Magnetic Effects of Electric Current — Class 10 Science practice questions

Question 1

Arjun is a class 10 student working on a science project. He builds a simple electromagnet by wrapping 50 turns of insulated copper wire around a soft iron nail and connecting it to a 6V battery. He tests its strength by counting how many iron pins it can pick up. He then modifies his setup in different ways and records his observations.

Observation 1: When he doubles the number of turns to 100, the electromagnet picks up significantly more pins.
Observation 2: When he replaces the soft iron nail with a steel rod (keeping everything else the same), the electromagnet picks up fewer pins during operation, but retains some magnetism even after the current is switched off.
Observation 3: When he reverses the terminals of the battery, the electromagnet still picks up iron pins.
Observation 4: When he removes the iron nail entirely and uses only the air-core coil, the electromagnet picks up far fewer pins than before.

(a) Based on Observation 1, state the relationship between the number of turns of the coil and the strength of an electromagnet. [1 mark]

(b) Arjun's friend suggests that a steel rod would make a better electromagnet core than soft iron. Based on Observation 2, do you agree? Give one reason to support your answer. [1 mark]

(c) Explain the science behind Observation 3 and Observation 4. [2 marks]

Accepted Answer

(a) The strength of an electromagnet is directly proportional to the number of turns of the coil — as the number of turns increases, the strength of the electromagnet increases. [1 mark]

(b) No, a steel rod does not make a better electromagnet core than soft iron. Although steel becomes magnetised in the presence of current, it retains its magnetism even after the current is switched off (it is a permanent magnet material). Soft iron, on the other hand, is easily magnetised when current flows and loses its magnetism almost completely when the current is switched off — this makes it ideal for an electromagnet, which must be switchable. [1 mark]

(c) Explanation of Observation 3: When the terminals of the battery are reversed, the direction of current through the coil reverses. This reverses the direction of the magnetic field produced by the electromagnet — the North and South poles of the electromagnet are interchanged. However, the electromagnet can still attract iron pins because iron is a ferromagnetic material and is attracted to both poles of any magnet, regardless of polarity. Therefore, reversing the current reverses the poles but does not stop the electromagnet from picking up iron pins. [1 mark]

Explanation of Observation 4: The soft iron core plays a critical role in strengthening the magnetic field of the electromagnet. When current flows through the coil, it magnetises the soft iron core. The magnetic field of the core adds to the magnetic field produced by the current-carrying coil, resulting in a much stronger combined field. Without the iron core (air-core coil), only the relatively weak magnetic field of the current-carrying coil is present, and no additional magnetisation occurs — hence the electromagnet picks up far fewer pins. [1 mark]

Question 2

Read the following passage and answer the questions that follow:

Riya is doing a science project on electromagnets. She winds 50 turns of insulated copper wire around a soft iron nail and connects the coil to a battery through a switch. When she closes the switch, the nail picks up iron filings. She notices that the nail picks up more iron filings when she increases the number of turns to 100. Her teacher tells her that the direction of the magnetic field at each end of the coil can be found using the right-hand thumb rule. The teacher also explains that soft iron is used as the core (instead of steel) for a reason.

(a) What type of device has Riya made? Name the principle on which it works.
(b) Riya observes that increasing the number of turns increases the strength of her device. State ONE other way she can increase its strength without changing the number of turns.
(c) Why is soft iron used as the core of Riya's device instead of steel? Give TWO reasons.

Accepted Answer

(a) Riya has made an Electromagnet. [½ mark]
It works on the principle that a current-carrying conductor (solenoid/coil) produces a magnetic field around it, and this field can magnetise a magnetic material placed inside it. [½ mark]

(b) She can increase the strength of the electromagnet by increasing the current flowing through the coil. [1 mark]
(Any ONE of the following is also accepted: use a stronger battery; use a thicker wire to reduce resistance and allow more current.)

(c) Soft iron is used as the core instead of steel for the following two reasons: [1 + 1 = 2 marks]
(i) Soft iron is easily and quickly magnetised when current flows through the coil, making the electromagnet strong during use.
(ii) Soft iron loses its magnetism immediately when the current is switched off (it is easily demagnetised). Steel, on the other hand, retains its magnetism even after the current is turned off, making it unsuitable — the device would no longer act as a controllable electromagnet.

Question 3

A science teacher demonstrates an experiment to her class. She takes a long cylindrical cardboard tube and winds 200 turns of insulated copper wire tightly over it. She connects this coil to a battery and a switch. When the switch is closed, a compass needle placed near one end of the coil deflects strongly. She then makes the following changes one at a time, each time returning to the original setup:

(i) She replaces the battery with one of higher voltage.
(ii) She inserts a soft iron rod into the cardboard tube.
(iii) She replaces the soft iron rod with a steel rod.
(iv) She doubles the length of the tube but keeps the total number of turns the same (so turns are more spread out).

Based on this scenario, answer the following:

(a) Name the device the teacher has made and state the principle on which it works. [1]
(b) Which ONE of the changes (i) to (iv) would she regret and why? [1]
(c) A student suggests that the device can be used as a lifting magnet to pick up iron filings, but argues it would be more useful than a permanent bar magnet for this purpose. Give TWO reasons to justify the student's argument. [2]

OR

(c) The teacher notices that when she reverses the terminals of the battery connected to the coil, the end of the coil that was behaving as the North pole now behaves as the South pole. Explain why this happens and state the rule used to determine the polarity of each end of the coil. [2]

Accepted Answer

(a) The teacher has made an electromagnet.

An electromagnet is formed by wrapping a current-carrying insulated copper wire in the form of a coil around a magnetic material like a soft iron core. It works on the principle that a current-carrying solenoid produces a magnetic field similar to that of a bar magnet, with a definite North pole and South pole at its two ends.

[½ mark for naming the device; ½ mark for stating the principle]

(b) She would regret change (iii) — replacing the soft iron rod with a steel rod.

Soft iron is used as the core of an electromagnet because it is easily magnetised when current flows and easily demagnetised when current is switched off. Steel, once magnetised, retains its magnetism even after the current is turned off (it is a permanent magnet material). This means the device would not function as a controllable electromagnet — it could not be switched off effectively.

[½ mark for identifying change (iii); ½ mark for the correct reason]

(c) Two reasons why an electromagnet is more useful than a permanent bar magnet as a lifting magnet:

(i) The strength of an electromagnet can be controlled — by increasing or decreasing the current, the magnetic force can be increased or decreased as required. A permanent magnet has a fixed strength that cannot be adjusted.

(ii) An electromagnet can be switched off by breaking the circuit, allowing the picked-up iron filings to be released at a desired location. A permanent magnet always retains its magnetism and cannot be switched off, making it difficult to release the material.

[1 mark for each distinct reason; both must be genuine contrasts with a permanent magnet]

OR

(c) When the terminals of the battery are reversed, the direction of current flowing through every turn of the coil is reversed. According to the Right-Hand Thumb Rule for a solenoid — if the solenoid is held in the right hand with the fingers curling in the direction of current flow in the coils, the thumb points toward the North pole — reversing the current causes the fingers to curl in the opposite direction, so the thumb now points toward the opposite end. The end that previously produced field lines emerging outward (North pole) now has field lines entering inward, making it the South pole, and vice versa.

[1 mark for explaining that reversing current reverses the direction of the magnetic field at each end; 1 mark for correctly stating the Right-Hand Thumb Rule for a solenoid and applying it to determine polarity]

Question 4

Read the following passage and answer the questions that follow.

A science teacher sets up a demonstration for her class. She takes a rectangular coil of insulated copper wire and places it between the poles of a strong horseshoe magnet, with the North pole on the left and the South pole on the right. The coil is connected to a battery through a split-ring commutator and two carbon brushes. When the key is closed, the coil begins to rotate continuously in one direction. The teacher then tells the students: 'This device converts one form of energy into another and is the working principle behind every electric fan, mixer, and pump you use at home.'

She then reverses the connections of the battery and asks the students to predict what will happen to the direction of rotation.

(a) Name the device described in the passage and state the energy conversion it performs.
(b) Name the rule used to determine the direction of force acting on the current-carrying coil sides placed in the magnetic field. State what each finger of that hand represents.
(c) What is the specific role of the split-ring commutator in this device? What would happen to the motion of the coil if the commutator were replaced by two continuous slip rings?
OR
(c) When the teacher reverses the battery connections, in which direction will the coil rotate compared to its original direction? Give a reason for your answer using the relevant rule.

Accepted Answer

(a) The device described is an Electric Motor. It converts electrical energy into mechanical (kinetic) energy. [1 mark]

(b) The rule used is Fleming's Left Hand Rule. Stretch the thumb, forefinger, and middle finger of the left hand such that they are mutually perpendicular. The forefinger points in the direction of the magnetic field, the middle finger points in the direction of current, and the thumb points in the direction of the force (motion) acting on the conductor. [1 mark]

(c) The split-ring commutator reverses the direction of current flowing through the coil after every half rotation. This ensures that the force acting on each side of the coil always remains in the same direction, thereby maintaining continuous rotation in one direction. [1 mark]

If the commutator were replaced by two continuous slip rings (as in an AC generator), the current in the coil would not be reversed at the half-rotation point. The force on each coil side would reverse every half rotation, causing the coil to oscillate back and forth rather than rotate continuously in one direction. [1 mark]

OR

(c) When the battery connections are reversed, the direction of current through the coil reverses. By Fleming's Left Hand Rule, if the direction of current is reversed while the magnetic field remains unchanged, the direction of the force on each side of the coil also reverses. Therefore, the coil will rotate in the opposite direction compared to its original direction of rotation. [2 marks]

Question 5

A school science club is setting up a simple electromagnetic crane to lift small iron nails. They wind 200 turns of insulated copper wire around a soft iron rod and connect it to a battery. When the switch is closed, the crane successfully lifts the nails. When the switch is opened, the nails fall back down immediately.

Based on this scenario, answer the following questions:
(a) What type of device have the students made?
(b) Why do the nails fall back down immediately when the switch is opened?
(c) The students want to make the crane lift heavier iron objects. Suggest TWO ways they can increase the strength of this device.

Accepted Answer

(a) The students have made an electromagnet.
[An electromagnet is formed by wrapping a current-carrying insulated copper wire in the form of a coil around a magnetic material like a soft iron core.]

(b) Soft iron is used as the core material. Soft iron is easily magnetised when current flows through the coil, but it loses its magnetism almost immediately when the current is switched off. Since the soft iron core does not retain magnetism, the magnetic field disappears the moment the switch is opened, and the nails are no longer attracted — so they fall back down immediately.

(c) The students can increase the strength of the electromagnet by:
(i) Increasing the number of turns of wire wound around the soft iron core — more turns produce a stronger magnetic field for the same current.
(ii) Increasing the current flowing through the coil — a larger current produces a stronger magnetic field around the core.

[Award 1 mark for (a); 1 mark for (b); 1 mark each for any two correct methods in (c).]

Question 6

Read the following passage and answer the questions that follow.

A school science club is setting up a demonstration to show how magnetic fields are produced by electric current. They arrange three setups:

• Setup 1: A long straight copper wire connected to a battery, with a compass needle placed 2 cm below the wire.
• Setup 2: The same wire wound into a coil of 50 turns around a soft iron rod, connected to the same battery.
• Setup 3: Setup 2 repeated, but the current is doubled by changing the battery.

The students observe that the compass needle deflects in Setup 1, the soft iron rod becomes a strong magnet in Setup 2, and the magnet in Setup 3 is even stronger than in Setup 2.

(a) Name the scientist who first demonstrated that a current-carrying conductor produces a magnetic field, and state the shape of the magnetic field lines around the straight wire in Setup 1.

(b) What is the device formed in Setup 2 called? State ONE reason why a soft iron rod is used as the core instead of a steel rod.

(c) The students in Setup 3 doubled the current. State TWO ways (other than changing the current) by which the strength of the device in Setup 2 could be further increased. Give a reason for any ONE of these ways.

Accepted Answer

(a) Hans Christian Oersted first demonstrated in 1820 that a current-carrying conductor produces a magnetic field around it. The magnetic field lines around the straight wire in Setup 1 are concentric circles, with the wire at the centre. [1 mark]

(b) The device formed in Setup 2 is called an Electromagnet. [½ mark]
Soft iron is used as the core because soft iron is easily magnetised when current flows through the coil and easily demagnetised when the current is switched off. Steel, once magnetised, remains permanently magnetised and cannot be easily demagnetised, making it unsuitable for an electromagnet. [½ mark]

(c) Two ways to increase the strength of the electromagnet (other than increasing current):

(i) Increasing the number of turns of wire wound around the core.
(ii) Using a core material with higher magnetic permeability (a better soft magnetic material). [1 mark — ½ mark each]

Reason for (i): When the number of turns is increased, each additional turn contributes its own magnetic field in the same direction along the axis of the coil. These individual fields add up, resulting in a stronger net magnetic field at the core. [1 mark]

Question 7

A science teacher demonstrates an experiment in the classroom. She takes a long straight copper wire and connects it to a battery through a switch. She places a magnetic compass needle directly below the wire, parallel to it. When she closes the switch, the compass needle deflects. When she opens the switch, the needle returns to its original north-south direction. She then reverses the connections of the battery and closes the switch again.

Based on this scenario, answer the following questions:
(a) Name the scientist who first demonstrated that a current-carrying conductor produces a magnetic field.
(b) What shape are the magnetic field lines formed around the straight current-carrying copper wire in this experiment?
(c) (i) In which direction will the compass needle deflect when the battery connections are reversed?
    (ii) What does this observation tell us about the relationship between the direction of current and the direction of the magnetic field produced?

Accepted Answer

(a) Hans Christian Oersted first demonstrated, in 1820, that a current-carrying conductor produces a magnetic field around it.
[1 mark]

(b) The magnetic field lines formed around a long straight current-carrying conductor are concentric circles, with the wire at the centre. The circles lie in a plane perpendicular to the wire. The field lines are more closely packed (stronger field) near the wire and more widely spaced (weaker field) farther away from it.
[1 mark]

(c) (i) When the battery connections are reversed, the direction of current in the wire is reversed. The compass needle will deflect in the opposite direction — that is, it will deflect to the opposite side compared to its deflection in the first case.
[1 mark]

(ii) This observation tells us that the direction of the magnetic field produced by a current-carrying conductor depends on the direction of the current flowing through it. When the current is reversed, the magnetic field around the wire is also reversed in direction. This can be determined using the Right-Hand Thumb Rule: if the thumb of the right hand points in the direction of current, the curled fingers show the direction of the magnetic field lines around the wire.
[1 mark]

Question 8

Read the following passage and answer the questions that follow:

Riya is doing a science project on electromagnets. She winds 50 turns of insulated copper wire around a soft iron nail and connects it to a battery through a switch and a rheostat. When she closes the switch, the electromagnet picks up iron filings. She notices that when she slides the rheostat to increase resistance in the circuit, the electromagnet picks up fewer iron filings. Her teacher then asks her to replace the soft iron nail with a steel rod of the same size. Riya observes that the steel rod becomes a strong magnet when current flows, but it does not lose its magnetism even after the switch is opened.

(a) Name the device in Riya's circuit that is used to change the resistance without changing the voltage source.

(b) Why does the electromagnet pick up fewer iron filings when the resistance in the circuit is increased?

(c) Why does Riya's teacher prefer soft iron over steel as the core material for an electromagnet? State TWO reasons.

Accepted Answer

(a) Rheostat (variable resistance).
[1 mark]

(b) When resistance in the circuit is increased, by Ohm's law (V = IR), the current flowing through the coil decreases. The strength of an electromagnet depends directly on the current flowing through its coils — less current means a weaker magnetic field is produced. A weaker magnetic field exerts a smaller force on the iron filings, so fewer are picked up.
[1 mark]

(c) Soft iron is preferred over steel as the core of an electromagnet for the following two reasons:

(i) Soft iron is easily magnetised and easily demagnetised. When current flows through the coil, soft iron becomes a strong magnet; when the current is switched off, soft iron loses its magnetism almost immediately. This makes the electromagnet controllable — it can be switched on and off as needed.

(ii) Steel retains its magnetism even after the current is switched off (as Riya observed). This means a steel-core electromagnet cannot be turned off — it becomes a permanent magnet. Soft iron does not retain magnetism, making it suitable for use as an electromagnet core.
[1 + 1 = 2 marks]

OR (c)
(i) Soft iron has high magnetic permeability — it gets magnetised much more strongly than steel for the same current, producing a more powerful electromagnet.
(ii) Soft iron is easily demagnetised when the current stops, allowing the electromagnet to release attracted objects immediately. Steel cannot do this as it retains residual magnetism.

Question 9

Name the rule used to find the direction of force experienced by a current-carrying conductor placed in a magnetic field.

Accepted Answer

Fleming's Left Hand Rule is used to find the direction of force experienced by a current-carrying conductor placed in a magnetic field.

Question 10

A straight wire carries a current flowing vertically upward (out of the page). What is the shape and direction of the magnetic field lines produced around this wire at the same horizontal level?

Accepted Answer

Option (C) is correct.

Explanation: According to the Right Hand Thumb Rule, if the right hand holds the wire with the thumb pointing in the direction of current (upward, i.e., out of the page), the curled fingers indicate the direction of the magnetic field lines around the wire. The fingers curl in the anticlockwise direction when viewed from the front (the side facing the observer). The magnetic field lines around a straight current-carrying conductor are always concentric circles centred on the wire — they are never straight lines. Therefore, the field lines are concentric circles directed anticlockwise.

Question 11

Name the rule used to find the direction of force experienced by a current-carrying conductor placed in a magnetic field.

Accepted Answer

Option (B) is correct.
Explanation: Fleming's Left Hand Rule gives the direction of force (motion) on a current-carrying conductor placed in a magnetic field. When the thumb, forefinger, and middle finger of the left hand are stretched mutually perpendicular, the forefinger points in the direction of the magnetic field, the middle finger in the direction of current, and the thumb points in the direction of the force acting on the conductor. This rule applies to the electric motor, which converts electrical energy into mechanical energy.

Question 12

A current-carrying straight conductor is held vertically and the current flows in the upward direction. Using the Right Hand Thumb Rule, what is the direction of the magnetic field lines produced around the conductor when viewed from above (i.e., looking downward along the conductor)?

Accepted Answer

Option (B) is correct.
Explanation: According to the Right Hand Thumb Rule, if the right hand holds the conductor with the thumb pointing in the direction of current (upward), the fingers curl in the direction of the magnetic field lines around the conductor. When the current is directed upward and the conductor is viewed from above, the fingers curl in the anticlockwise direction. The magnetic field lines form concentric circles around the conductor, and their direction is anticlockwise when viewed from the end from which current is coming towards the observer.

Question 13

Which of the following correctly describes the shape of magnetic field lines produced around a straight current-carrying conductor?

Accepted Answer

Option (B) is correct.

Explanation: When electric current flows through a straight conductor, it produces a magnetic field around it, as demonstrated by Oersted's experiment. The magnetic field lines take the shape of concentric circles in a plane perpendicular to the conductor, with the conductor at the centre. The circles are more closely spaced near the conductor (stronger field) and become more widely spaced farther away (weaker field). The direction of these circular field lines is given by the Right Hand Thumb Rule.

Question 14

Name the device used in an electric motor that reverses the direction of current through the coil after every half rotation. What is its function?

Accepted Answer

(a) The device is called a commutator (split ring).

(b) Its function is to reverse the direction of current flowing through the coil after every half rotation, so that the coil continues to rotate in the same direction continuously.

Question 15

Assertion (A): A current-carrying solenoid behaves like a bar magnet.
Reason (R): The magnetic field lines inside a current-carrying solenoid are parallel and uniform, similar to the field inside a bar magnet.

Accepted Answer

Option (A) is correct. The Assertion is true: a current-carrying solenoid does behave like a bar magnet — it has a distinct north pole at one end and a south pole at the other, and its external magnetic field pattern is identical to that of a bar magnet. The Reason is also true: the magnetic field lines inside a solenoid are parallel to the axis and uniformly spaced, indicating a strong, uniform field, exactly as inside a bar magnet. The Reason correctly explains the Assertion because it is precisely this uniform, axial field — produced by the superposition of fields from all the circular turns — that makes the solenoid equivalent to a bar magnet in its magnetic behaviour.

Question 16

A horizontal straight wire carries electric current in the direction from West to East. A compass needle is placed directly below this wire. (i) In which direction will the North pole of the compass needle deflect? (ii) State the rule used to determine this direction.

Accepted Answer

Step 1: Apply the Right-Hand Thumb Rule — point the right thumb in the direction of current (West to East); the curled fingers show the magnetic field around the wire.
Step 2: Directly below the wire the field is directed towards the North, so the North pole of the compass needle aligns with it.
∴ (i) The North pole of the compass needle deflects towards the North. (ii) The Right-Hand Thumb Rule (Maxwell's Corkscrew Rule) gives the direction of the magnetic field around a current-carrying conductor.

Question 17

A horizontal conducting wire carries current from South to North. A compass needle is placed directly below this wire. (a) In which direction will the North pole of the compass needle deflect? (b) State the rule used to determine the direction of the magnetic field around a current-carrying conductor.

Accepted Answer

Step 1: Apply the Right-Hand Thumb Rule — point the thumb of the right hand in the direction of current flow (South to North). The curling fingers below the wire point from East to West.
Step 2: Therefore, the magnetic field at the point below the wire is directed from East to West, so the North pole of the compass needle deflects towards the West.
(b) Right-Hand Thumb Rule: If the thumb of the right hand points in the direction of conventional current through a straight conductor, the curling fingers indicate the direction of the magnetic field lines around that conductor.
∴ (a) North pole deflects towards West; (b) Right-Hand Thumb Rule.

Question 18

A horizontal conducting wire carries current flowing from East to West. A compass needle is placed directly below this wire. (i) In which direction will the North pole of the compass needle deflect? (ii) State the rule used to determine the direction of the magnetic field around a current-carrying conductor.

Accepted Answer

Step 1: Apply the Right-Hand Thumb Rule — if the right hand's thumb points in the direction of conventional current (East to West), the curled fingers show the direction of the magnetic field around the wire.
Step 2: Below the wire, with current flowing from East to West, the magnetic field at that point is directed from North to South (i.e., pointing South). A compass needle aligns with the field, so its North pole deflects towards the South.
∴ (i) The North pole of the compass needle deflects towards the South. (ii) The Right-Hand Thumb Rule — if the thumb of the right hand points in the direction of current, the curled fingers give the direction of the magnetic field around the conductor.

Question 19

A horizontal straight wire carries current flowing from South to North. A magnetic compass is placed directly below the wire. (i) In which direction will the north pole of the compass needle deflect? (ii) What will happen to the deflection if the direction of current in the wire is reversed?

Accepted Answer

(i) The current flows from South to North. By the Right-Hand Thumb Rule, holding the wire in the right hand with the thumb pointing along the current (South to North), the fingers curl so that the magnetic field directly below the wire points towards the West. The compass needle aligns with this field, so its North pole deflects towards the West.

(ii) When the current is reversed (North to South), the magnetic field below the wire reverses, now pointing towards the East. Therefore the North pole of the compass needle deflects towards the East — opposite to part (i).

Question 20

A straight conducting wire is placed horizontally on a table and connected to a battery. A compass needle is placed directly below the wire. When current flows from South to North through the wire, state the direction in which the compass needle will deflect. Give a reason for your answer using the right-hand thumb rule.

Accepted Answer

**Direction of deflection:** The North pole of the compass needle will deflect towards the **West**.

**Reason:** By the right-hand thumb rule, if the right hand holds the wire with the thumb pointing in the direction of current (South to North, i.e., towards North), the fingers curl around the wire. Below the wire, the curling fingers point from East to West. Therefore, the magnetic field at the position of the compass needle (below the wire) is directed towards the West. Since a compass needle aligns itself along the magnetic field at that point, its North pole deflects towards the West.

Magnetic Effects of Electric Current: Class 10 Science Practice Questions

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