In DC unlike AC the electrons have a constant flow, thus having a constant voltage. Here we have what an oscilloscope would show us when connected to a DC circuit, as you see it’s just a straight line as again the voltage is constant. (this would be a 1.5V battery).
This experiment showed that there is a connection between electricity and magnetism. He proved this by showing that whenever a current was switched on through a wire, it made a compass needle turn so that it would be positioned perpendicularly to the wire. This would prove that the current had produced a magnetic field strong enough to cause the compass needle to turn.
Faraday´s First Law on electromagnetic induction states that any changes in the magnetic field of a coil of wire will cause an electromotive force to be induced in the coil. This force is called induced EMF and if the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.
Faraday´s Second Law on electromagnetic induction states that the magnitude of emf induced in the coil is equal to the rate of flux change that links with the coil. The flux linkage of the coil is the product of the number of turns in the coil and the flux associated with the coil.
Lenz's law states that the direction of the current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field that produced it.
Although more complex DC motors will have sets of coils as stators just like in the induction motor, simpler and smaller dc motors, will have permanent magnets as stators. These permanent magnets will then create a magnetic field inside of the stator, the same would happen with the sets of coils since these would be connected by an electric current which then will create an induced magnetic field.
The rotor which stays inside of the rotor is composed of many components of it’s own. First, we have the copper windings, these are connected to a commutator which will be indirect contact with 2 spring loaded brushes. These brushes will be the ones who the electric current will flow through. Since we will have an electric flowing through the copper windings it will create an induced magnetic field.
The magnetic field inside the rotor will interact with the one of the stator, consequently making all of the rotor rotate. Each time the brushes make contact with different commutator segments the polarity of the circuit will reverse which will reverse the polarity of the induced magnetic field as well. This reversing of the polarity will continue to happen creating a continuous rotation of the rotor.
Concluding, while the induction motor relied on its alternating current to reverse the polarity of the induced magnetic field to then make it rotate the DC motor runs on a constant voltage,thus it needs the help of the commutator and the brushes to “manually” reverse the polarity of the circuit.
