eLFH - Magnetism and Current Flashcards
Link between magnetism and electricity
Conductor carrying electrical charge or current creates a magnetic field - i.e. magnetic field can be created using electrical power
A wire moved through a magnetic field induces electric current in the wire proportional to its speed of movement - e.g. a dynamo
Magnetic flux
The lines drawn that represent the effect of a magnetic field around magnet or electrical conductor
Flux density
If magnetic flux lines are close together, flux density is high
Current induced in an electrical conductor moving through a magnetic flied with higher flux density is greater than that of low flux density
Flux density of Earth’s magnetic flied
Low flux density
But overall high total magnetic strength
Total flux definition
Flux density summed over an area
Units of flux density
Tesla
Force equation for force exerted by charge moving perpendicular to magnetic field
F = Q x U x B
F is force (Newtons)
Q is electric charge (Coulombs)
U is velocity perpendicular to magnetic field (m/sec)
B is magnetic flux density (tesla)
Magnetisation curve for ferromagnetic materials
As increasing magnetic field strength applied to ferromagnetic material, it passes through different regions of magnetic flux
Reversible to irreversible magnetism
Once the domains are irreversibly aligned (saturated) the metal is a magnet
Magnetic hysteresis definition
If applied magnetic flied to a material is removed, the magnetic flux density does not always return to zero
Material remains weakly magnetised
This is Hysteresis
Direction of magnetic field created by electrical current
Magnetic flied runs clockwise around the wire when looking in the direction if the current flow
Effect for coiling a wire on magnetic field and why
More intense magnetic field with coiled wire
Flux density increases as many times as there are coils
Neighbouring coils induce neighbouring reinforcing magnetic flux
Voltage equation for a coiled wire
Voltage induced by coil = number of turns in coil x rate of change of magnetic flux
Types of core that can be used in wire coil
Air
Iron
Why does iron core in coiled wire have greater flux density than air core
High magnetic permeability of iron
Equation for flux density
B is flux density
Mu is magnetic permeability
H is strength of magnetic field applied to material
Mutual inductance in a transformer
2 separate circuits coiled around same iron core
Electrical current in one circuit creates magnetic field which generates current in second coiled wire without it being connected to any mains power
Use of isolating transformers
Allows floating circuits to be made which are not earthed - only circuit connected to mains is earthed but circuit connected to patient is not
Therefore from floating circuit, cannot be electrocuted
Used to isolate individual instruments
Effect of adjusting number of coils on each side of a transformer
Can step up or step down voltage between input and output depending on number of coils on primary and secondary winding
V1 / V2 = N1 / N2
I1 x N1 = I2 x N2
Wire coils in transformer material and why
Copper
Reduce losses
Material used for core of transformers and why
Steel
high magnetic permeability to reduce hysteresis losses
Why is transformer laminated
Reduce eddy current loss
Eddy current definition
Opposing flows of electrical current that can develop due to variations in magnetic field or relative movement of the conductor
These flows induce own magnetic field that result in resistive losses
Function of Earth’s magnetic field
Protects against cosmic and solar radiation
