P15 - Electromagnetism Flashcards
What is the difference between permanent and induced magnets?
-permanent magnets produce their own magnetic field (eg nickel, cobalt, iron, steel)
-induced magnets are materials that become magnetic when placed in a magnetic field, and always experiences a non-contact force of attraction due to it (it will lose its magnetism once removed from a magnetic field)
eg induced magnetism is felt by a paperclip near a permanent bar magnet, and the paperclip experiences a force and is attracted to the bar magnet - however, paperclips aren’t permanent magnets and don’t attract each other
What is a magnetic field?
the region around a magnet where a non-contact force can act on another magnetic material, where its strength depends on the distance from the magnet
The field is strongest at a magnet’s poles
What is the direction of a magnetic field defined as?
it is the direction of the force that would act on another North pole placed at that point (always goes from North to South)
You can use a plotting compass to determine it’s direction too
Explain how a compass works:
-the end of the needle that points roughly to geographical North is the North (seeking) pole of the magnet
-since opposites attract, the geographical North pole is technically the magnetic South pole of the Earth’s magnetic field
The North-seeking and North pole are the same thing
Describe what happens to a wire’s surroundings when a current flows through it:
-a magnetic field is produced
-direction can be found with right-hand grip rule
What factors affect the magnetic field strength when current flows through a straight wire?
-amount of current through wire
-distance from wire
What is a solenoid, and how can you determine the direction of its magnetic field?
-a coil of wire
-if the current around it on one face goes anticlockwise (whether entering OR leaving the coil), that face is North, while if it goes clockwise, it is South
You can also use the right hand grip rule for little parts along the initial turn to see how the field line’s directions all point inwards or outwards, representing a South or North pole respectively
How can you increase the magnetic field strength of a solenoid?
-add iron core (turns the solenoid into an electromagnet)
-increase current in wire
-add more turns in solenoid
What happens to the rods when the switch is closed?
-the rods become induced magnets as they are in the solenoid’s magnetic field
-they have the same polarity at each end, and so they repel each other and move apart
What is the motor effect?
-when a current-carrying conductor is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other
-their relative orientations can be determined with Fleming’s left hand rule
provided that all of the components are perpendicular to each other
Explain how to use Fleming’s left hand rule to determine the direction the copper rod will move in:
-hold the left hand’s thumb, index, and middle finger at right angles to each other
-index finger represents direction of the permanent magnet’s magnetic field (downwards)
-middle finger represents direction of current (coming out of the page)
-thumb represents the direction of the force, and it ends up pointing right, meaning the rod moves right
What is magnetic flux density?
a measure (in Teslas, T) of the strength of a magnetic field at a point in space
What factors affect the strength of the force on a current-carrying conductor in a magnetic field?
-magnetic flux density
-current
-length of wire
The length of the wire that is in the part of the field where the field lines are parallel
Explain how an electric motor works:
-coil of wire carrying DC is placed in a magnetic field, and experiences a force in opposite directions on either side, causing it to rotate a 1/2 turn
-split-ring commutator causes the positive and negative connections of the wire to switch every 1/2 turn, and thus the current is reversed
-reversed current after the half turn means that the coil will continue to rotate in the same direction as seen previously
How could you increase the rotational speed of an electric motor?
-add more turns in the coil
-increase current
-increase magnetic flux density
What does a loudspeaker do?
uses the motor effect to convert variations in current in electrical circuits into pressure variations in air for sound waves
How does a loudspeaker operate?
moving coil loudspeaker
-AC flows through coil, creating a magnetic field around it that interacts with the permanent magnetic field causing the coil and cone to move (by the motor effect)
-current reverses and varies, meaning the force that the cone experiences is reversed and changes in magnitude
-cone moves in and out very quickly, causing pressure variations (compressions/rarefactions) in the air, which are sound waves
Same structure as microphones
How could you induce a current in a conductor? Why does this happen, and what is the name of this effect?
-move a conductor through a magnetic field
-if there is a change in magnetic field around a conductor, a PD is induced across its ends
-if the conductor is connected into a closed circuit, then a current is induced (the generator effect, ie electromagnetic induction)
This also works if the magnet is the one moving
What factors could you change to increase the induced PD across a conductor with the generator effect?
-increase the permanent magnet’s field strength
-move the wire/magnets quicker, so the magnetic field they experience changes faster
-add more turns/make the wire into a coil
Describe what happens when a magnet that has been dropped through a solenoid - refer to the effects on the magnet and the solenoid:
-magnet is dropped into coil
-due to the generator effect, a PD is induced and therefore creates a current in the coil’s circuit
-the current creates a magnetic field around the coil that opposes the magnet’s polarity and resists its motion, thus creating a force that repels the magnet
-the magnet drops slowly through the coil
-coil’s polarity flips when the magnet leaves, as it wants to resist the falling motion of the magnet by attracting it
If the magnetic field from the coil were in the same direction as the dropped magnet, it would accelerate the magnet, creating energy out of nowhere.
Give 3 uses of the generator effect:
-alternators, to produce AC
-dynamos, to produce DC
-microphones
What is the difference between the structure of an alternator and a dynamo?
-the 2 slip rings and brushes in alternators mean that the contacts don’t swap every half turn (like in a motor/dynamo), meaning the PD it induces alternates
-the split-ring commutator on a dynamo allows it to produce a direct PD, as the contacts switch every half turn
Give one requirement for dynamos and alternators to work:
both require their coils to be turned in one direction
Explain why an alternating current is induced in the coil of an alternator:
-coil moves through the permanent magnetic field
-change in MF, so a PD is induced across the coil
-complete circuit in alternator, so a current is induced in the coil
-every ½ turn the PD reverses direction as the magnetic field’s direction flips (relative to the coil)
-this means the induced current also reverses direction every ½ turn
