AFCAT EKT Electrical Mock Test - 4 - AFCAT MCQ

The current at a point in the circuit is the amount of charge that passes that point in one second. A current of 1 A is flowing in a circuit if a charge of 1 coulomb passes any point in the circuit every second.

Duct propagation in communications is opposite to free space propagation i.e. through guided medium like transmission line, waveguides et.al. In duct propagation line of sight  and diffraction zones like edges of building walls etc are not applicable

They are the most visible type of diode that emit a fairly narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red light for remote controls or laser type light when a forward current is passed through them.
The “Light Emitting Diode” or LED as it is more commonly called, is basically just a specialised type of diode as they have very similar electrical characteristics to a PN junction diode. This means that an LED will pass current in its forward direction but block the flow of current in the reverse direction.
Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light emitting diode produces a “cold” generation of light which leads to high efficiencies than the normal “light bulb” because most of the generated energy radiates away within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable and provide much longer lamp life than normal light sources.

The probability of error decreases as the energy per bit increases.

In an AC circuit, resonance occurs when capacitive reactance equals reactive reactance.
At resonance, X _L = X _C, Z = R

Open loop function is used when finding the stability of the system by using Nyquist plot.

A Cavity Resonator is a 1 port device, it is a short terminated rectangular or circular waveguide with a matched load.

A dc series motor should always be started on load. When the motor is connected across the supply mains without load, it draws small current from the supply mains flowing through the series field and armature, the speed tends to increase so that back emf may approach the applied voltage in magnitude. The increase in back emf weakens the armature current and hence the field current. This cause again increase in speed so in back emf. Thus the field continues to weaken and speed continues increase until the armature produced such centrifugal force that it is coming out from its shaft and gets damaged.

Reverberation is the time taken for the sound to fall below the minimum audibility measured from the instant when the source stopped sounding. Therefore if the reverberation time becomes too large it produces an echo.

The frequency deviation in phase modulated carrier is proportion to only amplitude of the modulating signal.

The charge carriers in an n-channel FET are electrons alone

Advantages of FM over AM are:

• Improved signal to noise ratio (about 25dB) w.r.t. to manmade interference.
• Smaller geographical interference between neighbouring stations.
• Less radiated power.
• Well defined service areas for given transmitter power.

Disadvantages of FM:

• Much more Bandwidth (as much as 20 times as much).
• More complicated receiver and transmitter.

The oscillation frequency of an RC phase shift oscillator using three identical RC sections is .1 √(6RC)

The Schottky diode is a semiconductor formed by the junction of a semiconductor with metal. It has a low forward voltage drop and near zero reverse recovery time hence used in very fast switching circuits.

In a matched junction the there is no reflection, reflection co-efficient is zero implying diagonal elements of S matrix are zero.

Multiplying matrix A by itself, we get 

Similarly, .

Power gain  is given by

 is the expression for directive gain.

The Zener Diode because of its stable output characteristic, is used as a constant voltage source, gate electrode on the FET corresponds to the base in bipolar transistor and the SCR is a silicon rectifier with a gate electrode to control when current flows from anode to cathode.

Diffusion is a physical process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules).

A transducer is any device used to convert energy from one form to another; typically when converting input energy into output energy. For transduction to occur, a change from one form of energy must also take place, such as a conversion from mechanical to electrical energy or vice versa.

The stator frame consists of laminations of silicon steel, usually with a thickness of about 0.5 millimetre. Lamination is necessary since a voltage is induced along the axial length of the steel as well as in the stator conductors. The laminations are insulated from each other usually by a varnish layer. This breaks up the conducting path in the steel and limits the losses (known as eddy current losses) in the steel.

Tunnel diode owing to heaving doping(typically 100 to several thousand times normal doping semiconductor diode) resulting in extremely small depletion layer width. Due to this extremely thin depletion region width the charge carriers can "tunnel" through depletion region

The mean and variance of a Poisson’s distribution are equal. So, if the mean is 5, the variance will also be 5.

If both inputs of JK flip flop are same, then it acts as T flip flop. If both the inputs are low, then the output will be same as the previous output. If both the inputs are high, then the output will be complement of the previous output.

The efficiency of the transformer is maximum at when copper loss is equal to iron loss.

When pulse –width modulation has the disadvantages compared with pulse- position modulation (PPM), that its pulses are of varying width and therefore of varying power content . This means the transmitter must be powerful enough to handle the maximum width pulses, although the average power transmitted is perhaps only half of the peak power. On the other hand, PWM will still work if synchronization between transmitter and receiver fails whereas pulse-position modulation will not.

By conservation of angular momentum

I’ω’ = Iω

(M + 2m) r²ω’ = Mr²ω

⇒ ω

We can apply Thevenin’s theorem to both AC and DC networks. The fact is that in the DC circuit we use Thevenin equivalent resistance, but in AC we have to find the equivalent Impedance. In AC network we have to use phasor sum of the voltage sources. All other conditions are similar to the DC source.

A full adder can be realized by using two half adders and one OR gate.

y = a sin t - at cos t
dy/dt = a cos t - (a cos t + at(-sin t)) = at sin t

x = a cos t + at sin t
dx/dt = -a sin t + (a sin t + at (cos t)) = at cos t

dy/dx = tan t