**In this post, we have discussed Electricity and Magnetism Short Questions Answer of Electricity and Magnetism Physics Class 12 NEB**

#### 1. What do you mean by electric current?

The rate of flow of electric charge through any cross-section of the given specimen is called the Electric Current. Let q be the total charge that flows through a given cross-section of a specimen in ‘t’ second, then-current through the specimen (I) is given by

I=q/t

Its unit is ampere in the SI system of the unit.

#### 2. Define one-ampere current?

The current flowing through a wire is said to be one ampere if one coulomb of charge flows through its any cross-section in one second. If the 2-ampere current is flowing through a wire, it means that 2C per second flows past any cross-section of the wire. We know, 1C = charge on 6.25×10^18 electrons. Thus, when we say that current through a wire is 1A, it means that 6.25 x 10^18 electrons. Thus, when we say that current through a wire is 1A, it means that 6.25 x 10^18 electrons per second flow past any cross-section of the wire.

#### 3. State Ohm’s law or the most fundamental law of electricity.

Ohm’s law states that if the physical condition i.e. temperature, pressure, mechanical strain, etc are remained unchanged, the current through a conductor is always directly proportional to the potential difference across its two ends. Let V and I be the potential difference across of conductor and current through it. Then

I α V

I = (1/R)V

where R is a constant and known as electrical resistance of the conductor.

4. Define the term resistance.

The resistance of a conductor is defined as its ability to oppose the flow of charge through it. It is equal to the ratio of the potential difference applied to the current flowing the conductor. Mathematically,

R =V/I

where R is the resistance of the conductor, V is the p.d. across it, and I is the current flowing through it. Its unit is Ohm (Ω) in the SI system of the unit.

#### 5. What is the cause of resistance?

A conductor has a large number of free electrons moving in a random direction with different velocities. When a potential difference is applied across it, an electric field is set up across it and the free electrons get accelerated in a direction. As the electrons move, the electrons collide against other electrons and the atoms or positive ions which slow down the motion of electrons. Hence collision of free electrons with other electrons and ions is the cause of resistance.

#### 6. Define the term conductance.

The reciprocal of resistance is known as the conductance i.e.

C=1/R

where C is the conductance and R is the resistance of the conductor. Its unit is ohm-1 or mho or Siemen. It is the measure of how much current is allowed through the circuit or device.

#### 7. Define the term resistivity (or specific resistance). What is its unit? [HSEB 2054]

Resistivity or specific resistance of a material of conductor is defined as the resistance offered by the conductor of unit cross-sectional area per unit length. Its unit is ohm meter (Ωm) in the SI system of units.

Mathematically, ρ = RA/l

where ρ, R, A, and l be the resistivity, resistance, cross-sectional area, and length of the conductor respectively.

#### 8. Define the term conductivity.

The reciprocal of resistivity of the material of a conductor is called the Conductivity of the Conductor. Mathematically, σ = 1/ρ

where σ be the conductivity and ρ be the resistivity of the material. Its unit is ohm-1 meter-1 or Ω-1m-1 or Siemen-1.

#### 9. What do you mean by the drift velocity of electrons?

Drift velocity of electrons is defined as the average velocity acquired by the electrons in a conductor when subjected to an electric field. The drift velocity of free electrons is of the order of 10^-5m/s. Let a current I pass through a conductor of cross-sectional area A. Then the current is related with drift velocity v of electron of charge e as I = nevA. Where n is the electron density.

#### 10. What is the relation between drift velocity and electric current?

The relation between drift velocity and an electric current is given by

I = nevA

where I is the current is the charge density, e be the charge of an electron, v be the drift velocity and A be the cross-sectional area of the conductor. This shows that the current in a conductor is directly proportional to the drift velocity of the electron in it.

#### 11. What do you mean by the term “relaxation time”?

If a potential difference is applied across the ends of a conductor, each electron starts accelerating towards the positive terminal. During this course, the electron collides again and again with positive ions of metal. The average time that an electron spends between two successive collisions (as shown in fig.) is called the Relaxation Time (t). Its value is of the order of 10-14 second.

#### 12. When a potential difference is applied across the ends of a conductor, the accelerating electrons towards the positive potential end collide with ions. What happens to an electron after collision with an ion?

When an accelerating electron inside an electric field collides with an ion, it moves off in some new and quite random direction. However, it still experiences the applied electric field, so it continues to accelerate again, gaining a velocity in the direction of the positive terminal. It again encounters an ion and loses its directed motions. This situation is repeated again and again for every free electron in a metal.

13. A large number of free electrons are present in metals. Why is there no current in the absence of an electric field across it?

In the absence of an electric field across a conductor, the free electrons move in a random direction. During their motion, an electron collides with other electrons and ions in the metal and it will have net displacement zero making drift velocity zero. We know I = nevA where I is the current and v is the drift velocity. Since v = 0 m/sec, I will be zero.

#### 14. On what factors resistance of a conductor depends? [HSEB 2053, 057]

The resistance of a conductor depends upon the length of the conductor (l), cross-sectional area (A), the temperature of the conductor, and the nature of the material used. For a given material at a constant temperature, the resistance (R) is directly proportional to the length and inversely proportional to the cross-sectional area.

i.e. R α l/A

or R = ρl/A

where ρ is known as the resistivity of the material and depends upon the temperature.

#### 15. On what factors the resistivity of a conductor depend?

The resistivity of a conductor does not depend upon the dimension of the conductor. It depends only upon the nature of the material i.e. charge density, the temperature of the conductor, and the average time of the relaxation.

#### 16. If the length of a conductor of resistivity ρ is made four times its initial length, what will be its new value of resistivity?

Since the resistivity of a material depends upon the nature of the material and its temperature at constant pressure but not on the dimension of the body, the change in length of a conductor does not change its resistivity.

#### 17. Why do we use connecting wires made of copper? [HSEB 2051, 056]

Copper is a good conductor of electricity. It has low resistivity and it is a diamagnetic substance. It does not get magnetized when current flows through it. Thus, it does not disturb the flow of the current. Hence the connecting wires are made of copper.

#### 18. You are given n wires each of resistance RΩ What is the ratio of maximum to minimum resistance that can be obtained from these wires? [HSEB 2063]

Maximum resistance can be obtained by joining all the given resistors in series but it will be minimum when they are connected in parallel. Therefore,

Rmax = R + R + . . . n times

or Rmax = nR

And 1/Rmin = 1/R+1/R+ … n times

1/Rmin = n/R

Rmin = R/n

Rmax/Rmin = nR/(R/n)

Rmax/Rmin = n^2/1

#### 19. What is the conventional direct current?

Before the discovery of the electron, it was believed that the positive charge would move in an electric circuit and conduct electricity in metals. So the people began to show the direction of current in the direction from a positive terminal to the negative terminal of the battery in the external circuit called the Conventional Directions as shown in the figure. But after the discovery of electrons, it is found that the conduction of electricity is due to the free electrons, not due to positive ions. Still, many people are adopting the traditional direction called Conventional Direction.

#### 21. What will happen to the drift velocity of free electrons when the potential difference across a conductor is increased?

We know the relation,

v(drift) = (eE/m) T

Where v be the drift velocity, e be the charge of an electron, E be the strength of electric field applied, T be the average relaxation time, and in be the mass of an electron. If V be the p.d. across the conductor and l be the length of the conductor.

E= V/I

v(drift) = (eV/ml) T

If the temperature remains constant, v(drift)∝ V. Hence if p.d. is increased, the drift velocity is also increased.

#### 22. How can we obtain a high value of current through the drift velocity of electrons and the charge on electrons are so small?

Though the drift velocity of electrons and the charge on electrons are so small, we can obtain a high value of current because the number of electrons per unit volume is very large. In a good conductor, it is about 10^28m-3.

#### 23. What do you mean by the temperature coefficient of resistance?

The change in resistance per unit resistance at 0°C per unit change in temperature is known as the temperature coefficient of resistance. Mathematically, it is given by

α = △R/RoθQ

Where △R be the change in resistance, △θ** **be the change in temperature, and Ro be the resistance at 0°C. Its unit is K-1 in the SI system of units.

#### 24. Batteries are always labeled with their emf, for instance, an AA flashlight battery is labeled “1.5 volts”. Would it also be appropriate to put a label on batteries stating how much current they provide? Why? Why not?

No, a battery does not always produce the same current, no matter what circuit it’s used in. The current in an electric circuit consisting of a cell of emf E and internal resistance r and a resistance R is given by

I = E/R + r

This equation shows that the current that a source of emf produces in a given circuit depends on the external resistance R as well as the internal resistance r of the cell. The greater the resistance, the less the current will produce. So, however, a battery has a fixed emf, it may not have the same magnitude of the current. Hence, it is not appropriate to label the current.

#### 25. Why do we use alloys like manganin, constantan, or nichrome standard resistance?

We use alloys like manganin, constantan, or nichrome to make standard resistance because they have a small value of temperature coefficient of resistance which makes their value almost constant for a range of temperature.

#### 26. What is a superconductor?

A conductor which loses all signs of resistance on being cooled to its critical temperature is called a Superconductor. In a superconductor, the carriers of current are mutually coherent electrons. At the critical temperature, the transition of the normal state of the conductor to the superconducting state takes place. A graph between resistivity and temperature for mercury is shown below. For mercury, the critical temperature is 4.2K.

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Niraj Raut is the CEO of Merocourse and writes blog posts for Ignited Nepal, Merocourse, and ProtoTrend. He studied B.Sc.CSIT at Tribhuvan University.