Magnetism and Current Flashcards
Describe the relationship between current, a conductor and a magnetic field?
A current flowing though a conductor induces a magnetic field around it in a clockwise manner (when looking in the direction of the current)
Conversely a current can be induced through a conductor by moving a conductor through a magnetic field. The current will be proportional to the speed the conductor moves through the magnetic field.
https://www.youtube.com/watch?v=WhATjUHgzxQ (helpful video)
What role does the earth’s magnetic field play?
It protects against solar and cosmic radiation
At a microscopic level how are ferromagnetic materials is magnetised?
‘By aligning there unpaired electron spins.’
The spin of an electron in its orbital is in essence an electrical charge in motion at a localised level. In most materials, neighbouring atoms spin also and cancel each other out.
Materials with unbalanced electron spinning are known as ferromagnetic materials i.e. iron.
These atoms align themselves in randomly oriented microscopic domains, so that within each domain there is an intense localised magnetic field.
Macroscopically the domain orientations cancel each other out and the material exhibits no magnetic field externally.
However if an external magnetic field is applied these unpaired electrons become aligned, therefore magnetising the feromagnetic materials.
What is magnetic flux?
It is the lines that are drawn to represent a magnetic field around a magnet. (imagine iron filings around a powerful magnet)
The closer the lines are together the higher the flux density.
Induced current is directly proportional to flux density, therefore the higher the flux density the greater the induced current.
What formula links flux density, force, charge and velocity?
F=QUB
F=force Q=charge U=velocity B= flux density
Magnetic flux is said to exist when a electrical charge moving perpendicular to a magnetic field at velocity U experiences a Force.
How can you calculate total flux from flux density?
δΦ= δB x δA
Φ=total flux
B=fluc density
A=area
What factors influence flux density?
As the strength of the magnetic field (H) increases the flux density (B) increases.
The permeability of the material (μ).
B= Hxμ
Draw a curve to describe the behaviour of ferromagnetic materials becoming magnetised?
H (magnetic field strength) on the x axis. B (flux density) on the y axis.
Sigmoid shaped curve
1. Initial slow slope represents reversible pathway of magnetism.
2. Steep slope represents onset of irreversible growth
3. Later slow slope represents domains becoming irreversibly aligned
What is the formula which describes the relationship between flux density (B), current (i), length of a wire (s) at a certain point?
Flux density is variable at different points (think back to the iron filings around a magnet)
The scientist ampere created a formula to calculate flux density at a given point.
δB = μ.i. δs. cos α/(4πr2*)
This formula can be integrated for the whole wire to give:
B = (μi)/(2πr)
Important take homes are:
B is directly proportional to permeability and current
B is inversely proportional to distance
B= flux density μ=material permeability s=length of wire r=distance between wire and point in question
2*=squared
What is the significance of coiling of a wire?
If the current-carrying wire is coiled, a more intense magnetic field is created, because each wire in the coil which carries current induces a magnetic field around it.
The close proximity of the coils to each other magnifies the flux density as many times as there are coils.
Aka the flux induced is proportional to the number of coil turns.
Explain the relationship between magnetic field and voltage and express this in a formula?
Voltage induced is directly proportional to the rate of change of magnetic field in the coil.
The voltage induced opposes the change of magnetic flux which induced it (reduce current change flowing in the coil).
As previously stated flux density is directly proportional to number of coils (N).
Therefore:
V= N (dФ/dt)
dФ= change in total flux
dt= change in time
What is the significance of the core material that a wire is coiled around?
By using a feromagnetic material with a high permeability you increase flux. (B=uH)
How does a 4 pole generator work?
There is a coil of wire in a circular circuit.
This circular circuit surrounds 2 magnets (therefore 4 poles 2 north and 2 south).
The central magnets spin as they spin there is a rotating magnetic field. As the coils of wire cross the magnetic field a current is generated.
Compare a 2 pole to a 4 pole generator?
2 pole generators only include 1 magnet. In 2 pole generators the coil will only cross the magnetic field twice so you will only have 1 AC cycle (the positive part of the sine wave for N and the negative part of the sine wave for S).
Therefore to create a certain frequency it will have to rotate twice as fast a 4 pole generator in which there will be 2 AC cycles for each rotation.
https://www.youtube.com/watch?v=iZzK7bPfvUs
Explain the influence of inductance in an AC electromagnetic circuit.
If an AC generator is used to drive a current across a coil of wire there will be alternating flux. (as flux is directly proportional to current)
The alternating flux will produce a voltage which is proportional to the rate of change of flux. V= N (dФ/dt)
where N = no. of coils.
This voltage opposes the change in current which had produced it.
Therefore, the voltage required to drive the current is proportional to the rate of change of the current.
V= L x (di/dt)
L=inductance (Henry) of the wire
