Embed Size (px)
Citation preview
3.1.2 – Using The Motor Effect
Topic 9.3
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.
Forces and Torque
● The torque or moment of a force is calculated using:
●
● Here d is the perpendicular distance between the line of force and the pivot.
● Torque is measured in newton-metres (Nm)
τ= fd
For
ce
d
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
Forc
e
dCosθd
θ
● 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
Current Carrying Loops
NS
● 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
Current Carrying Loops
NS
● 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
Current Carrying Loops
NS
τ= fd cosθ
τ=Fd2
cos0+Fd2
cos0
τ=Fdd
● 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:
Current Carrying Loops
NS
F=BIl sinχ
d
τ=BIld
l
● 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
Current Carrying Loops
NS
τ=BIA cosθ
d
l
τ=nBIAcosθ
● 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.
A Motor
τ=nBIAcosθ
NS
● 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.
A Motor
NS
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.
The DC Motor
● The DC motor consists of:● A circular magnetic
field● An iron rotor with wire
coils● A split ring
commutator
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
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.
The DC Motor
● The split ring commutator is used so as to reverse the current every 180 degrees and keep the motor spinning.
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.
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.
