Motors

Noadswood Science, 2013

Motors

To know how motors function and Fleming’s left hand rule

Thursday, April 20, 2023

Magnetic Fields

• A magnetic field is a region where magnetic materials and also wires carrying currents experience a force acting on them

• They can be represented using field diagrams (arrows from North to South)

• Strengths of a magnetic field can be increased using a magnetically “soft” iron core – these materials magnetise and demagnetise easily (electromagnets)

Magnetic Field

• When a current flows through a wire a magnetic field is created around the wire – the field is made up of concentric circles with the wire in the centre – this is shown by the right-hand thumb rule

Magnetic Field

• The magnetic field inside a coil of wire (solenoid) is strong and uniform, whilst outside the magnetic field is like that of a bar magnet

Electromagnets

• When electricity is passed through a coil of wire, the coil has a magnetic field around it – this is an electromagnet

• If the coil of wire is wrapped around a piece of iron, such as an iron nail, the magnetic field gets stronger

Electromagnets

• A magnetic field is produced when an electric current flows through a coil of wire - this is an electromagnet (a solenoid is a long coil of wire)

• Electromagnets strength can be increased via the following: -

– Wrapping the coil around an iron core

– Adding more turns to the coil

– Increasing the current flowing through the coil

Electromagnets

• The magnetic field around an electromagnet is just the same as the one around a bar magnet

• It can, however, be reversed by turning the battery around

• Unlike bar magnets, which are permanent magnets, the magnetism of electromagnets can be turned on and off just by closing or opening the switch

Electromagnets

• More turns = stronger electromagnet

• More current = stronger electromagnet

• Iron core = stronger electromagnet as it increases the magnetic field strength (plus iron is magnetically “soft” so it can be turned on/off (steel is magnetically “hard”, retaining its magnetism (good for permanent magnets))

Core

Crocodile clips

Connecting wires

Power pack

Insulated wires

The Motor Effect

• The motor effect is a force experienced by a current-carrying wire in a magnetic field

• The force gets bigger if either the current or the magnetic field is made bigger

• If the current of magnetic field direction is reversed then the direction of the force is reversed too (this can be worked out using Fleming’s left hand rule)

Fleming’s Left Hand Rule

• Fleming’s left hand rule shows which way a force will act: -– Thumb shows motion– First finger shows field– Second finger shows current

Fleming’s Left Hand Rule

• For example, using Fleming’s left hand rule which direction is the force on the wire…

Fleming’s Left Hand Rule

• Draw the current (+ve to –ve); apply the left hand rule and then draw in the direction of force (motion)

Simple Electric Motors

• Simple electric motors are affected by current and the magnetic field strength – if either of these is increased the speed of the motor increases

• The forces are those found which act on any current in a magnetic field, however as the coil is on a spindle the forces act one up and one down (causing rotation)

Simple Electric Motors

• The split-ring commutator swaps the contacts every half turn keeping the motor rotating in the same direction

• A simple motors direction can be reversed by swapping the polarity of the direct current or swapping the magnetic poles

Simple Electric Motor

• Which way is the coil turning in this simple motor?

Simple Electric Motor

• Which way is the coil turning in this simple motor?

• Charged particle in a cathode ray tube example.