Électromagnétisme Flashcards
What are properties of magnets?
- all magnet, any shape, hv 2 mag poles (N & S)
- like poles repel, unlike poles attract
- when no other magnet is near, freely suspended magnet aligns itself approx parallel to earth north-south axis
Define magnetic field
region of space where permanent magnet or moving charge or current carrying conductor will experience magnetic force
NOTE: mag field no effect on stationary charge
What is the magnetic equivalent of electric/grav field strength?
Magnetic flux density, B
Describe magnetic flux density B
- vector qty
- SI unit: Tesla (T)
How to visualise magnetic fields?
- Scatter iron fillings
- Use plotting compass
(Needle points to S pole of magnet, needle gives tangent of B-field line)
How are magnetic fields represented? What are some conventions?
By magnetic field lines
- direction: emerge fr N pole & enter S pole of magnet
- strength: closeness of lines indicate strength of field
- uniformity: parallel field lines evenly spaced indicate uniform field
Describe B fields
- direct n of mag flux density at pt is tangent to field line at that pt
B field line - do not intersect, touch each other
- have arrows drawn on each of them fr N to S pole
NOTE: within magnet, field line pt fr S to N
What is formula for magnetic flux density, B
B=μ0I/(2πd)
where,
μ0 is permeability free space,
I is current
d is distance
Eqn shows field weakens further away fr wire (flux lines further apart as d increase)
What is formula for magnetic flux density, B at centre of circular coil with N turns?
B = μNI/(2r)
where,
N is no of turns,
r is radius of loop
What is formula for magnetic field produced by solenoid?
B = μ0nI
where,
n is no. turns per unit length
Describe effect of ferrous core on magnetic field produced by solenoid
When ferrous core (eg iron bar) places within current-carrying solenoid, magnetic field produced by solenoid is stronger (shown by flux lines closer tgt)
What hand rules are there in electromagnetism?
- Maxwell right hand grip rule (2 variations)
- Fleming’s left-hand rule
What is formula for magnitude of magnetic force?
F = BIL
where,
B is component of mag flux density perpendicular to conductor,
I is current in conductor,
L is length of conductor within mag field
*If conductor placed at angle θ to mag field B, formula is
F = BILsin θ
where θ is angle btw conductor & e field
Define magnetic flux density. Give formula
of a mag field is force per unit current per unit length of conductor acting on straight current-carrying conductor placed at right angle to mag field
B = F/(IL),
where,
F is force act on conductor
L is length of conductor within field
I is conventional current flow thru conductor
For 2 parallel current-carrying conductors, what is magnitude of force per unit length on each wire AND direction of force?
- magnitude:
F/L = μ0(I1)(I2)/ (2πd)
where
I1 & I2 are currents in each wire,
d is dist btw wires,
L is length of wire
- direction:
- same direct n current –> attractive force btw wires
- opp direct n current –> repulsive force btw wires
What is the formula for magnetic force on a moving charge?
F = Bqv sinθ
where,
F is force acting on a moving charge,
B is magnetic flux density,
q is mag of charge,
v is velocity of charge,
θ is angle velocity makes w B field
Fleming’s Left Hand rule considers direction of what type of current?
Conventional current
(Same direction as positive charge, opposite direct n as negative charge)
Describe path taken by moving charge in a magnetic field
- since B force const mag & always at right angle to velocity, conditions met for circular motion
- Magnetic force on moving charge provides for centripetal force
ie
Fnet = ma
Bqv = mv²/r - eqn shows orbit radius is proportional to velocity of particle if charge & mass r const
Describe path taken by charge entering magnetic field at an angle
- if enter field other than right angle, charged particle takes helical path
Let v be og velocity - velocity component parallel to B field = v cosθ
=> unaffected by B field so particle continue motion parallel to B field (no net force) - velocity component perpendicular to B field = v sinθ
=> causes circular path perpendicular to B field
=> Combined effect: both velocity components cause helical path
Compare interaction of charge with magnetic and electric field
- When act
B field:
- exert mag force oni on moving charge
E field:
- exert e force on BOTH moving & stationary charge - Direct n
B field:
- mag force perpendicular to both B fiekd & direct n of motion of charge (Fleming’s Left hand rule)
E field:
- e force act same direct n as e field for +ve charge & opp direct n for -ve charge - Variable dependence
B field:
- mag force dependent on speed & direct n of motion of charge
E field:
- e force independent of both speed & direct n of motion of charge - Shape of motion
B field:
- circular motion obtained when charge enters B field perpendicularly
E field:
- parabolic motion obtained when charge enters E field perpendicularly
Describe motion of charge in crossed electric and magnetic fields
- Consider charge entering region where both uniform e field & B field act perpendicularly to each other
- charge experience e force, Fe and mag force Fb. Fe & Fe act in opp direct n
- If magnitude of Fe & Fb same, no net force so charge travels in straight line. Thus,
Fe=Fb
qE = Bqv - only charges w velocity equal to ratio of E to B will travel undeflected
- If velocity too high, Fb>Fe & particle move in CURVED path upwards (not parabolic)
- If velocity is too low, Fb<Fe & particle move in curved path downwards
What is a velocity selector?
- uses e field at right angle to mag field to allow charges of oni specific velocity to pass thru
- Only particles w velocity = E/B emerge undeflected thru all slits
- By varying magnitude of E & B, can select charges w one specific velocity to emerge fr all slits
