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
Describe the shapes seen on an oscilloscope connected to an alternator and dynamo, and how they change when they rotate faster:
-alternator has a sinusoidal shape
-dynamo also has sinusoidal shape, but is always positive (abs)
-increasing the rotational speed of the coils will increase the frequency and amplitude of the waves seen
Interpret the oscilloscope graph for an alternator, referring to the positions of the coil:
-0 PD is induced when the plane of the coil is perpendicular to the MF, as the rotational forces are parallel to it, thus inducing no PD
-max PD induced when plane of the coil is parallel to the MF, since the rotational forces are perpendicular to it, inducing the maximum PD
What does a microphone do?
uses the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits
How does a microphone work?
moving coil microphone
-sound waves cause the diaphragm to vibrate, which causes the coil to also vibrate
-coil of wire moves through magnet’s magnetic field, inducing a PD and then a current by the generator effect
-the frequency/amplitude of the sound waves will determine how much the diaphragm vibrates and in turn the frequency/amplitude of the AC induced
Same structure as loudspeakers
Describe the structure of a microphone/loudspeaker:
coil wrapped around one pole of the permanent magnet, and is connect to the diaphragm/cone that can slide with the coil in/out
What does a transformer consist of?
insulated primary coil and secondary coil both wound around the same rectangular iron core, but on opposite sides
Coils are insulated so that it can’t conduct the electricity through the iron
Explain how a transformer works:
-AC in primary coil makes an alternating magnetic field around the coil
-iron core is exposed to this field and is easily magnetised, so it produces its own alternating magnetic field
-by the generator effect, an alternating PD is induced across the secondary coil (and if the circuit is complete on the secondary, it induces AC)
Why is an iron core used for a transformer?
it is easily magnetised
Describe the structural difference between step-up and step-down transformers:
-step-up transformers have more turns on the secondary coil
-step-down transformers have less turns on the secondary coil
Describe the mathematical relations between the voltage, current, and number of turns in a transformer, with 2 equations:
-the ratio between the voltages on the primary and secondary are equal to the ratio between the number of turns on the primary and secondary
-the power input is equal to the power output, so the voltage times current on the primary equals the voltage times current on the secondary
assuming 100% efficiency
