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Choose the incorrect statement from the following regarding magnetic lines of field

(a) The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points

(b) Magnetic field lines are closed curves

(c) If magnetic field lines are parallel and equidistant, they represent zero field strength

(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines

 

Answer

Here, (a), (b) and (d) are correct.

 

The incorrect statement is (c) If magnetic field lines are parallel and equidistant, they represent zero field strength

 

Explanation

If the magnetic field lines are parallel and equidistant, it represents uniform (same at all points) magnetic field. 

 

If the key in the arrangement (Figure 13.1) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are

fig 13.1.jpg

 

(a) concentric circles

(b) elliptical in shape

(c) straight lines parallel to each other

(d) concentric circles near the point O but of elliptical shapes as we go away from it

 

Answer

concentric circles

Explanation

Magnetic Field in a Straight Current Carrying Conductor - Teachoo.png

The conductor in the given figure is a straight current carrying wire

The magnetic field lines produced by a straight current carrying conductors are in the form of concentric circles.

 

A uniform magnetic field exists in the plane of paper pointing from left to right as shown in Figure 13.3. In the field of an electron and a proton move as shown. The electron and the proton experience

figure 13.3 - Teachoo.jpg

(a) forces both pointing into the plane of paper

(b) forces both pointing out of the plane of paper

(c) forces pointing into the plane of paper and out of the plane of paper, respectively

(d) force pointing opposite and along the direction of the uniform magnetic field respectively

Answer

forces both pointing into the plane of paper

 

Explanation

 

For Proton and Electron

 

Proton

Electron

When proton moves upwards, 

i.e. Positively charged particle moves upwards

So, Direction of current is upwards

When electron moves downwards, 

i.e. Negatively charged particle moves downwards

So, Direction of current is upwards

So, for both Proton and Electron,

Direction of Current i s upwards i.e. from South to North

 

And, Magnetic field is from West to East (Left to Right )

Fig 13.3 NCERT Exemplar (Answer) Magnetic Effects Class 10 - Teachoo.png  

Applying Fleming’s left hand rule,

Taking the forefinger, middle finger and thumb of the left hand in mutually perpendicular direction

  • Pointing middle finger towards north direction (direction of current)
  • Fore-finger towards east/right direction (direction of magnetic field)
  • We get the direction of force is into the plane , since the thumb points into the plane.

 

So, direction of force for both electron and proton is

Into the plane

 

Choose the incorrect statement

(a) Fleming’s right-hand rule is a simple rule to know the direction of induced current

(b) The right-hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors

(c) The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically

(d) In India, the AC changes direction after every 1/50 second 

 

Answer

Here, (d) is the incorrect statement

Explanation

In India,

Frequency of AC current is 50 Hz

That means that the AC changes direction after every 1/100 second, not after 1/50 seconds

 

 

To convert an AC generator into DC generator

(a) split-ring type commutator must be used 

(b) slip rings and brushes must be used 

(c) a stronger magnetic field has to be used 

(d) a rectangular wire loop has to be used

Answer

 

(a) split-ring type commutator must be used

 

Explanation

Split rings are used to reverse the direction of current after every half revolution.

If split ring commutator will be used in AC generator, then the direction of current will be reversed after every half revolution. 

Hence, a direct current will be produced.

  1. Class 10
  2. Chapter 13 Class 10 - Magnetic Effects of Electric Current

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.