NCERT Science Class 10 Chapter 13 Magnetic Effects of Electric Current CBSE Board Sample Problems Long Answer (For CBSE, ICSE, IAS, NET, NRA 2022)

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Explain the underlying principle and working of an electric generator by drawing a labeled diagram. What is the function of brushes?


Electric generator labelled diagram is as follows:

Electric Generator Labelled Diagram
  • Principle: An electric generator works on the principle of electromagnetic induction.
  • Working: Let the in the beginnings brushes and are kept pressed separately on rings
  • and respectively. Let the axle attached to the rings is rotated such that arm AB of the coil moves up and arm CD moves down in the magnetic field.
  • Due to rotation of arms AB and CD induced current are set up in them.
  • As per Fleming՚s right hand rule, induced current in these arms along the directions AB and CD respectively and current flows into .
  • After half rotation, arm AB moves downward and arms CD upward to change the direction opposite to first case.
  • Thus, after every half rotation current changes its direction and an alternate current is obtained in the generator.
  • Brushes are kept pressed on the two slip rings separately.
  • Outer ends of the brushes are connected to the galvanometer.
  • Thus, brushes help in transferring current from the coil ABCD to external circuit.


Explain the principle, construction and working of an electric motor with a help of labeled diagram?


Principle - it is based on the principal that a current carrying conductor placed perpendicular to the magnetic field experiences a force.


  • Armature or coil- It consist of an insulated copper wire wound on a soft iron core.
  • Strong field magnet- two pole pieces of a strong magnet provides a strong magnetic field.
  • Split ring- it consist of two halves
  • ( and ) of a metallic ring which reverses the direction of the current in a coil.
  • Brushes- two carbon brushes touch the commutator (split ring) .
  • Battery - a battery is connected across the carbon brushes.
Construction and Working of an Electric Motor


a) State Fleming՚s Left hand rule.

b) Write the principle of working of electric motor.

c) Explain the function of the following part of electric motor

(i) Armature

(ii) Brushes

(iii) Split Ring


  • Fleming՚s left hand rule state that stretch the forefinger, the central finger and the thumb of left hand in mutually perpendicular to each other. Forefinger shows the direction of magnetic field, central finger in the direction of current, then thumb will point the direction of motion of conductor.
  • Electric motor works on the principle that a coil carrying current, when placed in a uniform magnetic field, experiences a force whose direction is given by Fleming՚s left hand rule. As a result coil starts rotating about its own axis.
  • Armature coil carries current. As a result to long arm of armature coil experience equal force in mutually opposite direction and under their influence coil being to rotate
  • Brushes draw current from split ring and supply it to the coil
  • Split Ring draw current from the battery. It change their contact with brushes after every half rotation as a result current supplied to the armature coil through brushes changes its direction after every half rotation


(i) With the help of a labeled circuit diagram describe an activity to illustrate the pattern of the magnetic field lines around a straight current carrying long conducting wire.

(ii) Name the rule that is used to find the direction of magnetic field associated with a current carrying straight conductor.

(iii) Is there a similar magnetic field produced around a thin beam of moving

(a) Alpha particles

(b) Neutrons

Justify your answer.


(i) Activity

  • Take a battery (12 V) , a variable resistance (or a rheostat) , an ammeter (O — 5 A) , a plug key, and a long straight thick copper wire.
  • Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down.
  • Connect the copper wire vertically between the points X and Y, as shown in diagram in series with the battery, a plug and key.
  • Sprinkle some iron filings uniformly on the cardboard. Keep the variable of the rheostat at a fixed position and note the current through the ammeter.
  • Close the key so that a current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight.
  • Gently tap the cardboard a few times. Observe the pattern of the iron filings.
  • It is observed that the iron filings align themselves showing a pattern of concentric circles around the copper wire. These represent the magnetic field lines.
Variable Resistance

(ii) Right hand thumb rule

(iii) (a) Yes, Alpha particles being, positively charged constitutes a current in the direction of motion.

(b) No, Neutron being electrically neutral constitutes no current.


Name a device which converts mechanical energy into electrical energy. Explain the underlying principle and working of this device with the help of a labeled diagram.


Electric generator converts mechanical energy into electrical energy.

Principle: Electromagnetic induction which states that electric current is induced in a closed circuit because of changing magnetic field.


Electric Generator Converts Mechanical Energy into Electrica …
  • When a coil of insulated copper wire is forced to rotate inside a magnetic field then electric current is induced. The following figure shows circuit diagram of a simple AC generator.
  • If axle is rotated clockwise, then the arm AB moves upwards and the arm CD moves downwards.
  • Since the arm AB and CD are moving in a magnetic field, a current will be induced due to electromagnetic induction. Arm AB is moving upwards and the magnetic field acts from left to right. Hence, according to Fleming՚s right hand rule, the direction of induced current will be from A to B.
  • Similarly, the direction of induced current in the arm CD will be from C to D.
  • The direction of current in the coil is ABC D. Hence, the galvanometer shows a deflection in a particular direction. After half a rotation, arm AB starts moving down whereas arm CD starts moving upward.
  • The direction of the induced current in the coil gets reversed as DCBA. As the direction of current gets reversed after each half rotation, the produced current is called an alternating current (AC) .


Whenever in a closed circuit (i.e.. , a coil) , the magnetic field lines change, an induced current is produced.

(i) Identify the instrument, Draw a neat and labelled diagram. (1 + 2)

(ii) Write the working of the instrument. (3)




Ciecuit Diagram
  • Working: As the armature is rotated about an axis, the magnetic flux linked with the armature changes. Therefore, an induced current is produced in the armature.
  • The armature abcd is rotating anticlockwise so that the arm ad moves inwards and bc moves outwards. Applying Fleming՚s right-hand rule, induced current in the armature and in the circuit is due to which G shows deflection towards the right.
  • After the armature has turned through , it occupies the position, with the armature rotating in the same direction (anti-lock wise) , bc moves inwards and ad moves outwards.
  • The induced current in the external circuit flows in the opposite direction due to which the direction of deflection in the galvanometer is towards left.