When a wire is placed in a magnetic field and current flows through it, the wire experiences a force. This force is the driving force that makes motors rotate.
Basic Knowledge
A current placed in a magnetic field experiences a force in a direction perpendicular to both the direction of the magnetic field and the direction of the current. The direction of this force is found using Fleming’s left-hand rule:
・Middle finger: direction of current
・Index finger: direction of the magnetic field
・Thumb: direction of the force (direction in which the wire moves)
Ways to increase the force:
① Increase the current
② Strengthen the magnetic field
③ Increase the length of the wire
Reversing either the current or the magnetic field also reverses the direction of the force. Reversing both returns the force to its original direction.
Fleming’s left-hand rule (middle finger = current, index finger = magnetic field, thumb = force)
Force exerted on a current by a magnetic field (current, magnetic field, and force are mutually perpendicular)
Motor (electric motor) (a device that rotates because of the force exerted on a current by a magnetic field)
Commutator (a component that switches the direction of current flowing through a motor coil)
Brush (a component that contacts the commutator to conduct current)
DC motor (a motor powered by direct current. It has a commutator)
Further Exploration (Background and Significance)
A motor uses the force exerted on a current in a magnetic field to produce rotational motion. Its basic structure is:
① Create a magnetic field with a permanent magnet (or electromagnet)
② Pass current through a coil
③ The forces on the two sides of the coil are opposite → rotational torque is produced
④ Switch the direction of current with the commutator each time the coil makes half a rotation, maintaining continuous rotation
Motors are the unsung heroes of modern society. They are at the heart of electric vehicles, air conditioners, washing machines, fans, elevators, Shinkansen trains, hard disks, CD players, drones, robots, and countless other machines.
Motors and generators are two sides of the same machine. Supplying current makes it a motor; rotating it from outside makes it a generator. This is one of the most important physical insights in middle-school science.
In the 1830s, Faraday and the English scientist Sturgeon established the basic principles, laying the foundation for the electrification revolution of the latter 19th century.
- Fleming’s left-hand rule (current, magnetic field, force)
- Increase force: increase current, strengthen the magnetic field, lengthen the wire
- Reversing either the current or magnetic field reverses the force
- Motor = rotation caused by the force on a current in a magnetic field
- The commutator switches the current direction for continuous rotation
- Motors are at the heart of modern society
- Motors and generators are two sides of the same machine
Points to Note (Easily Confused)
① Fleming’s left hand represents the “force on a current” (motor), while the right hand represents “induced current” (generator). It is easy to confuse the hands. ② Remember the order middle finger = current, index finger = magnetic field, thumb = force as “current, magnetic field, force.” ③ Both current and magnetic field are necessary. A force does not arise with only one of them. ④ Do not confuse the commutator (DC) with slip rings (AC).
Practice
- According to Fleming’s left-hand rule, what does the thumb represent?
- What happens to the direction of the force on a wire in a magnetic field when the current direction is reversed?
- What is the name of the component that switches the direction of current flowing through the coil in a motor?