In our daily lives we can come across several devices that make use of a basic principle of magnetism: the magnetic force. In our studies on magnetism we saw that when we place a charge in a magnetic field, a force called magnetic force arises on it .
In the case of a conductor carrying an electric current, when we place it in a magnetic field, a magnetic force appears on it. This force can be used in a range of devices such as ammeters, galvanometers and motors.
Most of the electric motors that are currently used work based on the effect of rotation of forces acting on coils placed in a magnetic field. The motor shown in the figure below is a direct current motor, such as the starter motors of automobiles or battery powered motors in toy cars.
The working principle of these motors consists of a conductor with a rectangular shape, which can generate around an axis and travel by an electric current i and immersed in a magnetic field of induction B . Magnetic forces acting on two branches create a couple of forces that tend to rotate the conductor about axis e .
To understand the operation of a galvanometer, let’s do the analysis in the figure below.
we have a rectangular loop CDEG that is placed inside a uniform magnetic field of induction B. If an electric current i is passed through this loop, with the indicated direction, it is noticed that the sides EG and DC will be under the action of magnetic forces of equal magnitude, which will cause torques in the loop, causing it to rotate around the axis OP, in the indicated direction.
To increase the effect of rotation, that is, to increase the sensitivity of the device, several turns are used, generally wound in a cylinder.