- current carrying rectangular coil is placed between 2 permanent magnets.
current carrying coils → experience an equal force on parts AB and CD of the wire.
- this force is created by the motor effect on each side of the coil. the forces produce a turning effect that is equal in magnitude on each wire.
- this causes the coil to turn in an anticlockwise direction as shown below
if the number of turns in the coil increases, the resultant force also increases → since it is the sum of the individual force acting on each turn of the coil.
If we add a soft iron cylinder into the core of the wire coil → the turning effect on the motor would also increase.
A soft iron core → concentrates the magnetic field lines and thus increases strength of magnetic fields.
a stronger magnetic field gives rise to a stronger force.
Definition
Turning effect can be increased by:
- increasing magnetic field strength.
- increasing number of turns per unit length in the coil
- increasing the current
- adding a soft iron cylinder to concentrate the magnetic field lines
in order to maintain the anticlockwise rotation → the current flowing in the coil must always flow clockwise.
- this is achieved through the use of a split ring commutator.
the split ring has 2 splits or gaps.
 |  |
|---|---|
| as the coil rotates → it momentarily disconnects itself from the carbon brushes. at this point → the current from the cell to the coil is cut off and the momentum of the coil carries it past this position | once the coil passes the vertical position, the split ring commutator comes into contact with carbon brushes again. - the current in the coil continues to flow in a clockwise manner. |
| direction of current flowing through parts AB and CD of the coil has reversed. | |
| in above fig → current flows from A to B and from C to D. | |
| after half a revolution → current flows from B to A and from D to C. |
the split ring commutator enables this to happen.
- therefore, we say that the function of the split ring commutator is to reverse the direction of the current in the coil every half a revolution
Carbon brushes: maintain good contact between split rings and external circuit Split Ring Commutator: reverse the direction of the current in the coil every half a revolution. forces produced near to the poles of the magnet will always be in the same direction so that the coil will rotate continuously in the same direction
principle of DC Motor answer technique (kind of)
- current enters through A and exits through B. applying Fleming’s left hand rule, a pair of forces is produced in the opposite direction causing the motor to rotate in clockwise direction
- after 90 degrees, contact between carbon brushes and commutators is broken, current is cut off, hence no force and no turning effect produced. coil continues to rotate due to the momemtumat /intertia of the coil
- a 180s degrees, current now enters B and exits through A (current reverses its direction). applying Fleming’s left hand rule, a pair of forces are produced that are exactly the same direction as in a. thus direction of rotation is maintained
Sample
when current flows in the coil with battery connected, magnetic field is set up in the coil, which interacts with the external magnetic field of the magnet. the combined magnetic field above the right coil is stronger producing a downward force on the right. similarly the combined magnetic field below the left coil is stronger producing a upward force on the left of the coil, causing coil to turn clockwise
Sample
explain why turning effect becomes smaller as coil becomes more vertical
- magnetic field strength of magnet decreases with distance from the centre, hence the combined magnetic field is weaker, leading to smalle turning effect
Sample
when motor turns there is an induced e.m.f in the coil.
- as the coil moves through the magnetic field of the magnet, it “cuts” the magnetic field lines, leading to a change of magnetic flux linkage to the coil
Important
please ensure that when you are talking about the split ring commutator, you state that it reverses direction of current for every **half a revolution