Electric motors are essential components found in large machinery like electric trains as well as tiny everyday gadgets like smartphones. Most electric motors operate on a fundamental electromagnetic principle: when a current-carrying conductor is placed inside an external magnetic field, it experiences a magnetic force. This topic explores how this force is formed through catapult fields, how to determine its direction using Fleming's left-hand rule, the factors determining its magnitude and rotational speed, and how brushed and brushless direct current (d.c.) motors function.
See how opposing and reinforcing magnetic field lines create strong and weak field regions resulting in a net force, alongside the hand orientation of Fleming's left-hand rule.
When a conductor carrying an electric current is placed in an external magnetic field, it experiences a magnetic force. Reversing the direction of the electric current reverses the direction of the force. Reversing the magnetic poles also reverses the direction of the force.
The force is produced by the interaction between the magnetic field produced by the electric current and the uniform magnetic field from the permanent magnets. The resultant combined field is known as a catapult field.
Used to determine the direction of the force acting on a current-carrying conductor. The Thumb points in the direction of the Force, the Index finger points in the direction of the Magnetic Field, and the Middle finger points in the direction of the electric Current.
Increasing the current increases the force acting on the conductor. Using stronger magnets or placing magnetic poles closer together also increases the force acting on the conductor.
Answer in your mind, then press “Check Answer”.
What is meant by a catapult field?
State the direction in which a current-carrying conductor moves within a catapult field.
Why is a split-ring commutator needed in a direct current motor?
The force on the wire is caused by electrostatic attraction or repulsion between charges.
The force is entirely magnetic, arising from the vector superposition and interaction of two magnetic fields: the field of the external magnet and the field generated by the moving charges in the wire.
A current-carrying wire will experience a force in any orientation within a magnetic field.
If the conductor is placed parallel to the magnetic field lines, no catapult field is formed and no magnetic force acts on the conductor. The force is maximum when the conductor is perpendicular to the field lines.
The commutator changes the d.c. power supply into an a.c. supply outside the motor.
The power supply remains direct current; the split-ring commutator acts internally as a mechanical switch to alternate the current direction within the rotating coil itself every half-turn.
Objective: Identify catapult field, Fleming's left-hand rule, commutator and motor characteristics.
1 Which of the following correctly pairs each finger in Fleming's left-hand rule with the physical quantity it represents?
2 What is the primary role of the split-ring commutator in a simple direct current motor?
3 In a catapult field, the resultant force acts on the conductor from the region of magnetic field to the region of weaker magnetic field.
4 In a brushless direct current motor, the is stationary while the permanent magnet rotates.
Drag each term to the correct meaning. If using a phone, tap the answer first, then tap the matching box.
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