magnetism and electromagnetism 6 Flashcards
what do all magnets have?
north and south poles
what are magnetic field lines?
used to show the size and direction of magnetic fields
always point from north to south
what happens if you place two permanent bar magnets near each other (south and north)?
creates a uniform field between magnets
how can a material become magnetised?
by placing an object in a magnetic field
domains align
what are some properties of magnetically soft materials?
easy to magnetise and demagnetise
iron
electromagnets
what are some properties of magnetically hard materials?
hard to magnetise and demagnetise
steel
permanent magnets
whats the law of magnetism?
opposite poles attract
like poles repel
do field lines go in or out of north and south poles?
south - in
north - out
practical
finding magnetic field lines using a compass:
compass shows pattern and direction
- carry out on a blank piece of paper
- bar magnet is placed in the centre of the paper
- start at the edge of the north pole and put a dot where the arrow of compass Is pointing
- repeat this method until you get to the south pole
practical
finding magnetic field lines using iron fillings:
iron fillings show the pattern
- place magnet under piece of paper
- sprinkle iron on top of paper
- iron become induced magnets and lines up with field
- tap paper until iron fillings create a clear pattern
how do you create a uniform field with two bar magnets?
place two magnets next to each other with the north and south pole facing
must be close but not too close
field lines will be evenly spaced and parallel
what do magnetic field lines never do?
intersect
what is a magnetic field?
a region where magnetic materials experience a force
like poles…
repel each other
opposite poles…
attract each other
how is magnetism induced?
- magnets affect magnetic materials and other magnets
- poles attract magnetic materials that aren’t magnets
- when magnetic materials are brought near to a magnet (into its magnetic field) that material acts as a magnet
- the original magnet has induced this magnetism
- the closer the magnet and the magnetic material get, the stronger the induced magnetism will be
how does a current-carrying wire create a magnetic field?
- an electric current in a conductor produces a magnetic field around it
- the larger the electric current, the stronger the magnetic field
- the direction of the magnetic field depends on the direction of the current
what is the magnetic field around a straight wire?
the field is made up of concentric circles with the wire in the centre
what is the magnetic field around a flat circular coil?
similar to that of a bar magnet
there are concentric ellipses (stretched circles) of magnetic field lines around the coil
what is the magnetic field around a solenoid?
the magnetic field inside a current-carrying solenoid (coil of wire) is strong and uniform
- outside the coil, the field is just like the one around a bar magnet
- this means that the ends of a solenoid act like the north and south poles of a bar magnet
this type of magnet is called an electromagnet
how can you increase the strength of a magnetic field around a solenoid?
by adding a magnetically soft iron core through the middle of the core
how does current in a magnetic field experience a force?
- when a current-carrying wire is put between magnetic poles, the two magnetic fields affect one another
- this results in a force acting on the wire
- this can cause the wire to move
- this is called the motor effect
- this is due to charged particles moving through a magnetic field experiencing a force as long as they are not moving parallel to the field lines
explain the motor effect:
- the wire has to be at 90 degrees to the magnetic field (if the wire runs along the magnetic field it won’t experience any force at all)
- the force always acts in the same direction relative to the magnetic field of the magnets and direction of the current in the wire
- the strength of the force increases with the strength of the magnetic field
- the force also increases with the amount of current passing through the conductor
- reversing the current or the magnetic field also reverses the direction of the force
what is fleming’s left-hand rule?
helps you work out motor effect
- LEFT hand
- first finger = direction of the field
- second finger = direction of current
- thumb = direction of the force
what 4 factors can speed up the motor effect?
- more current
- more turns on the coil
- stronger magnetic field
- a soft iron core in the coil
what is a dc electric motor?
- when a force is acting on two sides of the coil
- the forces are the usual ones that act on any current in a magnetic field
- the coil is a spindle and the forces act one up and one down it rotates
- the split-ring commutator is a clever way of swapping the contacts every half-turn to keep the motor rotating in the same direction
- the direction of the motor can be reversed either by swapping the polarity of the DC supply or swapping the magnetic fields over
how do speakers work due to the motor effect?
- AC electrical signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone
- the coil is surrounded by a permanent magnet so that the AC signals cause a force on the coil and make it move back and forth
-these movements make the cone vibrate and this creates sounds
what is electromagnetic induction?
the creation of voltage (and maybe current) in a wire which is experiencing a change in the magnetic field
what is the dynamo effect?
- using electromagnetic induction to generate electricity using energy from kinetic energy stores is called the dynamo effect
dynamo effect:
what are the two different situations where you get electromagnetic induction?
- electrical conductor (a coil of wire is often used) moves through a magnetic field
- the magnetic field through an electrical conductor changes (gets bigger or smaller or reverses)
dynamo effect:
how can you test the two different situations where you get electromagnetic induction?
you can test by connecting an ammeter to a conductor and moving the conductor through a magnetic field
if the direction of movement is reversed then the induced voltage/current will be reversed too
electromagnetic induction:
how can you get a bigger voltage?
- increase the strength of the magnet
- increase the number of turns of the coil
- increase the speed of movement
how do AC generators work?
- generators rotate a coil in a magnetic field
- their construction is pretty much like a motor
- as the coil spins, a current is induced in the coil, this current changes direction every half-turn
- instead of a split ring commutator AC generators have slip rings and brushes so the contracts don’t swap every half-turn
- power stations use AC generators to produce electricity, they just get the energy needed to turn the coil or magnetic field in different ways
how do transformers work?
- transformers change the voltage of an alternating current
- they all have two coils, the primary and secondary joined by an iron coil
- when an alternating voltage is applied across the primary coil, the magnetically soft iron core magnetises and demagnetises quickly = this induces an alternating voltage in the secondary coil
- the ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils
what do step-up transformers do?
step-up transformers increase the voltage, they have more turns on the secondary coil than the primary coil;
what do step-down transformers do?
step-down transformers decrease the voltage, they have more turns on the primary coil than the secondary
how do you calculate the output voltage from a transformer from the input voltage and the number of turns on each coil?
input (primary) voltage / output (secondary) volatge = number of turns on primary / number of turns on secondary
transformers are nearly 100% efficient so…
power in = power out
primary voltage x primary current = secondary voltage x secondary current
how have transformers made transmitting mains electricity more efficient?
step-up and step-down transformers are used when transmitting electricity across the country
- voltage produced by power stations is too low to be transmitted efficiently so the lower the voltage the higher the current for a given amount of power therefore the current causes wires to heat up
- a step-up transformer is used to boost the voltage before it is transmitted
- step-down transformers are used at the end of the journey to reduce the voltage so its safer and more useful
