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USING ELECTRICITY SUMMARY NOTES Section 6 — Movement from Electricity Electrodynamics (a) A magnetic field exists round a wire which is carrying an electric current. (b) An electromagnet is made up of a coil of wire wound on an iron core. It works as a magnet only when there is an electric current in the coil. (c) Electric bells and relays make use of the principle of the electromagnet. Current-carrying Conductor in a Magnetic Field When a wire carrying a current is placed in a magnetic field, a force acts on it. The direction of this force depends on the direction of the current and on the direction of the magnetic field. If either is reversed, the direction of the force is reversed. When the field and the current are at right angles to each other, the force is at right angles to both. The Electric Motor When a rectangular loop of wire carrying a current is placed in a magnetic field, forces act on the two sides of the loop. They act in opposite directions and tend to make the loop turn. If the loop is free to turn - (i), it will move until it reaches the position shown in (ii). At this position, the forces no longer have a turning effect. In practice, the loop will be carried slightly past this position due to its inertia. If the current through the loop is now reversed - (iii), the two forces will again have a turning effect causing the loop to make another half turn in the same direction. By reversing the current every half turn, the coil will keep rotating in the same direction. To do this automatically, a split ring commutator is used with sliding contacts, called brushes, touching it. By using a coil of wire instead of a single loop, a much greater turning force is produced. Commercial Motors Field Coils In a 'real' motor, the magnetic field is provided by fixed coils, usually called field coils, which carry the current. Advantage: This is a more compact, lighter and cheaper way of providing a magnetic field than using a permanent magnet. Brushes The brushes in commercial motors are made of carbon (graphite) Advantages: because carbon is a good conductor and it is soft enough to wear away as it slides over the commutator, without wear occurring to the commutator itself. The brushes are held against the commutators by springs so that they always make good contact with it. If the brushes wear away completely, they can be replaced. Rotating Coil and Commutator The commutator is made up of several segments with a separate coil connected to each pair of segments directly opposite each other. Advantage: The greatest turning force exists when a coil is at right angles to the direction of the magnetic field; so, by having several coils mounted at various angles to one another, one of the coils is always moving at approximately right angles to the magnetic field. This makes the rotation of the coil much smoother.