Electrical Questions

Electrical Questions

1.The number of micro amperes in 2 milli amperes is?
A.2 μA
B.20 μA
C.200 μA
D.2,000 μA
Ans: D

2.If the diameter of a conductor of 10 ohms is doubled, its new resistance would be?
A.20 ohms
B.5 ohms
C.40 ohms
D.2.5 ohms
Ans: D

3.What amount of current will flow through resistance of 10 ohms when it is connected across a potential
difference of 20 volts?
A.30 amperes
B.5 amperes
C.10 amperes
D.2 amperes
Ans: D

4.Resistance is measured in?
A.henries
B.ohms
C.hertz
D.watts
Ans: B

5.Eighteen thousand watts is the same as?
A.18 mW
B.18 MW
C.18 kW
D.18 μW
Ans: C

6.If an incandescent lamp of 80 ohms resistance takes a current of 0.75 ampere, what voltage is required to light it?
A.45 volts
B.60 volts
C.23 volts
D.None of these
Ans: B

7.Materials with lots of free electrons are called?
A.conductors
B.insulators
C.semiconductors
D.filters
Ans: A

8.A thermistor is a type of?
A.switch
B.resistor
C.battery
D.power supply
Ans: B

9. A constant load power means a uniform conversion of?
A.mechanical to electrical energy
B.electrical to mechanical energy
C.current to voltage
D.voltage to current
Ans: B

10.The minimum resistance value for a blue, gray, red, silver resistor is?
A.612 ohms
B.6,120 ohms
C.6,800 ohms
D.6,460 ohms
Ans: B

11.Current flows in a circuit when?
A.a switch is opened
B.a switch is closed
C.the switch is either open or closed
D.there is no voltage
Ans: B

12.Electrons in the outer orbit are called?
A.nuclei
B.valences
C.waves
D.shells
Ans: B

13.A material that does not allow current under normal conditions is a(n)?
A.insulator
B.conductor
C.semiconductor
D.valence
Ans: A

14.The formula to find I when the values of V and R are known is?
A.I = VR
B.I = R/V
C.V = IR
D.I = V/R
Ans: D

15.Which of the following is not an electrical quantity?
A.voltage
B.current
C.distance
D.power
Ans: C

DC Machines:

DC Machine

a DC generator or a DC motor can be broadly termed as a DC machine. 

Construction Of A DC Machine

The above figure shows the constructional details of a simple 4-pole DC machine. A DC machine consists two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.
4. Armature core: Armature core is the rotor of the machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.
   
5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. 
6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. 

Ohm’s Law

Statement of Ohm’s Law

Georg Simon Ohm 

The statement of Ohm’s law is simple, and it says that whenever a potential difference or voltage is applied across a resistor of a closed circuit, current starts flowing through it.This current is directly proportional to the voltage applied if temperature and all other factors remain constant. Thus we can mathematically express it as:

Now putting the constant of proportionality we get

This particular equation essentially presents the statement of this law where I is the current through the resistor, measured in Ampere (Ampere, or amps), when the electric potential difference V is applied across the resistor in unit of volt, and ohm(Ω) is the unit of measure for the resistance of the resistor R.

SYLLABUS OF E L E C T R I C I A N