Physics: Motors & Generators

1 Current Carrying Conductors

1.1Discuss the effect on the magnitude of the force on a current carrying conductor of variations in certain properties:

(F = Force (N) B = Magnetic field strength (T) I = Current (A) L = Length of the conductor (m) = Angle of the conductor to the magnetic field)Formula:

F = BILsin

Strength of the Magnetic Field:

Force is proportional to the strength of the magnetic field

Stronger magnetic field, greater force on conductor

Magnitude of Current in Conductor:

Increasing current means increasing the velocity of the electrons

Each moving charged particle experiences a force in proportion to its velocity

Length of Conductor:

The longer the section of conductor in a magnetic field, the more moving electrons simultaneously experience a force

Force is proportional to the length within the magnetic field

Shorter length, smaller force on conductor

Angle between direction of magnetic field and conductor:

Force is strongest when particle is moving at right angles to the magnetic field (90)

Force is zero when particle is moving parallel to the magnetic field (0)

Movement of electrons is along a length of conductor, magnitude of force varies with angle between conductor and magnetic field

As angle increases, force increases

2 Parallel Conductors

2.1Describe qualitatively the force between long parallel current carrying conductors:

Force between parallel conductors exists because magnetic fields due to current flowing through the conductors interact with each other.

Direction of Force (Attraction or Repulsion):

Depends on relative directions of the two currents

Currents flowing in the same direction, attractive force, towards each other

Currents flowing in opposite direction, repulsive force, away from each other

Magnitude of Force:

Depends on magnitude of current within wire

Increases or decreases with the product of the two currents

Also depends on distance of separation between the conductors

Increasing as the conductors are moved closer together

Relation to Length:

Force between conductors depends on length of parallel conductors

Larger for longer conductors

"Force per unit length" - varies only with magnitude of the two currents and the distance between them.

(F = Force (N) l = Length of parallel conductors (m) I1 & I2 = Currents in the conductors (A) d = Distance between the conductors (m)k = constant (2.0x10-7))Formula:

(F I1I2l d)

(k)

3 Current Carrying Coils

3.1Define torque as the turning moment of a force:

Turning force or turning moment of a force

Increased by increasing the applied force or perpendicular distance

( = Torque (Nm) F = Applied force perpendicular to axis of rotation (N) d = Perpendicular distance between line of action and pivot (m))Formula:

= Fd

3.2Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces:

Forces on the sides ab and cd:

Experience maximum force since the current in them is perpendicular to the magnetic field

Magnitude of the force does not change throughout its rotation

Using the right hand palm rule, the direction of the force on sides ab and cd can be deduced

The net result of these two forces is to produce a torque on the loop about the axis; in the diagram above, the torque is acting in an anticlockwise direction

Forces on the sides bc and ad:

The sides of the loop, ad and bc, experience no force because the current is parallel to the magnetic field

Magnitude of the force varies from zero to maximum

Zero when the plane of the coil is parallel to the magnetic field (i.e. as above)

Maximum when the plane of the coil is perpendicular to the magnetic field

Net torque:

Maximum when the plane of the coil is parallel to the magnetic field (i.e. as above)

Direction alternates through a complete rotation

Current-carrying loop orientated in a plane at right angles to a magnetic field will experience no net force

( = Torque (Nm) n = number of turns/loops of the coilB = Magnetic field strength (T) I = Current flowing through the loop (A) A = Area of the loop (m2) = Angle between the plane of the loop and the field)Formula:

= nBIAcos

3.3Identify that the motor effect is due to the force acting on a current carrying conductor in a magnetic field:

Force on a Current-Carrying Conductor The Motor Effect:

Force on a current carrying conductor in a magnetic field causes it to move relative to the magnetic field

This movement caused by the forces is referred to as the Motor Effect

4 Motor Effect Applications

4.1Describe the application of the motor effect in the galvanometer and the loudspeaker:

The Galvanometer:

Device used to measure magnitude and direction of small DC currents

The Motor Effect:

When current flows through the coil, the coil experiences a force due to the presence of the external magnetic field

The iron core of coil increases the magnitude of this force

Needle rotated until magnetic force on the coil is equalled by a counter-balancing restraining spring

Scale of galvanometer is linear, amount of deflection proportional to current flowing through coil

The Loudspeaker:

Device that transforms electrical energy into sound energy

The Motor Effect:

A current-carrying coil interacting with a permanent magnet experiences a force as a result of the motor effect

This force causes the coil to vibrate rapidly back-and-forth, in turn making the speaker cone vibrate and send sound waves into the air

When the magnitude of the current increases, so too does the force on the coil

When the force on the coil increases, it moves more and the produced sound is louder

5 DC Electric Motors

An electric motor is a device which converts electrical energy to useful mechanical energy (usually rotation)

5.1Describe the main features of a DC electric motor and the role of each feature & identify that the required magnetic fields can be produced either by current-carrying coils or permanent magnets:

External Magnets:1. Permanent Magnets:

Made of ferromagnetic metals two permanent magnets curved around the armature on opposite sides of the motor, opposite poles face each other

Role: Supplies the magnetic field producing the motor effect

2. Electromagnetic coils:

A coil of current-carrying wire wound around a soft iron core coils are shaped to fit around the armature

Role: Provides a stronger magnetic field which can be switched on and off when required

Armature:

Cylinder of laminated iron mounted on an axle which coils are wound onto and placed inside the magnetic field

Role: Maximises the torque that can act on the coils, thus making the motor run more efficiently laminations reduce eddy currents which might otherwise overheat the armature

Rotor Coils:

Turns of wire wound onto the armature the ends of the coils are connected to bars on the commutator

Role: Provides the torque as the current passing through the coils interacts with the magnetic field. Any torque acting on the coils is transferred to the rotor (which the coils are mounted on) and hence to the axle

Split-ring commutator:

Cylindrical ring of metal mounted on the axle at one end of the armature cut into an even number of separate bars in which each opposite pair of bars is connected to one coil

Role: Reverses the direction of current flow every half-revolution of the motor to ensure that the torque on each coil is always in the same direction

Brushes:

Compressed carbon blocks, connected to an external circuit, mounted on opposite sides of the commutator spring loaded to make close contact with the commutator bars

Role: Provides the contact to conduct current into and out of the coils they are also responsible for maximising the torque on the axle

Axle:

Cylindrical bar of hardened steel, through the centre of the armature and commutator

Role: Provides a centre of rotation for the motor. Useful work extracted from the motor via a pulley or cog mounted on the axle

6 Michael Faraday

6.1Outline Michael Faraday's discovery of the generation of an electric current by a moving magnet:

After discovering that an electric current produces a magnetic field, in 1820, Faradays ideas about conservation of energy led him to believe that since an electric current could cause a magnetic field, a moving magnetic field should be able to produce an electric current.

In 1831, Faraday attached two wires through a sliding contact to touch a rotating copper disk located between the poles of a horseshoe magnet. This induced a direct current and was the basis to an electric generator.

Faradays explanation was that the electric current was induced in the moving disk as it cut a number of lines of magnetic force coming from the magnet (the magnetic field). The wires allowed the current to flow in an external circuit where it could be detected.

7 Magnetic Flux

7.1Define magnetic field strength (B) as magnetic flux density:

Representing Magnetic Fields:

Magnetic flux lines 'flowing' out of the north pole and into the south pole

Lin