Magnetic fields (C23) Flashcards
What is a B field
A region in which a magnetic material experiences a force
Symbol for a magnetic field
B (vector)
Which way do field lines point
Direction North pole would move (N–>S)
What do equally spaced field lines mean
uniform field strength
Why are materials magnetic
Electrons have spin, giving a dipole
This can give regions (domains) a net dipole if the spin aligns
Domains with roughly aligned dipoles give a magnetic material
what is a soft magnetic material
Loses dipole after magnet is removed
What is a hard magnetic material
Domains remain aligned
How do current and magnetic field relate
RH rule
What does a wire into the page look like
Cross in circle
Wire out of the page
dot in circle
What is a solenoid
Coil with multiple turns
How do you work poles of a solenoid
Draw with dot and cross wires and look at general field
What does a iron core do
Reduces leakage, magnetic fields prefers to go through a magnetic field
How do you give solenoid stronger field
More turns
More current
Iron core
What is current
the movement of positive charge
Why does a current in a magnetic field produce a force
Field from wire and magnetic field interract (lines want to straighten) therefore force
How to know direction?
Flemings LH rule
Describe Flemmings LH rule
thumb motion
1st field
2nd current
Describe the relationship between force and magnetic flux density
Force is proportional to the current, magnetic flux density, and the length of the wire WITHIN the field
What is magnetic flux density
Strength of magnetic field
Symbol and Unit for magnetic flux density
B [Teslas] or [N/m/A]
Experiment to determine magnetic flux density
Scales and wire and magnetic
What is a Helmholtz coil
Solenoid pulled apart
~ uniform field in middle
if a electron is moving through a magnetic field…
Treat current going the other direction
When does F=BQv apply
when particle is moving perpendicular to field
Describe movement of charged particle in magnetic field
Force is perpendicular
Velocity does not change
Direction changes
Circular motion
What can act as a centripetal force
the force due to the magnetic field on a charged particle
Describe a velocity selector is like
Magnetic field and electric field with a charged particle going through it
How does a velocity selector work
only particles that are in equilibrium travel straight through
BQv=EQ
What is v in the velocity selector
E/B ratio of electric to magnetic fields
what is a potential difference not
a current
How does a spectrometer work
velocity is selected
go through a magnetic field
radius depends on mass of particle as everything else is constant
What is the principle of induction
a magnetic field and force can induce a current
Why does induction occur
conservation of energy- electrons in wire experience a magnetic force causing them to move a current to be induced
What is magnetic flux
product of magnetic flux density (perpendicular) and the cross sectional area of the coil
What is the unit and symbol for magnetic flux
phi
Weber (Wb)
SI for Wb
T/m^2
What is magnetic flux linkage
N*phi - product of the number of turns in the coil and the magnetic flux of each
What is Faraday’s Law
Magnitude of EMF produced is directly proportional to the rate of change of flux linkage
What is lenz’s Law
Direction of EMF induced is always in the direction to oppose the change producing it
Is it a n or s pole at the end of solenoid if the magnets n-pole is moving away
South
Why is the constant of proportionality in Faraday’s Law -1
Lenz’s Law
Why does Lenz’s Law occur
to ensure energy is conserived as work is done to move magnet
What does EMF look like on graph
Sinusoidal
Why is there alternating current from induction
As flux linkage = BAN cos(theta)
angle changes as inserted
Describe a Transformers
two coils of wire around a soft iron core
Why do transformers work
AC produces varying magnetic flux
this induces an EMF which induces a current in the wire
Turns on a step up transformer
More turns on secondary
Describe a step up transformer
raises voltage
turns on a step down transformer
more turns on primary
What do you remember about ideal transformers
Power is conserved - VI = VI
To improve transformers
laminated soft iron core (reduces eddy currents)
low resistance reduces energy as heat lost
