Explain the motor effect as the force on a current-carrying conductor in a magnetic field, and describe how this principle is used in electric motors and loudspeakers
A wire carrying current makes its own magnetic field. When the wire crosses a magnet's field, the two fields push on each other, so the wire feels a force called the motor effect. The force is largest at right angles and zero when the wire runs parallel to the field. The magnet also feels an equal and opposite reaction force.
The direction of the push depends on two things: which way the current flows, and which way the magnetic field points. Reverse either one, and the force flips to point the opposite way. Reverse both at once, and the force ends up pointing the same way it started.
In a simple battery-powered motor, a coil sits between a magnet's poles. Current runs in opposite directions along its two sides. The magnetic forces push one side up and the other down, forming a turning pair. A split-ring commutator switches the coil's connections every half turn to keep the turning effect in the same direction. Current can also heat the wire, but heating does not provide this turning effect.
A loudspeaker uses the same push. A coil attached to the speaker cone sits in the field of a fixed magnet. Current from an amplifier flows through the coil. As the current changes, the motor effect pushes the coil back and forth, and the cone moves with it, shoving the air to make sound.
For a wire of fixed length at right angles to a field, increasing the current or field strength increases the force. Increasing both increases it further. Keeping their directions unchanged keeps the force direction unchanged. Reversing only the current reverses the force, not its size. In a motor, larger forces give a larger turning effect, not a guaranteed higher running speed. Motor speed also depends on its circuit and load.
A motor turns electrical energy into movement energy as its coil spins. A loudspeaker turns electrical energy into cone movement, then sound in the air. Some energy is always wasted as heat in the coil. Useful energy out plus wasted heat equals electrical energy in. Heat wasted equals energy in minus useful energy out.
A current-carrying wire in a magnetic field feels a push called the motor effect, and reversing the current or the field reverses which way that push points.
Readings and practice questions written for this app. Videos by Khan Academy India, linked from YouTube.
You have seen Zara switch a magnetic push on and off with a coil, a nail, and a battery. Today she wants to know whether an invisible push can make something spin.
With the wire fixed in a stand at right angles to the magnet's field, Zara closes the switch. What does the current-carrying wire feel?
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