Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of Oersted’s experiment, magnetic field lines, Right-Hand Thumb Rule, solenoid electromagnetism, Lorentz magnetic force, Fleming’s Left-Hand Rule, domestic electric circuit safety, short-circuiting, and earthing. Chapter 12 of Class 10 Physics, “Magnetic Effects of Electric Current”, explores the fundamental connection between moving electrical charges and magnetic fields. It investigates the magnetic field geometry around straight wires, circular loops, and solenoids; analyzes the mechanical force experienced by current-carrying conductors in external magnetic fields (F = B I l \sin\theta); and details the vital safety protocols of household electrical systems (fuse operation, earthing of metallic appliances, live/neutral potential differences). To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.
Every question presented in the official NCERT textbook—ranging from all five in-text question sets (Pages 224, 228, 231, 233, and 237) to the complete chapter-end exercises (Questions 1 to 10 on Pages 240–241)—has been solved with exhaustive detail. Short-answer conceptual responses follow the official 30–40 word limit, while 3-mark and 5-mark directional rule problems use structured step-by-step box layouts. Key scoring terms, official CBSE board year tags, and comparative magnetic rule charts have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.
Master Formula & Concept Summary Tables
1. Master Hand Rules for Magnetic Effects
| Physical Rule Name | Hand Used | Finger Alignments | Physical Application / Direction Determined |
|---|---|---|---|
| Right-Hand Thumb Rule (Maxwell’s Corkscrew Rule) | Right Hand | • Thumb: Direction of electric current (I). • Curled Fingers: Encircle the wire in the direction of Magnetic Field Lines (B). | Determines direction of magnetic field lines around a straight current-carrying wire or circular loop. |
| Fleming’s Left-Hand Rule (Motor Rule) | Left Hand | • Forefinger: Magnetic Field (B, North to South). • Middle Finger: Electric Current (I, positive flow). • Thumb: Direction of Mechanical Force / Motion (F). | Determines the direction of force/deflection acting on a current-carrying conductor in an external magnetic field. |
| Clock Face Rule | Observation | • Clockwise Current: Represents a South Magnetic Pole (S). • Anticlockwise Current: Represents a North Magnetic Pole (N). | Identifies the magnetic polarity of the faces of a circular current-carrying coil or solenoid. |
2. Comparison: Bar Magnet vs. Current-Carrying Solenoid
| Feature / Parameter | Permanent Bar Magnet | Current-Carrying Solenoid (Electromagnet) |
|---|---|---|
| Nature of Magnetism | Permanent magnetism; cannot be easily demagnetized. | Temporary magnetism; exists only as long as electric current flows. |
| Magnetic Field Strength | Fixed and constant; cannot be increased or decreased. | Adjustable; strength increases by increasing current (I) or number of turns (n). |
| Magnetic Polarity | Fixed North and South poles; cannot be reversed. | Reversible polarity; reversing current direction reverses the N and S poles. |
| Internal Field Nature | Non-uniform inside the magnetic material. | Strong and completely uniform parallel field lines inside the core. |
3. Domestic Wiring Safety Features & Color Codes
| Wire Type | Insulation Color Standard (Old / New) | Electric Potential (V) | Primary Safety Function in Domestic Circuits |
|---|---|---|---|
| Live Wire (L) (Phase) | Red / Brown | 220 V (High Potential) | Carries high-voltage current from power grid into home appliances. |
| Neutral Wire (N) | Black / Blue | 0 V (Zero Potential) | Completes the electrical circuit by providing the return current path. |
| Earth Wire (E) (Ground) | Green / Yellow | 0 V (Connected to Earth plate) | Safety grounding path; safely channels leakage currents from metallic bodies into the ground. |
NCERT In-Text Questions: Set 1 (Page No. 224)
Question 1 Why does a compass needle get deflected when brought near a bar magnet? [CBSE 2024, 2020]
Answer: A compass needle is a small, freely pivoted permanent bar magnet with north and south magnetic poles:
- When brought near a bar magnet, the compass needle enters the magnetic field exerted by the bar magnet.
- The bar magnet’s magnetic field exerts mechanical forces of attraction and repulsion on the poles of the compass needle (like poles repel, unlike poles attract).
- These magnetic forces create a torque that rotates the needle until it aligns tangent to the magnetic field line at that point, producing visible deflection.
NCERT In-Text Questions: Set 2 (Page No. 228)
Question 1 Draw magnetic field lines around a bar magnet. [CBSE 2024, 2022]
Answer:
================================================================================ FIELD LINES AROUND A BAR MAGNET: -------------------------------------------------------------------------------- 1. Outside the Magnet: Continuous closed curved lines emerge from the NORTH (N) pole and merge into the SOUTH (S) pole. 2. Inside the Magnet: Field lines continue from the SOUTH (S) pole back to the NORTH (N) pole. 3. Density: Field lines are crowded closely near the poles where the magnetic field is strongest, and spread out in the middle where it is weaker. ================================================================================
Question 2 List the properties of magnetic lines of force. [CBSE 2024, 2023, 2020]
Answer: Magnetic field lines possess four fundamental physical properties:
- Continuous Closed Loops: They emerge from the North pole and enter the South pole outside the magnet, while continuing from the South pole to the North pole inside the magnet.
- Degree of Closeness Represents Strength: The relative strength of the magnetic field is shown by the crowding of field lines; field is strongest where lines are closest (at the poles).
- Direction of Field: The tangent drawn at any point on a field line gives the direction of the magnetic field vector at that point.
- Never Intersect: Two magnetic field lines never cross or intersect each other.
Question 3 Why don’t two magnetic lines of force intersect each other? [CBSE 2024, 2022]
Answer: Two magnetic field lines can never intersect each other because:
- The direction of the magnetic field at any point is given by the direction in which the north pole of a compass needle points.
- If two field lines were to intersect at a point, a compass needle placed at that point of intersection would have to point in two different directions simultaneously, which is physically impossible. Hence, field lines never cross.
NCERT In-Text Questions: Set 3 (Page No. 231)
Question 1 Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop. [CBSE 2024, 2020]
Answer:
================================================================================
RIGHT-HAND THUMB RULE DEDUCTION:
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1. Inside the Loop:
• Grasp any portion of the wire with the right hand such that the thumb points
along the clockwise current.
• The curled fingers curl and point DOWNWARD, entering perpendicularly INTO
the plane of the table.
• Therefore, the magnetic field inside the loop is directed PERPENDICULARLY
INTO THE TABLE (Downwards).
2. Outside the Loop:
• The curled fingers emerge upward from below the table.
• Therefore, the magnetic field outside the loop is directed PERPENDICULARLY
OUT OF THE TABLE (Upwards).
================================================================================
Question 2 The magnetic field in a given region is uniform. Draw a diagram to represent it. [CBSE 2023]
Answer: A uniform magnetic field is represented by a set of equidistant, parallel straight lines pointing in the same direction:
================================================================================
UNIFORM MAGNETIC FIELD REPRESENTATION:
--------------------------------------------------------------------------------
─────────────────────────────────────────►
─────────────────────────────────────────►
─────────────────────────────────────────►
─────────────────────────────────────────►
(Parallel, equidistant straight lines indicate constant magnitude and direction).
================================================================================
Question 3 Choose the correct option: The magnetic field inside a long straight solenoid-carrying current: [CBSE 2024, 2020] (a) is zero (b) decreases as we move towards its end (c) increases as we move towards its end (d) is the same at all points
Answer: (d) is the same at all points Explanation: The magnetic field lines inside a current-carrying solenoid are parallel, equidistant straight lines, indicating that the magnetic field is completely uniform and has the same magnitude and direction at all interior points.
NCERT In-Text Questions: Set 4 (Page No. 233)
Question 1 Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.) [CBSE 2024, 2022] (a) mass (b) speed (c) velocity (d) momentum
Answer: (c) velocity and (d) momentum Explanation: When a proton enters a magnetic field, the magnetic Lorentz force acts perpendicular to its direction of motion. This force changes the direction of motion continuously without altering its speed (kinetic energy). Because direction changes, both velocity (vector) and momentum (p = mv) change, while mass and speed remain constant.
Question 2 In Activity 12.7 (displacement of a current-carrying rod AB in a magnetic field), how do we think the displacement of rod AB will be affected if: [CBSE 2024, 2020] (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
Answer: The magnetic force acting on a current-carrying conductor is given by F = B I l \sin\theta:
- (i) If current (I) is increased: The magnetic force increases (F \propto I). Therefore, the displacement of rod AB increases.
- (ii) If a stronger horse-shoe magnet is used: The magnetic field strength (B) increases (F \propto B). Therefore, the displacement of rod AB increases.
- (iii) If length of rod AB (l) is increased: The length under the magnetic field increases (F \propto l). Therefore, the displacement of rod AB increases.
Question 3 A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is: [CBSE 2024, 2023] (a) towards south (b) towards east (c) downward (d) upward
Answer: (d) upward
================================================================================
DEDUCTION USING FLEMING'S LEFT-HAND RULE:
--------------------------------------------------------------------------------
1. Current Direction (Middle Finger):
• The alpha-particle is positively charged and moving towards WEST.
• Conventional electric current (I) is in the direction of positive charge motion
===> Point Middle Finger towards WEST.
2. Force / Deflection Direction (Thumb):
• The particle is deflected towards NORTH ===> Point Thumb towards NORTH.
3. Magnetic Field Direction (Forefinger):
• With the middle finger pointing West and thumb pointing North, the Forefinger
naturally points strictly UPWARD (out of the page).
FINAL ANSWER: The direction of the magnetic field is UPWARD.
================================================================================
NCERT In-Text Questions: Set 5 (Page No. 237)
Question 1 Name two safety measures commonly used in electric circuits and appliances. [CBSE 2024, 2022]
Answer:
- Electric Fuse (or Miniature Circuit Breaker – MCB): A safety device connected in series with the live wire that melts and interrupts excess current flow during overloading or short-circuiting.
- Earthing (Earth Wire): Grounding the metallic casing of high-power appliances (electric irons, refrigerators, geysers) via a low-resistance green earth wire to protect users from fatal electric shocks.
Question 2 An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain. [CBSE 2024, 2020]
Answer:
================================================================================
NUMERICAL DEDUCTION (CIRCUIT OVERLOADING):
--------------------------------------------------------------------------------
GIVEN DATA:
• Power of Electric Oven (P) = 2 kW = 2000 W
• Domestic Supply Voltage (V) = 220 V
• Current Rating of Circuit = 5 A
STEP 1: Calculating Current Drawn by the Oven (I)
Formula: P = V · I ===> I = P / V
I = 2000 / 220 = 200 / 22 = 9.09 A
STEP 2: Evaluation & Result
• The electric oven draws a current of 9.09 A, which is MUCH GREATER than the
safe current rating of the circuit (5 A).
• Result: The circuit becomes heavily OVERLOADED. If a standard 5 A fuse is used,
the excessive current will heat up the fuse wire, causing it to MELT and BLOW,
safely breaking the circuit. If no fuse is present, the wires will overheat and
catch fire.
================================================================================
Question 3 What precaution should be taken to avoid the overloading of domestic electric circuits? [CBSE 2023, 2020]
Answer: To prevent dangerous overloading in domestic circuits, three key precautions must be followed:
- Avoid Connecting Multiple Appliances to a Single Socket: Do not operate multiple high-power electrical appliances simultaneously from a single multi-plug extension board.
- Use Wires with Proper Current Ratings: Use appropriate separate circuits: a 5 A rating circuit for low-power loads (fans, bulbs, TVs) and a 15 A rating circuit for heavy heating loads (heaters, geysers, air conditioners).
- Install Safety Fuses / MCBs: Ensure proper electric fuses or MCBs with matching current ratings are connected in series with the live wire of every circuit.
NCERT Chapter-End Exercises (Page No. 240-241)
Question 1 Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire (b) The field consists of straight lines parallel to the wire (c) The field consists of radial lines originating from the wire (d) The field consists of concentric circles centred on the wire
Answer: (d) The field consists of concentric circles centred on the wire Explanation: According to the Right-Hand Thumb Rule and Oersted’s experiments, magnetic field lines surrounding a straight current-carrying wire form a series of concentric circular loops centered on the wire in planes perpendicular to the conductor.
Question 2 At the time of short circuit, the current in the circuit: (a) reduces substantially (b) does not change (c) increases heavily (d) vary continuously
Answer: (c) increases heavily Explanation: During a short circuit, the live wire comes into direct physical contact with the neutral wire. The electrical resistance of the circuit drops close to zero (R \approx 0). According to Ohm’s law (I = V/R), the electric current surges to an extremely large value almost instantaneously.
Question 3 State whether the following statements are true or false: [CBSE 2024, 2022] (a) An electric motor converts mechanical energy into electrical energy. (b) An electric generator works on the principle of electromagnetic induction. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines. (d) A wire with a green insulation is usually the live wire of an electric supply.
Answer:
- (a) False (An electric motor converts electrical energy into mechanical energy).
- (b) True (An electric generator operates on Faraday’s principle of electromagnetic induction).
- (c) True (Near the center of a circular current loop, magnetic field lines become uniform, parallel straight lines).
- (d) False (Green insulation is strictly used for the Earth wire; the Live wire is Red or Brown).
Question 4 List two methods of producing magnetic fields. [CBSE 2023]
Answer: Two common methods of producing magnetic fields are:
- Using Permanent Magnets: Utilizing natural or artificial permanent bar magnets, horseshoe magnets, or magnetic needles.
- Using Electric Current: Passing direct electric current through a conductor, such as a straight wire, circular loop, or a long helical Solenoid (creating an Electromagnet).
Question 5 How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain. [CBSE 2024, 2020]
Answer: A solenoid is a long helical coil of many circular turns of insulated copper wire:
- Magnetic Behavior: When current flows through the solenoid, magnetic fields produced by each individual turn add vectorially, creating a strong, uniform magnetic field inside the core. The overall external magnetic field pattern is identical to that of a permanent bar magnet, with one end acting as a North pole and the other as a South pole.
- Determining Polarity using a Bar Magnet:
- Suspend the current-carrying solenoid freely or bring a known North pole of a permanent bar magnet near one end of the solenoid.
- If the end of the solenoid is repelled by the bar magnet’s North pole, that end of the solenoid is a North pole (like poles repel).
- If the end is attracted, that end is a South pole (unlike poles attract).
Question 6 When is the force experienced by a current–carrying conductor placed in a magnetic field largest? [CBSE 2024, 2023]
Answer: The mechanical force acting on a current-carrying conductor is given by F = B I l \sin\theta.
The force experienced is largest (maximum) when the conductor is placed perpendicular to the direction of the magnetic field (\theta = 90^\circ, so \sin 90^\circ = 1, giving F_{max} = B I l).
Question 7 Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field? [CBSE 2024, 2020]
Answer:
================================================================================
DEDUCTION USING FLEMING'S LEFT-HAND RULE:
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1. Current Direction (Middle Finger):
• The electron beam (negatively charged) moves from BACK WALL to FRONT WALL.
• Conventional electric current (I) flows in the direction OPPOSITE to electron
motion ===> Current flows from FRONT WALL to BACK WALL (towards you).
• Point Middle Finger towards YOURSELF (Backwards).
2. Force / Deflection Direction (Thumb):
• The beam is deflected to your RIGHT side ===> Point Thumb towards your RIGHT.
3. Magnetic Field Direction (Forefinger):
• Aligning the left hand with Middle Finger pointing backwards and Thumb
pointing Right causes the Forefinger to point strictly VERTICALLY DOWNWARDS.
FINAL ANSWER:
The magnetic field is directed vertically DOWNWARDS (towards the floor).
================================================================================
Question 8 State the rule to determine the direction of a: [CBSE 2024, 2022] (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) induced current produced in a circuit through its motion in a magnetic field.
Answer:
- (i) Right-Hand Thumb Rule: Imagine holding the straight current-carrying wire in your right hand such that your stretched thumb points along the direction of current; then your fingers curled around the conductor point in the direction of the magnetic field lines.
- (ii) Fleming’s Left-Hand Rule: Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other. If the forefinger points along the magnetic field and the middle finger along the current, then the thumb points in the direction of mechanical force/motion.
- (iii) Fleming’s Right-Hand Rule: Stretch the thumb, forefinger, and middle finger of your right hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the thumb in the direction of conductor motion, then the middle finger points in the direction of induced electric current.
Question 9 When does an electric short circuit occur? [CBSE 2024, 2020]
Answer: An electric short circuit occurs when the Live wire (220\text{ V}) and Neutral wire (0\text{ V}) come into direct physical contact with each other. This happens when:
- The plastic insulation of electrical cables is damaged, torn, or aged.
- A fault occurs inside a connected electrical appliance.
- Direct contact drops circuit resistance to near zero, resulting in a sudden, massive surge of electric current that produces intense sparking and fire hazards.
Question 10 What is the function of an earth wire? Why is it necessary to earth metallic appliances? [CBSE 2024, 2023, 2020]
Answer:
- Function of Earth Wire: The earth wire (green/yellow insulation) provides a very low-resistance conducting safety path connecting the metallic body of household appliances to a metal plate buried deep inside the ground.
- Why Earthing is Necessary:
- If the live wire insulation inside an appliance (like an electric iron, toaster, refrigerator) wears out and touches the metallic body, the entire outer metal chassis becomes charged at 220 V.
- If an ungrounded user touches the metallic body, current passes through the human body to the ground, delivering a fatal shock.
- With proper earthing, the leakage current flows harmlessly through the low-resistance earth wire into the ground, triggering the fuse to blow and protecting the user from electrical shock.
Frequently Asked Questions (FAQs) – Class 10 Physics Chapter 12
Question 1: What was Hans Christian Oersted’s landmark discovery? [CBSE 2024] Answer: In 1820, Oersted discovered that an electric current flowing through a metallic wire produces a magnetic field around it, establishing the foundational link between electricity and magnetism.
Question 2: What is an Electromagnet? How can its strength be increased? [CBSE 2024, 2022] Answer: An electromagnet is a temporary magnet created by placing a soft iron core inside a current-carrying solenoid. Its strength increases by: (i) increasing electric current (I), (ii) increasing the number of turns (n), and (iii) using a high-permeability soft iron core.
Question 3: Why is soft iron used as the core of an electromagnet instead of steel? [CBSE 2023] Answer: Soft iron has high magnetic permeability and low retentivity; it magnetizes strongly when current is switched on and demagnetizes instantly when current is switched off. Steel retains permanent magnetism and cannot be easily turned off.
Question 4: What is the magnitude of force on a current-carrying conductor when placed parallel to a magnetic field? [CBSE 2024] Answer: When a conductor is parallel to the magnetic field (\theta = 0^\circ or 180^\circ), \sin\theta = 0, so the magnetic force experienced is Zero (F = 0).
Question 5: What is the difference between Direct Current (DC) and Alternating Current (AC)? [CBSE 2024, 2020] Answer:
- Direct Current (DC): Electric current that flows in a single constant direction with time (supplied by chemical batteries and solar cells).
- Alternating Current (AC): Electric current that periodically reverses its direction at regular time intervals (domestic supply in India: 220\text{ V}, frequency of 50\text{ Hz}, reversing direction every 1/100\text{ second}).
Question 6: What is the frequency of AC supply in India? [CBSE 2023] Answer: The frequency of domestic AC electricity in India is 50 Hz (50 cycles per second), meaning the current changes its direction 100 times every second.
Question 7: What is an Electric Fuse made of, and why? [CBSE 2024] Answer: A fuse wire is made of an alloy of Tin and Lead (Pb-Sn) because it has a low melting point and high resistance, allowing it to melt and break the circuit when current exceeds safe limits.
Question 8: Why is a fuse always connected in the Live wire and never in the Neutral wire? [CBSE 2024, 2022] Answer: If a fuse is placed in the neutral wire and blows during a fault, the appliance remains connected to the live wire at 220\text{ V}. Touching the appliance would still deliver a fatal shock. Placing the fuse in the live wire completely disconnects high voltage from the appliance when it blows.
Question 9: What is the Clock Face Rule used for? [CBSE 2023] Answer: The Clock Face Rule determines the magnetic polarity of a current-carrying circular coil: if current flows clockwise when viewed from a face, that face acts as a South Pole (S); if anticlockwise, it acts as a North Pole (N).
Question 10: What is Electromagnetic Induction? Who discovered it? [CBSE 2024] Answer: Electromagnetic induction is the phenomenon of generating an induced electric current in a closed circuit by changing the magnetic flux linked with the circuit, discovered by Michael Faraday in 1831.
Question 11: Does a stationary charge experience a force in a magnetic field? [CBSE 2023] Answer: No. Magnetic Lorentz force is F = q v B \sin\theta. For a stationary charge, velocity v = 0, so the magnetic force is strictly Zero (F = 0).
Question 12: What is the nature of magnetic field lines inside a current-carrying solenoid? [CBSE 2024, 2020] Answer: Inside a solenoid, the field lines are parallel, equidistant straight lines running from the south pole to the north pole, indicating that the magnetic field is completely uniform throughout the interior.
Question 13: What happens when an electric current is passed through a freely suspended current-carrying conductor? [CBSE 2022] Answer: The conductor aligns itself in the geographic North-South direction, behaving exactly like a freely suspended magnetic needle.
Question 14: What is the role of split-ring commutator in an electric motor? [CBSE 2024] Answer: The split-ring commutator reverses the direction of current flowing through the armature coil every half-rotation, ensuring continuous unidirectional rotation of the motor shaft.
Question 15: What is the potential difference between the live wire and neutral wire in India? [CBSE 2023] Answer: The potential difference is 220 Volts (V_{Live} = 220\text{ V}, V_{Neutral} = 0\text{ V}).
Mastering the NCERT Solutions for Class 10 Science Chapter 12, “Magnetic Effects of Electric Current”, equips students with the directional hand rules, circuit safety concepts, and solenoid principles required for the CBSE Board Examination. Review the Fleming’s Left-Hand Rule deductions, the domestic safety wiring protocols, and the 15 board-level FAQs above to secure full marks in your physics evaluations.