P2 - Magnetism and Electromagnetism Flashcards
Describe the differences between permanent and induced magnets
- permanent magnets induce their own magnetic field
- induced magnets become a magnet when placed in a magnetic field, when removed from the field they loose their magnetism quickly
How can a magnetic compass be used to plot the field around a bar magnet?
- place bar magnet on a piece of paper
- place compass at one end of magnet
- on paper, mark where the point of compass needle is
- move compass so the tail of the needle is at the point that has just been marked
- on paper, mark a new point where the needle now is
- release and connect the marks until the full field is plotted
What happens when a current flows in a conducting wire?
A magnetic field is produced around the wire
Right hand grip method
- grip wire with right hand, think pointing in direction of current
- finger curled around the wire will point in direction of magnetic field lines
What does the strength of a magnetic field depend on?
- size of current
- - distance from wire
What is a solenoid?
- wire looped in cylindrical coil
- this increases strength of magnetic field, creating a strong uniform field inside the solenoid
- adding an iron core can further increase the magnetic field strength (this would make it an electromagnet)
What does a solenoid increase magnetic field strength?
- concentrates longer piece of wire into a smaller area
- loops shape means that the magnetic field lines around wire are all in same direction
- similar to a bar magnet
How would you find the north pole of a solenoid?
right hand grip method
- hold solenoid in right hand with fingers following current direction
- thumb points to north pole
How does an electric bell work?
- when the switch is pushed the electromagnet is magnetised
- electromagnet attracts armature
- hammer strikes the gong, breaking the circuit
- armature springs back, completing the circuit again
- remagnetises electromagnet
- cycle repeats as button keeps being pushed
What is it called when a current carrying conductor is placed in a magnetic field and experiences a force?
the motor effect
How can the force in a field be increased?
- size of current
- length of conductor
- flux density
What is Fleming’s left hand rule?
Thumb = direction of force
second finger = magnetic field
middle finger = direction of current
(2 must be known to find another)
Explain what happens to a current carrying coil in a magnetic field
- rotates
- current going up one side is opposite to current coming back down
- used in electric motors
How is the direction of the coil kept in the same direction?
brush contacts at the commutator, so the motor keeps rotating and doesn’t stop in the upright position
(Fleming LH rule can be used on one side to see what direction the motor will rotate)
How do loudspeakers work?
- as current travels through coil it experiences a force due to magnetic field
- speaker cone moves
- current is alternating, so force direction changes creating oscillations
- increasing electrical power produces a larger force, so cone oscillates at high amplitude (louder)
- frequency of sound produced matches frequency of alternating current
- large cone is better for low pitch as size affects how quickly it can oscillate and size of oscillations
- kinetic energy is converted in to sound energy
What is the generator effect?
when conductors and magnetic fields interact, a potential difference can be induced by
- electrical conductor moving relative to magnetic field
- change in magnetic field around a conductor
How would you increase the size of an induced potential difference?
increasing speed of movement or size of magnetic field
An induced current generates a magnetic field that __________ the original change that produced it
opposes
What is the difference between an alternator and a dynamo?
Alternators generate an alternating current whereas dynamos generate a direct current
Describe how an alternator works
- rotating magnet is used with a fixed coil of wire
- as magnet rotates, the direction of the field alternates
- induces alternating potential and alternating current
Describe how a dynamo works
- the could rotates and the magnet is fixed
- as coil rotates, it generates a potential difference in one direction
- a split ring commutator means once it has passed upright the connections reverse so current output is always in the same direction
In an alternator or dynamo, when is the induced potential greatest?
when the magnetic field and coil are parallel ( magnetic field is being cut by rotating coil at fastest rate)
In dynamos and alternators, when is induced potential 0?
When the coil and field are perpendicular
How does a microphone work?
- sound waves hit microphone
- changes in air pressure related to sound wave cause microphone diaphragm to oscillate
- diaphragm and coil oscillate at some frequency as sound wave - bigger amplitude bigger vibration
- potential difference and current induced in coil with same frequency as incoming sound wave and an amplitude dependent on amplitude of wave
Describe a basic transformer
primary and secondary coil wrapped around a soft iron core
How does a transformer work?
- ac flows through primary coil (effectively a solenoid)
- ac current induces alternating magnetic field
- alternating magnetic field in iron core induced alternating potential difference in secondary coil
- if secondary coil is part of a compete circuit, ac will flow in to secondary coil
Compare a step up and step down transformer
step up
- increases potential difference
- more turns on secondary than primary
- Vs > Vp
step down
- reduces potential difference
- more turns on primary than secondary
Vs < Vp
