Section 22: Magnetic Fields Flashcards
what are the poles of the magnet
- north
- south
what do all magnets produce
magnetic field
what is a magnetic field
a region where all magnets or other magnetic materials experience force
what direction do the magnetic fields go
north to south
what happens if the lines are close or further away from eachother
the closer the lines are together, the stronger the magnetic field
the further away from eachother, the weaker the magnetic field
where is the magnetic field the strongest
the magnetic field is the strongest at the poles of the magnetic meaning the magnetic forces are the strongest at the pole
how do you get the shape of a magnetic field
place a card over a magnet and sprinkle iron fillings onto it. the fillings line up with the field lines
HOWEVER it wont show the DIRECTION of the field
what do magnetic fields cause
it causes forces between magnets
what happens between the 2 magnets
between the 2 magnets the magnetic force can be REPULSIVE or ATTRACTIVE.
what happens if you place the north and south poles of two bar magnets near eachother
it creates a UNIFORM FIELD between the 2 poles
do the same poles repel or attract
repel
do the opposite poles attract or repel
attract
how do you find the direction of magnetic fields
inside a compass is a tiny bar magnet called a needle. a compass needle ALWAYS lines up with the magnetic field that it is in.
how do you use a compass to find what the field around a magnet looks like
you can use a compass to build up a picture of what the field around a magnet looks like:
1. put a magnet on a piece of paper and draw around it.
2. place the compass on the paper near the magnet, the needle will point in the direction of the field line at this position.
3. mark the direction of the compass needle by drawing 2 dots at each end of the needle.
4. then move the compass so that the tail end of the needle is where the tip of the needle was in the previous position and put a dot by the tip of the needle. repeat this and then join up the marks youve made, you will end up with a drawing of one field line around the magnet.
5. repeat this method at different points around the magnet to get several field lines.
- make sure to draw the arrows from north to south.
what happens when youre not near a magnet
the compasses always points towards the earths north pole.
why does the compass always point towards the earths north pole
this is because earth generates its own magnetic field. this shows that the core of the earth must be magnetic
what are the 3 magnetic elements
- iron
- nickel
- cobalt
what happens if you put a magnetic material near a magnet
the magnetic material is attracted to the magnet. some alloys and compounds of the magnetic elements are magnetic as it contains the element. the magnetic force between a magnet and a magnetic material is always attractive.
what are the 2 types of magnets can a metal be
permanent of induced
what is a permanent magnet
- magnets that produce their own magnetic field ALL THE TIME
what is a induced magnet
also known as temporary.
- magnets that only produce a magnetic field while theyre in another magnetic field. if you put any magnetic materials into a magnetic field- it will become an INDUCED magnet
why is the force between a magnet and a magnetic material always attractive
the south pole of the magnet INDUCES a north pole material
what happens when you take away the magnetic field from the induced magnets
the induced magnet will return back to normal and stop producing a magnetic field
- how quickly they lose their magnetism depends on the materials theyre made from
how long do magnetically SOFT materials lose their magnetism
very quickly
- pure iron
- nickel-iron alloys
how long do magnetically HARD materials lose their magnetism
very slowly
- permanent magnets are made from magnetically hard materials
some uses of magnetic materials
- fridge doors
- doorbells
- MRI machines
how are magnetic materials used in fridge doors
a permanent magnetic strip in your fridge door to keep it closed
how are magnetic materials used in doorbells
they use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise.
how are magnetic materials used in MRI machines
they use magnetic fields to create images of the inside of your body without having to use ionising radiation
what does a moving charge create
- a magnetic field
when a current flows through a long, straight conductor, a magnetic field is created around it.
what is the magnetic field made up of
concentric circles perpendicular to the wire, with the wire in the centre.
how do you change the direction of the magnetic field
changing the direction of the current
how do you work out which way the magnetic field goes
using the right hand thumb rule
how do you know if the field is strong or weak
the larger the current through the wire, or the closer to the wire you are the stronger the field is
what is the motor effect
when a current carrying wire in the presence of a magnetic field experiences a force
what happens if you put a wire between magnetic poles
when a current carrying conductor is put between magnetic poles, the two magnetic fields interact. the result is a force on the wire
how to experience the full force
- the wire has to be 90° to the magnetic field. if the wire runs along the magnetic, it wont experience any force at all.
- the force will always act in the same direction relative to the magnetic field, and the direction in the wire.
- changing the direction of either the current or magnetic field will change the direction of the force
how do you change the direction of the force
changing the direction of either the magnetic field or the current
what does the force acting on the conductor in a magnetic field depend on
- the magnetic flux density (field) - how many field lines there are in a region
- this shows the strength of the magnetic field - the size of the current through the conductor
- the length of the conductor thats in the magnetic field
how do you find the force acting on the current when it is at 90° to the magnetic field
F=BIL
F= force (N)
B=magnetic flux (T-tesla OR N/Am)
I=current (A)
L=length (m)
what is a solenoid
a long coil of wire.
you can increase the strength of the magnetic field produced by a length of wire- by wrapping it onto a long coil with lots of loops called a solenoid
it is an example of an electromagnet
what happens inside the solenoid
- inside the solenoid, you get lots of field lines pointing in the same direction. the magnetic field is strong and almost uniform. the magnetic field is the strongest inside
what happens outside the solenoid
outside the solenoid, the overlapping field lines cancel eachother out, so the field is weak apart from the ends of the solenoid
what is the direction of the field in a solenoid
the direction of the field depends on the direction of the current.
what is an electromagnet
a magnet with a magnetic field that can be turned on and off using an electric current
how do you increase the field strength of a solenoid
you can increase the field strength of the solenoid even more by putting a block iron in the centre of the coil. this iron core becomes an induced magnet whenever the current is flowing
what happens if you change the magnetic field
it induces a potential difference in a conductor
electromagnetic induction
the induction of a potential difference in a wire which is experiencing a change in magnetic field
what are the situations where you get electromagnetic induction
there are 2 situations where you get electromagnetic induction:
- if an electrical conductor and a magnetic field moves relative to eachother.
- you can do this by moving/rotating either a magnet in a coil of wire or a conductor in a magnetic field.
- if you move or rotate the magnet in the opposite direction, then the potential difference/current will be reverse
- if you keep the magnet moving backwards and forwards, or keep it rotating in the same direction you produce an alternating current
what are the situations where you get electromagnetic induction
you can also get induced potential difference when the magnetic field through an electrical conductor changes (gets bigger or smaller)
this is what happens in a transformer
1. you can increase the size of the induced potential difference by increasing the strength of the magnetic field, increasing the speed of the movement or change of the field or having more turns per unit length on the coil of wire.
2. the induced potential difference or current always opposes the change that made it:
- when a current is induced in a wire, that current produces its own magnetic field
- the magnetic field created by an induced current always acts against the change that made it.
so it is basically trying to return things to the way they were
what do transformers do
they change the potential difference but only for alternating currents.
- they use induction to change the size of the potential difference of an alternating current
features of a transformer
- primary coil
- secondary coil
- an iron coil in the middle of the coils
what happens when an alternating p.d. is applied across the primary coil
it produces an alternating magnetic field
what is the iron core
it is a magnetic material that is easily magnetised and demagnetised as the coil is producing an alternating magnetic field, the magnetesiation in the core also alternates.
what happens if you change the magnetic field
it induces a potential difference in the secondary coil
what is a step up transformer
increases the potential difference- steps the transformer up
- they have more turns on the secondary coil
what is a step down transformer
decreases the potential difference- steps the transformer down
- they have more turns on the primary coil
how do you assume the input power is equal to the output power
Vp x Ip = Vs x Is
Vp= p.d. in primary
Ip = current in primary
what do transformers in the national grid do
produces a high p.d. difference and a low current
1. once the electricity has been generated, it goes into the national grid
2. the national grid has to transfer loads on energy each second which means it transmits electricity at a high power
3. electrical power=current x p.d. so to transmit the huge amounts of power needed, you need a high potential difference or a high current
4. but a high current makes the wire heat up, so loads of energy is wasted to thermal stores
5. to reduce these losses and make the national grid more efficient, high voltage, low resistance cables and transformers are used.
6. STEP UP transformers at POWER STATIONS boost the p.d. up really high and keep the current really low
7. STEP DOWN transformers decreases it and brings it back down to safe, usable levels at the consumers end
what is the national grid
a network of wires and transformers that connects uk power stations to consumers
