4.1 - Force on a Current-carrying Conductor in a Magnetic Field

Force and Catapult Field

A current-carrying conductor in a magnetic field experiences a force. Its direction depends on the directions of the current and the magnetic field.
The field produced by current in the conductor interacts with the permanent magnet’s field. Their resultant field is the catapult field, which exerts a resultant force on the conductor.
Where the two fields have the same direction, the resultant field is stronger; where they oppose each other, it is weaker.
The force acts towards the weaker field region. A dot represents current out of the paper; a cross represents current into the paper.
Use the right hand grip rule for the magnetic field around a straight conductor. The permanent magnet’s field runs from north to south.
In a force demonstration, an uninsulated copper wire rests on two uninsulated copper rods between Magnadur magnets on a U-shaped steel yoke. A low-voltage direct current supply provides the current.

Hold the thumb, index finger and middle finger perpendicular to one another.

Force increases with larger current or a stronger magnetic field. Bringing magnetic poles closer together strengthens the field.
A copper frame swings higher when the force is larger. A conductor on uninsulated tracks travels further when the force is larger.
An electronic balance can indicate force magnitude: an upward force on the copper wire produces an equal downward reaction on the magnets and balance pan. A larger force gives a larger balance reading.
Reset the electronic balance to zero before switching on the current for each comparison.
Comparisons use one magnet pair at 1 V and 2 V, then two magnet pairs at 1 V. These compare increased current and increased field strength; the balance readings are recorded in grams.

A rectangular current-carrying coil rotates about its axis in a magnetic field.
Catapult fields at opposite sides of the coil produce equal forces in opposite directions. This pair of forces turns the coil.
This turning effect is the working principle of a direct current motor, which changes electrical energy into kinetic energy.
Small motors occur in toys, portable drills and computer hard disks; larger motors occur in electric vehicles, lifts and factory rollers.

The split ring commutator rotates with the rectangular coil. The carbon brushes remain fixed and maintain contact with the commutator.
In the illustrated first half rotation, current flows $A \to B \to C \to D$ : side AB experiences a downward force and side CD an upward force.
After half a turn, each brush contacts the other commutator half. Current in the coil reverses to $D \to C \to B \to A$ : AB experiences an upward force and CD a downward force.
These changes keep the coil rotating in the same direction in successive half rotations.

Rotation becomes faster when the current in the coil increases, the magnetic field becomes stronger, or the coil has more turns.
A simple motor comparison uses a copper coil supported by large paper clips, Magnadur magnets and a direct current supply. Increasing the supply from 4.0 V to 6.0 V, adding a magnet and increasing the coil turns demonstrate these effects.
Precautions: switch off immediately after observing; allow current to flow briefly so that the copper coil does not overheat.

Brushless motor	Brushed motor
Coil is stationary; magnet rotates.	Magnet is stationary; coil rotates.
No carbon brushes or friction between brushes and commutator.	Brush–commutator friction wears out the carbon brushes.
No sparking at the commutator.	Sparking occurs at the commutator.
Quieter operation.	Louder operation.

A simple homopolar motor uses a neodymium magnet, an AA dry cell and copper wire.
Michael Faraday constructed and demonstrated a homopolar motor in London in 1821.