Torque is a force that causes rotation, defined with the symbol τ and measured in newton meters (nm). In electric motors, a coil experiences changing torque as it turns, influenced by the angle between the magnetic field and the coil plane. Maximum torque occurs at θ = 0° and minimum at θ = 90°, with the torque behavior resembling a cosine graph during rotation.

1. HSC Physics
9.3 Motors and Generators
TORQUE

2. Torque is the tendency of a
force to rotate an object
about an axis, fulcrum or
pivot. Just as a force is a
push or a pull, a torque can
be thought of as a twist.
Torque is a force that
produces rotation.
Torque has the symbol, 𝞃,
and units of newton metres
(Nm).
If the force is not
perpendicular to the
distance d it must be
resolved into a
perpendicular force using
trigonometry as shown.

3. In electric motors, it is a
conductor wound into a coil
or loop that turns when
torque is applied.
The force in the equation
for torque comes from the
motor effect:
F = BILsinθ

4. To see how the torque
changes as the coil turns, it
is best visualise the coil in
only two dimensions.

5. As the coil turns the torque
changes because the angle
between the magnetic field
lines and the plane of the
coil (θ) is also changing.
The torque is maximum
when the θ = 0 and zero
when θ = 90°.
The force stays constant
during rotation but the
torque changes following a
cos graph.

6. If a radial magnetic field is
used instead, the field lines
are curved between the
north and south poles of
the magnet.
The angle θ between the
plane of the coil and the
field lines is kept closer to
zero as the coil rotates.
If θ is small the torque will
be kept larger for longer
during a rotation.