Fleming's Left-hand Rule
For the maximum force, the current and magnetic field are at right angles, and the motion is at right angles as well. This is summed up in Fleming's Left Hand Rule:
ThuMb = Motion
First Finger = Field
SeCond Finger = Current
The Simple Electric Motor
The electric motor uses the interaction of a magnetic field on a current to produce a force. This is true from the smallest toy motor to a large traction motor.
The main parts of the motor are:
the magnets that provide the magnetic field. In this motor they are permanent magnets, but in many motors they are electromagnets.
the armature, which is a coil of wire free to turn. The armature in this motor is made of a single coil of wire. It has two poles.
The commutator, which connects the armature to the brushes. The current passes to the armature by the commutator.
The brushes that bring the current to the commutator.
When the current is turned on, the force from the current and the magnetic field makes the left hand side of the armature go up, while the right hand side goes down. The motor turns clockwise.
The Simple Electric Motor (2)
The force which a simple motor can produce can be increased by:
increasing the current through the armature;
increasing the number of turns in the coils on the armature;
increasing the magnetic field strength. In a motor where the magnetic field strength is provided by electromagnets, the magnetic field strength can be changed quite easily.
This is a common problem with two pole motors, which have a single coil of wire.
Often a two pole motor needs a spin to get it going. Most toy motors have three poles. Therefore they can start whatever position the poles are. Larger motors have many more.
Electric motors can be made more efficient by having more poles, and circular magnets, which means that the magnetic field remains at 90 degrees all the time.
The Motor Effect (experiment)
The carbon rod is NOT magnetic.
When no current flows, the rod is stationary
When we turn on the current, the rod experiences a force that makes it move
The direction of the force is determined by Fleming's Left Hand Rule
In the exam you need to know that:
When a conductor carrying an electric current is placed in a magnetic field, it may experience a force.
The size of the force can be increased by:
- increasing the strength of the magnetic field
- increasing the size of the current.
The conductor will not experience a force if it is parallel to the magnetic field.
The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.