CBSE Class 10 Science Magnetic effects of electric current Sure Shot Questions Set 02

These 15 most expected Class 10 Science board-style questions on Magnetic Effects of Electric Current come with full step-by-step answers, a simple-words line and a teacher's note. They cover field lines, the right-hand thumb rule, solenoids and electromagnets, Fleming's rules, the motor and generator, electromagnetic induction, AC and DC, and domestic circuit safety.

Most Expected Questions

Question 1. What are magnetic field lines? Justify the following statements: (a) Two magnetic field lines never intersect each other. (b) Magnetic field lines are closed curves.
Answer:
Magnetic field lines are imaginary lines that represent the magnetic field around a magnet; the tangent at any point gives the direction of the field there.
Properties: (i) outside the magnet they go from the North pole to the South pole and inside from South to North; (ii) they are closer where the field is stronger; (iii) they never cross each other.
No intersection: if two lines crossed, the compass needle at the point of crossing would point in two directions, which is impossible.
Closed curves: the lines leave the North pole, reach the South pole outside the magnet and continue from South to North inside it, so each line forms a closed loop.
In simple words: Field lines show direction and strength, never cross, and form closed loops.

Teacher's Note:
1. Write the three properties as a numbered list.
2. Give the reason for no crossing in terms of the compass needle.

Question 2. (a) State the right-hand thumb rule. (b) How will the magnetic field around a straight current-carrying conductor be affected on (i) increasing the current, (ii) reversing the direction of the current?
Answer:
(a) Hold the conductor in the right hand so that the stretched thumb points in the direction of the current; the direction in which the fingers wrap around the conductor gives the direction of the magnetic field.
(b)(i) The strength of the field increases.
(b)(ii) The direction of the field is reversed.
In simple words: Thumb along the current, fingers show the field circles.

Teacher's Note:
1. Draw concentric circles around the wire in the exam.
2. State both effects.

Question 3. What is a solenoid? Draw the pattern of magnetic field lines of (i) a current carrying solenoid and (ii) a bar magnet. List two distinguishing features between the two fields.
Answer:
Similarity: the field lines of a solenoid have the same pattern as those of a bar magnet; one end behaves as a north pole and the other as a south pole. Inside the solenoid the field lines are parallel, so the field is uniform.
Differences: (i) a bar magnet is a permanent magnet, whereas an electromagnet is magnetic only while current flows; (ii) the strength of an electromagnet can be changed by changing the current or the number of turns, whereas that of a bar magnet cannot be changed; (iii) an electromagnet is generally stronger.
In simple words: A solenoid acts like a bar magnet but only while current flows.

Teacher's Note:
1. Name the similarity first and then give two differences.
2. Mention the role of the soft iron core in an electromagnet.

Question 4. Name, state and explain with an example the rule used to determine the direction of force experienced by a current carrying conductor placed in a uniform magnetic field.
Answer:
Rule: stretch the forefinger, middle finger and thumb of the left hand at right angles to each other. 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.
Example: when an electron enters a magnetic field at right angles, the force on it is perpendicular to both the field and the current (which is opposite to its motion), as given by the rule. The rule is used in the electric motor.
In simple words: Forefinger: field; middle finger: current; thumb: force.

Teacher's Note:
1. Name the fingers in order.
2. Check the direction by turning your left hand.

Question 5. (a) Write the principle of working of an electric motor. (b) Explain the function of the following parts of an electric motor. (i) Armature (ii) Brushes (iii) Split ring
Answer:
Principle: a current-carrying conductor (coil) placed in a magnetic field experiences a force, and the forces on the two opposite sides of a rectangular coil rotate it. The direction of force is given by Fleming's left-hand rule.
(i) Armature: the rectangular coil (wound on a soft iron core in commercial motors) that rotates in the field.
(ii) Brushes: two fixed carbon or metal contacts that touch the split ring and pass the current from the battery to the coil.
(iii) Split ring (commutator): reverses the direction of current in the coil after every half rotation, so the forces keep turning the coil in the same direction.
In simple words: A coil in a magnetic field rotates when current flows; the split ring keeps it turning.

Teacher's Note:
1. Always begin with the principle.
2. Labelled diagram carries a mark.

Question 6. (i) What is meant by electromagnetic induction? Name one device which works on the principle of electromagnetic induction. (ii) Describe three different ways to produce induced current in a coil of wire.
Answer:
Electromagnetic induction: the production of an electric current in a coil or conductor by a changing magnetic field linked with it.
Experiment: connect a coil of insulated wire to a galvanometer and push a bar magnet into the coil; the needle deflects. Pulling it out gives a deflection in the opposite direction. When the magnet is at rest, there is no deflection.
To increase the current: move the magnet faster, use a stronger magnet or use a coil with more turns.
In simple words: A changing field induces a current; no change means no current.

Teacher's Note:
1. Mention the three observations.
2. State that motion of the coil gives the same result.

Question 7. State and explain Fleming's right hand rule for the direction of induced current.
Answer:
Rule: stretch the thumb, forefinger and middle finger of the right hand at right angles to each other. If the forefinger points along the magnetic field and the thumb along the motion of the conductor, the middle finger shows the direction of the induced current.
Difference: the right-hand rule gives the direction of the induced current when a conductor moves in a field. The left-hand rule gives the direction of the force on a conductor carrying current in a field.
Applications: right-hand rule for an electric generator; left-hand rule for an electric motor.
In simple words: Right hand for generators, left hand for motors.

Teacher's Note:
1. Compare using a small table in the exam if marks permit.
2. Remember: in the right-hand rule the thumb is motion, in the left-hand rule the thumb is force.

Question 8. Explain with the help of a diagram, the principle and working of an a.c. generator.
Answer:
Principle: electromagnetic induction; when a coil rotates in a magnetic field, the magnetic field linked with it changes and a current is induced.
Construction: a rectangular armature coil rotates between the poles of a permanent magnet. The ends of the coil are connected to two slip rings, which touch two brushes connected to the external circuit.
Working: when the axle is rotated by mechanical means, the sides of the coil cut the field lines and a current is induced. After every half rotation the direction of the induced current reverses, so the generator gives alternating current.
Using a split-ring commutator in place of the slip rings makes it a DC generator.
In simple words: A rotating coil in a magnetic field produces AC.

Teacher's Note:
1. Write the energy change: mechanical to electrical.
2. Mention the slip rings clearly.

Question 9. Define alternating current and direct current. Explain why alternating current is preferred over direct current for transmission over long distances.
Answer:
AC: a current that reverses its direction periodically. DC: a current that always flows in the same direction.
Reason: AC can be transmitted over long distances without much loss of electrical energy, whereas DC suffers a large loss.
In simple words: AC reverses, DC does not; AC travels farther with less loss.

Teacher's Note:
1. Define both in one line each.
2. State the frequency with unit.

Question 10. Give reason for the following: The burnt out fuse should be replaced by another fuse of identical rating.
Answer:
A fuse is a short piece of wire of a material with a low melting point that is connected in series in a circuit.
When the current exceeds the safe value, the fuse wire gets hot because of the heating effect of the current, melts and breaks the circuit.
It is connected in the live wire so that the appliance is disconnected from the high potential when the fuse melts.
A fuse of higher rating will not melt at the dangerous current; one of lower rating will melt too soon. So the same rating must be used.
In simple words: A fuse melts and cuts off the circuit when the current is too high.

Teacher's Note:
1. Mention the heating effect.
2. Put the fuse in the live wire.

Question 11. Distinguish between short circuiting and overloading.
Answer:
Short circuit: the live wire comes in direct contact with the neutral wire (because of damaged insulation or a fault in an appliance), the resistance becomes very small and a very large current flows.
Overloading: too many appliances (or a high-power appliance) are connected to the same circuit so that the total current drawn exceeds the rated value of the wires.
In simple words: Short circuit is a direct contact of live and neutral; overloading is too much load.

Teacher's Note:
1. Give the cause for each.
2. Mention fuse or MCB in the precautions.

Question 12. Mention and explain the function of an earth wire. Why is it necessary to earth metallic appliances?
Answer:
Earthing means connecting the metal body of an appliance to the earth through the earth wire.
If the live wire touches the metal casing because of a fault, the casing would be at 220 V and a person touching it would get a severe shock.
With earthing, the leakage current flows to the earth through the earth wire because it offers a very low-resistance path, so the person is protected.
In simple words: The earth wire sends a leaking current into the ground and prevents shock.

Teacher's Note:
1. Write the example of a metal-bodied appliance such as a refrigerator or an iron.
2. Mention the low resistance path.

Question 13. Describe an activity with labelled diagram to show that a force acts on a current carrying conductor placed in a magnetic field and its direction of current through conductor. Name the rule which determines the direction of this force.
Answer:
Suspend a small aluminium rod horizontally with two connecting wires between the poles of a strong horseshoe magnet, so that the field is vertical. Connect the rod in series with a battery, a key and a rheostat.
When a current is passed through the rod, the rod is displaced (moves sideways) in a direction perpendicular to both the current and the field.
On reversing the current, the displacement is in the opposite direction. Reversing the magnet also reverses the displacement.
The direction is found by Fleming's left-hand rule.
In simple words: The rod moves in the field when current flows, and reverses if current is reversed.

Teacher's Note:
1. Draw the setup and label the magnet, the rod, the battery and the key.
2. Say that the force is greatest when the rod is at right angles to the field.

Question 14. How does the strength of the magnetic field at the centre of a current carrying circular coil depend on the (i) radius of the coil, (ii) number of turns in the coil, and (iii) strength of the current flowing in the coil?
Answer:
Current: the field is directly proportional to the current, so it increases when the current increases.
Number of turns: the field is directly proportional to the number of turns, because the current in every turn has the same direction and the fields add up.
Radius of the coil: the field is inversely proportional to the radius, so it decreases when the radius increases.
At the centre the field lines are nearly straight and perpendicular to the plane of the coil.
In simple words: More current or turns give a stronger field; a bigger radius gives a weaker field.

Teacher's Note:
1. Write B ∝ N I / r.
2. Mention why the turns add up.

Question 15. What do you mean by an electromagnet? Show how it is made. Give two uses of electromagnets.
Answer:
An electromagnet is a magnet formed by a soft iron core placed inside a current-carrying solenoid; it is a magnet only while current flows.
Making: wind many turns of insulated copper wire closely on a soft iron rod and connect the wire to a battery through a key.
Increasing the strength: increase the current and increase the number of turns of the coil.
Uses: electric bells, cranes that lift scrap iron, and loudspeakers.
In simple words: A coil with a soft iron core becomes a switchable magnet.

Teacher's Note:
1. Include the core in your answer.
2. Mention that the strength depends on current and turns.