Electromagnetism Flashcards
Features of Magnets
-North and South Poles
-Same Poles repel
-Opposite poles attract
Permanent Magnets
Always magnetic, always have poles
Induced Magnets
<Materials that are “magnetic” but do not have fixed poles -
<These can be made into temporary magnets by ‘stroking’ them with a permanent magnet
What do Induced Magnets align and give examples
o These align the domains in the material all in the same
direction, creating a temporary magnet
o Iron, Nickel, Cobalt
Magnetic Fields
- Field Lines point from North to South
- Strength decreases with distance from the magnet
- Direction always points to south pole and away from north pole, at any point
- Use Plotting Compasses
o Small compasses which show the direction of the magnetic field at a certain point
Earth’s Core
- The core is magnetic, and creates a large magnetic field around the Earth
- We know this because a freely suspended magnetic compass will align itself with the earth’s field lines and point North
- It doesn’t point to the Geographic North pole – it is over North Canada
- Also, the compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth’s ‘North pole’
o However this cannot be right, as like poles repel
o So in fact, Earth’s magnetic pole above Canada is a magnetic South Pole! (and the geographic south pole is close to the Magnetic North Pole)
What does a current produce around the wire?
Current produces a magnetic field around the wire
How is the direction of the current dictated?
<The direction is dictated by the “right hand grip rule”
<Plotting compasses on a piece of paper through which a wire is pierced shows this
Describe the relationship between current and the strength of a magnetic field
Greater current, stronger magnetic field
Describe the relationship between distance and the strength of a magnetic field
Greater distance from wire, weaker field
What is the magnetic field shape of a solenoid like?
Magnetic field shape is similar to a bar magnet
Explain the function of a solenoid
It enhances the magnetic effect as coiling the wire causes the field to align and form a giant single field, rather than lots of them all perpendicular to the direction of the current
What does having an iron core in the centre of a solenoid do?
Having an iron core in the centre increases its strength as it is easier for magnetic field lines to pass through than air
Name the factors that affect the strength of a magnetic field of a solenoid
· Size of current
· Length
. Cross sectional area
. Number of turns (coils)
· Using a soft iron core
Explain the motor effect?
<Two magnets will interact, feeling a magnetic force of attraction/repulsion
<So a magnet and a wire will also exert a force, as the two magnetic fields (generated by the magnet and the current in the wire) will also interact
o The magnetic field around a wire is circular, but the magnetic field between two magnets is straight
o When the two interact, the wire is pushed away from the field between the poles (at right angles to the wire direction and the field direction)
- To visualise
o Fixed permanent magnets have field lines along the x axis, as the
magnets are at A and B and the field lines are shown
o Wire is along y axis, where current is moving up from C to
D
o The Force felt on the wire is at right angles to both the direction of the
current and magnetic field lines
Along the z axis
To visualise go on pmt to see the image and explain from memory
- To visualise
o Fixed permanent magnets have field lines along the x axis, as the
magnets are at A and B and the field lines are shown
o Wire is along y axis, where current is moving up from C to D
o The Force felt on the wire is at right angles to both the direction of the current and magnetic field lines
Along the z axis
Explain how to use Fleming’s Left Hand Rule
-Each direction is 90° to each other
-Use this to work out the unknown factor out of the three (usually the direction of the force felt)
-Remember current is conventional current, which moves in opposite direction to the electrons
What does thumb, 1st finger and 2nd finger represent?
thumb: force
1st finger: field
2nd finger: current
Equation for force and short form too with units
Force = (magnetic flux density) x (current)
× (length)
F = BIL
<Where Magnetic Flux Density is measured in Tesla
o And it is the number of flux lines per metre squared
Explain how electric motors work
Direct current passes through
<Permanent Magnets lie in fixed positions
<In between, a coil of current-carrying wire lies on an axis
-Force on one side moves that side up
-Force on the other side (where current is flowing in opposite direction) moves down
-This can be verified using Fleming’s Left Hand Rule
<Hence it rotates
Electromagnetic Induction
<When there is a relative movement between a conductor and a magnetic field, a potential
difference is induced across the conductor.
<This happens if the magnetic field changes as well
<A current flows if the conductor forms a complete circuit.
<This current will produce its own magnetic field, which oppose the change inducing it
Explain how electric generators (dynamos) work
*Same setup as a motor, with a coil of wire able to rotate between two permanent magnets
*A turbine spins turning the coil of wire
The movement of the wire causes the wire to cut through the magnetic field
*It experiences a change in magnetic field
*This creates a potential difference
*If the coil of wire is connected to a complete circuit, an alternating current (AC) will flow – this is a basic alternator (picture)
*Direct current (DC) current is produced if the ends, A and D in diagram above, are connected to a split ring commutator
*This reverses the current each half-rotation so current remains positive – this system is called a dynamo
Explain how Transformers work
< AC in first coil creates a changing magnetic field
< This changing magnetic field cuts through the secondary
coil
< This induces a current in the secondary coil
o Which is also AC
o If primary current was DC, magnetic field it
produces will be constant, not inducing anything in the secondary coil
< More coils on secondary: Step up transformer, as voltage will be increased, as changing field will cut through more of the secondary wire inducing a larger pd
< Fewer coils on secondary: Step down transformer, as smaller pd forms on secondary
number of coils on primary pd of primary
number of coils on secondary. pd of secondary
Equation to work out number of coils or p.d of primary and secondary coils
-Also when does this work?
number of coils on primary / number of coils on secondary
pd of primary / pd of secondary
–> This only works with current too if the transformer is 100% efficient. Unless it states this, assume not and just use this to find voltage
Explain how dynamic microphones work
<They produce a current which is proportional to the sound signal
<Fixed magnet is at the centre, and the coil of wire around the magnet is free to move
<Pressure variations in the sound waves cause the coil to move, and as it moves current is induced in the coil (because it cuts the magnetic field)
<This current is then sent to a loudspeaker
Explain how loudspeakers work
<The setup is identical to the microphone, however, working in reverse
<The current flows into the coil
<The magnetic field from magnet and from current interact, causing the coil to move
<The cone therefore moves
<Producing pressure variations, making sound
