Transcript 1. Motors use the effect of forces on current-carrying

1. Motors use the effect of
forces on current-carrying
conductors in magnetic fields
Jacaranda
REVIEW OF MAGNETIC FIELDS
P.108-110
Identify that moving charged
particles in a magnetic field
experience a force
This happens
because the magnetic
field created by the
moving charged
particle interacts
with the existing field
Account for the motor effect due to
the force acting on a current-carrying
conductor in a magnetic field
It depends on the field
strength, the charge
size and the velocity.
e.g. Conventional current, alpha particles
e.g. Electron flow, beta particles
N.B. If the velocity has a component parallel
to the magnetic field, that component is not
subject to the force and is retained while the
other component is accelerated.
Record observations, from teacher
devised demonstration, of the deflection
of an electron stream in a Cathode Ray
Tube, by a permanent magnet.
Jacaranda
Experiment 6.1
Identify data sources, gather, analyse and present
information to discuss the Van Allen radiation belts as
examples of motion of charged particles in a field
Jacaranda
PHYSICS FACT P.124
Research material from the internet and other sources
on Van Allen radiation belts
Discuss the effect, on the magnitude of the force on a current-carrying
conductor, of variations in:
– the strength of the magnetic field in which it is located
– the magnitude of the current in the conductor
– the length of the conductor in the external magnetic field
Solve problems and analyse
information about the force on
current-carrying conductors in
magnetic fields using F = BI l
– the angle between the direction of the external magnetic field
and the direction of the length of the conductor
F  BIl sin 
B
N.B. magnetic field
may be created by
permanent magnet
or electromagnet

I
M.Edwards
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where  is the angle between the magnetic field and the conductor
So F is directly proportional to B, I, l and sin 
Perform a first-hand investigation to
demonstrate the motor effect
Jacaranda
Experiment 6.2
Describe qualitatively and quantitatively
the force on long parallel currentcarrying conductors in magnetic fields
using
Use the same
approach to show
that opposite
currents repel
F
I1 I 2
k
l
d
I2
The right-hand grip rule gives the
direction of the magnetic field
surrounding conductor 1.
B
l
d
M.Edwards
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F
I1
This shows the field that
conductor 2 experiences
due to conductor 1
The right-hand push rule gives
the direction of the force
experienced by conductor 2
F
I2
Conductor 2 has the same
effect on conductor 1, so
they ATTRACT.
F
F
I1
M.Edwards
25/4/02
Describe qualitatively and quantitatively
the force on long parallel currentcarrying conductors in magnetic fields
using
F
I1 I 2
k
l
d
k  2.0 10 7 NA2
Conductor 2 experiences a
magnetic field due to conductor 1
B
Conductor 2 experiences a
force due to this magnetic field
I1
d
F  BI 2l
l
d
Substituting for B:
M.Edwards
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F
I1
Bk
I2
rearranging gives
Force per unit length:
I1 I 2 l
F k
d
F
I1 I 2
k
l
d
F
I1 I 2
k
l
d
k  2.0 10 7 NA2
Two straight current-carrying conductors are placed parallel
to each other, 20 cm apart.
I1 = 2 A
I2 = 5 A
20 cm
I1
I2
M.Edwards
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(a)
Determine the force per unit length of the 2 A wire
on the 5 A wire.
(b)
Describe what will happen to the magnitude of the
force as the 2 A wire is rotated 90o until it is
perpendicular with the 5 A wire.
F
I1 I 2
k
l
d
k  2.0 10 7 NA2
20 cm
I1
I2
M.Edwards
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(a)
(b)
F/l = kI1I2/d
F/l = (2.0 x 10-7 x 2 x 5) / 0.2
F/l = 1 x 10-3 N/m (attraction).
1 mark
The force will decrease as the 2 A wire is
rotated and will be zero when the two wires
are perpendicular to each other.
1 mark
(OPTIONAL EXERCISE)
Undertake a first hand investigation, devised by the
teacher, to observe Oersted’s experiment. Students record
the method, their observations, and the significance of the
experiment, after teacher led discussion.
Jacaranda
Experiment 6.3
Record information, after teacher led discussion, to explain
the force between the wires as an example of the motor effect
D.C. ELECTRIC MOTORS
1. Motors use the effect of
forces on current-carrying
conductors in magnetic fields
Define torque as the turning
moment of a force using:
  Fd
M.Edwards
25/4/02
So total Torque,
and for n loops
l
  BIld
  BIl (2d )
Torque from
each side
  BIA
I
F
M.Edwards
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d
B
F
Torque for each
loop
  nBIA
If the coil is at an angle  to
a uniform magnetic field
F
d
F  BIl
Substituting:
Increasing F or d
increases the
turning effect
(torque)


M.Edwards
25/4/02
Identify the forces experienced
by a current-carrying loop in a
magnetic field and describe the
net result of the forces
Solve problems and analyse
information about simple motors
using:
  nBIA cos
Describe how the required magnetic fields can be produced
either by current-carrying coils or permanent magnets
Where are the permanent magnets placed? Why?
What is the shape and direction of a magnetic field produced by a current-carrying coil?
Can we place an electromagnet in the same circuit as
the motor? How?
What are the advantages and disadvantages of
permanent and electromagnets?
Identify data sources, gather and
process information to qualitatively Jacaranda
describe the application of the motor PHYSICS IN FOCUS P.112
effect in:
Student exploration of
– the galvanometer large galvanometers
– the loudspeaker
Student exploration of
car stereo speakers
E.g.
ammeter:
The galvanometer can be converted into an ammeter by using a
very small resistor in parallel with the galvanometer and changing
the scale appropriately.
Most of the current will flow through the small resistor, with the
remainder through the galvanometer. This allows larger currents to
be measured without the needle moving off the scale.
voltmeter: Make sure you find the answer to this too!
Describe the main
features of a DC
electric motor
magnets (stator)
The coil, armature and axle rotate
and are known as the rotor.
The magnets are stationary
and are known as the stator.
coil
axle
F
N
brush
d
S
armature
brush
M.Edwards
25/4/02
Source of EMF
Split-ring commutator
Internet DC motor animation by Walter Fendt
HSC Practical Research Task
Discuss the importance of
the invention of the
commutator for developing
electric motors
Gather and process secondary
information to analyse the function
of the parts of a commutator
side A
side B
F
N
S
I
Conventional current in side B is
currently out of the page
The right hand push rule tells us
this rotor will move anticlockwise.
F
M.Edwards
25/4/02


A
B
I
When  reaches 90
degrees, the split
ring will contact the
other brush, reverse
polarity and current
in side B will be into
the page
M.Edwards
25/4/02
Describe the role of the metal split
ring and the brushes in the
operation of the commutator
So the rotor
continues to rotate
in an anticlockwise
direction
B
F
M.Edwards
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A
HSC Practical Research Task