles cant be altered
temporary magnets- temporarily magnetise, made of soft materials, magnetism can be varied in strength, magnet poles can be altered
state difference between magnetic or non magnetic materials
magnetic materials can also repulse along with attracting, non magnetic can only attract not repulse
what is the direction of a magnetic field at a point
the direction of a magnetic field at a
point is the direction of the force on the N pole of a magnet at that point
what are magnets
objects which experience attraction and repulsion
like poles repulse
unlike poles attract
describe magnetic materials
Experience a force when placed in a magnetic field
Are attracted to a magnet when unmagnetised
Can be magnetised to form a magnet
are not repulsed
describe non magnetic materials
Non-magnetic materials do not experience a force when placed in a magnetic field
describe uses for permenant magnets
compasses- navigation
toys, fridge magnets
describe electromagnets
Electromagnets use electricity to create a magnet from a current-carrying wire
They have the advantage that they can be magnetised and demagnetised, literally at the flick of a switch
They can be switched on and off
Soft iron is the metal normally used for this
It can easily become a temporary magnet
describe uses for electromagnets
mri scanners, speakers/ headphones,
how is the relative strength of a magnetic field represented in a drawing
by the proximity of the lines
closer= stronger
what are magnetic forces due to
interactions between magnetic fields
how do you test whether a material is a magnet
should be brought close to a known magnet
If it can be repelled by the known magnet then the material itself is a magnet
If it can only be attracted and not repelled then it is a magnetic material
difference between parmenant magnet and induced magnets
permenant- normally made of steel, produce its own magnetic field
It will not lose its magnetism
induced-One end of the material will become a north pole,The other end will become a south pole
Magnetic materials will always be attracted to a permanent magnet
This means that the end of the material closest to the magnet will have the opposite pole to magnets pole closest to the material
also can lose magnetism easily
induced magnetism
When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet
uniform magnetic field
same strength and direction at all point
equal spacing between lines and all should point same direction
what are two ways used to draw magnetic field lines
with iron fillings, or with compasses, put around magnet and draw
what are the two types of electric charges an object may have
positive or negative
what is law of attraction and repulsion
oppoistes attract like repell
electric charge is measure in…
coulumbs (C)
electrostatic repulsion is caused by…
force between charges
what are expirments to show electrosatic charges
charge a rod by friction eg. glass using a cloth or silk tie, then suspend it and put it near another charged material, see if repulses or attracts
charging of solids by friction involves
only a transfer of….
negative charge aka, electrons, only negative charges are able to move
what do all charged objects create
an electric field around tiself, shown by electric field lines, Fields lines always point away from positive charges and towards negative charges
The direction of the field lines in an electric field is described as:
The direction of the force on a positive charge at that point
electric field
a region in which an
electric charge experiences a force
Field Lines Between Two Oppositely Charged Parallel Conducting Plates
always from positive to negative, paraller straight lines
key difference between conductors and insulators
Conductors allow charge carriers to freely move
Insulators do not allow charge carriers to move
due to the difference in their internal structure
describe conductors
conductors carry charge easily, due to their strcture of postiive ions and sea of delocolised electrons
positive ions vibrate and electrons move freely, thus both carry charge easily
usually made of metals
describe insultors
do not carry charge qucikly, but carries some, as no delocolised electrons only way to carry charge is through ion vibration hitting neighbouring ion, (static electricity)
how do you test for conductors vs insulators
using gold leaf electroscope, charge plate of gle, gold leaf stands up, touch gle plate with item being tested;
leaf falls quickly- good conductor
leaf remains up- insultor/ bad conductor
leaf falls slowly- poor conductor
electric current
the amount of charge passing a point in a circuit every second
current (amps)= charge (culoumbs)/ time (seconds)
describe dc current
used in cells/ bateries
electrons flow in one direction only, from the negative terminal to the positive terminal
in graph of current X time, line is parallel to x axis, remains same the whole time
describe ac current
mains electricity or generators
The direction of electron flow changes direction regularly
in graph for current X time, ac current line is squigly pattern switching from positive to negative current
current is measure in
ammeters in amps which measure the amount of charge passing through them per unit time,
always connected in series
movement of convectional current
from psotive to negative
movement of electrons
from negative to positive
analogue ammeters
with a needle and scale
Typical ranges are 0.1-1.0 A and 1.0-5.0 A
could igve “zero errors” or parallax errors
digital ammeters
with an electronic read out
can measure very small currents
more accurate, more specific and accurate
may flicker between values
in metals currents are formed because of
flow of electrons which are delocolised, moving through the material
emf
The electrical work done by a source in moving a unit charge around a complete circuit
measure in volts
emf= work done ( joules )/ charge (culoumbs)
potential difference
The work done by a unit charge passing through a component measure in volts (v)
v=work done (joules)/ charge (culoumbs)
how is potential difference measure
using voltmeter either digital or analogue, connected in parallel with component being tested
equation for resistance
resistance (in ohms)= voltage (v)/ current (amps)
relationshiop between length and diameter area with resistance
bigger the legnth the bigger the resistance, direct porportion
bigger the diameter smaller the reisstance, inversely porpoprtional
state ohms law
current is porportional to voltage at a constant temp
describe voltage current graphs
for a reistor, it obeys ohms law, straight line from origin,
for a lamp, as the temp increases, ohms law isnt obeyed, graph is a curve (current in x and voltage in y, curve facing up, vs, current in y and voltage in x, curve facing down)
what happens to resistance and why when we icnrease temp
resistance increases, because metal ions oscillate more making harder for conduction electrons to travel
describe how electrical circuits transfer energy
from source of electrical energy such as a cell, or mains supply, to the circuit components and then into the surroundings as thermal energy
equations for power
power(watts)=voltage (volts) * current (amps)
power(watts)=work(joules/ work= change in energy)/ time (sec)
power= currnet. ˆ2 * resistance
power= voltage ^2/ resistance
equation for electrical energy
electrical energy (joules) =voltage (volts) * current (amps) * time (sec)
what is meant by killowhat hour
another unit for electrical energy used for payment of electricity
to calculate:
watts * time (seconds)
voltage in series
=voltage 1 + voltage 2
current in series
same everywhere
resistance in series
= reistance 1 + resistance 2
voltage in parallel
same verywhere
current in parallel
= current 1 +current 2
resistance in parallel
= resistance1 * resistance2/ resistance 1 +resistance 2
advantages of parallel circuits
all components can be switched off independatly
if one lamp breaks others still work
resistance is smaller so current in cell is greater
as resistance increases what happens to voltage in constant currnet
increases
what does a potential divider do
controls output voltage altering ratio fo resitance accros differnet components eg. light dimmers
equation for potential dividers
Vout= (r1/ r1 + r2) * Vin
transducers
device that changes forms of energy eg. thermistors and LDR
thermistor
type of transducer
high temp- low resistance
low temp- high resistance
LDR
type of transducer
light- low resistance
dark- high resistance
diode
semi conductor allows curent to pass in one direction only, follows direction of convectional current to work
half wave retrification
when diode converts ac-dc, but only receives positive current= 1/2 wave
in graph of voltage against time, current waves only in positive side however with straight line in 0 between each full wave
full wave retrification
diode converts ac-dc, in graph plotting voltage agaisnt time, no space between waves of ac only in positve side
damaged insulation
short ciruit can cause heat and fire
risk of electric shock
overheated cables
too much current causes heat
damp conditions
water conducts electricity better than air but less well than metals
excess current
from overloading of plugs,
extension leads, single and multiple sockets
when using a mains supply can lead to overheating cables, and fires
what are fuses
thin metal wires, melt or blow when too much current flow through them, thus cutting off the circuit
symbol= rectangle with line in the middle
trip switches
opens a switch when too much current flows, works faster than a fuse, circuit can be reset,
too much current coil becomes a strong magnet
when is no earth wire needed
when electrical device has plastic insulation in between wires and case= double insulation
mains electricity 3 wires
live wire- has alternating voltage, fuse and switch, brown
neutral wire- kept at 0V, blue
earth wire- connects case of electrical apparatus, together with fuse earth wire will cut off electricity supply if fault with electrical device, to prevent shock, yellow and green
what do all current carrying conductors or moving electrons have
a magnetic field around them
how do you draw magnetic fields around a wire
magnetic field stronger closest to the wire thus when drawing lines are closer together
greater current results in a stronger field
right hand grip rule 1
thumb-convectional current from positive to negative
fingers- magnetic field direction
fight hand grip rule 2
for solenoids
thumb= points to north pole
fingers= conventional currne tin coil
how do you reverse magnetic field direction
reverse cell
wrap wire in opposite direction
how do you icnrease srength of magnet
more turns of coil
more current
wrap wire on an iron core
whenever a current is at right angles to magnetic field
experiences a force at right angles to both, due to interaction between the currents magnetic field and permenant magnetic field, work out the direction of the force using flemings left hand rule
flemings left hand rule
first finger= n-s
second finger= convectional current
thumb= force
force between 2 current carrying wires
like current attract
opposites repel
magnetic field of one wire itneracts with the magnetic field of the second wire cuaisng wires to either attract or repel
what is a relay
a magnetic switch, when a circuit with small current turn on another with large current
1- switch is closed, small current magnitises coil
2- iron reeds are attracted and close, current flows to other circuit
if relay not used thick heavy expensive cables would be used instead
how do you make a permenant magnet
place steel rod inside a coil use high dc current for a time, turn off current and then remove rod
how to demagnetise a magnet
place magnet inside a coilusing ac current then slowly remove magnet from coil with ac left on
describe dc electric mottor
current + magnetic field = force
sides of coil have magnetic field which itneracts with permenant magnet producing a force
calculate force direction with felmmings eft hand rule
when coil vertical to magnetic field lines, no current, keeps moving due to inertia/momentum
max when horizontal/ parallel to magnetic field lines of permanent magnets
commutator= changes direction of current in coil every 1/2 turnso coil rotates in same direction and constantly
brushes- connects commutator to circuit
how do you reverse rotation of dc mottor
reverse current or magnetic field
how do you increase rotation speed of dc mottor
more current
more turns on coil
stronger magnetic field
describe the loudpseaker
converts electrical energy to sound energy
ac in coil makes alternating magnetic field in coil
which interacts with permenant magnetic field
makes paper cone move in and out producing sound waves
high freuqency ac- high frequency sound waves
large voltage ac- loud sound waves
what happens to a static electric when out in between two oppositely charged plates
accelerates towards positive plate
what happens to a moving electric when entering in between two oppositely charged plates
curves towards the psoitive plate
what happens to moving electrons in magnetic fields
flemings left hand rule (moves opposite to convectional current)
if into page (current), curves down
describe what happens when we move a current carrying wire through two magnets
movement of conductor + magnetism= induced current
conductor cuts the permenant magnetic field lines produced by permenant magnets, inducing a current, maximum induced when 90 degrees to magnetic field lines
no induced current when movement parallel to magnetic field lines
use flemings right hand rule
how do you reverce induced current direction
reverse wire cutting movement
reverse magnetic field direction
how do you increase induced current production
stronger magnetic field
use coil of wire
move wire more quickly
flemings right hand rule
first finger=north to south
second finger= convectional current
thumb= force on wire
describe a magnet mving into a solenoid
when either magnet or solenoid move, current is induced
magnetic field lines are cut by the coil whcih induces a current
when magnet is moved faster more field lines are cut per second increases the current produced, when magnet is pulled out induced current in opposite direction
more turns of coil or stronger magnetic field increases induced current due to more field line cutting
what is lenzs law
direction of idnuced emf in the coil opposes change causing it
pull magnet of coil out of solenoid, magnets and solenoids poles are opposite, when pulling magnet in, poles are the same
describe ac generator
movement of coil + magnetic field= induced emf
sides of coil cut magnetic field lines inducing an emf
the emf changes direction because for half a turn of the coil
emf changes size because there is a maximum induced emf when moving coil is horizontal= max rate of field line cutting, coil is vertical, no induced emf, sides of coil parallel to magnetic field lines, no cutting of lines
slip rings- connect coil to brush
how do you increase size of induced emf in ac generator
increase magnetic field strength
increase number of coils
increase rate of rotation of coil
why is a transformer used
to increase or decrease alternating p.d
so that power lins can carry a very high voltage and low current which results in low heat lost, most electricity reaches destination, so cheaper, less heavy cables used
explain how a transformer works
alternating p.d in primary circuit, produces an alternating amgentic field in primary coil
alternating magnetic field travels in iron core and cuts secondary coil , inducing an alternating p.d in secondary circuit
what are formulas used in transformers
number of turns primary coil/ number of truns secondary coil= primary voltage/ secondary voltage
voltage primary * current primary= voltage secondary * current secondary
describe mutual induction
when you close switch pulse of current flows out ammeter deflects, then zero current when switched is opened, current flows in opposite direction
this is because magnetic field lines grow and cut coil
