Force on a Current-carrying Conductor in a Magnetic Field

Catapult Force

When a current-carrying conductor is placed in a magnetic field, the interaction between the two magnetic fields will produce a force on the conductor, which called a catapult force.

Fleming's Left Hand Rule (Motor Rule)

The direction of the force can be determined by Fleming's left hand rule

The fore finger, middle finger and the thumb are perpendicularly to each other. The forefinger points along the direction of the magnetic field, middle finger points in the current direction and the thumb points along the direction of the force.

Strength of the Catapult Force

The strength of the force can be increased by:

• Increase the current
• Using a stronger magnet

The Catapult Field

The interaction of the two magnetic fields (the magnetic field of the current-carrying wire and the magnetic field of the permanent magnet) produces a resultant field known as catapult field as shown in the figure above.

The non-uniform field produces the catapult force from the stronger field to the weaker field.

Force between 2 current carrying conductor

When 2 current carrying conductors are placed close to each other, a force will be generated between them.

If the current in both conductors flow in the same direction, they will attract each other, whereas if the currents are in opposite direction, they will repel each other.

Force between 2 current carrying conductor - How it's produced and the field pattern?

This force is due to the interaction between the magnetic field of the 2 conductor. The figure above shows the catapult field produced by 2 current carrying conductors when their current is in the same direction or opposite direction.

Turning Effect of a Current-carrying Coil in a Magnetic Field

If a current carrying coil is placed in a magnetic field, a pair of forces will be produced on the coil. This is due to the interaction of the magnetic field of the permanent magnet and the magnetic filed of the current carrying coil.

Direct Current Motor

Electric motor converts electrical energy to kinetic energy. It consist a rectangular coil of wire placed between 2 permanent magnets. The coil are soldered to a copper split ring known as commutator. 2 carbon brushes are held against the commutator.

The function of the commutator is to change the direction of the current in the coil and hence change the direction of the couple (the 2 forces in opposite direction) in every half revolution. This is to make sure that the coil can rotate continuously.