Electricity and Magnetism

Mr D. Patterson

Outcomes• explain the torque produced by the force on a

rectangular coil carrying a current in a magnetic field—this will include applying the relationships: for perpendicular cases

Electricity and magnetism• A current carrying

conductor, in an external magnetic field, feels a force in a direction indicated by using the right hand rule

So what’s the use?

• Basis for the electric motor

Electricity + Magnetism = Motion

Parts of a motor

• http://newton.physics.uiowa.edu/~umallik/adventure/nov_06-04/motor.gif

Parts of a motor• The rotor:– Coil– Soft Iron Core

Parts of a motor• The stator:– Permanent magnetOr – field coils (electromagnet)

Parts of a motor• Other parts:– Commutator– Carbon brushes

Rotation

There is an external magnetic field provided by a permanent magnet or an electromagnet

Rotation

While a current runs through the coil, a force will be applied to the sections of the coil that are perpendicular to the magnetic field. This creates a torque and the coil rotates

Use the right hand rule to check

RotationAs the coil rotates, the magnitude and direction of the current and magnetic field remain constant so the force on each side of the coil will also remain the same.

If left like this, the coil will stop rotating as the forces no longer provide a torque.

Rotation

Just before the coil reaches this position the current is removed, which allows the momentum of the coil’s rotation to rotate the coil past this point.

The current is then quickly reapplied but in the opposite direction. This now changes the direction of the forces acting on each side of the coil which allows rotation to continue

Rotation

Every half rotation the current must be removed and then reapplied in the reverse direction to ensure continuous rotation.

The current is reversed through the special design of the commutator

Apply Calculations• Force on a current carrying wire in a magnetic field:

• Force on each side of a motor:

• F is force (N)• N is number of coils in rotor• I is current in rotor (A)• is the length of the rotor coil perpendicular to B• B is the magnetic flux density (T)

Applying Calculations• The torque acting on each side of the rotor coils:

• is the torque (Nm)• F is the force (N)• r is the radius of the coil

N S

Example• A motor is made up of 20 turns of square coil with a side

length of 8 cm and has a current of 425 mA running through it. The external magnetic flux density is 300 mT.

• What is the torque acting on the motor’s coil when the coil – lines up with magnetic field and – when it is perpendicular with the magnetic field.