A d.c. motor is a device that turns electrical energy into mechanical motion. Picture a tiny wheel (the rotor) that spins when you push a button. Inside the wheel is a coil of wire that carries current. When this current‑carrying coil sits inside a magnetic field, it feels a force that tries to rotate it. This is the turning effect.
The force on a current‑carrying wire in a magnetic field is given by the Lorentz force law:
\$\mathbf{F}=I\,\mathbf{L}\times\mathbf{B}\$
For a coil, the forces on opposite sides push in opposite directions, creating a torque that makes the coil spin.
The torque (turning effect) on a coil is:
\$\tau = N\,I\,A\,B\,\sin\theta\$
| Factor | Effect on Torque | Analogy |
|---|---|---|
| \$N\$ – Number of turns | Directly proportional (\$\uparrow\$) | More loops = stronger pull, like a stronger rope |
| \$I\$ – Current | Directly proportional (\$\uparrow\$) | More electrons = stronger push, like turning up a fan |
| \$B\$ – Magnetic field | Directly proportional (\$\uparrow\$) | Stronger magnet = stronger force, like a magnet pulling a metal nail |
When you press the button on a toy fan, the battery sends current through a coil inside the fan. The coil is placed in the fan’s magnetic field, so it starts spinning. If you replace the coil with one that has more turns, or use a stronger battery (higher \$I\$), the fan spins faster. If you also use a stronger magnet, the fan will spin even quicker. This is exactly how the torque equation works in practice.
The turning effect of a d.c. motor increases when you increase the number of turns, the current, or the magnetic field strength. All three factors multiply together in the torque formula.