Teaching Notes for Unit 9.3 Motors and Generators Topic of NSW HSC Physics Course

1. Topic 9.3
3.1.2 – Using The Motor Effect

2. Forces and Torque
● An unbalanced force on a body will always
cause an acceleration on a body.
● If this is in a straight line it is called a translation.
● However, if the object is anchored so it cannot
move then the force could cause a rotation.
● The turning effect of a force is called its torque.

3. Forces and Torque
● The torque or moment of a
force is calculated using:
●
τ= fd
● Here d is the perpendicular
distance between the line of
force and the pivot.
● Torque is measured in newton-metres
(Nm)
Force
d

4. Forces and Torque
● If the force is not at 90o
to the object, then the
perpendicular distance
needs to be calculated.
● This is so that the
perpendicular
component of the force
is used in calculations
τ= fd cosθ
Force
d
θ
dCosθ

5. Current Carrying Loops
● When a loop of wire
carrying a current is
placed in a magnetic
field as shown it
experiences forces on
its long edges as
shown.
● These forces cause a
rotation of the coil
S N

6. Current Carrying Loops
● If an axle (a pivot) is
introduced then the coil
can be made to rotate
around it.
● Both forces are identical
in magnitude as they
are in the same field
and are caused by the
same current
S N

7. Current Carrying Loops
● If the coil is d metres
wide then the total torque
on the loop is:
●
●
●
●
● Θ=0 as the coil is in the
plane of the magnets
S N
τ= fd cosθ
τ=Fd2
cos0+ Fd2
cos0
τ=Fd
d

8. Current Carrying Loops
● The force acting on a
current carrying wire
in a magnetic field is
given by:
●
● Therefore as the wire
and the magnet are
parallel (χ=90) the
torque on the coil is:
S N
F=BIl sin χ
d
τ=BIld
l

9. Current Carrying Loops
● The product ld is
equal to the area of
the coil A
●
● Where θ is the angle
of rotation of the coil
● If n coils are used
then this becomes
l
S N
τ=BIAcosθ
d
τ=nBIAcosθ

10. A Motor
● When the current is switched on
the left hand side of the coil moves
up and the coil turns clockwise.
● As it turns θ gets larger and the
torque becomes less.
● When the coil is at 90o then it will
stop turning as the torque is now
zero because the perpendicular
distance is now zero.
τ=nBIAcosθ
S N

11. A Motor
● If inertia takes the coil
past the vertical then
the torque will
actually act to return
the coil to the vertical
● To make the coil spin
requires the direction
of the current to be
changed.
S N

12. The Split Ring Commutator
● A device called a split ring
commutator allows the
current to be reversed every
180o.
● The commutator consists of
two conductive arcs
separated by an insulator.
● Two brushes, usually carbon,
allow electricity to flow into
the commutator and then into
the coils of the motor.

13. The DC Motor
● The DC motor
consists of:
● A circular magnetic
field
● An iron rotor with wire
coils
● A split ring
commutator

14. The DC Motor
● A circular magnetic
field is used so as to
try to keep θ as low
as possible and so
maximise the torque.
● This field can be
created using
permanent magnets
or electromagnets

15. The DC Motor
● An iron armature
(rotor) is used to as to
increase the magnetic
effect of the rotor
windings.
● DC motors can have
one winding, or
multiple windings
known as poles.

16. The DC Motor
● The split ring
commutator is used
so as to reverse the
current every 180
degrees and keep the
motor spinning.

17. The Galvanometer
● Because the torque on a coil is
directly proportional to the
current through it, it can be used
as an ammeter.
● The commutator is removed and
replaced with balancing springs.
● This means that the angle
becomes directly proportional to
the torque and hence to the
current.

18. Loudspeakers
● A loudspeaker works by using
the motor effect in a circular
magnetic field
● An audio signal is applied to the
coil which causes a force which
varies in time with the signal.
● The cone then moves in and out
in time with the signal making
sound waves.