Power a motor. Reverse its direction. Add a load and watch speed, current and turning force find a new balance.
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A magnetic field pushes on current-carrying wires. Arrange those pushes around an axle and they create torque. A brushed motor switches its rotor connections as it turns, keeping the turning effect going.
At the same supply voltage, what happens when this motor’s load increases?
Slower rotation produces less back EMF. More voltage is left across the winding resistance, so current and motor torque increase. Compare light and heavy drag after the same running time.
Permanent magnets belong to the stator, which stays fixed. The rotor carries coils and a shaft. Brushes stay still against a segmented commutator attached to the rotor. The 3D view separates these roles with a transparent enclosure.
As the rotor moves through the magnetic field it also acts as a generator. Its back electromotive force, or back EMF, opposes the supply. Current falls as speed builds. Reverse the supply and the average torque and rotation reverse.
The virtual load is a viscous brake: its opposing torque is proportional to speed. More drag leads to a lower settled speed and a larger current. A real fan often has a more strongly speed-dependent load, so this is a motor rig with a fan-shaped shaft marker, not an airflow prediction.
I = (V − kω)/R; τ = kI; J dω/dt = τ − bω. Here k = 0.03 N·m/A = 0.03 V·s/rad, R = 2 Ω and J = 0.002 kg·m². These are illustrative constants. Inductance, brush drop and torque ripple are omitted.
For a start from rest, ω∞ = (kV/R)/(b + k²/R) and T = J/(b + k²/R). Then ω(t) = ω∞(1 − exp(−t/T)). The shaft angle is the integral of this expression. Negative supply produces the mirrored response.
VI = I²R + τω. Mechanical power changes rotational kinetic energy and is dissipated by drag. The steady balance is kI = bω. A stalled real motor can overheat; this model does not calculate temperature or safe operation.
Draw two semicircular contacts with gaps between them. Mark two fixed brush positions on opposite sides. Use one pencil for the positive brush and one for the negative.
Place your paper arrow across the circle to stand for the rotor. Rotate it half a turn. Notice that each rotor contact now meets the opposite fixed brush.
Sketch a wire loop connected to the contacts. Trace how its current reverses relative to the loop each half-turn. Compare with the motor’s mechanism view. This drawing shows switching, not an operating motor.
Can you keep a turning effect pointing the same way as a coil turns over?
A conceptual paper model: it does not create a magnetic force or reproduce a real commutator’s contact timing. No electrical supplies are needed.
University Physics Volume 2 §11.5, Fig. 11.15 and current-loop torque derivation. Supports component roles, not the exact three-coil geometry.
OpenStax · Magnetic torqueCollege Physics §23.6, motor circuit and loading discussion. The constants and exact zero-inductance solution are Brytalearn teaching choices.
OpenStax · Back EMFUniversity Physics Volume 1 §10.8, rotational work and power. Supports the mechanical and energy balance.
OpenStax · Rotational powerIndependent subject review is pending.
Read the sources and model assumptionsClose a switch. Follow a complete path. Give current a second route and discover what changes.
Turn once. Count what happens. Discover gear ratios, real-world uses, and why different jobs need different gears.
Interactive explorationFollow a transparent car around a bend, open its rear axle, and discover how a differential shares motion, torque and a low-grip limit.