The DC MotorCambridge IGCSE Physics: Revision notes
Section 1
Why Does a Current-Carrying Coil in a Magnetic Field Turn?
When a coil carrying an electric current sits inside a magnetic field, each side of the coil is a current-carrying conductor at right angles to the field, so each side experiences a force. Because the two long sides of the coil carry current in opposite directions, the forces on them act in opposite directions too — this pair of forces produces a turning effect (a couple) that rotates the coil.
The size of this turning effect can be increased by:
- increasing the number of turns on the coil
- increasing the current flowing through the coil
- increasing the strength of the magnetic field the coil sits in
Three separate ways to make the motor turn faster: more turns, more current, stronger field. Learn all three — exam questions often ask for two or three at once.
Section 2
What Is Inside a DC Motor?
A simple d.c. electric motor consists of a rectangular coil of wire mounted on an axle, free to rotate between the poles of a magnet. Current reaches the spinning coil through two carbon brushes that press against a split-ring commutator fixed to the axle.
The split-ring commutator is a ring split into two halves, each connected to one end of the coil. As the coil rotates past the vertical position, the commutator swaps which half of the coil connects to which brush — this reverses the direction of current in the coil at exactly the right moment.
Without the commutator reversing the current every half-turn, the coil would feel forces alternately clockwise and anticlockwise and only rock back and forth instead of spinning continuously.
Section 3
Which Way Does the Force Act on the Wire?
A current-carrying conductor placed in a magnetic field experiences a force whenever the current is not parallel to the field. The force, the magnetic field, and the current are all mutually at right angles to one another.
The direction of the force can be found from the relative directions of the current and the field: if either the current direction or the field direction is reversed, the direction of the force reverses too. Reversing both the current and the field at the same time leaves the direction of the force unchanged.
In a motor, this force acts on both sides of the coil, in opposite directions on each side, which is exactly what produces the turning effect.
Must Know
- A current-carrying coil in a magnetic field experiences a turning effect because opposite sides of the coil feel forces in opposite directions
- Turning effect increases with more turns, more current, or a stronger magnetic field
- The split-ring commutator reverses the current in the coil every half turn so it keeps spinning the same way
- Carbon brushes maintain contact between the fixed circuit and the rotating commutator
- Reversing either the current or the field alone reverses the direction of the force; reversing both leaves it unchanged
That's the notes covered.
Carry on to the next subtopic.