Topic 6 - Electric and Magnetic Fields Flashcards
What is a build up of static caused by?
Frictional forces
how does rubbing an insulator transfer electrons and create a static charge
negatively charged electrons are scraped off one material onto another
- as its an insulator the electrons aren’t free to move
- this builds up static electricity
- the material becomes electrically charged with a positive static.
Which charged particles are always moved
negative electrons
Electrically charged particles exert a ……on each other
force
how do forces of attraction change with distance
further means weaker
How can an electrically charged object attract uncharged objects (with example)
Rubbing a balloon on your head transfers electrons to the balloon leaving it negatively charged
-if you hold the balloon against a wall it will stick because the negatively charges on the wall are repelled and positive charges are attracted to negative balloon
How does attraction by induction occur
If a negatively charged object is used to charge a neutral object by induction, then the neutral object will create a positive charge.
(The charged object that is brought near will always repel like charges and attract opposite charges.)
What is earthing
sending current down to the earth
provides easy route for static charge so none builds up
How do spark occur
If a potential difference is large enough, electrons can jump a gap between charged object and earth making a spark
eg)lightning
Too much static causes….
sparks
Why might you get an electric shock getting out of a car
static charge builds up on car
static charge jumps to earthed conductor nearby being you and you get a shock
How do electrostatic sprayers work
- spray gun is charged, so paint droplets become negatively charged
- object being painted has an opposite charge to attract the fine sprays of paint
- gives an even coat as areas with paint become negative so theres no build up
- no paint shadows
why is static dangerous in fueling cars/aircraft
as fuel flows out of pipe into tank, static can build up which can lead to sparks which might cause an explosion in dusty places
Why is static on planes dangerous and how does it occur
- as a plane flys through the air, friction between the air and plane builds up causing plane to become charged
- this static can interfere with communication equipment
How does lighting form
raindrops and ice bump causing friction and leaves top of cloud positively charged and bottom negative which creates huge voltage and spark which jumps across gap to earth as lightning
-damage homes, start fires
How can objects be earthed to stop charge building up
by connecting charged object to the ground using conductor through earthing
- provides easy route for static charge
- no build up of static so no sparks
How do electrons flow to ground by earthing if ground charge is:
negative
positive
Negative= flow down the conductor
Positive= flow up the conductor
What do electric charges do
Create an electric field around any charged object
stronger as you get closer
How is the direction of an electric field defined
Field lines go FROM positive TO negative
What happens to a charged object is placed in an electric field? and what is it caused by?
it feels a force caused by two charged objects interacting
what happens field lines between charged objects point in opposite directions
They repel
How do you get a uniform field
between two positively charged parallel plates
How can sparking be explained with electric fields (and therefore static)
- When an object is statically charged it creates its own electric field
- interactions in this field cause sparking
eg) plastic comb becomes charged and produces an electric field which can interact with paper without touching them
What is ionisation in a electric field
Electrons in the air particles to be removed
When is air not an insulator
when its ionised enough that it becomes more conductive and sparks are made
What do magnets produce
magnetic fields
Where do field lines go to in a magent
north to south
the closer the field lines the …..the magnetic field
stronger
where is the magnetic field strongest at
the poles
What are the 3 main magnetic elements
iron, nickel, cobalt
some alloys containing these will also be magnetic like steel
What is a permanent magnet
produces its own magentic field
What is an induced magnet
only produces a field when in another magnetic field
putting any magnetic material into a magnetic field it becomes an induced magnet
What is magnetic induction
force between magnet and magnetic material is always attractive
What does magnetically soft mean and give examples
Lose their magnetism very quickly
pure iron and nickel iron alloys
what does magentically hard mean and give examples
lose there magnetism more slowly
permanent magnets are made from magnetically hard materials
Name some uses of magnetic fields
Doorbells- electromagnets that turn on and off rapidly to repeatedly attract and release an arm which strikes a metal bell
MRI machines- magnetic fields to creat images inside your body without having to use ionsing radiation
A moving charge creates a
magnetic field
explain the right hand rule
thumb represents current curled up fingers represent the direction of the magnetic field at right angles to the current
in order to experience full force in a motor the wire needs to be at……
90 degrees to the magnetic field
if it runs along field it wont experience any force
What is flemmings left hand rule
Used to explain the motor effect
Thumb = motion
first finger = field
second finger = current
How can you find the size of a force
Force = magnetic flux density x current x length (of conductor)
Describe how a motor works
- forces (which act on current in magnetic field) act on two arms of a current carrying coil
- they act in opposite directions so coil rotates
- the split ring commutator swaps the contacts every half turn to keep motor rotating in same direction
how can the direction of a motor be reversed
by swapping the polarity of the D.C supply or swapping the magnetic poles over
What is a solenoid
describe the field lines:
a long coil of current carrying wire
- the field lines around each separate coil of wire line up in same direction so the field is strong and uniform
why is a solenoid an electromagnet
-its an electromagnet as field can be turned off using electric current
how can you increase strength of solenoid
by putting a block of iron in the center of coil which becomes an induced magnet
what is electromagnetic induction in terms of Potential difference
induction of potential difference in a wire which is experiencing a change in magnetic field
what two situations can you get electromagnetic induction
when electrical conductor (coil of wire) and magnetic field are relative to each other
-this can be done by rotating/ moving/ changing polarity of magnet
when magnetic field through conductor changes (like in a transformer)
-this can be done by increasing STRENGTH of magnetic field, increasing SPEED, increasing TURNS ON COIL
How does a transformer work
-use electromagnetic induction to change size of Potential difference of an alternating current by: increasing number of coils on secondary coil than primary coil in step down
How do dynamos work
- generate a direct current by applying a force to rotate coil in magentic field (like in a motor)
- current is induced in coil which changes direction every half turn due to split ring communicator to keep current flowing in same direction
How do Alternators work
- similar way to dynamos, force applied to rotate coil in magnetic field with an induced current in coil
- but instead of split ring has slip rings and brushes so the contacts dont swap every half turn which produces and alternating current
How do microphones work
- using electromagnetic induction
- sound waves hit diaphragm which is attached to coil of wire which makes both move creating a current in the coil
-
How do loudspeaker work
-coil wrapped around pole of permanent magnet
so a.c signal causes a force on the coil
-when current reversed the force acts i different direction
-this moves the paper cone as it vibrates it as pressure causes sound waves
Describe how a power station generates electricity for the national grid
- burning fuel heats water which creates steam
- steam powers turbine
- turbine connected to powerful magnet inside generator
- the magnet spins, inducing a large voltage and a.c current and single output generated
- generator joined to national grid
How are transformers used
The high current heats up wires so energy can be wasted so to reduce this transformers are used and high voltage cables.
- Step up make Voltage really high at 400,000 V and keep current low (usually in less comercial areas like across country
- step down decrease voltage to safe usable levels for consumers like in towns and homes
