Context and question: Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. Sparks are created by the brushes making and breaking circuits through the rotor coils as the brushes cross the insulating gaps between commutator sections. Depending on the commutator design, this may include the brushes shorting together adjacent sections and hence coil ends momentarily while crossing the gaps. Furthermore, the inductance of the rotor coils causes the voltage across each to rise when its circuit is opened, increasing the sparking of the brushes. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity, limits the output of the motor. The making and breaking of electric contact also generates electrical noise; sparking generates RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance (on larger motors) or replacement (on small motors). The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the whole rotor.
What don't brushes need to be in contact with?
Answer: unanswerable
Context and question: Between 1949 and the 1980s, telephone communications in Greece were a state monopoly by the Hellenic Telecommunications Organization, better known by its acronym, OTE. Despite the liberalization of telephone communications in the country in the 1980s, OTE still dominates the Greek market in its field and has emerged as one of the largest telecommunications companies in Southeast Europe. Since 2011, the company's major shareholder is Deutsche Telekom with a 40% stake, while the Greek state continues to own 10% of the company's shares. OTE owns several subsidiaries across the Balkans, including Cosmote, Greece's top mobile telecommunications provider, Cosmote Romania and Albanian Mobile Communications.
Who ran the phones in Greece between 1949 and the 1980s?
Answer: state
Context and question: Niels Bohr introduced the first quantized model of the atom in 1913, in an attempt to overcome a major shortcoming of Rutherford's classical model. In classical electrodynamics, a charge moving in a circle should radiate electromagnetic radiation. If that charge were to be an electron orbiting a nucleus, the radiation would cause it to lose energy and spiral down into the nucleus. Bohr solved this paradox with explicit reference to Planck's work: an electron in a Bohr atom could only have certain defined energies En
Why was the first quantized model of the atom destroyed?
Answer:
unanswerable