This Physics quiz is called 'Electricity - The Motor Effect' and it has been written by teachers to help you if you are studying the subject at senior high school. Playing educational quizzes is one of the most efficienct ways to learn if you are in the 11th or 12th grade - aged 16 to 18.
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When an electric current passes through a wire, it creates a magnetic field around the wire. If this magnetic field is placed in another magnetic field, they interact, causing the wire to move. This is known as the motor effect. Electric motors that work off direct current are found in many devices we use in everyday life. They are energy transfer devices that convert electrical energy into rotational kinetic energy. As with any energy transfer, it is never a perfect conversion, some energy is always wasted. In the case of an electric motor, the wasted energies are heat (from friction and the heating effect of an electric current) and sound.
The motor effect is sometimes called the catapult effect since that is the effect on a single current-carrying wire in a magnetic field.
If either the current or the magnetic field is reversed, the wire is catapulted in the opposite direction. The reason that a motor turns is because the wire is in the form of a coil wound onto a block called an armature. As the current flows through the coil, it is flowing in opposite directions on each side. One part of the coil is being catapulted out of the magnetic field in one direction; the other side is being catapulted in the other direction so the coil is able to turn on an axle. Fleming's left hand rule is a favorite with the senior high school examiners. It is used to predict in which direction a current-carrying wire will move in a magnetic field so make sure that you revise it well.
If the two ends of the wire used to make the coil were connected directly to the electricity supply, they would soon twist together, preventing the coil from turning. To overcome this problem, a commutator is used. In a DC motor, this is a split-ring commutator. This consists of two semi-circular pieces of copper, separated by a thin strip of an insulating material. The commutator is connected to the electricity supply by two carbon brushes. These are held against the commutator using springs.
The discovery of the motor effect is usually credited to Michael Faraday, however, as is often the case in science, it was based on earlier work by others. The theory had already been developed by André Ampère and the commutator was developed a few years after Faraday's demonstration of the motor effect by the Hungarian scientist Ányos Jedlik. It took about 10 years after the invention of the commutator for useful DC motors to be built and about 50 years before the world's first commercially successful electric motor to be built.