Electric MotorElectric Motor

Magnetic Force On A Current – Magnetic Force On A Current – Carrying Conductor Carrying Conductor

The magnetic force (F) the conductor The magnetic force (F) the conductor experiences is equal to the product of its experiences is equal to the product of its length (L) within the field, the current I in length (L) within the field, the current I in the conductor, the external magnetic field the conductor, the external magnetic field B and the sine of the angle between the B and the sine of the angle between the conductor and the magnetic field. In shortconductor and the magnetic field. In short

F= BIL (sinF= BIL (sin))

The force on a current-carrying The force on a current-carrying conductor in a magnetic fieldconductor in a magnetic field: :

When a current-carrying conductor is placed in a When a current-carrying conductor is placed in a magnetic field, there is an interaction between the magnetic field, there is an interaction between the magnetic field produced by the current and the magnetic field produced by the current and the permanent field, which leads to a permanent field, which leads to a forceforce being being experienced by the conductor:experienced by the conductor:

The magnitude of the force on the conductor depends The magnitude of the force on the conductor depends on the magnitude of the current which it carries. The on the magnitude of the current which it carries. The force is a maximum when the current flows force is a maximum when the current flows perpendicularperpendicular to the field (as shown in diagram A on to the field (as shown in diagram A on the left below), and it is zero when it flows the left below), and it is zero when it flows parallelparallel to to the field (as in diagram B, on the right):the field (as in diagram B, on the right):

Fleming’s left-hand ruleFleming’s left-hand rule

The directional relationship The directional relationship of I in the conductor, the of I in the conductor, the external magnetic field and external magnetic field and the force the conductor the force the conductor experiencesexperiences

I

F

B

Motion of a current-carrying loop in a Motion of a current-carrying loop in a magnetic fieldmagnetic field

N SL R

I

F

F Rotation

Commutator (rotates with coil)

brushes

Vertical position of the loopVertical position of the loop::

N S

Rotation

Electric MotorElectric Motor

An electromagnet is the basis of an An electromagnet is the basis of an electric motor electric motor An electric motor is all about magnets and An electric motor is all about magnets and magnetism: A motor uses magnetism: A motor uses magnetsmagnets to to create motion. create motion. Opposites attract and likes repel. Inside an Opposites attract and likes repel. Inside an electric motor, these attracting and electric motor, these attracting and repelling forces create repelling forces create rotational motionrotational motion. . A motor is consist of two magnets.A motor is consist of two magnets.

Parts of the MotorParts of the Motor

Armature or rotor Armature or rotor

Commutator Commutator

Brushes Brushes

Axle Axle

Field magnet Field magnet

DC power supply of some sortDC power supply of some sort

Motor IllustrationMotor Illustration

ArmatureArmatureThe armature is an The armature is an electromagnet made by electromagnet made by coiling thin wire around coiling thin wire around two or more poles of a two or more poles of a metal core.metal core. The armature has an The armature has an axleaxle, , and the commutator is and the commutator is attached to the axle. attached to the axle. When you run electricity into When you run electricity into this electromagnet, it creates this electromagnet, it creates a magnetic field in the a magnetic field in the armature that attracts and armature that attracts and repels the magnets in the repels the magnets in the stator. So the armature spins stator. So the armature spins through 180 degrees.through 180 degrees.To keep it spinning, you have To keep it spinning, you have to change the poles of the to change the poles of the electromagnet.electromagnet.

Commutator and BrushesCommutator and Brushes

Commutator is simply a pair of plates Commutator is simply a pair of plates attached to the axle. These plates provide attached to the axle. These plates provide the two connections for the coil of the the two connections for the coil of the electromagnet. electromagnet. Commutator and brushes work together to Commutator and brushes work together to let current flow to the electromagnet, and let current flow to the electromagnet, and also to flip the direction that the electrons also to flip the direction that the electrons are flowing at just the right moment. are flowing at just the right moment.

The contacts of the commutator are attached to The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of the magnet. The brushes are just two pieces of springy metal or carbon that make contact with springy metal or carbon that make contact with the contacts of the commutator. the contacts of the commutator.

Spinning ArmatureSpinning Armature

Example of MotorExample of Motor

Answer the questionsAnswer the questions

A current-carrying coil in a magnetic field experiences a turning effect.

How can the turning effect be increased?A increase the number of turns on the coilB reduce the size of the currentC reverse the direction of the magnetic fieldD use thinner wire for the coil

What are the directions of the force in the left What are the directions of the force in the left and right loop?and right loop?

A student sets up the apparatus shown in order to make a relay.

Which metal should be used to make the core? A aluminium B copper

C iron D steel