A wire carrying an electric current induces a concentric circular magnetic field around it.
According to the right-hand grip rule, if the thumb points in the direction of current flow, the fingers will indicate the direction of the magnetic field lines around a current carry-ing conductor.
Fleming’s left-hand rule gives the direction of the magnetic force (represented by the thumb) which is 90 degrees to the magnetic field and the conventional current (represented by the second finget). The direction of motion of negatively charged particles is taken to be the opposite of positively charged particles.
A current-carrying conductor (or moving charged particle) will experience a magnetic force in a magnetic field unless the direction of current (or motion of charged particle) and the magnetic field are parallel. Stationary charged particles do not experience magnetic forces.
Magnetic field lines are drawn to represent magnetic fields. Lines that are close together indicate a greater magnetic field strength where a charged particle experiences a greater magnetic force.
An induced magnetic field is strongest near the conductor. It weakens as the distance from the conductor increases.Magnetic field strength also increases as the current in the conductor increases.
A charged particle moving in a magnetic field perpendicular to its direction of motion experiences a magnetic force that causes it to deflect in a circular path.
A pair of parallel wires attracts each other if the current in both wires are in the same direction. A pair of wires repels each other if the currents are in opposite directions.
A rectangular coil carrying an electric current in a magnetic field experiences a turning effect due to the magnetic forces acting on each side of the coil. This turning effect is used in the action of a simple d.c. motor.
The turning effect and speed of rotation of the d.c. motor is increased by:
- increasing the strength of the magnet;
- increasing the number of turns on the coil or increasing current;
- using the soft iron core within the coil.
THINGS TO NOTE
Similarity between a solenoid and a bar magnet.
When current flows through a solenoid, the solenoid creates a magnetic field around it. The solenoid behaves like a bar magnet.
Variation of net moment in a d.c motor.
The net moment acting on the coil in the d.c. motor depends on the orientation of the coil.