Moving-coil loudspeaker The moving-coil loudspeaker used in radio receivers and record players works by the force exerted on a current-carrying coil situated in a magnetic field (Fig. 38.9). Varying electric currents which correspond with the sound to be reproduced are passed through a short cylindrical voice coil which is free to move in the radial magnetic field set […]
Back e.m.f. in an electric motor When an electric motor is running it also acts as a dynamo (page 496) and so sets up a back e.mf. E; in opposition to the e.m.f E applied to the motor to drive it. The work done by the current against the back e.mJ. becomes transferred to useful work done […]
Efficiency of an electric motor If a practical electric motor is regarded as a machine for transferring electric energy to mechanical energy its efficiency will depend on its design. In this connection, the factors leading to energy losses are the same as those involved in the design of a transformer described in chapter 43. The only additional energy […]
Practical d.c. electric motors The simple electric motor as described above is not very powerful or efficient but it c~n ?e improved by increasing the number of turns in the rotating coil and also wmdmg them on a soft-iron armature. If there are n turns of wire instead of one turn the force on the side of the […]
The simple electric motor Fig. 38.6 illustrates the construction of a simple direct current (d.c.) electric motor. It consists of a rectangular coil of wire mounted on a spindle so that it can rotate between the curved pole pieces of a U-shaped permanent magnet. The two ends of the coil are soldered respectively to the two halves of […]
Use of mercury in the laboratory In the experiment just described the sparking which occurs when the spokes of the wheel leave the mercury hastens the production of mercury vapour, which is poisonous. So, the experiment should be done in a fume chamber or under an extraction hood. In all experiments involving the use of mercury care should […]
Barlow’s wheel Peter Barlow utilized the force on a conductor in a magnetic field in order to produce continuous rotation. His apparatus is illustrated in Fig. 38.5. It consists of a star-shaped wheel made of copper which is able to rotate in a vertical plane with its points dipping into a pool of mercury. A wire is taken […]
Force on a conductor explained by properties of magnetic field lines Fleming’s left-hand rule merely tells us the direction in which a current carrying wire moves in a magnetic field: it does not explain the motion. An explanation can, however, be given which is based on the properties of magnetic field lines. Faraday’s concept of magnetic field […]
Fleming’s left-hand rule (motor rule) Place the forefinger, second finger, and thumb of the left hand mutually at right angles. Then, if the Forefinger points in the direction of the Field and the second finger in the direction of the Current, the thumb will point in the direction of the Motion (Fig. 8.3).
The kicking wire experiment. Force on a conductor in a magnetic field The experiment illustrated in Fig. 38.2 has been designed to show how the direction of the force on a wire carrying a current is related to the direction of the magnetic field in which the wire is situated. A straight wire with a flexible connection […]