The question asks about the material used for the yoke of small DC machines. The yoke serves two primary purposes: providing mechanical support and acting as a part of the magnetic circuit. The choice of material depends on these functions and cost-effectiveness.
Correct Option: A) Cast iron is commonly used for the yoke of small DC machines due to its good mechanical properties, adequate magnetic properties, and cost-effectiveness.
The question asks about the excitation method for a self-excited generator. Understanding the fundamental principle of self-excitation in DC generators is key to answering this question. Self-excited generators do not require an external power source for their field winding; instead, they use their own generated output to create the magnetic field.
C) by its own current — This is correct. Self-excited generators derive the current for their field windings from their own armature output. This current is DC, which is necessary to create a steady magnetic field.
The question asks about the principle behind an electric heater's operation. We need to identify which effect of electric current is primarily utilized in an electric heater. This involves understanding the different effects of electric current and how they are applied in various devices.
B) heating — Electric heaters utilize the heating effect of current. When current passes through the high-resistance heating element, electrical energy is converted into thermal energy, producing heat.
The question asks to define the unit "one B.O.T". B.O.T stands for Board of Trade Unit, which is a common unit for measuring electrical energy consumption. We need to relate this unit to the standard unit of electrical energy, kilowatt-hour (kWh).
By definition:
\[ 1 \text{ B.O.T} = 1 \text{ kilowatt-hour} \]\[ 1 \text{ B.O.T} = 1 \text{ kW} \times 1 \text{ h} \]\[ 1 \text{ B.O.T} = 1 \text{ kWh} \]Therefore, one B.O.T is equal to 1 kWh.
A) 1 kWh — This is the correct definition. One Board of Trade Unit (B.O.T) is precisely equal to one kilowatt-hour (kWh).
The question asks to identify the type of chemical bond formed by the sharing of electrons between two or more atoms. We need to recall the definitions of different types of chemical bonds.
Correct Option: C) Covalent Bond is formed by the mutual sharing of electrons between two or more atoms to achieve stability.
To determine the number of atoms in a face-centred cubic (FCC) unit cell, we need to consider the contribution of atoms located at different positions within the unit cell: corners and face centres. Each position contributes a specific fraction of an atom to that particular unit cell.
D) 4 — As calculated, a face-centred cubic unit cell contains a total of 4 atoms.
The question asks to identify what kind of impurity acts as a donor in semiconductors. This relates to the concept of doping in semiconductors, specifically n-type doping, where extra electrons are introduced into the semiconductor material.
C) a pentavalent impurity — Pentavalent impurities have five valence electrons. When they replace a tetravalent semiconductor atom, four electrons form covalent bonds, and the fifth electron is loosely bound and can easily be donated to the conduction band, increasing the number of free electrons. This makes them donor impurities.
The question asks about the resistance offered by a PIN diode when it is reverse biased. Understanding the structure and operating principles of a PIN diode, especially its intrinsic (I) layer, is crucial to determine its resistance characteristics under reverse bias.
D) infinite resistance — In reverse bias, the intrinsic layer of a PIN diode is almost completely depleted of charge carriers, making it behave like an insulator. This results in a very high, ideally infinite, resistance to current flow.
The question asks to identify the diode known for ultra-high switching speed. We need to evaluate the characteristics of each given diode type concerning their switching capabilities.
D) Tunnel diode is known for its ultra-high switching speed because its operation relies on quantum mechanical tunneling, which is a very fast process and does not involve minority carrier storage effects that limit the speed of other diodes.
To understand the normal operation of a transistor, it's crucial to know the biasing conditions of its two p-n junctions: the emitter-base junction and the collector-base junction. These biasing conditions determine how the transistor amplifies or switches signals.
Correct Option: A) For normal operation (active region) of a transistor, the collector-base junction is always reverse biased. This ensures that the collector effectively collects the carriers injected from the emitter and diffused through the base, leading to current amplification.