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When two different magnetic fields (for example from a solenoid and a permanent magnet) interact with one another, a force is experienced from a region of stronger magnetic field to a region of weaker magnetic field.
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Left-hand Rule: Directions of current, magnetic field and force are all perpendicular to each other.
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Two parallel wires with current in same direction are attracted to each other.
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Two parallel wires with current in opposite directions are repelled from each other.
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Working of loudspeaker
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Cone produces compressions and rarefactions in the air because it vibrates with the coil.
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The coil vibrates because
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Current in coil produces magnetic field
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Poles of coil are reversed when the current changes direction
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The coil experiences attraction and repulsion from the N (or S)-pole of permanent magnet.
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Working of D.C. Motor
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When current flows through coil, it becomes a magnet.
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Its magnetic field interacts with the magnetic field of the permanent magnet. Hence it experiences forces.
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To keep the coil rotating, the forces need to be reversed after every half turn. Therefore,
split-ring commutator reverses the direction of current after every half turn.
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