Physics Class 12

Physics Class 12 Chapter 4 Subjective Questions

Moving Charges and Magnetism

  1. What type of magnetic field is produced due to a current carrying straight conductor?


The magnetic field lines due to a current carrying straight condutor are the concentric circular loops lying in a plane perpendicular to the straight conductor with their centre on the conductor.

  1. What is the S.I. unit of permeability? Also write its dimensional formula.


The S.I. unit of permeability is weber ampere-1 metre-1 (wb A-1 m-1) or tesla ampere-1 metre (TA-1 m). The dimensional formula of permeability is (MLT-2 A-2].

  1. In what respect does a wire carrying a current different from a wire carrying no current?


A current carrying wire produces magnetic field because when the current flows through it, the electrons move inside it along a definite direction.
On the otherhand, in a wire carrying no current, electrons are in random motion and thus such a wire producees no magnetic field.

  1. State the principle of a moving coil galvanometer.


It states that when a current carrying coil or loop is placed in a uniform magnetic field, it experiences a deflecting torque given by τ = NIAB sin θ = N MB sin θ
where M = IA is the magnetic moment of the coil and 0 is the angle made by the normal to the plane of the coil with the direction of magnetic field.

  1. Equal currents are flowing through the two infinitely long parallel conductors. What shall be the magnetic field at a point mid way between them when the currents:
    (a) in them are flowing in the same direction?
    (b) in them are flowing in the opposite direction?


The magnetic field at a point midway between two infinitely long parallel conductors shall be :

(a) Zero when the currents in them are flowing in the same direction because magnetic fields due to these conductors are equal and opposite and thus cancel out each other.

(b) Double the magnetic field due to one current carrying conductor because the magnetic fields due to these conductors are equal and act in the same direction.

  1. What are the primary function of electric and magnetic field in a cyclotron?


The magnetic field makes the charged particle to cross the gap between the two dees again and again by making it to move along a circular path, while the oscillating electric field, applied across the dees accelerates the charged particle again and again.

  1. Mention the factors on which the magnitude of the force experienced by a charged particle depend?


It depends upon the following factors :

Magnitude of the charge q.

Magnitude of the magnetic field in which the charge particle is moving.

Magnitude of the velocity of the moving charge.

Sine of the angle made by the velocity vector with the direction of the magnetic field.

  1. State Fleming’s Left hand rule.


It states that if we stretch the thumb, first (fore) finger and central finger of our left hand in mutually perpendicular directions such that the first finger points in the direction of magnetic field, central finger points in the direction of electric current or velocity of the charge, then the thumb represents the direction of force experienced by the current carrying conductor (or the charged particle).

  1. Define magnetic field induction at a point in terms of the force experienced by a charge moving in a magnetic field and hence define one Tesla.


Definition of B :
We know that F = Bq υ sin q
If. sin θ = 1 i.e., θ = 90°, υ = 1, q = 1, then F = B x 1 x 1 x 1
or B = F
Thus B is defined to be numerically equal to the force experienced by a unit charge moving perpendicular to the magnetic field with unit velocity.

Definition of IT :
If q = 1 C, υ = 1 ms-1 θ = 90°, F = 1 N,
Then B = 1T
∴ 1T = 1 N C-1 m-1 s = 1 N-1 A-1 m-1.
Thus the magnetic field at a’ point is said to be 1 T if 1 C charge moving perpendicular to the magnetic field with a velocity of 1 ms-1 experiences a force of 1 N at that point.

  1. State some important features of Biot-Savart’s law.


Following are some important features of the law :

It is applicable only to very small length current carrying conductors.

This law can’t be easily verified experimentally.

This law is analogous to Coulomb’s law in electrostatics.

 It is valid for a symmetrical current distribution.

The direction of dB is perpendicular to both Idl (current element) and r (position vector).

dB = 0 on the axis of a thin linear current carrying conductor.

dB = 0 i.e., minimum when 0 = 0 or 180° and maximum when 0 = 90°.

  1. How will you identify whether the magnetic field at any point is due to earth or due to some current carrying conductor?


If the magnetic field is due to earth’s magnetism only then the magnetic needle will always lie along the N-S direction On the other hand if the magnetic field is due to a current carrying conductor, then its direction may differ from N-S direction and it will come back to N-S direction when the current passing through the conductor is switched off.

  1. An electron beam is deflected in a given electric or magnetic field. How will you detect wether it has been deflected in electric or magnetic field?


When the electron beam is passed through electric field or two parallel plates having a potential difference V across them, then the ‘ electrons are deflected towards the positive plate and the path of the electron beam is parabola. On the other hand, when the electron beam enters into a uniform magnetic field perpendicularly then its path is circular. If it enters at any . angle θ (≠ 0° and 180°), then the path will be helical.

  1. Answer the following questions:

(a) Why does the picture of TV screen becomes distorted when a magnet is brought near the screen?

(b) Why neutrons cannot be accelerated by a cyclotron?


(a) The picture on TV screen becomes distorted when a magnet is brought near the screen because it exerts a magnetic force on the electron beam of T.V. tube.

(b) Neutrons cannot be accelerated by a cyclotron because they are neutral.

  1. Define current element. What is its significance?


It is defined as the product of current I and the length of the current dl.
∴ Current element = I dl
Its significance is that it produces the magnetic field in the same way as a stationary charge produces an electric field.

  1. Is the source of magnetic field analogue to the source of electric field? Why?


No, they are not analogue because source of the magnetic field cannot be a monopole whereas the source of electric field can be either of the positive or negative charges.

  1. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Explain.


Ammeter reads slightly lesser than the actual currents when it is connected in a circuit because it has a small resistance which slightly decreases circuit current!

  1. How will you convert a moving coil galvanometer into:

(a) an ammeter

(b) a voltmeter


A galvanometer is converted in to:

(a) an ammeter by connecting a low resistance in parallel to the galvanometer. This low resistance is called shunt.

(b) a voltmeter by connecting a high resistance in series with the galvanometer.

  1. Why should the ammeter have a low resistance?


An ammeter should have a low resistance so that it may not change the resistance of the circuit when it is connected in series.

  1. What is the nature of the path of a charged particle moving in a uniform electrostatic field with initial velocity—

(a) parallel to the field

(b) perpendicular to the field

(c) at an arbitrary angle with the direction of field.


The path of the charged particle moving

(a) parallel to the electric field is a straight line

(b) perpendicular to the electric field is parabolic.

(c) at an arbitrary angle with the direction of field is parabolic.

  1. Out of an ammeter and a voltmeter which of the two has a high resistance and why?


Voltmeter has high resistance. This is because galvanometer is converted into voltmeter by connecting a high resistance in series with it.

  1. Why does a solenoid contract when current is passed through it?


When the current is passed through a solenoid, the current in the adjacent turns of the solenoid flows in the same direction. We know that current flowing in the same direction between two conductors causes a force of attraction. So the turns of the solenoid will attract each other and hence the solenoid will contract.

  1. Compare a voltmeter and ammeter.


(i) An ammeter is used to measure current while a voltmeter is used to measure the potential difference

(ii) An ammeter is a low resistance instrument while a voltmeter is a high resistance instrument.

(iii) An ammeter is always connected in series while a voltmeter is always connected in parrallel.

  1. If a student by mistake connects a voltmeter in series with or an ammeter in parrallel or a circuit, what will happen?


When the voltmeter is connected in series, the resistance of the circuit becomes high. Hence the current in the circuit will be reduced, so the voltmeter will not read the required potential difference. When the ammeter is connected in parallel in a circuit, the resistance is reduced considerably. So a large current would flow which can damage the ammeter.

  1. What is a galvanometer? Why it cannot read strong currents?


Galvanometer is an instrument to measure weak currents of the order of milli amperes. It is quite sensitive instrument and gives large deflection for weak currents and its resistance is also small. If a strong current is passed through a galvanometer it would produce a very large defelection which would be unmeasurable. Strong currents may bum coil by producing large heat (I2R).

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