Electrical Basics MCQ Quiz - Objective Question with Answer for Electrical Basics - Download Free PDF

Concept:

• The total number of protons and neutrons in the nucleus of an atom is known as its Mass Number and is denoted by A.
• Protons and neutrons are situated in the nucleus of an atom and contribute most to the mass of the elements as the mass of electrons is negligible.
• The atomic number of an atom is the same as a number of proton.

Additional Information

Mass number(A) = Number of protons (Z) + Number of neutrons (N)

It is represented by:

​

Mistake PointsIt is conventional to count the number of protons as atomic numbers and not the number of the neutron.

The number of neutrons can be changed and as so nuclear bombs are made. so we count only the number of protons and not neutrons.

CONCEPT:

• Ohm’s law: At constant temperature, the potential difference across a current-carrying wire is directly proportional to the current flowing through it.

          i.e. V = IR

Where V = potential difference, R = resistance and I = current.

CALCULATION:

Given V = 18 V and R = 3 Ω,

• According to ohm's law:

⇒ ​V = IR

⇒ I = V/R

⇒ I = 18/3 = 6 A

Aluminum Vs. Copper Conductivity

• Aluminum makes a fantastic conductor of both electricity and heat. Aluminum weighs 30 percent lighter than copper and is 61 percent less conductive. This means aluminum with half the weight of copper has the same conductivity and electrical resistance.
• Aluminum cable has a lower current carrying capacity (by about 40%) and greater resistance (requiring a 50% larger cross-sectional area on average) than the equivalent copper conductor.
• Aluminum is generally more inexpensive when compared to copper conductors, hence preferred over copper conductors. 

Kirchhoff's circuit laws

1.) Kirchhoff's current law (KCL)

It states that the total current entering a junction is equal to the total current leaving the node.

Incoming current = Outgoing current

\(I_1+I_2+I_3=I_4+I_5\)

2.) Kirchhoff's Voltage law (KVL)

It states that the sum of the total voltage drop in a closed loop is equal to zero.

\(-V_1+V_2+V_3+V_4=0\)

\(V_1=V_2+V_3+V_4\)

Concept

The power factor is defined as the cosine of the angle between voltage and current.

Power factor = cosϕ 

Power factor is also defined as the ratio of active and apparent power.

\(cos \space ϕ={P\over S}\)

Explanation

For a DC circuit, the voltage and current remain in the same phase, hence the angle between them is zero.

Power factor = cos ϕ  

cos ϕ = cos 0° = 1

cos ϕ = unity

Concept:

The resistance of copper wire is given by:

\(R = \rho \frac{l}{A}\)

ρ = Resistivity of the wire

l = length of the wire

A = Area of the cross-section of the wire

Calculation:

Given Resistance of copper wire is 'R'

For a constant cross-sectional area, the resistance is directly proportional to length, i.e.

R ∝ l

If the length is doubled, the resistance will also be doubled. 

So the resistance of the new wire is:

R' = 2R

Four such wires connected in parallel is as shown:

The equivalent resistance is therefore R/2

 SI units:

• Conductivity is the reciprocal of resistivity.
• The resistance offered by a wire of unit length and unit area of cross-section is called resistivity or specific resistance (r).
• Resistance is the ratio of applied potential difference (V) to the current (I) flowing through the conductor.
• Conductance is the reciprocal of resistance.

Concept:

The resistance of the conductor is given by:

\(R = \frac{{\rho l}}{A}\)

Where ρ = resistivity of the material

l = length of conductor

A = area of cross-section

Calculation:

For the second conductor:

\(R' = \frac{{\rho 2l}}{{A'}}\)

Given R’ = R

\(\frac{{\rho l}}{A} = \frac{{\rho 2l}}{{A'}}\)

A chopper is a static device that is used to obtain a variable dc voltage from a constant dc voltage source. Also known as a dc-to-dc converter.

Types of Choppers:

• Type A Chopper or First-Quadrant Chopper
• Type B Chopper or Second-Quadrant Chopper
• Type C Chopper or Two-quadrant type-A Chopper
• Type D Chopper or Two-quadrant type-B Chopper
• Type E Chopper or fourth-quadrant Chopper

The circuit diagram of Type B Chopper is as shown:

• When the chopper is ON, 'E' voltage drives a current through L in a direction opposite to that shown in the figure.
• During the ON period of the chopper, the inductance L stores energy.
• When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is returned to the supply.
• Therefore the average output voltage is positive and the average output current is negative as shown:

• In this chopper, power flows from load to source.
• Class B chopper is used for regenerative braking of dc motor.
• Class B chopper is a step-up chopper.

Ideal (Perfect) Conductor: 

• The ideal conductor needs a very small amount of potential difference (voltage) to carry on an extremely high current.
• This means that it has infinite conductivity and zero resistance.
• The voltage drop is also zero.
• This type of conductor generally not exist.

Properties of ideal (perfect) conductor:

• The electric and magnetic field (i.e. net field) inside the conductor is zero.
• The net charge density inside the conductor is zero and any net charge resides on the surface of the conductor.
• An ideal conductor always has an equipotential surface.
• Outside the conductor, there is a field present.

5 Resistors of 10 Ω connected in parallel with 1 A flowing through each of them is as shown:

Since the voltage source is parallel to the resistance, the voltage across each resistance will be:

V_10 Ω = 10 × 1 V

V10 Ω = 10 V

Now, when the resistors are connected in series, the circuit will be:

The series combination of these resistances will give a total resistance of:

R_total = 10 + 10 + 10 + 10 + 10 Ω 

R_total = 50 Ω 

The current through resistors in series will be equal, i.e.

\(I=\frac{10}{50}A\)

I = 0.2 A

Concept

The power factor is defined as the cosine of the angle between voltage and current.

Power factor = cosϕ 

Power factor is also defined as the ratio of active and apparent power.

\(cos \space ϕ={P\over S}\)

Explanation

For a DC circuit, the voltage and current remain in the same phase, hence the angle between them is zero.

Power factor = cos ϕ  

cos ϕ = cos 0° = 1

cos ϕ = unity

• A valence electron is an outer shell electron that is associated with an atom. These electrons can participate in the formation of a chemical bond.
• There are four electrons in the outer most orbit of a carbon atom as shown:

Concept:

The capacitance of a parallel plate capacitor is given by:

\(C = \frac{{ϵ A}}{d}\)

Where ϵ is the dielectric constant. For air, the value of ϵ is:

ϵ = 8.854 × 10^-14 C²/N-cm²

A is the area of the cross-section of the plates 

d is the distance between the plates

Calculation:

With d = 0.1 cm, A = 10 cm², and ϵ = 8.854 × 10^-14 C²/N-cm², the value of capacitance will be:

\(C = \frac{{8.854× 10^{-14}× 10}}{0.1}\)

C = 8.854 × 10^-12 F

C = 8.85 pF

Concept:

The power dissipated by a resistor is given by:

\(P=I^2R=\frac{V^2}{R}\)

I = Current flowing across the resistor

V = Voltage across the resistor

Calculation:

Given P = 100 Watts and R = 50 Ω

\(I=\sqrt {\frac{P}{R}}=\sqrt{\frac{100}{50}}\)

I = 1.414 A