Topic 9 - Magnetism Flashcards
magnetic field
- a region where magnets and magnetic materials experience a non contact force acting on them
2 poles of magnets
- North and South pole
- magnetic field lines go from north to south
how does field of magnet vary
- magnetic filed is strongest at the poles
- magnetic field stronger closer to the magnet, as field lines are closer together
what is the force between a magnetic material ( or induced magnet ) and a permanent magnet
always attractive
what is an induced magnet
- magnetic materials that turn into magnets when they’re in a magnetic field
- and lose their magnetism when you take away the magnetic field
what is a permanent magnet
a magnet that generates its’ own magnetic field
compass
- they have a tiny bar magnet and the north pole of this magnet is attracted to the south pole of any magnet it is near
how to prove the earth has a magnetic core
- when compasses are not near a magnet they always point north
- which proves that the Earth generates its own magnetic field
- so has a magnetic core
magnetic materials
- iron ( steel )
- cobalt
- nickel
electromagnetism
- when a current flows through a wire a magnetic field is created around it
right hand thumb rule
- find the direction of magnetic field depending on current
solenoid
-cylindrical coil of wire
- acts as a magnet when current flows through it
- field lines around each loop line up with each other and form magnetic field
how to increase strength of magnetic field around solenoid
- increase number of coils
- increase current through wire
- soft iron core ( combined magnetic fields )
motor effect
- a current carrying conductor and a magnet exert a force on each other
- while the conductor is inside the magnet’s magnetic field
how does a dc motor work ( P1)
- a loop of wire carrying a current generates a magnetic field around it. this is placed inside another m field
- two side arms experience forces in opposite directions which causes the same moment and so will start to rotate
how does a dc motor work ( P2)
- when the wire loop reaches a vertical position the forces will be acting one up and one down, so the loop gets stuck
- the split ring commutator reverses the contacts of the loop every half turn, which reverses the current
how does a dc motor work ( P3)
- therefore the forces acting on each arm revreses every half turn allowing rotation to continue in the same direction
what is a split-ring commutator
- conducting ring with gap between 2 halves
- it reverses the direction of the current every half turn ensuring that the loop continues rotating same direction
how does the split ring commutator fulfil its purpose
- as it rotates, the part of the commutator touching contacts changes every half turn
how can you increase the speed of a simple electric motor
- increasing current
- increasing strength of magnetic field
how do headphones work
- when a current flows through the coil it generates a magnetic field
- and interacts with the permanent magnetic field
how do headphones work
- producing a resultant force which causes the coil and cone to move
- when the direction of the current chnages the force changes, meaning the cone moves backwards and forwards
- this makes the air around cone vibrate creating variations in sound pressure
what is the generator effect?
- induction of a potential difference across a conductor experiencing a change in an external magnetic field
- which induces a current if the circuit is complete
2 ways to induce p.d in ends of conductor:
- moving electric conductor in a magnetic field
- moving the magnetic field relative to the electrical conductor
what happens if you move the magnet/conductor in opposite direction
- direction of the p.d and current reverses
how to change size of induced p.d
- change speed of movement, cutting more magnetic lines per second
- increasing strength of magnetic field
-coiling the wire into a solenoid
how do alternators produce a.c
- as the coil moves through the magnetic field it cuts through the magnetic field
- this induces a p.d
- as the coil is connected to a complete circuit, a current is also induced
- every half turn the induced p.d reverses direction
- so every half the direction of current reverses, producing a.c
what are the purpose of slip rings?
to make sure the wires don’t get twisted
provides a connection
how does dynamo produce d.c
- as the coil moves through the magnetic field it cuts through the magnetic field
- this induces a p.d
- as the coil is connected to a complete circuit, a current is also induced
- The split-ring commutator reverses the connections every half-turn, ensuring the current always flows in the same direction
- inducing p.d / current in same direction
microphones
- sound (waves) cause the diaphragm to vibrate
the diaphragm causes the coil / wire to vibrate - the coil / wire moves through the magnetic field and cuts magnetic field lines
- a potential difference is induced, therefore a current is produced (across the ends of the coil / wire)
what do transformers do?
- changes the p.d of electrical supply using generator effect
- only works on a.c
how transformers works?
- when a.c flows through primary coil, it produces an alternating magnetic field around it
- this magnetises the soft iron core
- the changing magnetic field cuts through the secondary coil
- inducing an alternating p.d in the secondary coil ( and a.c if connected to circuit )
step up transformers
- across secondary coil p.d is larger and more loops/turns
step down transformers
- across secondary coil p.d is smaller and less loops/turns
