# RRB JE Electrical question paper with solution 2012 | RRB JE Electrical

Ques.1. What is the value of knee voltage of silicon diode?

1. 0.3 V
2. 0.33 V
3. 0.7 V
4. 1.1 V

Explanation:-

Knee voltage:- It is the forward voltage at which the current through the p-n junction starts to increase rapidly. When a diode is forward biased, it conducts current very slowly until we overcome the potential barrier. The value of potential barrier and hence the knee voltage for silicon p-n junction is 0.7 V and that for germanium is 0.3 V.

As the applied forward voltage exceeds the knee voltage, the current through the p-n junction starts increasing rapidly. Therefore, in order to get useful current through a p-n junction, the applied voltage must be more than the knee voltage.

Ques.2. There are 20 Volts across the inductor and 15 volts across the resistance in the A.C supplied series R-L circuit. What would be the supply voltage?

1. 20 volts
2. 15 volts
3. 25 volts
4. 17.5 volts

Explanation:-

The voltage across the inductor leads the voltage across the resistor by 90 degrees. Therefore by applying Pythagoras Theorem we ger

V2 = VR2 + VL2

Where

VR = Voltage across resistor = 20 V

VL = Voltage across inductor = 15 V

V2 = 202 + 152

V = 25 V

Ques.3. A transformer mainly transforms

1. Current
2. Voltage
3. Frequency
4. Power

Explanation:-

A transformer is a static device which converts the alternating current energy from one voltage level to another voltage level.

Ques.4. What is the power factor of a pure resistor circuit?

1. One
2. Zero
4. Lagging

Explanation:-

In case of pure resisitve load the voltage and current are in same phase hence the power factor is unity.

Ques.5. ________ is used to heat the non-conductor

1. Eddy current heating
2. Arc heating
3. Induction heating
4. Dielectric heating

Explanation:-

Dielectric heating, also known as electronic heating, RF (radio frequency) heating, and high-frequency heating, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material.

Dielectric heating (also called High-frequency capacitive heating) is employed for heating insulators like wood, plastics, and ceramics etc.

Ques.6. What is the S.I unit of magnetic flux density?

1. Gauss
2. Tesla
3. Oersted
4. Weber

Explanation:-

Magnetic flux density is defined as being the amount of magnetic flux crossing unit area at right angles to the direction of the flux. The appropriate basic SI unit is thus the weber per square meter (Wb/m2). This unit has been given the name testa (T).

Ques.7. Which type of oscillator is most stable in a simple circuit?

1. Crystalline oscillator
2. Clapp oscillator
3. Colpitts oscillator
4. Armstrong oscillator

Explanation:-

The crystal is usually made of the quartz material and provides a high degree of frequency stability and accuracy or we can say that crystal oscillator is basically a tuned oscillator. It uses a piezoelectric crystal (when an ac voltage is applied across a crystal it starts vibrating at the frequency of supply voltage, the effect is known as piezoelectric effect and the crystal which exhibit this effect is known as piezoelectric crystals, conversely, when these crystals are placed under mechanical strain to vibrate, they produce an ac voltage.

The stability of time and frequency standards is closely related to their quality factor or Q. The Q of an oscillator is its resonance frequency divided by its resonance width. A high-Q resonator will not oscillate at all unless it is near its resonance frequency. Quartz oscillator achieves the highest Q of any mechanical type device. A typical Q for a quartz oscillator ranges from 10to 106. The maximum Q for a high—stability quartz oscillator can be estimated as Q = 16 million/f, where f is the resonance frequency in MHZ.

Ques.8. What is the proper use of a signal generator?

1. Designing
2. Testing
3. Repairing
4. All of the above

Explanation:-

A signal generator is a device that supplies a standard voltage of known amplitude, frequency and waveform for test and measuring purposes. Signal generators are classified according to the shape of the output waveform, e.g. sine wave oscillators, sine square generators, sawtooth generators, etc. They may also be classified according to the range of frequencies they generate

Among the most common types of signal generators are function waveform generators which provide standard waveforms such as sine, square, pulse, exponential rise and fall, cardiac waveforms. These generators use direct digital synthesis techniques to create waveforms over a wide range of frequency characteristics from down to 1µHz frequency resolution.

The application of the signal generators in troubleshooting electronic equipment are almost unlimited. Signal generators are used for signal tracing, testing and adjusting amplifier response, alignment of radio and television receivers, and testing digital circuits, particularly with pulse generators. They may also be used for the precise measurement of time anal frequency.

Ques.9. The electrostatic instrument relay for their operation upon the

1. Current
2. Voltage
3. Fact that a force exist between the two plates having opposite charges
4. Power

Answer.3. Fact that a force exists between the two plates having opposite charges

Explanation:-

### Electrostatic Instrument

These are the only instruments that are directly voltage sensitive and do not depend on a current for their operation. Electrostatic instruments are almost always used as voltmeters and that too more as a laboratory rather than as industrial instruments.

Operating Principle

The operating principle of an electrostatic instrument depends on the force of attraction between two or more electrically charged conductors between which a potential difference is maintained, and this force gives rise to a deflecting torque. The electrostatic mechanism resembles a variable capacitor, where the force existing between the two parallel plates is a function of the potential difference applied to them.

Basically, there are two types of electrostatic (also known as electrometers instruments):

(ii) Attracted-disc type

The following are the advantages of electrostatic instruments:

1. They can be manufactured with very high accuracy.
2. They can be used on either a.c. or d.c. and over a fairly large range of frequencies.
3. The instruments may be calibrated with d.c, and yet the calibration would be valid for a.c. also since the deflection is independent of the waveform of the applied voltage.
4. Since no iron is used for their construction, they are free from hysteresis, eddy current losses and temperature errors.
5. Their power loss is negligible.
6. They are unaffected by stray magnetic field although they have to be guarded against any stray electrostatic field.
7. Once the discs are charged, no more current is drawn from the circuit and the instrument represents infinite impedance.
8. They can be used upto 1000 kHz frequency without any serious loss of accuracy.
9. They do not draw any continuous current on d.c. circuits and that drawn on d.c. circuits are extremely small. Hence, such voltmeters do not cause any disturbance to the circuit to which they are connected.

Limitations or Electrostatic Instruments

The following are the limitations of electrostatic instruments:

1. Their use is limited to certain special applications, particularly in a.c. circuits of relatively high voltage where the current taken by other instruments would result in erroneous indications.
2. Low voltage voltmeters are liable to friction errors.
3. Since the deflecting torque is proportional to the square of the voltage, their scales are not uniform although some uniformity can be obtained by suitably shaping the quadrants of the instruments.
4. They are expensive and are not likely to be durable.
5. They are, inherently, laboratory-type rather than industrial-type instruments.

Ques.10. The unit of resistivity is

1. Ohm
2. Ohm-meter
3. Ohm/meter
4. Meter/ohm