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P6E Magnetic fields and wires A wire carrying an electric current produces a magnetic field. The shape of this field depends upon the shape of the wire. A straight wire The magnetic field around a straight wire consists of ‘concentric circles’ (circles around the same centre). These are at right angles to the direction in which the electric current flows. Study the animation below to make sure you understand this. Note that the direction of the magnetic field is reversed if the direction of the electric current is reversed. A single turn in a coil of wire The magnetic field around a coil of wire with a single turn also consists of concentric circles, but there are two sets of them acting in opposite directions. A solenoid A solenoid consists of a long piece of wire made into several coils. Its magnetic field is the same as the magnetic field produced by a simple bar magnet. Current upwards Current downwards No current Uses of electric motors Electric motors transfer electrical energy in two main ways - to the load (as useful work) and to the surroundings (mostly as waste heat energy). Motors are found in a variety of everyday applications. For example, they are used in: Current away fans electric drills food processors washing machines CD and DVD players car windscreen wipers You should be able to recall uses like these. DC electric motors A force can act on a wire carrying an electric current when it is in a magnetic field. This does not happen if the wire is parallel to the magnetic field, but it does happen when the wire is at right angles to the magnetic field. The direction the wire moves depends on the direction of the current and the direction of the magnetic field. The direction of the wire is reversed if: the direction of the current is reversed the direction of the magnetic field is reversed Current torwards A turning force is produced if the wire is made into a coil. This effect is used in simple DC (direct current) electric motors. The turning force in a motor increases when the number of turns on the coil is increased. It also increases when: the electric current is increased the strength of the magnetic field is increased DC (direct current) electric motors Fleming’s left-hand rule Fleming’s left hand rule is a simple way to predict the relative directions of the movement of the wire, the magnetic field and the electric current. You need to point your thumb, forefinger (index finger) and second finger at right angles to each other for this to work. Then, as you can see in the diagram below, your thumb shows the movement of the wire, your forefinger that of the field and your second finger that of the current. More about motors In a simple DC (direct current) electric motor, the direction of the electric current must be changed every half-turn. This means that the force on the coil is maintained as the coil turns. The coil’s momentum carries it on at the start of each half-turn. The use of a split-ring commutator allows the coil to make electric contact with the DC supply, while ensuring that the current changes direction every half-turn. Practical electric motors use curved magnets rather than flat ones. These produce a radial magnetic field - which means that the electric current stays at right angles to the magnetic field for longer.