Force on each side: the two sides of the coil that run along the axle are at right angles to the field, so each one feels a force F = nIlB. The currents in the two sides go in opposite directions, so one side is pushed up and the other is pushed down. Use the right-hand rule (or Fleming's left-hand rule) to check the directions in the end view.

Torque: the two forces make a turning effect about the axle.

τ = nIAB sinθ

Here θ is the angle between the normal to the coil (gold arrow) and the field. The torque is largest when the plane of the coil is parallel to the field (θ = 90°) and zero when the plane of the coil is at right angles to the field (θ = 0° or 180°). Some books measure the angle between the plane of the coil and the field instead, and write τ = nIAB cosφ. Both give the same answer.

Commutator: every half turn, the split ring swaps which half touches each brush. This reverses the current in the coil, so the torque keeps acting the same way. While the brushes touch the gaps, no current flows and the coil carries on by its own momentum. If the coil stops in that position, the motor cannot start by itself.

Coil: 4.0 cm wide, 5.0 cm long, so A = 2.0 × 10^-3 m²

In this model the motor turns against friction, so it settles at a steady speed where the motor torque, averaged over one turn, balances the friction. A real motor also produces a back emf that reduces the current as it speeds up. This model leaves the back emf out and keeps the current fixed.