Electromagnetic effects Flashcards
What always have magnetic fields around them?
current carrying conductors or moving electrons
What is the pattern of a magnetic field in straight wires?
circular and perpendicular to the wire, getting weaker with distance
How can you find the direction of a magnetic field in straight wires?
Right hand grip rule n1: your thumb is conventional current, and the rest of your fingers are the magnetic field direction
How do you know if the conventional current is going into or out of the paper?
cross: current into the page
dot: current out of the page
In a single wire, how can you increase the strength of the magnetic field?
greater current
What is the direction/pattern of the magnetic field in a solenoid?
the same as in a bar magnet
What is the relationship between current and direction of magnetic field on a solenoid?
the general direction of the current flow indicates the direction of the magnetic field
How can you find out the direction of the magnetic field in a solenoid?
right hand grip rule n2: your thumb points to the the north pole, the rest of your fingers are conventional current
How can you reverse magnetic field direction in a solenoid?
- reverse current/cell
- wrap wire in opposite direction
How can you increase the strength of the magnet in a solenoid?
- more turns of coil
- more current
- wrap wire on an iron core
Describe an experiment to identify the pattern/direction of a magnetic field in straight wires and solenoids.
- use a compass to determine the direction of a field at a single point
- draw an arrow in the direction of the field
- move the compass to different points and repeat
- you will have a diagram of the magnetic field
What is a relay?
electromagnetic switch, a circuit with a small current turns on another circuit with a large current
How does a relay work?
- switch is closed
- small current magnetised the coil
- iron reeds are attracted and close
- large current flows
What is the energy conversion in a loudspeaker?
it converts electrical energy into sound energy
How does a loudspeaker work?
- alternating current in coil creates an alternating magnetic field in coil
- this alternating magnetic field will interact with the permanent magnetic field
- this interaction will make the papercone move in/out producing sound waves
If you use high frequency ac in a loudspeaker, what will happen?
high frequency sound waves
If you use large voltage ac in a loudspeaker, what will happen?
loud sound waves
Why are relays useful?
the turn on larger current circuit by turning on a smaller current circuit: this way, we do not need to use thick, heavy, expensive cables to carry the large current
What happens if you supply a solenoid with AC?
the current will change, so the direction of the magnetic field will also change every half cycle
What is the variation of magnetic field strength in straight wires?
strongest closest to wire
What is the variation of magnetic field strength in solenoids?
uniform magnetic field inside coil
What happens if you increase the size of the current in a straight wire?
results in a stronger magnetic field
What happens if you increase the size of the current in a solenoid?
results in a stronger magnetic field
What happens if you reverse the current in a solenoid?
reverses magnetic field direction
What happens if you reverse the current in a straight wire?
reverses magnetic field direction
What happens whenever a current is at a right angle to a magnetic field?
it experiences a force, perpendicular to both the current and magnetic field
Why is a force experienced when a current carrying conductor is in a magnetic field?
interaction between the current’s magnetic field and the permanent magnetic field
How can we work out the direction of the force when there is a current carrying conductor in a magnetic field?
Fleming’s left hand rule:
First finger is magnetic field (N to S)
Second finger is conventional current
Thumb is the thrust/force
How can you reverse the force direction (current carrying conductor in a magnetic field)?
reverse current or reverse magnetic field direction
How can you increase the strength of the force (current carrying conductor in a magnetic field)?
stronger magnets, magnets closer together or increase current
What happens if you reverse the current in a current-carrying conductor in a magnetic field?
you reverse the force
What happens if you reverse the direction of the magnetic field in a current-carrying conductor in a magnetic field?
you reverse the force
How do you make a permanent magnet?
- place steel rod inside coil
- use high dc current for a time
- leave steel inside, turn off current
How can you demagnetise a magnet?
- place magnet inside coil
- use ac current
- slowly remove magnet with ac current still on
Describe an experiment to show the force in a current-carrying conductor in a magnetic field.
- Place a wire carrying current in between two permanent magnets
- turn on power supply
- observe force on wire
Will any moving charged particle in a magnetic field experience a force?
yes
Determine the direction of the force on charged particles in a magnetic field when the field is into the page.
positive particle: up
negative particle: down
Determine the direction of the current on charged particles in a magnetic field when the field is into the page.
positive particle: left to right
negative particle: right to left
What is the basic principle of a d.c motor?
a current + a magnetic field will create a force
How does a d.c motor work?
the sides of the coil have a magnetic field, which interacts with permanent magnetic field to produce a force
How can we identify the force direction in a d.c motor?
Fleming’s left hand rule: first finger is field, second finger is current and thumb is thrust
What causes the turning effect in a d.c motor?
a current-carrying coil in a magnetic field
Why do we have split-ring commutators on a d.c motor?
to reverse the direction of the current every half turn so that the coil rotates in the same direction
What would happen without split-rings?
the coil would flip backwards and forwards (the changing direction of current caused by the coil rotating through 180 would cause this)
Why do we need (carbon) brushes in d.c motors? Why are they good for this?
connect commutator to the circuit: low friction, good conductors
What do split rings do to the current?
reverse the direction of the current every half turn
What do brushes ensure?
that the wires don’t get twisted
How can the turning effect in a d/c motor be increased?
- increasing number of turns on coil
- increasing current
- increasing strength of magnetic field
What may happen if a conductor moves rapidly across a magnetic field?
creates induced emf or current or p.d
What is the basic principle of electromagnetic induction?
movement of a conductor + magnetism -> induced emf/current/pd
What would happen if a conductor cutting a magnetic field is not moving?
there will be no induced current
When is there a maximum of induced current in electromagnetic induction?
when the conductor is at 90 degrees to magnetic fields
What happens when a conductor is moving parallel to a magnetic field?
there will be no induced current
What are two situations in electromagnetic induction when there will be no induced current?
- when there is no movement
- when the conductor is moving parallel to the magnetic field
How can you reverse the current direction in electromagnetic induction?
- reverse wire movement direction
- reverse magnetic field direction
How can you increase the size of induced current in electromagnetic induction?
- stronger magnetic field
- move wire faster
- use a coil of wire
Describe an experiment to demonstrate electromagnetic induction.
- connect a wire to an ammeter or a galvanometer
- place two permanent magnets on either side of the wire
- move the wire
- watch what happens to ammeter/galvanometer
What is a galvanometer?
a very sensitive ammeter
Why does electromagnetic induction work?
the conductor cuts the magnetic field lines, forcing electrons in the wire to move as a current
How do we identify the induced current direction in electromagnetic induction?
Fleming’s right hand rule: first finger is field, second finger is current and thumb is thrust
What are two ways to induce an e.m.f (electromagnetic induction)?
- moving a magnet so its field lines are cut by a wire
- moving a wire across a magnetic field so that it cuts it
What is Lenz’s Law?
the direction of the induced emf (electromagnetic induction) opposes the change that creates it
How does the a.c generator work?
- coil made to rotate
- sides of coil cut magnetic field lines, inducing emf
- emf changes direction every half turn, because for half a turn one side moves up and for the other half turn the same side moves down
- the emf changes size because there is a max. induced emf when the coil is perpendicular
What is the simple idea behind ac generators?
movement of coil + magnetic field -> induced emf/current
How is the graph of induced emf vs time of an ac motor?
it goes up and down
What does a peak in an induced emf vs time graph of an ac motor mean?
long sides of coil are perpendicular to magnetic field, maximum emf is induced
What do the zeros in an induced emf vs time graph of an ac motor mean?
top and bottom sides of coil are parallel to magnetic field, so no emf is induced
What does a trough in an induced emf vs time graph of an ac motor mean?
long sides of coil move perpendicular to field in opposite direction: induced emf is maximum but in opposite direction
What is the function of slip rings in ac generators?
allow direction of induced emf to alternate, ergo causing an alternating current
connect coil to brush
What are slip rings?
cylindrical conductors that make constant contact with the coil during rotation
What is the function of carbon brushes in ac generators?
make electrical connection between rotating coil and circuit, avoiding wires becoming twisted
