Important Short Questions and Answers: Electric Circuits and Measurements

ELECTRICAL CIRCUITS & MEASUREMENTS

1.     What is meant by charge?

Charge is an electrical property of the atomic particles which matter consists. The charge of an electron is so small. Charge in motion represents current. The unit of charge is coulomb.

2.     What is meant by Current?

The flow of free electrons in a conductor is called current. Unit is ampere (A). I =

Q/t

3.  What is meant by Voltage?

The poterntial difference between two points is called as voltage. Unit is Volts (V). V=W/Q , W=work done in joules & Q = charge in coulombs

4.  State Ohm’s Law.

The potential difference across any two ends of a conductor is directly proportional to the current flowing between the two ends provided the temperature of the conductor remains constant.

5.  State Krichoff’s Voltage Law

KVL states that the algebraic sum of voltages in a closed path is zero.

6.  State Krichoff’s current Law.

KCL states that the algebraic sum of currents in a node is zero.

7.     Give short notes on resistor.

It is a property of a substance3 which opposes the flow of electrons. It is denoted by R and its unit is Ohm

8.     Distinguish between a Branch and a node of a circuit.

A pair of network which connects the various points of the network is called branch A point at which two or more elements are joined together is called node.

9.           Distinguish between a mesh and a loop of a circuit.

A mesh is a loop that does not contain other loops. All meshes are loop, but all loops are

not meshes. A loop is any closed path of branches

10.            Write down the formula for a star connected network is converted into a delta network?

RA=( R1 R2)/( R1 +R2+ R3) RB=( R1 R3)/( R1 +R2+ R3) RC=( R2 R3)/( R1 +R2+ R3)

11.      Write down the formula for a delta connected network is converted into a star network?

R1=( RARB+RBRC+RCRA)/RC R2=( RARB+RBRC+RCRA)/RB R3=( RARB+RBRC+RCRA)/RA

12.      Define line currents and phase currents?

The currents flowing in the lines are called as line currents The currents flowing through phase are called phase currents

13.            Define line voltage and phase voltage?

The voltage across one phase and neutral is called line voltage & the voltage between two lines is called phase voltage

14. Give the phase value & Line value of a star connected system.

VL= 3Vph

15. Give the phase value and line valued of a delta connected system.

IL= 3Iph

16. What is the power equation for a star connected system?

P= 3I V cosΦ L L W

17. What is the power equation for a delta connected system?

P= 3I V cosΦ L L W

18. What is meant by Real power?

Real power means the useful power transfer from source to load. Unit is watts.

19. What is meant by apparent power?

Apparent power is the product of voltage and current and it is not true power. Unit is VA

20. What is reactive power?

If we consider the circuit as purely inductive the output power is reactive power. Its unit is VAR

21. Define Instrument.

Instrument is defined as a device for determining the value or magnitude of a quantity or variable.

22. Mention the two main differences between an ammeter and a voltmeter.

Ammeter Voltmeter

It is a current measuring device it is a voltage measuring device

Always connected in series with circuit Always connected in parallel with circuit

The resistance is very small The resistance is very high

23. What is control system?

A system consists of a number of components connected together to perform a specific function . In a system when the output quantity is controlled by varying the input quantity then the system is called control system.

24. What are the two major types of control system?

The two major types of control system are open loop and closed loop

25. Define open loop control system.

The control system in which the output quantity has no effect upon the input quantity are called open loop control system. This means that the output is not feedback to the input for correction.

26. .Define closed loop control system.

The control system in which the output has an effect upon the input quantity so as to maintain the desired output value are called closed loop control system

27. Mention the errors in Moving iron instruments.

Hysteresis error

Temperature error

Stray magnetic field error

Frequency error

Eddy current error

28.            Mention any two precautions to be taken while using an Ammeter.

It should never be connected across any source. The polarity must be observed correctly.

First use the highest range and then decrease the voltage range until the sufficient deflection is obtained.

29.            Define Form factor and Crest factor.

Form factor= RMS value / Average Value

Crest(peak) factor=Maximum Value / RMS value

30. Which type of instrument is called as universal instrument?

The moving iron instrument are known as universal instruments, because these instruments can be used for AC and DC.

31.            What are the applications of MI instruments?

i) Used as multirange ammeters and voltmeters.

ii) Used as in expensive indicators such as charging and discharging current indicators in automobiles.

iii)Extensively used in industries for measurement of AC voltage and current where errors of the order of 5% to 10% are accepetable.

32. What is meant by eddy current damping?

When the conductor moves in a magnetic field an emf is induced in it and if a closed path is provided ,a current flows known as eddy current. This current intersect with the magnetic field to produce an electromagnetic torque , which opposes the deflecting torque.

33.            How is electrical power measured?

i)  Using Voltmeter-ammeter method for DC circuits. ii)Using Watt meters for AC circuits.

34.            What do you mean by compensation coil in a wattmeter?

By connecting a compensating coil in series with a pressure coil, The error caused by the pressure coil flowing in the current coil can be neutralized.

35.What are the three types of power used in a a.c circuit?

35.            Define average value.

The average value of an alternating current is that value of steady direct current which transfers the same charge as the alternating current flowing for the same time.

37. Define RMS value.

The effective value of an alternating current is that value of steady ,direct current which produces the same heat as that produced by the alternating current when passed which produces the same heat as that produced by the alternating current when passed through the same resistance for the same interval of time.

38. Define reactive power.

The power consumed by a pure reactance (XL or Xc ) in a a.c circuit is called reactive power.