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La Physique H2 > Électromagnétisme > Flashcards

Électromagnétisme Flashcards

1
Q

What are properties of magnets?

A
  • 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
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2
Q

Define magnetic field

A

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

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3
Q

What is the magnetic equivalent of electric/grav field strength?

A

Magnetic flux density, B

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4
Q

Describe magnetic flux density B

A
  • vector qty
  • SI unit: Tesla (T)
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5
Q

How to visualise magnetic fields?

A
  • Scatter iron fillings
  • Use plotting compass
    (Needle points to S pole of magnet, needle gives tangent of B-field line)
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6
Q

How are magnetic fields represented? What are some conventions?

A

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

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7
Q

Describe B fields

A
  • 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

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8
Q

What is formula for magnetic flux density, B

A

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)

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9
Q

What is formula for magnetic flux density, B at centre of circular coil with N turns?

A

B = μNI/(2r)

where,
N is no of turns,
r is radius of loop

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10
Q

What is formula for magnetic field produced by solenoid?

A

B = μ0nI

where,
n is no. turns per unit length

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11
Q

Describe effect of ferrous core on magnetic field produced by solenoid

A

When ferrous core (eg iron bar) places within current-carrying solenoid, magnetic field produced by solenoid is stronger (shown by flux lines closer tgt)

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12
Q

What hand rules are there in electromagnetism?

A
  1. Maxwell right hand grip rule (2 variations)
  2. Fleming’s left-hand rule
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13
Q

What is formula for magnitude of magnetic force?

A

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

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14
Q

Define magnetic flux density. Give formula

A

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

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15
Q

For 2 parallel current-carrying conductors, what is magnitude of force per unit length on each wire AND direction of force?

A
  1. 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

  1. direction:
    - same direct n current –> attractive force btw wires
    - opp direct n current –> repulsive force btw wires
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16
Q

What is the formula for magnetic force on a moving charge?

A

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

17
Q

Fleming’s Left Hand rule considers direction of what type of current?

A

Conventional current
(Same direction as positive charge, opposite direct n as negative charge)

18
Q

Describe path taken by moving charge in a magnetic field

A
  • 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
19
Q

Describe path taken by charge entering magnetic field at an angle

A
  • 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

20
Q

Compare motion of charge in magnetic and electric field

A
  1. When act
    B field:
    - exert mag force oni on moving charge
    E field:
    - exert e force on BOTH moving & stationary charge
  2. 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
  3. 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
  4. 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
21
Q

Describe motion of charge in crossed electric and magnetic fields

A
  • 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
22
Q

What is a velocity selector?

A
  • 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

La Physique H2 (19 decks)

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