CBSE Class 10 Science HOTs Magnetic Effects of Electric Current Set 05

This Class 10 Science page gives 17 higher order thinking questions (HOTS) from the chapter Magnetic Effects of Electric Current, each with a step-by-step answer, a simple-words summary and a teacher's note. The questions test reasoning on the compass near a wire, forces on moving charges, fields of coils and solenoids, permanent magnets, induced current, AC frequency, switches and fuses.

Higher Order Thinking Questions

Question 1. A compass needle is placed near a current carrying straight conductor. State your observation for the following cases and give reasons for the same in each case. (a) Magnitude of electric current is increased. (b) The compass needle is displaced away from the conductor.
Answer:
(a) The deflection of the needle increases, because the magnetic field of a straight current-carrying conductor is directly proportional to the current (B ∝ I).
(b) The deflection decreases, because the field is inversely proportional to the distance from the conductor (B ∝ 1/r).
In simple words: More current gives a stronger field; more distance gives a weaker field.

Teacher's Note:
1. Write B ∝ I/r once and use it for both parts.
2. Always give the reason along with the observation.

Question 2. It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.
Answer:
(i) Yes. Alpha particles carry positive charge, so a beam of them is an electric current, and every current produces a magnetic field.
(ii) No. Neutrons carry no charge, so moving neutrons do not form a current and produce no magnetic field.
In simple words: Only moving charges make a magnetic field.

Teacher's Note:
1. A moving charge is equivalent to a current.
2. Neutral particles never give a magnetic field by their motion.

Question 3. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from the back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field?
Answer:
Electrons are negatively charged, so the conventional current points opposite to their motion, that is, from the front wall towards the back wall.
The force on the beam is towards your right.
Apply Fleming's left-hand rule: thumb (force) to the right, middle finger (current) towards the back wall. The forefinger (field) then points vertically downwards.
In simple words: The field points straight down.

Teacher's Note:
1. Reverse the direction of the electron beam to get the current direction.
2. Then use the left-hand rule with the thumb as force.

Question 4. Two parallel straight conductors carry the same current in the same (downward) direction. The point X is equidistant from the two conductors and lies between them. Draw the pattern of the magnetic field lines around them showing their directions. What is the magnitude of the magnetic field at X? Give justification for your answer.
Answer:
Each wire has circular field lines around it. By the right-hand thumb rule, for a current directed downwards the field lines are clockwise circles when seen from above.
At the point X midway between the wires, the two fields have the same magnitude (same current, same distance) but point in opposite directions.
They cancel, so the magnetic field at X is zero.
In simple words: Equal and opposite fields add up to nothing.

Teacher's Note:
1. Use the right-hand thumb rule for each wire separately.
2. In the exam, draw two sets of concentric circles.

Question 5. Give reasons for the following. (i) There is either a convergence or a divergence of magnetic field lines near the ends of a current carrying straight solenoid. (ii) The current carrying solenoid when suspended freely rests along a particular direction.
Answer:
(i) A solenoid behaves like a bar magnet and has a similar field pattern. Its ends act as poles: the lines converge at the south pole end and diverge from the north pole end.
(ii) A freely suspended bar magnet aligns itself in the North-South direction, and the solenoid behaves in the same way, so it rests along the North-South direction. Reversing the current swaps its poles.
In simple words: A solenoid acts like a bar magnet with two poles.

Teacher's Note:
1. Compare the solenoid with a bar magnet in both parts.
2. Mention that the converging end is the south pole.

Question 6. A magnetic field is non-uniform but its direction is constant (East to West) is set-up in a chamber. A charged particle enters the chamber and travels undeflected along a straight path with constant speed. What do you say about the initial velocity of the particle?
Answer:
A charged particle feels no magnetic force only when it moves along the field, parallel or antiparallel to it.
The particle is undeflected, so the force on it is zero. Therefore its initial velocity must be along the East-West line (either towards the east or towards the west).
In simple words: The particle moves along the field lines, so it feels no force.

Teacher's Note:
1. Recall that force is zero when the velocity is parallel to the field.
2. The non-uniformity of the field does not change this.

Question 7. A circular coil is carrying a current of 100 A in the anti-clockwise direction when seen from upward direction. Find the pole of magnetic field when seen from 1.0 m below.
Answer:
Anticlockwise when seen from above is clockwise when seen from below.
By the clock-face rule, a face where the current appears clockwise is a south pole.
So the face seen from below is a south pole; the distance of 1.0 m does not change the answer.
In simple words: Clockwise seen from below means a south pole there.

Teacher's Note:
1. Check from which side the coil is being viewed.
2. Anticlockwise gives north, clockwise gives south.

Question 8. If a current carrying straight conductor is placed in the east-west direction, find the direction of the force experienced by the conductor due to the Earth's magnetic field.
Answer:
Near the equator the Earth's magnetic field is horizontal and points from the geographic south to the north.
The force on a conductor is perpendicular to both the current and the field, so for an east-west conductor the force is vertical.
By Fleming's left-hand rule it is upwards if the current flows from west to east and downwards if it flows from east to west.
In simple words: The force is vertical, up or down, depending on the current direction.

Teacher's Note:
1. State the assumption about the field direction.
2. Use the left-hand rule with the forefinger pointing north.

Question 9. State the effect of a magnetic field on the path of a moving charged particle.
Answer:
A charged particle moving in a magnetic field may experience a force in a direction perpendicular to both the direction of the field and the direction of motion of the particle.
This force deflects the particle from its straight path. If the particle moves along the field, there is no force and no deflection.
In simple words: The field bends the path of a moving charge.

Teacher's Note:
1. Mention that the force is perpendicular to both directions.
2. Add the special case of zero force.

Question 10. (i) Alternating current has a frequency of 50 Hz. What is meant by this statement? How many times does it change its direction in one second? Give reason for your answer. (ii) Mention the frequency of D.C. that is given by a cell.
Answer:
(i) A frequency of 50 Hz means that the current completes 50 cycles in one second. In each cycle it reverses its direction twice, so it changes direction 2 × 50 = 100 times in one second.
(ii) The frequency of direct current is zero, because its direction does not change with time.
In simple words: 50 Hz means 100 reversals every second; DC has zero frequency.

Teacher's Note:
1. Remember two reversals in one cycle.
2. DC never reverses, so the frequency is zero.

Question 11. An electron does not suffer any deflection while passing through a region. Are you sure that there is no magnetic field? Is the reverse definite?
Answer:
No, we cannot be sure. An electron feels no force when it moves parallel or antiparallel to the magnetic field, so a field may exist along its direction of motion.
The reverse is not definite either: if the electron is deflected, a magnetic field (or an electric field) must be present, because only a force can change its path.
In simple words: No deflection does not prove there is no field.

Teacher's Note:
1. Recall the zero-force case for motion along the field.
2. Mention both directions of the argument.

Question 12. Why is pure iron not used for making permanent magnets? Name one material used for making permanent magnets. Describe how permanent magnets are made electrically. State two examples of electrical instruments made by using permanent magnets.
Answer:
Pure (soft) iron loses its magnetism almost as soon as the magnetising field is removed, so it cannot retain magnetism for long. It is suitable for electromagnets.
Alloys of iron such as steel and Alnico retain their magnetism for a long time and are used for permanent magnets.
Method: place a hard steel rod inside a solenoid and pass a strong current; the rod becomes magnetised and stays a magnet after the current is switched off.
Instruments: galvanometer and ammeter (also loudspeaker).
In simple words: Soft iron makes temporary magnets, steel alloys make permanent ones.

Teacher's Note:
1. Contrast soft iron with steel.
2. Name Alnico as the alloy.

Question 13. How will the magnetic field intensity at the centre of a circular coil carrying current change, if the current through the coil is doubled and the radius is halved?
Answer:
The field at the centre of a coil is directly proportional to the current and inversely proportional to the radius (B ∝ I/r).
The current becomes 2 times and the radius becomes 1/2, so the field becomes 2 × 2 = 4 times the original value.
In simple words: Double current and half radius give four times the field.

Teacher's Note:
1. Write the proportionality first.
2. Multiply the separate factors.

Question 14. The magnetic field lines come closer to one another near the poles of a magnet but they are widely separated at other places. Give reason.
Answer:
The closeness of field lines shows the strength of the magnetic field.
Near the poles the field is strong, so the lines are crowded. Away from the poles the field is weak, so the lines are widely separated.
In simple words: Crowded lines mean a strong field.

Teacher's Note:
1. Link closeness of lines to field strength.
2. Name the places where the field is strongest.

Question 15. If you hold a coil of wire next to a magnet, no current will flow in the coil. Give reason.
Answer:
A current is induced in a coil only when the magnetic field linked with the coil changes, that is, when there is relative motion between the coil and the magnet.
A coil held at rest next to a magnet has a steady field, so there is no induced current. Moving the magnet into or out of the coil produces a current.
In simple words: No change of field means no induced current.

Teacher's Note:
1. Use the idea of relative motion.
2. Say what to do to get a current.

Question 16. Why should all the switches be put in the live wire of the A.C. circuit?
Answer:
When the switch is turned off, the appliance is disconnected from the live wire, so there is no risk of an electric shock.
If the switch were in the neutral wire, the appliance would stay connected to the live wire (220 V) even when switched off, which would be dangerous.
In simple words: Switch in the live wire means the appliance is safe when switched off.

Teacher's Note:
1. The live wire is at high potential.
2. Mention the shock risk.

Question 17. In household circuits, is a fuse wire connected in series or in parallel? Why?
Answer:
A fuse wire is connected in series with the circuit (in the live wire).
In series the whole current of the circuit flows through the fuse, so when the current exceeds the safe value the fuse melts and breaks the circuit. A fuse connected in parallel would not carry the main current and would not protect the circuit.
In simple words: Series connection lets the fuse carry the entire current.

Teacher's Note:
1. Mention the live wire.
2. State what happens when the current is too high.