Electrical and Electronics Measurements MCQ Quiz - Objective Question with Answer for Electrical and Electronics Measurements - Download Free PDF

Explanation:

The Hot-Wire Ammeter

• A hot-wire ammeter is a device used to measure current in a circuit. It operates based on the principle of thermal expansion of a wire when heated by the electric current passing through it. The instrument is designed to measure the effective value of the current in both alternating current (AC) and direct current (DC) circuits.
• When an electric current flows through the wire in the hot-wire ammeter, the wire heats up due to the resistive power loss. The heat causes the wire to expand, and this expansion is mechanically translated into a movement, which is measured and displayed on a calibrated scale. Since the heating effect of the current is proportional to the square of the current and independent of its direction, the hot-wire ammeter can measure the effective value of AC or DC current without differentiation.

Advantages:

• Can measure both AC and DC currents without requiring separate instruments.
• Insensitive to the waveform of AC, as it measures the effective (RMS) value of the current.
• Simple construction and operation, relying only on the thermal expansion of the wire.

Disadvantages:

• Limited accuracy compared to other types of ammeters, such as moving-coil ammeters.
• Slow response time due to the thermal inertia of the wire.
• Susceptible to environmental factors such as temperature variations and vibrations.

Applications: Hot-wire ammeters are typically used in situations where measuring the effective value of current is more important than waveform analysis. They are suitable for measuring currents in AC circuits with complex waveforms and for DC circuits where simplicity and reliability are prioritized.

Explanation:

Moving Coil Voltmeter

Definition: A moving coil voltmeter is an instrument used to measure the voltage (potential difference) in an electrical circuit. It operates on the principle that a current-carrying conductor placed in a magnetic field experiences a torque, causing the conductor to move. This movement is translated into a readable voltage measurement on a calibrated scale.

Working Principle: The moving coil voltmeter works based on the interaction between the magnetic field of a permanent magnet and the current passing through a rectangular coil. The coil is wound on a lightweight aluminum or copper frame and is suspended in the magnetic field of a permanent magnet. When a current flows through the coil, it experiences a torque due to the Lorentz force, causing it to rotate. This rotation is proportional to the voltage applied across the meter, and the pointer attached to the coil moves over a calibrated scale to indicate the voltage value.

Correct Option Analysis:

The correct option is:

Option 2: Only d.c. voltage

A moving coil voltmeter is designed to measure only direct current (d.c.) voltage. This is because the moving coil mechanism relies on the direction of current flow to generate a consistent torque. In the case of alternating current (a.c.), the current direction changes periodically, causing the torque to reverse as well. This results in the pointer oscillating around the zero mark rather than providing a stable reading. Therefore, a moving coil voltmeter is inherently suitable for measuring d.c. voltage only.

Advantages of Using a Moving Coil Voltmeter for D.C. Voltage:

• High sensitivity and accuracy due to the precision of the moving coil mechanism.
• Linear scale, making it easier to read and interpret the voltage measurement.
• Low power consumption as the instrument requires minimal current to operate.

Disadvantages:

• Cannot measure a.c. voltage without the addition of a rectifier circuit.
• Fragile components, such as the moving coil and suspension system, require careful handling.
• Limited range of voltage measurement, typically requiring external shunts or multipliers for higher voltages.

Applications:

• Used in laboratories and industries to measure d.c. voltage in electronic circuits.
• Common in battery testing and d.c. power supply measurements.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Only a.c. voltage

This option is incorrect. A moving coil voltmeter cannot measure a.c. voltage directly due to the alternating nature of the current, which causes the pointer to oscillate instead of providing a steady reading. However, with the addition of a rectifier circuit, the moving coil voltmeter can be adapted to measure a.c. voltage by converting it into d.c. voltage first. Without such modifications, the moving coil voltmeter is not suitable for a.c. voltage measurement.

Option 3: Both a.c. and d.c. voltage

This option is incorrect as well. While a moving coil voltmeter can measure d.c. voltage directly, it cannot measure a.c. voltage without additional components like a rectifier. Therefore, it is not capable of measuring both a.c. and d.c. voltage in its basic form.

Option 4: Nothing

This option is incorrect. A moving coil voltmeter is a precise and reliable instrument for measuring d.c. voltage. As explained earlier, it operates effectively within its designed scope, which is limited to d.c. voltage measurement.

Option 5:

The question provides no content for option 5, making it irrelevant to the analysis. It has no bearing on the discussion of the moving coil voltmeter's functionality.

Conclusion:

The moving coil voltmeter is a highly sensitive and accurate instrument designed specifically for measuring d.c. voltage. Its operation is based on the interaction between a magnetic field and current in a coil, which results in a proportional deflection of the pointer. While it cannot measure a.c. voltage directly due to the oscillatory nature of alternating current, it remains an indispensable tool for d.c. voltage measurements in various applications, such as laboratories and industries. The correct answer is option 2, as the instrument is inherently suited for measuring only d.c. voltage.

The correct answer is: 1) The propagation speed of the signal in the line

Explanation:

In an electrical delay line used in oscilloscopes, the delay time is determined by:

• The physical length of the delay line

• The propagation speed of the signal through the line (which depends on the medium's properties, such as its dielectric constant and inductance/capacitance per unit length).

Option Analysis

2. The resistance of the delay line → Affects signal attenuation, not the delay time.

3. The frequency of the input signal → The delay line is typically designed to be frequency-independent for a wide range of signals.

4. The speed of the electron beam → This relates to the CRT display, not the delay line.

Thus, the correct choice is 1) The propagation speed of the signal in the line.

Explanation:

Measuring Time Delay in an Oscilloscope

Definition: An oscilloscope is an electronic instrument used to measure and visualize varying electrical signals. It displays the waveform of the signal, allowing for analysis of its properties such as amplitude, frequency, and time delay. The time delay in an oscilloscope refers to the interval between specific points in the waveform, such as between peaks or zero crossings.

Working Principle: Oscilloscopes work by sampling the input signal and plotting it on a screen as a function of time. The horizontal axis represents time, while the vertical axis represents the signal’s amplitude. By analyzing the waveform, one can measure various parameters, including the time delay.

Correct Option Analysis:

The correct option is:

Option 1: Measuring the time between two successive peaks of the signal.

To measure the time delay in an oscilloscope, the most straightforward and accurate method is to observe the time interval between two successive peaks of the signal. This approach utilizes the waveform's visual representation to determine the period or frequency of the signal. By identifying the time difference between consecutive peaks, one can accurately measure the time delay.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Measuring the amplitude of the signal.

This option is incorrect for measuring time delay. The amplitude of the signal refers to its strength or magnitude, not the time interval between points in the waveform. While amplitude is an important parameter, it does not provide information about the time delay.

Option 3: Counting the number of weak signals.

This option is also incorrect. Counting the number of weak signals does not relate to measuring time delay. Weak signals might refer to low amplitude portions of the waveform, but this does not help in determining the time interval between specific points in the signal.

Option 4: Measuring the decibel of the signal.

This option is incorrect as well. Decibels are a unit of measurement for the intensity of the signal, typically used in audio and sound engineering. Measuring the decibel level of the signal does not provide information about the time delay.

Conclusion:

Understanding the correct method to measure time delay in an oscilloscope is crucial for accurate signal analysis. By measuring the time between two successive peaks of the signal, one can effectively determine the time delay. Other options such as measuring amplitude, counting weak signals, or measuring decibels do not provide the necessary information for time delay measurement. Proper analysis and understanding of the waveform are essential for utilizing an oscilloscope effectively in various applications.

Explanation:

Why is shielding used in oscilloscope probes?

Definition: Shielding in oscilloscope probes refers to the use of conductive materials to encase the probe and its components to protect against external electromagnetic interference. This interference, often coming from various electronic devices and environmental factors, can adversely affect the accuracy and reliability of measurements taken by the oscilloscope.

Working Principle: Shielding works on the principle of electromagnetic compatibility (EMC). It ensures that the probe is less susceptible to external electromagnetic fields, thereby maintaining the integrity of the signal being measured. The shielding typically involves a conductive material, such as metal, which encloses the probe and acts as a barrier to electromagnetic interference.

Correct Option Analysis:

The correct option is:

Option 2: To reduce external electromagnetic interference.

This option correctly describes the primary purpose of shielding in oscilloscope probes. The shielding effectively minimizes the impact of external electromagnetic interference, ensuring that the oscilloscope provides accurate and reliable measurements.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: To control the electron beam in the oscilloscope.

This option is incorrect because shielding is not used to control the electron beam in the oscilloscope. The electron beam is controlled by the internal components of the oscilloscope, such as the electron gun and deflection plates, which direct the beam to create the visual representation of the signal on the screen.

Option 3: To increase the probe’s sensitivity.

This option is also incorrect. While shielding can help improve the accuracy of measurements by reducing interference, it does not directly increase the sensitivity of the probe. The sensitivity of an oscilloscope probe is determined by its design and the quality of its components.

Option 4: To amplify weak signals.

This option is incorrect as well. Shielding does not amplify signals. Signal amplification is typically achieved through the use of amplifiers within the oscilloscope or external amplifier circuits. Shielding is solely for the purpose of reducing interference.

Ammeter:

• It is used to measure the current.
• An ideal ammeter has zero internal resistance and thus it provides the path for maximum current.
• It is always connected in series as it measures current.
• The range of ammeter can be extended by using a low shunt resistance.

Voltmeter:

• It is used to measure the voltage.
• An ideal voltmeter has infinite resistance and thus it provides the path for minimum current.
• It is always connected in parallel as it measures voltage.
• The range of voltmeter can be extended by using a high series resistance.

Concept:

Static error (E) = Am – At

Am = Measured value

At = True value

Relative Static Error: The ratio of absolute error to the true value is called relative static error.

\(R.S.E = \frac{{\left| {{A_m} - {A_t}} \right|}}{{{A_t}}} \times 100\)

Limiting Error:

The maximum allowable error in the measurement is specified in terms of true value, is known as limiting error. It will give a range of errors. It is always with respect to true value, so it is a variable error.

Guaranteed Accuracy Error:

The allowable error in measurement is specified in terms of full-scale value is known as a guaranteed accuracy error. It is a variable error seen by the instrument since it is with respect to full-scale value.

Application:

Given-

A_m = 125 V, A_t = 125.5 V

∴ Static error (E) = 125 - 125.5

E = 0.5 V

The essential features are possessed by an indicating instrument deflecting, controlling, and a damping device.

• Deflecting device: The deflection device produces deflecting torque which causes the moving system to move from its zero position.
• Controlling device: The controlling device produces the controlling torque (Tc) which opposes the deflecting torque and increases with the deflection of the moving system. It also brings the pointer back to zero when the deflecting torque is removed.
• Damping device: This device produces damping torque this torque is necessary to bring the pointer to rest quickly. This damping torque (Td)  is used to reduce the oscillation.

• A Galvanometer is used to detect the direction of the current.
• This device is used for detecting and measuring a small amount of electric current.
• Johann Schweigger invented the device in 1820.
   

Moving iron meter has large magnetic reluctance as compared to PMMC meter. That’s why more power is required to operate the moving iron meter.

Advantages of moving iron:

• It is a universal instrument which can be used for the measurement of AC and DC quantities
• These instruments can withstand large loads and are not damaged even under severe overload conditions
• It is very cheap due to the simple construction

Disadvantages of moving iron:

• These instruments suffer from error due to hysteresis, frequency change and stray losses
• The reading of the instrument is affected by temperature variation

Note: In terms of accuracy PMMC meter has the highest accuracy. The order of accuracy is given below.

Induction < Moving iron < PMMC instruments

The correct answer is 1 KWh.

Concept:

Electric bulb: It is an electric device that converts electric energy into heat and light energy.

• Resistance of the bulb \(R = \frac{{{{\left( {Rated\;voltage} \right)}^2}}}{{\left( {Rated\;power} \right)}}\)
• If we consider the bulb as a resistor then, we can easily find the current and voltage drops.
• \(Power\;consumed = {i^2}R\)
• Power consumed ∝ Brightness

The rate of work done by the electric current is called as electric power.

The difference of potential between two points is called a potential difference.

• Electric energy (E) = electric power (P) × time (t)

Unit of electric energy = unit of electric power × unit of time = kilowatt × hour = kilowatt-hour

Key Points

• Given that
  • Power of the bulb = 100 W
  • Time = 10 hours

We know that,

Energy = Power × Time

⇒ Energy consume = 100 w × 10 hrs = 1000 watt - hr = 1 KWh.

In a two-wattmeter method,

The reading of first wattmeter (W₁) = V_L I_L cos (30 – ϕ)

The reading of second wattmeter (W₂) = V_L I_L cos (30 + ϕ)

Total power in the circuit (P) = W₁ + W₂ = √3 V­­_LI_L cos ϕ

Total reactive power in the circuit \(Q = √ 3 \left( {{W_1} - {W_2}} \right) =\sqrt3 {V_L}{I_L}\sin \phi\)

Power factor = cos ϕ

\(\phi = {\rm{ta}}{{\rm{n}}^{ - 1}}\left( {\frac{{√ 3 \left( {{W_1} - {W_2}} \right)}}{{{W_1} + {W_2}}}} \right)\)

Energy meter:

Energy meter or Watt-hour meter is used to measure the energy in kWh.

It is an integrating type instrument.

Its working principle is similar to the transformer.

There are three essential mechanisms required in the energy meter named Driving torque, Braking torque, and registered mechanism.

Driving torque:

This torque is required to revolve the disc or rotate the disc.

It is obtained by the electromagnetic induction effect.

Braking torque:

It is required to rotate the disc at a constant speed.

It is obtained by using a permanent magnet placed inside the energy meter near the Aluminum disc.

Eddy currents have induced in the magnet due to the movement of the rotating disc through the magnetic field. This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk. The speed of the disk can be controlled by changing flux.

Breaking torque of induction type single-phase energy meter is:

\({T_b} = k\frac{{{\phi ^2}}}{{{R_e}}}N \times R\)

K = constant

ϕ = flux

N = speed in rpm

R = radius of the disc

R_e = resistance in path of current (i.e. disc)

The braking torque of induction type single-phase energy meter is directly proportional to the square of the flux.

Registered mechanism:

It registers the no. of rotations or revolutions of the disc which is proportional to the energy consumed in kWh.

Meter constant = (No. of revolutions / kWh)

Points to remember:

Creeping:

Sometimes the disc of the energy meter makes the slow but continuous rotation at no load i.e. when the potential coil is excited but with no current flowing in the load called creeping error

This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields, etc

It can be reduced by making two opposite holes on the disc.

Concept-When both pairs of the deflection plates (horizontal deflection plates and vertical deflection plates) of CRO (Cathode Ray Oscilloscope) are connected to two sinusoidal voltages, the patterns appear at CRO screen are called the Lissajous pattern. Shape of these Lissajous pattern changes with changes of phase difference between signal and ration of frequencies applied to the deflection plates (traces) of CRO.

Case – 1: When ø=0 or  ø=360

when the angle is   ø = 0 or   ø = 360, the Lissajous pattern is of the shape of straight line  passing through origin from first quadrant to third quadrant.

Case – 2: When, 0 < ø < 90 or 270 < ø <360 : –

when the angle is in the range of 0 < ø < 90 or 270 < ø < 360, the Lissajous pattern is of the shape of Ellipse having major axis passing through origin from first quadrant to third quadrant.

Case – 3: When ø=90

when the angle is   ø = 90  the Lissajous pattern is of the shape of circle.

Case – 4: When 90  < ø < 180 or 180 < ø < 270

when the angle is in the range of 0 < ø < 90 or 270 < ø < 360, the Lissajous Pattern is of the shape of Ellipse having major axis passing through origin from second quadrant to fourth quadrant.

Case – 5: When ø=180

when the angle is   ø = 180  the Lissajous pattern is of the shape of straight line  passing through origin from second  quadrant to  fourth quadrant.

Solution:-

Let at horizontal plate voltage graph is

Option-1:- It is of case (1) so at vertical plate voltage graph will be with phase difference ϕ = 0 or 360°

Option-2:- It is of case (2) so 0 < ϕ < 90° so at vertical plate voltage graph will have phase difference of 0 < ϕ < 90°

Option-3:- It is of case (3) so ϕ = 90° ↑ vertical voltage graph of have ϕ = 90° w.r.t horizontal voltage graph

Option-4:- It is of case (4) so 90° < ϕ < 180°, vertical voltage graph will have phase difference 90° < ϕ < 180° w.r.t. horizontal voltage graph