Class 10 Science · Chapter 12

Magnetic Effects of Electric Current

279 Class 10 Science questions on Magnetic Effects of Electric Current from the NCERT textbook, NCERT Exemplar and CBSE board papers 2018–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 14 of 14

1. Q261 CBSE 2019
   What is the ​principle ​of an electric motor ? State the rule which is ​applied to determine ​the direction of force experienced by a ​current carrying conductor when ​kept in a ​magnetic field.
   Show answer

   A motor works on the force on a current-carrying conductor in a magnetic field; the direction is found by Fleming's left-hand rule.

   Principle: an electric motor works on the principle that a current-carrying conductor (its coil) placed in a magnetic field experiences a force. The forces on the two opposite sides of the coil act in opposite directions and make it rotate, converting electrical energy into mechanical energy. Rule: Fleming's left-hand rule. Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force (motion) on the conductor.

   Step-by-step solution →
2. Q262 CBSE 2019
   Draw the pattern of the field lines ​of the magnetic field ​around a current carrying straight conductor passing ​through and ​held perpendicular to a horizontal cardboard. State right-hand thumb ​rule and explain how this ​rule is useful to determine the direction of the magnetic field in the above case, if the ​direction of current in the conductor ​is vertically downwards.
   Show answer

   Field lines are concentric circles round the wire; for a downward current the right-hand thumb rule gives clockwise circles seen from above.

   Pattern (see the figure): the magnetic field lines are concentric circles on the cardboard, centred on the conductor, closer together near the conductor and farther apart away from it. Right-hand thumb rule: Imagine that you are holding the current-carrying straight conductor in your right hand so that the thumb points in the direction of the current; then the fingers wrapped round the conductor give the direction of the magnetic field lines. Use in this case: the current is vertically downwards, so point the right thumb downwards along the conductor. Seen from above the cardboard the fingers curl clockwise, so the magnetic field lines are concentric circles in the clockwise direction (when seen from above).

   Step-by-step solution →
3. Q263 CBSE 2019
   State right-hand thumb rule to determine the ​direction of magnetic field around a current carrying conductor. ​Apply ​this rule to ​find the ​direction ​of magnetic field inside and outside a circular loop of wire lying in the plane ​of a table and current is flowing through ​it clockwise.
   Show answer

   For a clockwise loop the field is into the table inside the loop and out of the table outside it.

   Right-hand thumb rule: Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points in the direction of the current. Then the direction in which your fingers wrap around the conductor gives the direction of the magnetic field lines around it. Applying it to the loop (current clockwise as seen from above): gripping each part of the loop with the thumb along the current, the fingers go down into the table on the side facing the centre and come up on the outer side. So: Inside the loop: the magnetic field is perpendicular to the plane of the table and directed into it (downwards). Outside the loop: the magnetic field is perpendicular to the plane of the table and directed out of it (upwards).

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4. Q264 CBSE 2019
   A coil of insulated copper ​wire is connected to ​a ​galvanometer. What would happen ​if a strong bar magnet is

   1. (a)pushed into the coil ?
   2. (b)withdrawn from inside the ​coil ?
   3. (c)held stationary inside the ​coil ? Give justification for ​each ​observation.
   Show answer

   Pushing in and withdrawing the magnet give opposite momentary deflections; a stationary magnet gives none, as the linked field does not change.

   (a) The galvanometer needle shows a momentary deflection in one direction, because pushing the magnet in increases the magnetic field linked with the coil, and this change induces a current in the coil. (b) The needle is deflected in the opposite direction, because withdrawing the magnet decreases the magnetic field linked with the coil, so a current is induced in the opposite direction. (c) The galvanometer shows no deflection, because when the magnet is stationary the magnetic field linked with the coil does not change, so no current is induced.

   Step-by-step solution →
5. Q265 CBSE 2019
   Two circular ​coils P ​and Q are kept close to each other, of which coil P carries a current. What will you observe in the galvanometer connected ​across the coil Q

   1. (a)if current in the coil P is ​changed ?
   2. (b)if ​both the coils are moved in the same direction ​with the same speed ? Give reason to justify ​your answer ​in each case.
   Show answer

   The galvanometer deflects when the current in P changes, but not when both coils move together.

   (a) The galvanometer shows a deflection (momentarily, while the current in P is changing). Reason: changing the current in P changes its magnetic field, so the magnetic field lines linked with coil Q change and a current is induced in Q (electromagnetic induction). (b) The galvanometer shows no deflection. Reason: both coils move together, so there is no relative motion between them; the magnetic field lines linked with Q do not change and no current is induced in Q.

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6. Q266 CBSE 2019
   The following diagram shows two parallel straight ​conductors carrying same current. Copy the diagram and draw the ​pattern of the magnetic field lines around them ​showing their directions. ​What is the magnitude of magnetic ​field at a point ‘X’ which ​is ​equidistant from the conductors ? ​Give justification for your answer.

   Show answer

   The field at X is zero ($B_X=0$): the equal fields of the two conductors are opposite in direction at X.

   Drawing: Copy the two conductors with the current flowing downwards in both. Round each conductor draw concentric circular field lines in planes perpendicular to it (drawn as rings round the conductor). By the right-hand thumb rule, with the thumb pointing down along the current, the lines go round clockwise when seen from above; so between the conductors the field lines of the left conductor come out of the page and those of the right conductor go into it. Magnitude of the magnetic field at X: Zero. Justification: X is at the same distance from two conductors carrying the same current, so the magnetic fields of the two conductors at X are equal in magnitude. As the currents are in the same direction, the two fields at X, a point between the conductors, are in opposite directions. Equal and opposite fields cancel, so the net magnetic field at X is zero ($B_X=0$); X is a neutral point.

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7. Q267 CBSE 2019
   Magnetic field lines are shown in ​the given diagram. A student makes a statement that the magnetic field ​at ‘A’ is stronger than at ​‘B’. Justify this ​statement. Also redraw ​the diagram ​and mark the direction of ​magnetic field lines.

   Show answer

   The field is stronger at A, where field lines are crowded; outside the magnet the arrows run from N to S.

   The relative strength of a magnetic field is shown by the closeness of its field lines: the field is stronger where the lines are crowded together. Near A, close to the north pole, the field lines converge and are crowded; at B, far above the magnet, they are widely spaced. Hence the magnetic field at A is stronger than at B. Direction: outside the magnet the field lines emerge from the north pole and merge at the south pole, so in the redrawn diagram every line carries an arrow pointing from N, over the top of the magnet, to S (see the figure); inside the magnet the lines run from S to N.

   Step-by-step solution →
8. Q268 CBSE 2019
   1. (a)​Draw magnetic field lines produced ​around a current carrying straight ​conductor passing through a ​cardboard. Name, state and apply the rule to ​mark the direction of these field lines.
   2. (b)How ​will the strength of the magnetic field ​change when the point where magnetic field is to be determined is moved away from the straight wire carrying constant current? Justify your answer.
   Show answer

   The field lines are concentric circles whose direction follows the right-hand thumb rule; the field weakens as the distance from the wire grows.

   (a) The field lines are concentric circles centred on the wire, lying in the plane of the cardboard (see the diagram in step 1). Rule: the right-hand thumb rule. Imagine holding the current-carrying straight conductor in your right hand so that the thumb points in the direction of the current; then the fingers wrapped around the conductor give the direction of the magnetic field lines. Applying it: for a current flowing up through the cardboard, the field lines run anticlockwise when seen from above; if the current flows down, they run clockwise. (b) The strength of the magnetic field decreases as the point is moved away from the wire. Justification: the concentric circles representing the field become larger and larger, and more widely spaced, as the distance from the wire increases, so the field there is weaker; a compass needle moved away from the wire is deflected less.

   Step-by-step solution →
9. Q269 CBSE 2019
   1. (a)With the ​help of labelled circuit diagram, describe an activity to illustrate the ​pattern of magnetic field ​lines due to ​a straight ​conductor carrying current in vertically downward direction.
   2. (b)​State right hand thumb rule and explain how this rule is useful to determine the direction of magnetic ​field in the above situation.
   Show answer

   Iron filings form concentric circles round the wire; for a downward current the field is clockwise seen from above.

   (a) Activity: Pass a thick straight copper wire vertically through the centre of a horizontal rectangular cardboard. Connect it in series with a battery, a plug key, a rheostat and an ammeter so that the current flows downward through the wire (upper end towards the positive terminal). Sprinkle iron filings uniformly on the cardboard, close the key and tap the cardboard gently. The iron filings arrange themselves in concentric circles around the wire: these are the magnetic field lines of the straight conductor. A compass placed on a circle points along it and shows that, for the downward current, the field lines run clockwise when viewed from above. (b) Right-hand thumb rule: Imagine holding the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current; then the fingers wrapped around the conductor give the direction of the magnetic field lines. In this situation, the thumb points vertically downward along the current, and the fingers curl clockwise as seen from above. So the magnetic field lines are concentric circles around the wire in the clockwise direction when viewed from above.

   Step-by-step solution →
10. Q270 CBSE 2019
    1. (a)Explain with ​the help of a diagram the distribution of magnetic field due to a current through a circular loop. Why is ​it that if a current carrying coil has $n$ turns, the ​magnetic ​field produced at any point is $n$ times ​as large as that produced ​by a single turn ?
    2. (b)What does the direction of thumb indicate in the right hand thumb rule ? In what way is this rule different ​from ​the Fleming’s left hand rule ?
    Show answer

    A loop's field lines are circles that turn straight at the centre; $n$ turns give $n$ times the field; the thumb shows the current.

    (a) Every part of the circular loop acts like a short straight wire, so the magnetic field lines close to the wire are small concentric circles. Moving towards the centre of the loop, these circles become larger, and at the centre their arcs appear as straight lines. By the right-hand thumb rule every section of the loop makes the field lines inside the loop point in the same direction, so the field at the centre is strong, nearly uniform and perpendicular to the plane of the loop, and the lines return round the outside of the loop, as for a short bar magnet. The diagram shows the loop standing at right angles to the page, with the current coming out of the page at one side and going into it at the other, circles round each of these points, and straight, parallel lines through the centre. The field produced at a point by a current-carrying wire is directly proportional to the current. In a coil of $n$ turns the current flows in the same direction in every turn, so the fields of all the turns point the same way and add up: the field is $n$ times that of a single turn. (b) In the right-hand thumb rule the thumb indicates the direction of the current (the curled fingers give the direction of the magnetic field lines). The right-hand thumb rule gives the direction of the magnetic field produced by a current, whereas Fleming's left-hand rule gives the direction of the force (motion) on a current-carrying conductor placed in a magnetic field; in Fleming's rule the thumb indicates the force (motion), the forefinger the field and the middle finger the current.

    Step-by-step solution →
11. Q271 CBSE 2019
    What is a solenoid ? ​Draw the pattern of magnetic field lines of ​

    1. (i)a ​current carrying solenoid and ​
    2. (ii)​a bar ​magnet. List two ​distinguishing features between the two fields.
    Show answer

    A solenoid is a closely wound coil; its field is uniform inside and like a bar magnet's outside, but depends on the current.

    A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. (i) Current-carrying solenoid: inside, the field lines are parallel straight lines along the axis (a uniform field); outside, they come out of one end (N), curve round and enter the other end (S), as for a bar magnet. The end at which the current appears anticlockwise is N. (ii) Bar magnet: the field lines emerge from the N pole and merge at the S pole outside the magnet, and run from S to N inside it; they are closed curves, never cross, and are crowded near the poles. Distinguishing features (any two): (1) The solenoid's field exists only while a current flows, whereas the bar magnet's field is permanent. (2) The strength of the solenoid's field can be changed by changing the current, whereas that of a bar magnet is fixed. (3) The direction of the solenoid's field (its polarity) is reversed by reversing the current, whereas the poles of a bar magnet are fixed.

    Step-by-step solution →
12. Q272 CBSE 2019
    Draw the ​pattern ​of magnetic field lines produced around a current carrying straight conductor passing perpendicularly through a horizontal cardboard. State and apply right-hand thumb rule to ​mark the direction of the field lines. How will the strength of the magnetic field change ​when the point ​where ​magnetic field is to be determined is moved away from the ​straight conductor ​? Give reason to justify your answer.
    Show answer

    The field lines are concentric circles whose direction follows the right-hand thumb rule; the field weakens with distance because $B\propto \frac{1}{r}$.

    Pattern: when a current passes through the straight conductor held vertically through the horizontal cardboard, iron filings on the cardboard arrange themselves in concentric circles centred on the conductor. The field lines are these concentric circles, in the plane of the cardboard; they are crowded near the conductor and farther apart away from it (see the diagram). Right-hand thumb rule: imagine holding the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current; the fingers wrapped around the conductor then give the direction of the magnetic field lines. Applying it: for a current flowing vertically upward through the cardboard, the fingers curl anticlockwise as seen from above, so the circles are marked anticlockwise (seen from above). For a downward current they would be clockwise. Change with distance: the magnetic field becomes weaker as the point is moved away from the conductor. Reason: the field due to a straight conductor is inversely proportional to the distance from it, $B\propto \frac{1}{r}$ for a given current, so doubling the distance halves the field; accordingly the field lines are farther apart away from the wire.

    Step-by-step solution →
13. Q273 CBSE 2019
    1. (a)Name and ​state the rule to ​determine the ​direction of force experienced ​by a current carrying ​straight conductor placed ​in a uniform magnetic field which is perpendicular to it.
    2. (b)Draw ​a labelled ​diagram of an electric motor.
    Show answer

    Fleming's left-hand rule gives the force on a current-carrying conductor; the motor has a coil between magnet poles, a split ring and brushes.

    (a) Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force on the conductor (the direction of its motion). (b) Electric motor - the labelled diagram is the figure 'A simple electric motor (labelled)'. It shows a rectangular coil ABCD of insulated copper wire (the armature) between the poles N and S of a magnet, with arms AB and CD perpendicular to the field; the two ends of the coil joined to the halves P and Q of a split ring (commutator) on the axle; carbon brushes X and Y pressing on P and Q; and a battery and key K joined to the brushes. Labels: N, S, coil ABCD, axle, split ring P and Q, brushes X and Y, battery, key K.

    Step-by-step solution →
14. Q274 CBSE 2019
    1. (a)Why don’t two magnetic field ​lines ever intersect each ​other ? Explain.
    2. (b)“The magnetic field is ​said to be uniform inside a current ​carrying solenoid.” Why ​?
    3. (c)State Fleming’s left hand rule.
    4. (d)Enlist two factors ​that enhance the power of commercial motors.
    Show answer

    Field lines never cross; the field inside a solenoid is uniform; Fleming's left-hand rule gives the force; an electromagnet and many turns strengthen motors.

    (a) The tangent to a field line at any point gives the direction of the magnetic field at that point. If two field lines intersected, there would be two tangents, and hence two directions of the magnetic field, at the point of intersection - a compass needle placed there would point in two directions at once, which is not possible. So two field lines never intersect. (b) Inside a current-carrying solenoid the field lines are parallel straight lines, equally spaced. So the field has the same strength (magnitude) and the same direction at all points inside: it is uniform. (c) Stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb gives the direction of the force on (motion of) the conductor. (d) Any two of: (i) an electromagnet in place of a permanent magnet; (ii) a large number of turns of conducting wire in the coil; (iii) the coil wound on a soft iron core (armature).

    Step-by-step solution →
15. Q275 CBSE 2018
    State ​the principle ​of an electric motor and write the ​role of split rings in ​it. Write the energy transformations that takes place when ​an ​electric motor ​works.
    Show answer

    A motor works on the force on a current-carrying conductor in a magnetic field; split rings reverse the current; electrical energy becomes mechanical.

    Principle: an electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a force. The two arms of the coil carry currents in opposite directions, so they are pushed in opposite directions and the coil rotates. Role of the split rings: they act as a commutator, reversing the direction of the current through the coil (armature) after every half rotation, so that the coil keeps rotating in the same direction. Energy transformation: electrical energy is converted into mechanical energy.

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16. Q276 CBSE 2018
    We know that an electric ​current through a metallic conductor ​produces ​a magnetic field around it. Explain giving reason whether a similar ​magnetic field will ​be produced around a thin beam of moving

    1. (i)alpha particles,
    2. (ii)neutrons. Name the rule ​applied to determine the direction of magnetic field produced around a ​current carrying straight conductor.
    Show answer

    Moving alpha particles (charged) form a current and give a magnetic field; neutrons (uncharged) do not; the right-hand thumb rule gives the direction.

    (i) Yes. Alpha particles are positively charged (each carries a charge of $+2e$). A beam of moving alpha particles is a flow of charge, that is, an electric current in the direction of motion of the particles. Like the current in a metallic conductor, it produces a similar magnetic field around the beam. (ii) No. Neutrons carry no electric charge, so a beam of moving neutrons is not a flow of charge and is no current; no such magnetic field is produced around it. Rule: the right-hand thumb rule (Maxwell's corkscrew rule). If a current-carrying straight conductor is held in the right hand with the thumb pointing in the direction of the current, the fingers wrapped around the conductor give the direction of the magnetic field lines.

    Step-by-step solution →
17. Q277 CBSE 2018
    Describe an activity to ​show that ​“an electric ​current is ​induced in a ​coil ​when magnetic ​field around it is changed”.
    Show answer

    Moving a magnet relative to a coil deflects the galvanometer; a current is induced only while the magnetic field around the coil changes.

    Activity: (i) Take a coil of insulated copper wire having a large number of turns and connect its two ends to a galvanometer. (ii) Take a strong bar magnet and move its north pole towards one end of the coil. The galvanometer needle deflects momentarily, say to the right, showing that a current flows in the coil. (iii) Hold the magnet still near the coil: the deflection drops to zero. (iv) Pull the north pole away from the coil: the needle deflects to the left, showing a current in the opposite direction. (v) Keep the magnet fixed and move the coil towards it or away from it: the needle deflects in the same way, and when the coil is stationary with respect to the magnet the deflection drops to zero. Conclusion: a current is induced in the coil only while the magnetic field around it is changing, that is, while the magnet and the coil move relative to each other.

    Step-by-step solution →
18. Q278 CBSE 2018
    1. (a)​State Fleming’s left hand rule.
    2. (b)Write the principle of working ​of an ​electric ​motor.
    3. (c)Explain the ​function of the ​following parts of an electric motor. (i) Armature (ii) Brushes (iii) Split ring
    Show answer

    A motor works because a current-carrying coil in a magnetic field feels a force; the split ring reverses the current each half turn.

    (a) Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb points in the direction of the force (motion) acting on the conductor. (b) Principle: an electric motor works on the principle that a current-carrying coil (conductor) placed in a magnetic field experiences a force. The forces on the two arms of the coil are in opposite directions, so the coil rotates, converting electrical energy into mechanical energy. (c) (i) Armature: the coil wound on a soft iron core; it carries the current and rotates in the magnetic field, turning the axle, and the soft iron core enhances the power of the motor. (ii) Brushes: they press against the split ring and allow easy transfer of current between the external circuit (battery) and the rotating coil. (iii) Split ring: it reverses the direction of the current in the coil after every half rotation, so that the coil keeps rotating continuously in the same direction.

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19. Q279 CBSE 2018
    1. (a)What are magnetic ​field lines ? How is the direction of magnetic field at a point in a magnetic field determined using field lines ?
    2. (b)Two circular coils ‘X’ and ‘Y’ are placed close to each other. If the ​current ​in the coil ‘X’ is ​changed, ​will some current be induced in the coil ‘Y’ ? ​Give ​reason.
    3. (c)State ‘Fleming’s right hand rule”.
    Show answer

    Field direction is along the tangent to a field line; changing current in X induces current in Y; Fleming's right-hand rule applies.

    (a) Magnetic field lines are the curves along which a free north pole (the north pole of a small compass) would move in a magnetic field; iron filings arrange themselves along them around a magnet. They are closed curves, running from N to S outside a magnet and from S to N inside it, and they never cross. The direction of the magnetic field at a point is given by the tangent to the field line at that point, in the sense of the arrow on the line; a compass needle placed there points its north pole along this direction. (b) Yes. The magnetic field of coil X passes through coil Y. When the current in X is changed, its magnetic field changes, so the magnetic field linked with Y changes, and this change induces a current in Y, whose ends are joined in a closed circuit, for example through a galvanometer (electromagnetic induction). The induced current lasts only while the current in X is changing. (c) Fleming's right-hand rule: stretch the thumb, forefinger and middle finger of the right hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the thumb in the direction of motion of the conductor, then the middle finger gives the direction of the induced current.

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