Network Theory Questions and Answers – Basic Network Concepts

 1. Energy per unit charge is termed as

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

Answer.2. Voltage

Explanation:- 

An electric potential (also called the electric field potential, potential drop, or electrostatic potential) is the amount of work needed to move a unit positive charge from a reference point to a specific point inside the field without producing any acceleration. Typically, the reference point is Earth or a point at Infinity, although any point beyond the influence of the electric field charge can be used.

Mathematically, it can be expressed as

Electrical potential (V) = Work Done/Charge = W/Q

 

2. The rate of flow of charge per unit time is called

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

Answer.3. Current

Explanation

The flow of electrons is called an electric current hence the current can be measured by the number of electrons passing through the material per second. In other words, the rate of flow of charge per unit time is called current. Current is measured in coulombs per second and one coulomb per second is called Ampere.

The relation between current and charge can be given as

I = Q/T Ampere

I = Average current flowing

Q = Total charge Transform

T = Time required for transfer of charge

 

3. The direction of convection electric current is taken from

  1. Lower Potential to higher potential
  2. Higher potential to Lower Potential
  3. Higher potential to higher potential
  4. Half potential to full potential

Answer.2. Higher potential to Lower Potential

Explanation:-

Conventionally, the conductor which has excess electrons is considered to be a body at lower potential while the conductor with a deficiency of electrons is considered to be a body at higher potential. When these charged conductors are joined together, the electrons flow from lower potential to higher potential.

The flow of charge through a conductor constitutes an electric current.

But with the development of atomic structure, it came to our knowledge that positive charge cannot move through a conductor. It is only the negative charge (electrons) that flows from a lower potential to a higher potential. But we still continue the old convention that electric current flows from a higher potential to a lower potential. This current is called conventional electric current.

And the flow of electrons from a lower potential to higher potential is called electronic current. Thus we can conclude that conventional current flows from a higher potential to a lower potential, i.e. from a positively charged body to a negatively charged body.

 

4. The device used to measure potential diffrence is

  1. Ammeter
  2. Thermometer
  3. Voltmeter
  4. Potentiometer

Answer.3. Voltmeter

Explanation:-

Electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit charge from one point to the other. The SI unit of electric potential is volt.

Voltmeter

  • The device which is used to measure the potential difference between the two ends of the conductor is called voltmeter.
  • The voltmeter is connected parallel across the two ends of the conductor where potential difference is to be determined.
  • A voltmeter must have high resistance.

 

5. A conductor is said to have resistance of one ohm if a potential difference of one volt across its terminals causes a current of X ampere to flow through it. What will be the value of X?

  1. 5
  2. 3
  3. 2
  4. 1

Answer.4. 1

Explanation:-

Resistance is the property of a substance due to which it opposes the flow of electric current through it.

A conductor is said to have resistance of one ohm if a potential difference of one volt across its terminals causes a current of one ampere to flow through it.

 

6. The device used to measure the current is

  1. Ammeter
  2. Thermometer
  3. Voltmeter
  4. Potentiometer

Answer.1. Ammeter

Explanation

An ammeter is a device which is used to measure electric current. An ammeter is always connected in series with the circuit.

 

7. For reference purpose the potential of the earth is taken as

  1. Infinite
  2. Positive
  3. Zero
  4. Negative

Answer.3. Zero

Explanation:-

Since the size of earth is large and it is a good conductor, the potential of earth remains constant regardless of the electrons taken from it or supplied to it. For this reason, earth is taken to be zero potential surface.

Thus, a body whose potential is higher than that of the earth is said to have positive potential and one whose potential is lower than that of the earth is said to have a negative potential. Hence, if a positively charged body be connected to the earth, the charge will flow from the body to the earth (the body being at a higher potential than the earth until the potential of the body also becomes the same as that of the earth. Now if a negatively charged body be connected to the earth, the charge will flow from the earth to the body, because now the potential of the earth is higher than that of the body and again the potential of the body becomes the same as that of the earth.

 

8. The conductors have transport phenomena of electrons due to

  1. Electric field
  2. Magnetic field
  3. Electromagnetic field
  4. Inductive field

Answer.1. Electric field

Explanation:-

Conductors are substances through which electric charges can flow easily. They are characterized by the presence of a large number of free electrons. The number density of free electrons in a conductor is the same throughout the conductor. This is because free electrons experience repulsive force between them and the conductor allows the movement of free electrons.

In absence of applied potential difference electrons in the conductor have random motion. The average displacement and average velocity are zero. There is no flow of current due to the thermal motion of free electrons in a conductor. When two ends of conductors are joined to a battery then one end is at higher potential and another at lower potential. This produces an electric field inside the conductor from a point of higher to lower potential i.e. E = V/L.

The field exerts an electric force on the free-electron causing acceleration of each electron.

 

9. The specific resistance of a conductor depends upon

  1.  The dimension of the conductor
  2.  Composition of conductor material
  3.  The resistance of the conductor
  4.  Both (a) and (b)

Answer.2.  Composition of conductor material

Explanation:-

For any given material at a certain given temperature, the resistance is given as

Resistance

where ρ is a constant depending on the nature of the material of the conductor and is known as its specific resistance or resistivity.

Specific Resistance depends only on temperature and material of the conductor but not on its dimensions of the conductor, on which resistance depends, and mechanical deformation such as stretching, etc. As ρ depends only on the material of a conductor at a given temperature, hence it is a characteristic constant.

 

10. A 25 Ω resistor has a voltage of 150 sin377 t. Find the corresponding power.

  1. 900 sin2 337 t
  2. 90 sin2 337 t
  3. 900 sin2 377 t
  4. 9 sin2 337 t

Answer. 1. 900 sin2 337 t

Given,

R = 25 Ω

V = 150sin377t

Power, P =?

Current I = ?

I = V/R = (150sin377t)/25

I = 6sin377t A

Now power P = V.I

P = (150sin377t) × (6sin377t)

P = 900 sin2 337 t

 

11. Determine the total charge entering a terminal between t = 1 s and t = 2 s, if the current passing the terminal is (3t2 – t) A.

  1. 3.5 C
  2. 6.5 C
  3. 4.5 C
  4. 5.5 C

Answer.4. 5.5 C

Explanation

The charge is given as

q = ∫ i dt

Current passing the terminal = (3t2 – t) A

\begin{array}{l} \int\limits_1^2 {(3t2 - t)dt} \\ \\ = \left[ {{t^3} - \frac{{{t^2}}}{2}} \right]_1^2 \end{array}

 

= (8 − 1) −½(4 − 1) = 5.5 C

 

12.  The S.I unit of Electric energy is

  1. Watt
  2. Volts
  3. Ampere
  4. Joul

Answer.4. Joul

Explanation:-

Electrical Power

The rate at which the electric work is done in an electric circuit is called electric power. The SI unit of power is joule per second or watt.

Power

Note:- 

Electrical Energy

Electric energy is the total amount of electrical work done in an electrical circuit. Electric energy can also be defined as the product of power and time. The S.I Unit of Electrical- Energy is joule or watt-sec.

Ampere is the Unit of Current

Volt is the S.I unit of Potential Difference.

 

13. The Unit of inductance is _____

  1. Weber
  2. Henry
  3. Farad
  4. Tesla

Answer.2. Henry

Explanation:-

The unit of inductance is the henry (symbol H) and is defined as follows: ‘A circuit is said to possess an inductance of 1 H when an e.m.f. of 1 V is induced in that circuit by a current changing at the rate of 1 A/s’.

 

14. Which property of an electrical conductor opposes a change in the current?

  1. Resistance
  2. Capacitance
  3. Conductance
  4. Inductance

Answer.4. Inductance

Explanation

An inductor is a device that temporarily stores energy in the form of a magnetic field. It is usually a coil of wire. One of the basic properties of electromagnetism is that when you have current flowing through the wire it creates a small magnetic field around it.

One current first starts to flow through the inductor a magnetic field start to expand then after some time the magnetic field becomes constant then we have some energy stored in the magnetic field.

Once a constant magnetic field is generated in the Inductor, it will not change any further. As magnetic flux = N x I (Turns x Current), Inductor will draw a constant current to maintain the magnetic field.

Once the current stop flowing the magnetic field start to collapse and the magnetic energy turned back into electric energy.

So when the current flowing through the inductor changes, the magnetic field also changes in the inductor, and emf (electromotive force) is induced in the inductor as per Faraday’s law of electromagnetic induction.

According to Lenz’s law, the direction of electromotive force(emf) opposes the change of current that created it.  V= -L x dI/dt (rate of change of current).

So the inductor opposes any change of current through them.

 

15. Farad is the S.I units of____ 

  1. Inductance
  2. Resistance
  3. Capacitance
  4. Reluctance

Answer.3. Capacitance

Explanation

The Farad is the practical and the Sl unit of capacitance. The unit, named after Michael Faraday (1791-1867), was first suggested by Latimer Clark in 1867. The capacitor has a capacitance of 1 farad when a charge of 1 coulomb raises the potential between its plates to 1 volt.

 

16. Which property of an electrical conductor opposes a change in the current?

  1. Resistance
  2. Capacitance
  3. Conductance
  4. Inductance

Answer.2. Capacitance

Explanation

The amount of voltage (charge separation) that a capacitor can store is reflected by its capacitance (C), which is the ratio of the charge (Q) on either plate to the voltage (V) between the two plates (C = Q/V).

Also, the amount of charge that can be loaded onto a capacitor plate is proportional to the voltage and the capacitance of the capacitor (i.e., Q = VC).

In other words, capacitance is the amount of charge that you get for a given voltage. As the voltage increases, the charge increases.

The amount of charge stored is proportional to the surface area of overlap (A) of the two plates of the capacitor (i.e., the larger the surface area of overlap of the plates, the greater the magnitude of charge that can be accommodated.

Capacitor charging and discharging is not instantaneous – it takes time. In addition, charging and discharging are not linear, but rather occur at exponential rates. The time requirement for capacitor charging and discharging is reflected by its time.

The capacitor opposes the change in voltage. When we close the switch at time = 0, the voltage on the initially uncharged capacitor increases. With an ideal capacitor, the final voltage is equal to the battery voltage. The “rise time” is a function of the capacitor size and the value of the resistor.

Energy from the battery is transferred to the capacitor. We say that work is done in this process. Our universe does not allow infinite changes in energy because there is a limit to the rate at which work can be done.

All real circuits have resistance. Even if we connect a capacitor directly to the battery, we still have resistance. This hidden resistance is known as “battery internal resistance” and capacitor “equivalent series resistance.”

Since there is always resistance, there will always be a limit to the amount of current that can flow in this circuit. This imposes a limit on the time it takes to transfer energy from the battery to the capacitor. Yes, the resistor changes some of the battery’s energy to heat.

 

17. Which of the following is the dimensional formula for conductance or conductivity?

  1. M−1 L−2 T3 A2
  2. M L3 T3 A2
  3. M2 L2 T3 A2
  4. M L2 T3 A2

Answer.1. M−1 L−2 T3 A2

Explanation:-

The conductance is given as

Conductance = 1 ⁄ Resistance

Firstly consider the resistance.

Ohm’s law states that electric current flowing through the conductor is directly proportional to the potential difference between its two ends when the temperature and other physical parameters of the conductor remain unchanged.

IR

⇒ VI

Now V has units of (electric field)*(distance).

But the electric field has units (force)/(charge).

Also, charge has dimensions of (current)(time) and force has dimensions (mass)(length)/(time)2.

Thus, dimensions of V is,

[VLMLT2 ⁄ AT

⇒ [V= M L² T ⁻³ A⁻¹

Dimensional formula for I = A

Put these values in equation (1), we get:

The dimensional formula for R = M L2 T−3 A−1/ A

Dimensional formula for R = [M L2 T −3 A−2]

Now the Dimensional formula of conductance will be

G = 1 ⁄ [M L2 T −3 A−2]

G = [M−1 L−2 T3 A2]

18. Which of the following is the dimension of resistance?

  1. [M.L2.T-2]
  2. [M.L2.T-2.I-2]
  3. [M L2 T -3 A-2]
  4. [M L2 T -2 A⁻1]

Answer.3. [M L2 T -3 A-2]

Explanation

Firstly consider the resistance.

Ohm’s law states that electric current flowing through the conductor is directly proportional to the potential difference between its two ends when the temperature and other physical parameters of the conductor remain unchanged.

IR

⇒ VI

Now V has units of (electric field)*(distance).

But the electric field has units (force)/(charge).

Also, charge has dimensions of (current)(time) and force has dimensions (mass)(length)/(time)^2.

Thus, dimensions of V is,

[VLMLT2 ⁄ AT

⇒ [V= M L² T ⁻³ A⁻¹

Dimensional formula for I = A

Put these values in equation (1), we get:

The dimensional formula for R = M L2 T−3 A−1/ A

Dimensional formula for R = [M L2 T −3 A−2]

 

19. Energy stored in an inductor is _______

  1.  LI
  2. LI2
  3. LI/2
  4. LI2/2

Answer.4. LI2/2

The energy stored in the magnetic field of an inductor can be expressed as

W = 1/2 L I2

where

W = energy stored (joules, J)

L = inductance (henrys, H)

I = current (amps, A)

 

20. An inductor of 3mH has a current i = 5(1 – e-5000t). Find the corresponding maximum energy stored.

  1. 375 J
  2. 37.5 mJ
  3. 37.5 kJ
  4. 3.75 mJ

Answer.2. 37.5 mJ

Explanation:-

Given

L = 3 mH, i = 5(1 – e-5000t)

V = L di/dt=3×103d[5(1e5000t)]/dt

= 75e5000t

I = i(∞) = 5(1 – e-∞) = 5 A

E = 1/2 LI2 = 0.5 × 3 × 10-3 × 52 = 37.5 mJ

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