CBSE Class 10 Science Magnetic Effects of Electric Current Assignment Set 09

This Class 10 Science assignment for Magnetic Effects of Electric Current offers 15 chapter-wise practice questions with marks, ranging from 1-mark recall to 5-mark descriptive answers, each with a worked answer and teacher's note. The set focuses on magnetic fields and field lines, fields due to straight wires, circular coils and solenoids, and the force on moving charges and conductors.

Very Short Answer Questions (1 Mark)

Question 1. State the direction of magnetic field inside a bar magnet. (1 mark)
Answer:
Inside the magnet the field lines are directed from the South pole to the North pole. (Outside the magnet they go from the North pole to the South pole, so the lines are closed curves.)
In simple words: Inside a magnet the lines go from south to north.

Teacher's Note:
1. Say both inside and outside.
2. This is why field lines are closed curves.

Question 2. A horizontal power line carries current from east to west. What is the direction of the magnetic field at a point directly below the line and a point directly above it? (1 mark)
Answer:
Use the right-hand thumb rule with the thumb pointing towards the west.
The field is directed from north to south at a point below the wire.
It is directed from south to north at a point above the wire.
In simple words: Below: north to south; above: south to north.

Teacher's Note:
1. Place the thumb along the current and curl the fingers around the wire.
2. Check the direction below and above separately.

Question 3. What kind of core is used in an electromagnet? (1 mark)
Answer:
A core of soft iron is used.
In simple words: Soft iron makes a strong, temporary magnet.

Teacher's Note:
1. Soft iron loses its magnetism when the current is switched off.
2. Do not write steel.

Question 4. What does the divergence of the magnetic field lines near the ends of a current-carrying straight solenoid indicate? (1 mark)
Answer:
It indicates that the magnetic field is weaker at the ends than in the middle of the solenoid, where the lines are close, parallel and uniform.
In simple words: Spreading lines mean a weaker field.

Teacher's Note:
1. Closeness of lines shows strength.
2. Inside the solenoid the field is uniform.

Question 5. List three sources of magnetic fields. (1 mark)
Answer:
(i) A permanent magnet, (ii) a current-carrying conductor (wire, coil or solenoid), (iii) an electromagnet. Moving charged particles are also a source.
In simple words: Magnets and currents produce magnetic fields.

Teacher's Note:
1. Give three clearly different sources.
2. All of them involve either a magnet or moving charge.

Short Answer Questions (2 and 3 Marks)

Question 6. Why does a compass needle get deflected when it is brought near a bar magnet? (2 marks)
Answer:
A compass needle is a small pivoted bar magnet.
The magnetic field of the bar magnet exerts a force on its poles, so the needle turns and aligns itself along the field at that point.
In simple words: A small magnet turns in the field of a bigger magnet.

Teacher's Note:
1. Mention the needle is itself a magnet.
2. It aligns with the local field.

Question 7. State how the magnetic field produced by a straight current-carrying conductor at a point depends on (a) the current through the conductor, (b) the distance of the point from the conductor. (2 marks)
Answer:
(a) The field is directly proportional to the current.
(b) The field is inversely proportional to the distance of the point from the wire.
In simple words: More current, stronger field; more distance, weaker field.

Teacher's Note:
1. Use the words directly and inversely proportional.
2. The idea B ∝ I/r covers both.

Question 8. What is meant by a solenoid? How does a current-carrying solenoid behave? Give its main use. (2 marks)
Answer:
A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
A current-carrying solenoid behaves like a bar magnet; one end acts as a north pole and the other as a south pole.
Its strong field is used to magnetise a piece of soft iron placed inside it, which makes an electromagnet.
In simple words: A solenoid acts as a bar magnet when current flows.

Teacher's Note:
1. Define first, then explain behaviour.
2. Electromagnet is the standard use.

Question 9. Find the direction of the magnetic field at the centre of a current-carrying circular coil held (i) vertically in a North-South plane, with an observer looking at it from the east seeing the current flow anticlockwise; (ii) vertically in an East-West plane, with an observer looking from the south seeing the current flow anticlockwise; (iii) horizontally, with an observer looking from below seeing the current flow clockwise. (3 marks)
Answer:
Anticlockwise current as seen by an observer means the field at the centre points towards the observer; clockwise current means the field points away from the observer.
(i) Towards the east, that is from west to east.
(ii) Towards the south, that is from north to south.
(iii) Away from the observer who is below, so the field is vertically upwards.
In simple words: Anticlockwise gives field towards you; clockwise gives field away from you.

Teacher's Note:
1. Use the clock-face rule or the right-hand thumb rule.
2. Be careful with where the observer stands.

Question 10. State whether an alpha particle will experience a force in a magnetic field if (i) it is at rest, (ii) it moves parallel to the field lines, (iii) it moves perpendicular to the field lines. Justify each answer. (3 marks)
Answer:
(i) No force. A charge at rest does not experience a magnetic force.
(ii) No force. A moving charge feels a force only when its velocity has a component across the field; for motion along the field the force is zero.
(iii) Yes. The particle feels a force perpendicular to both its velocity and the field; the force is maximum in this case.
In simple words: Force acts only on a moving charge that crosses the field.

Teacher's Note:
1. Explain with the idea of angle between velocity and field.
2. The direction of force comes from Fleming's left-hand rule.

Question 11. State three factors on which the strength of the magnetic field produced by a current-carrying solenoid depends. (3 marks)
Answer:
(i) The number of turns per unit length of the solenoid (more closely wound turns give a stronger field).
(ii) The current flowing through it.
(iii) The nature of the core material placed inside; a soft iron core greatly increases the field.
In simple words: Turns, current and core decide the strength.

Teacher's Note:
1. Do not include the radius of the solenoid; the field inside does not depend on it.
2. Mention soft iron as the best core.

Question 12. Describe an activity to show that the direction of the magnetic field around a straight current-carrying wire reverses on reversing the current. (3 marks)
Answer:
Pass a thick copper wire vertically through a cardboard and connect it to a battery, a key and a rheostat. Place a compass at a point on the cardboard.
Close the key and note the direction in which the needle points. Move the compass around the wire and the needle follows a circle around the wire.
Reverse the battery connections. The needle now deflects in the opposite direction, which shows that the direction of the field reverses when the current is reversed.
In simple words: Reverse the current and the compass needle turns the opposite way.

Teacher's Note:
1. Mention the circuit parts.
2. State the observation and the conclusion.

Long Answer Questions (5 Marks)

Question 13. A current-carrying conductor is placed in a magnetic field. (i) List the factors on which the magnitude of the force on it depends. (ii) When is the force maximum? (iii) Name the rule that gives the direction of force. (iv) If the force was initially acting from right to left, how will its direction change if (a) the field is reversed, (b) the current is reversed? (5 marks)
Answer:
(i) The force depends on the current in the conductor, the strength of the magnetic field, the length of the conductor in the field and the angle between the conductor and the field.
(ii) The force is largest when the current is perpendicular (at right angles) to the magnetic field.
(iii) Fleming's left-hand rule.
(iv)(a) The force is reversed and acts from left to right.
(iv)(b) The force is again reversed and acts from left to right.
In simple words: Force depends on current, field, length and angle; reversing either the field or the current reverses the force.

Teacher's Note:
1. Remember that reversing both would bring back the original direction.
2. Name the rule clearly.

Question 14. Describe an activity to study the magnetic field produced by a current-carrying circular coil. State the main conclusions. (5 marks)
Answer:
Pass a circular coil through a cardboard so that its centre is on the cardboard. Connect the coil to a battery and key and sprinkle iron filings on the board; tap the board lightly. The filings arrange in a pattern, and plotting with a compass gives the direction.
Conclusions: (i) The lines near the wire are nearly concentric circles. (ii) At the centre they are nearly straight, parallel lines, so the field there is almost uniform. (iii) The field at the centre is perpendicular to the plane of the coil. (iv) The strength at the centre is directly proportional to the current and the number of turns and inversely proportional to the radius of the coil.
In simple words: Filings and a compass show circles near the wire and straight lines at the centre.

Teacher's Note:
1. Name the apparatus and the observation.
2. Write at least three conclusions.

Question 15. A straight wire carries current. Point R is at a distance r1 from the wire and point S is at a distance r2, with r1 greater than r2. (i) Where is the magnetic field stronger? Why? (ii) What happens to the direction of the field if the battery terminals are reversed? (iii) State the rule used to find the direction of the field around the wire. (5 marks)
Answer:
(i) The field is stronger at S, the nearer point, because the field is inversely proportional to the distance from the wire (B ∝ 1/r).
(ii) The direction of the current is reversed, so the direction of the magnetic field is also reversed.
(iii) Right-hand thumb rule: hold the wire in the right hand with the thumb pointing in the direction of the current; the curled fingers show the direction of the magnetic field.
Equivalent: Maxwell's corkscrew rule, where the direction of turning of the screw shows the direction of the field if the screw advances in the direction of current.
In simple words: The nearer point has the stronger field, and reversing the current reverses the field.

Teacher's Note:
1. Use the formula B ∝ I/r to compare distances.
2. State the rule in full.