About This Simulation: The Two-Pole DC Motor

In a current-carrying coil placed within a magnetic field, the coil experiences a turning effect due to the interaction between the magnetic field and the electric current. This simulation recreates the classic bench model: a rectangular coil on a vertical axle sits in the near-uniform field between the North (red) and South (blue) poles of two magnets. Current from a variable DC power supply reaches the coil through two carbon brushes pressing on a split-ring commutator. Each vertical side of the coil experiences a force F = (I × B) L — equal in size, opposite in direction on the two sides — forming a couple that rotates the coil. The action of the split-ring commutator in this two-pole, single-coil motor is crucial: every half revolution it reverses the current in the coil just as the coil passes the vertical, so the turning effect always acts in the same rotational sense.

Concepts Illustrated

• Force on a current-carrying conductor: F = (I × B) L, where I and B are vectors and L is the scalar length of wire in the field; magnitude F = BIL sinθ.
• Fleming's left-hand rule (F-B-I): apply it to each vertical side of the coil and check against the magenta force arrows.
• Turning effect of a couple: the two coil sides carry current in opposite directions, so their forces form a couple whose torque varies with coil orientation.
• The split-ring commutator: reverses the coil current every half turn, keeping the torque one-way — remove this idea and the coil would just oscillate.
• Back-EMF: the rotating coil is also a generator; its induced EMF opposes the supply, so the coil current falls as the motor speeds up (I = (V − ε_back)/R).
• The dead spot: at the commutation position the brushes bridge the gaps and the torque is zero — a real motor relies on inertia (or a push!) to carry it through.

Check Your Understanding

1. With the North pole on the right and current flowing up the near side of the coil, use Fleming's left-hand rule to predict the direction of the force on that side. Check against the magenta arrow.
   Answer

   Point the First finger with the Field (from N on the right to S on the left), the seCond finger with the Current (up), and the thuMb gives the Force — horizontal, perpendicular to both, pushing that side of the coil around the axle.
2. Why does the coil need a split-ring commutator, and what would happen if it were replaced with two plain slip rings?
   Answer

   Without commutation the force on each coil side would reverse its turning effect every half revolution, so the coil would swing back and forth about the vertical position instead of rotating continuously. The split ring reverses the coil current at exactly the right moment so the torque always acts in the same sense.
3. The instructions say to "give the motor a slight push if it does not start immediately". Explain the physics of why a push is sometimes needed.
   Answer

   If the coil happens to rest at the commutation (vertical) position, the brushes sit on the gaps of the split ring — no current flows and the torque is zero. A small push moves the coil off the dead spot so current flows and the magnetic torque can take over.
4. As the motor speeds up, the current reading falls even though the supply voltage is unchanged. Why?
   Answer

   The rotating coil cuts field lines, so by electromagnetic induction it generates a back-EMF that opposes the supply (Lenz's law). The net driving voltage is V − ε_back, so I = (V − ε_back)/R decreases as ω (and hence ε_back) grows.
5. Predict: what happens to the direction of rotation if you (a) reverse the supply polarity only, (b) swap the magnet poles only, (c) do both?
   Answer

   (a) Reverses the rotation — current direction flips, so all forces flip. (b) Also reverses the rotation — field direction flips. (c) Both flips cancel: the motor spins in the original direction.